Patent Application
20120100628
Certain organic compounds can exhibit nonlinear optical properties, e.g., nonlinear absorption and/or nonlinear refraction, when excited with a laser. However, the absorption cross-section or triplet excited-state lifetime of conventional organic compounds can be limited, which makes it difficult to realize applications such as organic light-emitting diodes, and optical-switching or optical-sensing devices with conventional organic compounds.
In one aspect, the invention provides a ligand of formula (I):
wherein:
R10 is H or —ORa;
R2 and R3 are each independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —(O—CH2—CH2)n—OCH3;
R4 is H, halo, aryl, heterocyclyl, arylalkyl, heterocyclylalkyl, arylalkynyl, arylalkenyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf;
X is C or N;
each n is independently an integer from 1-12;
each Ra, Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl or C2-C24 alkynyl, aryl, heterocyclyl, arylalkyl, and heterocyclylalkyl; and
Y is a bond, —CH═CH—, —CH═CH-Ph-, or —C≡C—;
wherein when the ligand of formula (I) bears a charge, it further comprises one or more counterions.
In another aspect, the invention provides a ligand of formula (II):
wherein:
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
R2 and R3 are each independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —(O—CH2—CH2)m—OCH3;
R4 is H, halo, aryl, heterocyclyl, arylalkynyl, arylalkyl, arylalkenyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, or —CH═N—NH—Rf;
each R15 is independently selected from H, halo, aryl, heterocyclyl, arylalkynyl, C1-C24 alkyl, C2-C24 alkenyl, or C2-C24 alkynyl;
each m is independently an integer from 1-12; and
each Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, heterocyclylalkyl and aryl,
wherein when the ligand of formula (II) bears a charge, it further comprises one or more counterions.
In yet another aspect, the invention provides a ligand of formula (III):
wherein:
each R1 is independently a group of the following formula:
each n equals to 0 or 1;
each R2 and R3 is independently C1-C24 alkyl, C2-C24 alkenyl or C2-C24 alkynyl, or —(O—CH2—CH2)m—OCH3;
each R4 is independently H, halo, aryl, arylalkynyl, heterocyclyl, arylalkyl, arylalkenyl, heterocyclylalkyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf; and
each Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, heterocyclylalkyl and aryl;
each m is independently an integer from 1-12; and
wherein the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings; and
wherein when the ligand of formula (III) bears a charge, it further comprises one or more counterions.
In a further aspect, the invention provides a metal complex of formula (IV):
wherein:
R10 is H or —ORa;
R2 and R3 are each independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —(O—CH2—CH2)m—OCH3;
R4 is H, halo, aryl, heterocyclyl, arylalkyl, heterocyclylalkyl, arylalkynyl, arylalkenyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf;
A is selected from halo,
X=C or N;
each m is independently an integer from 1-12;
each Ra, Rb, Rc, Rd, Re, Rf, and Rg is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl or C2-C24 alkynyl, aryl, heterocyclyl, arylalkyl, arylalkynyl and heterocyclylalkyl; and
Y is a bond, —CH═CH—, —CH═CH-Ph-, or —C≡C—;
M is a metal ion; and
wherein when the complex of formula (IV) bears a charge, it further comprises one or more counterions.
In one aspect, the invention provides a metal complex of formula (V):
wherein:
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
R2 and R3 are each independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —(O—CH2—CH2)m—OCH3;
R4 is H, halo, aryl, heterocyclyl, arylalkynyl, arylalkyl, arylalkenyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, or —CH═N—NH—Rf;
R15 is each independently selected from H, halo, aryl, heterocyclyl, arylalkynyl, C1-C24 alkyl, C2-C24 alkenyl, or C2-C24 alkynyl;
A is selected from halo and —C≡C—Rg;
M is a metal ion;
each m is independently an integer from 1-12; and
each Rb, Rc, Rd, Re, Rf, and Rg is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, arylalkynyl, heterocyclylalkyl and aryl,
wherein when the complex of formula (V) bears a charge, it further comprises one or more counterions.
In another aspect, the invention provides a metal complex of formula (VI):
wherein:
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
each R1 is independently a group of the following formula:
each n equals to 0 or 1
each R2 and R3 is independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, or —(O—CH2—CH2)m—OCH3;
each m is independently an integer from 1-12;
each R4 is independently H, halo, aryl, arylalkynyl, heterocyclyl, arylalkyl, arylalkenyl, heterocyclylalkyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf; and
each Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, heterocyclylalkyl and aryl;
A is selected from halo and —C≡C—Rg;
each Rg is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, arylalkynyl, heterocyclylalkyl and aryl; and
M is a metal ion;
wherein when the complex of formula (VI) bears a charge, it further comprises one or more counterions.
In a further aspect, the invention provides a metal complex of formula (VII):
wherein
M is a metal ion;
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
each R1 is independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, arylalkynyl or —(O—CH2—CH2)m—OCH3;
each R2 and R3 is independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, or —(O—CH2—CH2)n—OCH3;
each R4 is independently H, halo, aryl, arylalkynyl, heterocyclyl, arylalkyl, arylalkenyl, heterocyclylalkyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf;
each m is independently an integer from 1-12;
each Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, arylalkyl, heterocyclylalkyl and aryl; and
wherein when the complex of formula (VII) bears a charge, it further comprises one or more counterions.
In another aspect, the invention provides a metal complex of formula (VIII):
wherein
M is a metal ion;
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
each R1 is independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, arylalkynyl or —(O—CH2—CH2)m—OCH3;
each R11 is independently H, halo, aryl, arylalkynyl, heterocyclyl, arylalkyl, arylalkenyl, heterocyclylalkyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, and C2-C24 alkynyl;
each m is independently an integer from 1-12;
each Rb, Rc, Rd, and Re is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, arylalkyl, heterocyclylalkyl and aryl; and
wherein when the complex of formula (VIII) bears a charge, it further comprises one or more counterions.
In another aspect, the invention provides optical-switching devices, organic light emitting diodes, chemical sensors such as for organic vapors, ion sensors such as for zinc, and pH sensors using the ligands and metal complexes described herein.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.
a) shows UV-vis absorption spectrum of complex F-15 in CH2Cl2 solution, with the inset showing the normalized UV-vis absorption spectra of F-15 and 35 in CH2Cl2 solution.
b) shows expansion of the UV-vis spectrum of F-15 between 520 nm and 600 nm in CH2Cl2.
Metal complexes, including platinum and zinc complexes, having ligands bearing substituted fluorenyl substituents are described herein. These complexes may exhibit broad and strong reverse saturable absorption in the visible spectral region and two-photon absorption in the near-IR region, and may have good solubilities in organic solvents. In addition, various ligands are described herein. The complexes and the ligands may be useful as components of optical-switching devices, organic light emitting diodes (OLED), and chemical sensors, such as pH sensors and zinc sensors. The emission properties and nonlinear transmission properties of the metal complexes can be tuned by introducing different substituents on the terdentate (i.e. terpyridine (N̂N̂N) or phenylbipyridine (ĈN̂N)) or diimine (i.e. bipyridine (N̂N)) ligand and using different co-ligands. They can also be altered through inter-/intra-molecular interactions, such as the metal-metal or π-π interactions. Though not wishing to be bound by a particular theory, the unique photophysical properties and the variety applications of the metal complexes could be due to their square-planar configuration, the intramolecular charge transfer characteristics, and the heavy-atom effect from the metal that enhances the intersystem crossing (ISC) rate to the triplet excited state.
In addition to emission studies, another powerful tool in understanding the excited-state characteristics of these complexes is the transient absorption measurement of the excited state, which can predict the nonlinear absorption of the compound. Time-resolved transient difference absorption study not only provides valuable information on the excited-state absorption spectrum but also on the lifetime of the excited state. In general, a positive band in a transient absorption spectrum suggests stronger excited-state absorption than that of the ground state in the respective spectral region, which could cause reverse saturable absorption.
The nonlinear absorption of the square-planar metal terpyridyl, phenylbipyridyl or diimine complexes would also be useful for numerous other applications wherein the following characteristics are desired: a broadband excited-state absorption, a long-lived triplet excited state, an ease of structural modification, and a thermal and photochemical stability of complexes.
In addition, metal complexes such as those of the present invention can be fabricated as Langmuir-Blodgett (LB) films. Due to the highly ordered nature of the LB films, the intermolecular interaction of the metal complexes in LB films may be different from that in solutions. Moreover, for potential device applications, it is important to be able to transfer the complexes into thin film forms. An LB film is a convenient method to fabricate highly ordered thin films. The presence of a long chain alkoxyl substituent on the ĈN̂N ligand could make the complex amphiphilic, which allows for preparation of LB films of the complex.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.
As used herein, “alkyl” refers to a straight chain or branched, noncyclic or cyclic, saturated aliphatic hydrocarbon chain containing the indicated number of carbon atoms. Representative saturated straight chain alkyl groups include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, and the like; while saturated branched alkyls include isopropyl, sec-butyl, isobutyl, tert-butyl, isopentyl, and the like. Representative saturated cyclic alkyls include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and the like; while unsaturated cyclic alkyls include cyclopentenyl and cyclohexenyl, and the like. An alkyl group may be substituted or unsubstituted (e.g., by one or more substituents).
As used herein, “alkenyl” refers to an alkyl, as defined above, containing at least one double bond between adjacent carbon atoms. Alkenyls include both cis and trans isomers. Representative straight chain and branched alkenyls include ethylenyl, propylenyl, 1-butenyl, 2-butenyl, isobutylenyl, 1-pentenyl, 2-pentenyl, 3-methyl-1-butenyl, 2-methyl-2-butenyl, 2,3-dimethyl-2-butenyl, and the like. An alkenyl group may be substituted or unsubstituted (e.g., by one or more substituents).
As used herein, “alkynyl” refers to any alkyl or alkenyl, as defined above, which additionally contains at least one triple bond between adjacent carbons. Representative straight chain and branched alkynyls include acetylenyl, propynyl, 1-butynyl, 2-butynyl, 1-pentynyl, 2-pentynyl, 3-methyl-1-butynyl, and the like. An alkynyl group may be substituted or unsubstituted (e.g., by one or more substituents).
As used herein, an “aryl” group is an aromatic monocyclic, bicyclic, tricyclic or tetracyclic hydrocarbon ring system, wherein any ring atom capable of substitution can be substituted (e.g., by one or more substituents). Examples of aryl moieties include, but are not limited to, substituted or unsubstituted phenyl, naphthyl, anthracenyl, phenanthracenyl, fluorenyl and pyrenyl. An aryl moiety may also be a “heteroaryl” moiety. Heteroaryl refers to an aromatic monocyclic, bicyclic, tricyclic or tetracyclic ring system having at least one heteroatom selected from O, N, or S. Any ring atom capable of substitution can be substituted (e.g., by one or more substituents). Exemplary heteroaryls include, but are not limited to, furanyl, thienyl, thiazolyl, pyrrolyl, imidazolyl, pyrazolyl, triazolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, oxadiazolyl, tetrazolyl, oxadiazolyl, oxatriazolyl, isoxazinyl, pyridyl, pyridazinyl, pyrimidyl, pyrazinyl, pyrimidyl, benzofuranyl, isobenzofuranyl, indolyl, isoindolyl, purinyl (e.g., adenine, guanine), benzothiophenyl, benzimidazolyl, quinolinyl, isoquinolinyl, benzodiazinyl, pyridopyridinyl, quinoxalinyl, carbazolyl, dibenzothiophenyl, dibenzofuranyl, acridinyl, phenazinyl, benzothiazolyl, phenothiazinyl, naphthalimide, and naphthalene diimide.
As used herein, an “arylalkyl” group refers to an alkyl moiety in which an alkyl hydrogen atom is replaced by an aryl group. Arylalkyl includes groups in which more than one hydrogen atom has been replaced by an aryl group. Examples of arylalkyl groups include, but are not limited to, benzyl, 2-phenylethyl, 3-phenylpropyl, 9-fluorenyl, benzhydryl, and trityl groups.
As used herein, the term “counterion” refers to an atom or group having a formal charge that is present to balance the charge of an ionic species in order to maintain electronic neutrality. Counterions can be positively charged (cations) or negatively charged (anions). Exemplary negatively charged counterions include halides (e.g., fluoride, chloride, bromide and iodide), N3−, PO43−, HPO42−, H2PO4−, SO42−, HSO4−, NO3−, ClO4−, CO32, HCO3−, CrO42−, Cr2O72−, CN−, OH−, Cr2O42−, MnO4−, BF4−, B(C6H5)4−, PF6−, HCOO−, CH3COO−, CF3COO−, CF3SO3−, PtCl42−, and the like.
As used herein, “halo” is fluoro, chloro, bromo or iodo.
As used herein, “heterocyclyl” refers to a nonaromatic monocyclic, bicyclic, tricyclic or tetracyclic ring system having at least one heteroatom selected from O, N, or S. Any ring atom capable of substitution can be substituted (e.g., by one or more substituents). Examples of heterocyclyl groups include, but are not limited to, tetrahydrofuranyl, tetrahydropyranyl, piperidinyl, morpholino, pyrrolinyl, pyrimidinyl, pyrrolidinyl and phenothiazinyl.
As used herein, a “heterocyclylalkyl” group refers to an alkyl moiety in which an alkyl hydrogen atom is replaced by a heterocyclyl group. Heterocyclylalkyl includes groups in which more than one hydrogen atom has been replaced by a heterocyclyl group.
As used herein, a “substituent” refers to a group “substituted” on an alkyl, alkenyl, alkynyl, aryl or heteroaryl group at any atom of that group that is capable of substitution. Suitable substituents include, without limitation, alkyl, alkenyl, alkynyl, cycloalkyl, haloalkyl (e.g., perfluoroalkyl such as CF3), aryl, heteroaryl, arylalkyl, heteroarylalkyl, heterocyclyl, cycloalkenyl, heterocycloalkenyl, alkoxy, haloalkoxy (e.g., perfluoroalkoxy such as OCF3), halo, hydroxy, carboxy, carboxylate, cyano, nitro, amino, alkylamino, dialkylamino, SO3H, sulfate, phosphate, methylenedioxy (—O—CH2—O— wherein oxygens are attached to vicinal atoms), ethylenedioxy, oxo, thioxo (e.g., C═S), imino (alkyl, aryl, arylalkyl), S(O)nalkyl (where n is 0-2), S(O)naryl (where n is 0-2), S(O)nheteroaryl (where n is 0-2), S(O)nheterocyclyl (where n is 0-2), amine (mono-, di-, alkyl, cycloalkyl, arylalkyl, heteroarylalkyl, aryl, heteroaryl, and combinations thereof), ester (alkyl, arylalkyl, heteroarylalkyl, aryl, heteroaryl), amide (mono-, di-, alkyl, arylalkyl, heteroarylalkyl, aryl, heteroaryl, and combinations thereof), sulfonamide (mono-, di-, alkyl, arylalkyl, heteroarylalkyl, and combinations thereof). Substituents on a group may be any one single substituent, or each independently any subset of the aforementioned substituents. A substituent may itself be substituted with any one of the above substituents.
In one aspect, the invention features a ligand of the following formula (I):
wherein:
R10 is H or —ORa;
R2 and R3 are each independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —(O—CH2—CH2)n—OCH3;
R4 is H, halo, aryl, heterocyclyl, arylalkyl, heterocyclylalkyl, arylalkynyl, arylalkenyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf;
X is C or N;
each n is independently an integer from 1-12;
each Ra, Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl or C2-C24 alkynyl, aryl, heterocyclyl, arylalkyl, and heterocyclylalkyl; and
Y is a bond, —CH═CH—, —CH═CH-Ph-, or —C≡C—;
wherein when the ligand of formula (I) bears a charge, it further comprises one or more counterions.
In some embodiments, the ligand has one of the following formulae:
In another aspect, the invention features a ligand of the following formula (II):
wherein:
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
R2 and R3 are each independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —(O—CH2—CH2)m—OCH3;
R4 is H, halo, aryl, heterocyclyl, arylalkynyl, arylalkyl, arylalkenyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, or —CH═N—NH—Rf;
each R15 is independently selected from H, halo, aryl, heterocyclyl, arylalkynyl, C1-C24 alkyl, C2-C24 alkenyl, or C2-C24 alkynyl;
each m is independently an integer from 1-12; and
each Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, heterocyclylalkyl and aryl,
wherein when the ligand of formula (II) bears a charge, it further comprises one or more counterions.
In some embodiments, the ligand has one of the following formulae:
In another aspect, the invention features a ligand of formula (III):
wherein:
each R1 is independently a group of the following formula:
each n equals to 0 or 1;
each R2 and R3 is independently C1-C24 alkyl, C2-C24 alkenyl or C2-C24 alkynyl, or —(O—CH2—CH2)m—OCH3;
each R4 is independently H, halo, aryl, arylalkynyl, heterocyclyl, arylalkyl, arylalkenyl, heterocyclylalkyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf; and
each Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, heterocyclylalkyl and aryl;
each m is independently an integer from 1-12; and
wherein the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings; and
wherein when the ligand of formula (III) bears a charge, it further comprises one or more counterions.
In some embodiments, the ligand has one of the following formulae:
In Formulae (I), (II) and (III), R2 and R3 are suitably a branched alkyl, such as
In certain embodiments, Y is suitably a bond or —C≡C— and X is
suitably C or N. In certain embodiments, R4 is
naphthalimide, naphthalene diimide, or —NRcRd. Rc and Rd are suitably alkyl, or aryl, such as phenyl.
In one aspect, the invention features a metal complex of the following formula (IV):
wherein:
R10 is H or —ORa;
R2 and R3 are each independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —(O—CH2—CH2)m—OCH3;
R4 is H, halo, aryl, heterocyclyl, arylalkyl, heterocyclylalkyl, arylalkynyl, arylalkenyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf;
A is selected from halo,
X=C or N;
each m is independently an integer from 1-12;
each Ra, Rb, Rc, Rd, Re, Rf, and Rg is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl or C2-C24 alkynyl, aryl, heterocyclyl, arylalkyl, arylalkynyl and heterocyclylalkyl; and
Y is a bond, —CH═CH—, —CH═CH-Ph-, or —C≡C—;
M is a metal ion, such as Pt2+, Pd2+, Ni2+, Zn2+, Cu2+, or Au3+; and
wherein when the complex of formula (IV) bears a charge, it further comprises one or more counterions.
In some embodiments, the complex has one of the following formulae:
In another aspect, the invention features a metal complex of the following formula (V):
wherein:
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
R2 and R3 are each independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —(O—CH2—CH2)m—OCH3;
R4 is H, halo, aryl, heterocyclyl, arylalkynyl, arylalkyl, arylalkenyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, or —CH═N—NH—Rf;
R15 is each independently selected from H, halo, aryl, heterocyclyl, arylalkynyl, C1-C24 alkyl, C2-C24 alkenyl, or C2-C24 alkynyl;
A is selected from halo and —C≡C—Rg;
M is a metal ion, such as Pt2+, Pd2+, Ni2+, Zn2+, Cu2+, or Au3+;
each m is independently an integer from 1-12; and
each Rb, Rc, Rd, Re, Rf, and Rg is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, heterocyclylalkyl, arylalkynyl and aryl,
wherein when the complex of formula (V) bears a charge, it further comprises one or more counterions.
In some embodiments, the compound has one of the following formulae:
In another aspect, the invention features a metal complex of formula (VI):
wherein:
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
each R1 is independently a group of the following formula:
each n equals to 0 or 1
each R2 and R3 is independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, or —(O—CH2—CH2)m—OCH3;
each m is independently an integer from 1-12;
each R4 is independently H, halo, aryl, arylalkynyl, heterocyclyl, arylalkyl, arylalkenyl, heterocyclylalkyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf; and
each Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, heterocyclylalkyl and aryl;
A is selected from halo and —C≡C—Rg;
each Rg is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, heterocyclyl, arylalkyl, heterocyclylalkyl, arylalkynyl and aryl; and
M is a metal ion, such as Pt2+, Pd2+, Ni2+, Zn2+, Cu2+, or Au3+;
wherein when the complex of formula (VI) bears a charge, it further comprises one or more counterions.
In some embodiments, the complex has one of the following formulae:
In another aspect, the invention features a metal complex of formula (VII):
wherein
M is a metal ion, such as Pt2+, Pd2+, Ni2+, Zn2+, Cu2+, or Au3+;
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
each R1 is independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, arylalkynyl or —(O—CH2—CH2)m—OCH3;
each R2 and R3 is independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, or —(O—CH2—CH2)n—OCH3;
each R4 is independently H, halo, aryl, arylalkynyl, heterocyclyl, arylalkyl, arylalkenyl, heterocyclylalkyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl or —CH═N—NH—Rf;
each m is independently an integer from 1-12;
each Rb, Rc, Rd, Re, and Rf is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, arylalkyl, heterocyclylalkyl and aryl; and
wherein when the complex of formula (VII) bears a charge, it further comprises one or more counterions.
In some embodiments, the complex has the following formula:
In another aspect, the invention features a metal complex of formula (VIII):
wherein
M is a metal ion, such as Pt2+, Pd2+, Ni2+, Zn2+, Cu2+, or Au3+;
the dashed line represents the presence or absence of an optionally substituted aromatic ring or fused aromatic rings;
each R1 is independently C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, arylalkynyl or —(O—CH2—CH2)m—OCH3;
each R11 is independently H, halo, aryl, arylalkynyl, heterocyclyl, arylalkyl, arylalkenyl, heterocyclylalkyl, —C(O)Rb, —NRcRd, —ORe, —NO2, —CHO, C1-C24 alkyl, C2-C24 alkenyl, and C2-C24 alkynyl;
each m is independently an integer from 1-12;
each Rb, Rc, Rd, and Re is independently selected from H, C1-C24 alkyl, C2-C24 alkenyl, C2-C24 alkynyl, arylalkyl, heterocyclylalkyl and aryl, and
wherein when the complex of formula (VIII) bears a charge, it further comprises one or more counterions.
In Formulae (IV), (V), (VI), (VII) and (VIII) suitably, R1 is independently a branched alkyl, such as
n equals to 0 or 1;
R2 and R3 are independently a branched alkyl, such as
In certain embodiments, R11 is Br, I, H, CHO, NO2, OCH3 or —NRcRd. A is Cl or
In certain embodiments, R4 is
naphthalimide, naphthalene diimide, or —NRcRd. Rc and Rd are suitably alkyl or aryl, such as phenyl. Y is suitably a bond or —C≡C—. and X is suitably C or N. In certain embodiments, M is platinum.
Suitable complexes and ligands according to the present invention include those shown in
The ĈN̂N ligands with Y as a single bond in series I could be synthesized by using fluorene as the starting material. Bromonation and alkylation of fluorene, followed by reaction with BuLi at −78° C. yield the fluorenylaldehyde precusor, which reacts with 2-acetylpyridine to afford 1-(fluoren-2-yl)prop-2-en-1-one. Reaction of 1-(fluoren-2-yl)prop-2-en-1-one, N-(benzoylmethyl)-pyridinium bromide and NH4OAc results in the formation of ĈN̂N ligands. Alternately, the ligands with Y as a single bond could be synthesized via Suzuki coupling reaction from 7-R4-fluoren-2-yl borate and 4-bromoterpyridine or 4-bromo-6-phenyl-2,2′-bipyridine. The ligands with Y as a double bond could be obtained by Heck reaction using 2-bromofluorene derivative and 4-vinylterpyridine or 4-vinyl-6-phenyl-2,2′-bipyridine as the starting materials. The ligands with Y as a triple bond could be synthesized via Sonogashira coupling reaction from a 2-ethynylfluorene derivative and 4-OTf-terpyridine or 4-OTf-6-phenyl-2,2′-bipyridine
The ligands II could be synthesized by Suzuki coupling reactions from the corresponding 7-R4-fluoren-2-yl borate and 6-bromo-2,2′-bipyridine; while 6-bromo-2,2′-bipyridine could be synthesized following literature procedures. Shin, D.; Switzer, C. Chem. Comm. 2007, 42, 4401. The fluorene borates could be synthesized by treating the corresponding 2-bromofluorene with BuLi in THF at −78° C. under argon, followed by addition of one equivalent isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane.
The ligands III with a triple bond connection between the substituted fluorene and the bipyridine could be synthesized by Sonogashira coupling reaction from the 4,4′-dibromo-2,2′-bipyridine or 5,5′-dibromo-2,2′-bipyridine and the 2-ethynyl-7-R4-fluorene. The ligands III with a single bond connection between the substituted fluorene and the bipyridine could be synthesized by Suzuki coupling reaction from the 4,4′-dibromo-2,2′-bipyridine or 5,5′-dibromo-2,2′-bipyridine and the 7-R4-fluoren-2yl borate.
The platinum(II) complexes IV-VIII could be synthesized using the corresponding terdentate (ĈN̂N or N̂N̂N) ligands or diimine ligands and K2PtCl4 or Pt(DMSO)2Cl2 salts to afford the platinum chloride complexes. In the case that the complexes bearing acetylide ligand(s), the corresponding platinum chloride complexes are used to react with the acetylide ligands in the presence of CuI as the catalyst and base.
The metal complexes and ligands according to the present invention may be used in many different end products. For example, the metal complexes may be used as ion sensors, such as for zinc, as organic vapor sensors, in optical-switching devices, and in organic light emitting diodes. For example, the ligands may be used as pH sensors, ion sensors, and in organic light emitting diodes.
The optical-switching device includes a pair of transparent substrates separated by a cavity therebetween. A nonlinear optical material including the synthesized metal complexes substantially fills the cavity between the substrates. If the incident light intensity is below a given level, the optical-switching device passes the incident light through. On the other hand, if the incident light intensity is above the given level, the optical-switching device does not pass the incident light through. Alternatively, one pump beam, which is at the wavelength of the absorption band maximum of the UV-vis absorption spectrum of the material, can be used to control the transmission of another beam (probe beam), which is not absorbed by the material via one-photon absorption. Without the pump beam, the probe beam will pass through; with the pump beam on, the probe beam does not pass through the device. Put another way, light controls light in the optical-switching devices.
In other embodiments, the synthesized metal complexes described above are used as light-emitting materials in an organic light-emitting diode (OLED). The OLED is constituted of an organic compound layer including the synthesized metal complexes, interposed between an anode and a cathode. When a DC voltage is applied to the OLED with the anode as a positive electrode and the cathode as a negative electrode, light is emitted.
In still other embodiments, the synthesized metal complexes described above are used as chemical sensors. The chemical sensor includes a fibrous substrate and a coating solution including a metal complex impregnated to the substrate. When contacted with organic vapors, the chemical sensor indicates the presence of the organic vapors by a vapochromic response, that is, by changing color. Mathew, I.; Sun, W. Dalton Trans. 2010, 39, 5885 (incorporated by reference herein). Non-limiting examples of the vapors that can be sensed by the disclosed chemical sensor include methanol, ethanol, iso-propanol, diethyl ether, acetonitrile, hexanes, acetone, benzene, dichloromethane, chloroform, and a combination thereof.
In still other embodiments, the synthesized metal complexes described above are used as ion sensors. The ion sensors include the dimethyl sulfoxide (DMSO) solution in which the ligands and metal complexes described herein are dissolved. When contacted with anions such as F−, H2PO4− and OAc−, the color of the solutions can change; whereas addition of NO3−, Cl−, Br− and I− (all as tetra-n-butylammonium salts, TBA salts) may have no effect on the solution color. B. Zhang, Y. Li, W. Sun, “Anion-Sensitive 2,4-Dinitrophenylhydrazone-Containing Terpyridine Derivative and Its Platinum Chloride Complex,” Eur. J. Inorg. Chem. (incorporated by reference herein).
All of the reagents and solvents were purchased from Alfa Aesar or Aldrich. Solvents were used as received unless otherwise stated. 1H NMR spectra were measured on a Varian 400 MHz VNMR spectrometer. ESI-HRMS analyses were conducted on a Bruker Daltonics BioTOF III mass spectrometer. Elemental analyses were performed by NuMega Resonance Labs, Inc. in San Diego, Calif.
Compounds 1 and 2 were synthesized according to the reported procedures. McGarrah, J. E.; Kim, Y. J.; Hissler, M.; Eisenberg, supra; Sae-Lim, C.; Sandman, D. J.; Foxman, B. M.; Sukwattanasinitt, A. M. J. Macromolecular Sci., Part A: Pure Appl. Chem. 2006, 43, 1929 (incorporated by reference herein). The synthesis of complex 3 was reported previously. Shao, P.; Li, Y.; Yi, J.; Pritchett, T. M; Sun, W., Inorg. Chem. 2010, 49, 4507-4517 (incorporated herein by reference).
Complex 4: Complex 3 (0.18 g, 0.20 mmol), compound 1 (0.08 g, 0.25 mmol), KOH (0.02 g, 0.36 mmol), and catalytic amount of CuI were dissolved in a mixed solvent of CH2Cl2 and CH3OH (v/v=50 mL/30 mL). The reaction mixture was stirred at room temperature for 18 hrs. After removal of the solvent, the residue was dissolved in CH2Cl2, and then washed with brine three times to remove KOH. The CH2Cl2 layer was combined and dried over Na2SO4. Solvent was then removed and the crude product was purified by chromatography on a silica gel column using CH2Cl2 as the eluent. The product was recrystallized from CH2Cl2/ethanol to yield 98 mg dark red solid (yield: 39%). 1H NMR (400 MHz, CDCl3, 25° C., TMS): δ 9.26 (d, 1H, J=4.8 Hz), 8.03 (t, 1H, J=7.6 Hz), 7.97 (d, 1H, J=8.4 Hz), 7.80 (d, 1H, J=8.0 Hz), 7.74 (t, 1H, J=2.8 Hz), 7.67 (t, 2H, J=3.8 Hz), 7.60 (d, 2H, J=6.4 Hz), 7.57 (d, 1H, J=2.8 Hz), 7.51 (d, 3H, J=7.2 Hz), 7.40 (d, 1H, J=7.6 Hz), 7.35 (d, 3H, J=3.2 Hz), 7.18 (d, 2H, J=7.6 Hz), 7.06 (dt, 2H, J=7.6 Hz and 1.2 Hz), 6.97 (tt, 2H, J=7.2 Hz and 1.2 Hz), 6.84 (t, 2H, J=7.6 Hz), 6.67 (d, 2H, J=8.0 Hz), 6.61 (dd, 1H, J=7.6 Hz and 1.2 Hz), 5.06 (s, 2H), 3.93 (d, 2H, J=5.6 Hz), 2.02 (t, 4H, J=8 Hz), 1.72 (m, 1H), 1.51 (d, 1H, J=1.6 Hz), 1.50-1.37 (m, 4H), 1.29-1.26 (m, 5H), 1.31-1.05 (m, 5H), 1.31-1.05 (m, 12H), 0.89-0.83 (m, 6H), 0.75 (t, 5H, J=7.2 Hz), 0.65 (s, 3H). ESI-HRMS: m/z calcd for [C70H73N3OPtS+Na]+: 1222.5034; found, 1222.5048. Anal. Calcd (%) for C70H73N3OPtS.CH3OH: C, 69.16; H, 6.74; N, 3.46; found: C, 69.24, H, 6.30, N, 3.41.
Complex 5: Complex 3 (0.18 g, 0.20 mmol), compound 2 (0.11 g, 0.30 mmol), KOH (0.02 g, 0.36 mmol), and catalytic amount of CuI were dissolved in a mixed solvent of CH2Cl2 and CH3OH (v/v=50 mL/30 mL). The reaction mixture was stirred at room temperature for 18 hrs. After removal of the solvent, the residue was dissolved in CH2Cl2, and then washed with brine three times to remove KOH. The CH2Cl2 layer was combined and dried over Na2SO4. Solvent was then removed and the crude product was purified by chromatography on a silica gel column using CH2Cl2 as the eluent. The product was recrystallized from CH2Cl2/ethanol to yield 102 mg dark red solid (yield: 46%). 1H NMR (400 MHz, CDCl3, 25° C., TMS): δ 8.58 (d, 1H, J=4.8 Hz), 7.74 (d, 2H, J=8.4 Hz), 7.67 (d, 2H, J=7.6 Hz), 7.58 (t, 2H, J=3.8 Hz), 7.55 (s, 1H), 7.84 (d, 2H, J=8.4 Hz), 7.33-7.29 (m, 4H), 7.14-7.05 (m, 6H), 6.90 (dd, 1H, J=8.8 Hz and 4.8 Hz), 6.83 (t, 2H, J=7.6 Hz), 6.36 (dd, 1H, J=8.4 Hz and 2.4 Hz), 4.84 (s, 2H), 3.43 (d, 2H, J=5.2 Hz), 1.97 (t, 4H, J=8.0 Hz), 1.47-1.43 (m, 1H), 1.36-1.16 (m, 10H), 1.09-1.00 (m, 12H), 0.84-0.75 (m, 6H), 0.69 (t, 5H, J=6.8 Hz), 0.62 (s, 3H). ESI-HRMS: m/z calcd for [C64H69N3OPtS+Na]+, 1145.4706; found, 1145.4662. Anal. Calcd (%) for C64H69N3OPtS.CH3OH: C, 67.15; H, 6.59; N, 3.54; found: C, 67.57; H, 6.37; N, 3.64.
Two platinum 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine complexes (4 and 5) with phenothiazinyl (PTZ) acetylide ligand were characterized. Their UV-vis absorption and emission characteristics in solution and in LB film were systematically investigated. The triplet transient difference absorption and nonlinear absorption properties were also studied for these complexes. Both complexes exhibit a broad metal-to-ligand charge transfer/intraligand charge transfer/ligand-to-ligand charge transfer (1MLCT/1ILCT/1LLCT) absorption band between 400 and 500 nm and a 3MLCT/3ILCT/3π,π* emission band at ˜594 nm at room temperature, which blue shifts at 77 K. Both UV-vis absorption and emission spectra show negative solvatochromic effect. The triplet excited-state lifetime at room temperature for complex 4 is ca. 1.2 μs, which is longer than that for complex 5 (˜600 ns). The emission quantum yield of complex 4 in toluene is 0.18, while it is 0.053 for complex 5. Both of the complexes also exhibit broad and moderately strong triplet transient absorption from the near-UV to the near-IR spectral region. However, 5 exhibits stronger reverse saturable absorption than complex 4 does at 532 nm for ns laser pulses. This is attributed to the weaker ground-state absorption but stronger triplet excited-state absorption at 532 nm for 5 than for 4, which leads to a larger ratio of excited-state absorption cross-section to ground-state absorption for 5 than 4. In addition, LB films of 4 and 5 were prepared and characterized by AFM technique. The UV-vis absorption and emission spectra of the LB films of 4 and 5 were also investigated and compared with those obtained in solution.
The UV-vis absorption spectra were measured on an Agilent 8453 spectrophotometer in a 1-cm quartz cuvette in HPLC-grade solvent. The steady state emission spectra were obtained using a SPEX fluorolog-3 fluorometer/phosphorometer. The emission quantum yields were measured by the comparative method (Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991) (incorporated by reference herein) in degassed toluene solution. A degassed [Ru(bpy)3]Cl2 in aqueous solution (Φem=0.042, λex=436 nm) (Van Houten, J.; Watts, R. J. Am. Chem. Soc. 1976, 98, 4853) (incorporated by reference herein) was used as the reference. The excited-state lifetimes and the triplet transient difference absorption spectra were obtained in toluene solutions on an Edinburgh LP920 laser flash photolysis spectrometer. The third harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant, pulse width (fwhm)=4.1 ns, the repetition rate was set to 1 Hz) was used as the excitation source. Each sample was purged with Ar for 30 min prior to each measurement. The triplet excited-state absorption coefficients were measured using partial saturation method. Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref Data 1986, 15, 1 (incorporated by reference herein).
The experimental setup was described previously. Sun, W.; Zhu, H.-J.; Barron, P. Chem. Mater. 2006, 18, 2602 (incorporated by reference herein). The second harmonic (λ=532 nm) of a 4.1 ns (fwhm), 10 Hz, Q-switched Quantel Brilliant Nd:YAG laser was used as the light source. The laser beam was focused by a f=30 cm plano-convex lens to the center of a 2 mm thick quartz cuvette that contained the sample solution. The radius of the beam waist was approximately 75 μm. Two Molectron J4-09 pyroelectric probes and an EPM2000 energy/power meter were used to monitor the incident and output energies.
Surface pressure-mean molecular area isotherm measurement and LB film preparation were carried out on a KSV minitrough (Teflon coating, 7.5×30×1 cm3). The trough and the barriers were cleaned using ethanol and ultra pure water with a resistance of 18 MΩ·cm. Both 4 and 5 were dissolved in HPLC-grade CHCl3 solvent for the surface pressure—mean molecular area measurement (c=1.16×10−3 mol/mL for 4 and 0.78 mmol/mL for 5). 50 μl of solution was spread on ultra pure water subphase at 25±1° C. and was left for 25 mins. to allow for CHCl3 to evaporate. The compression rate was kept at 5 mm/min The isotherm measurement for each sample was repeated at least twice.
The LB films of 4 and 5 were deposited on hydrophilic glass slides using a dipping method with the transfer ratio in a range of 0.6-1. The glass slides were cleaned using detergent and water first, followed by soaking in concentrated H2SO4 for at least 1 hour. This would make the surfaces hydrophilic before the deposition. The slides were then cleaned by ultra pure water.
Surface morphology of the films prepared was studied by AFM technique using the tapping mode of a Veeco DI-3100 with a silicon nitride probe. Each sample was repeated three times to verify that the observed AFM images are not from the defective surface.
The electronic absorption spectra of 4 and 5 in dichloromethane solution are shown in
The charge transfer nature of the low-energy absorption band is supported by the negative solvatochromic effect for these two complexes in different polarity solvents (demonstrated in
aFrom Shao, P.; Li, Y.; Yi, J.; Pritchett, T. M. W. Sun, supra.
bThis Example.
cIn CH2Cl2.
dIn CH2Cl2 at a concentration of 5 × 10−5 mol/L.
eMeasured at a toluene solution with A = 0.1 at 436 nm.
fIn butyronitrile glassy solution at 77 K. c = 5 × 10−5 mol/L for 3, F-3 and F-4, and 3.5 × 10−5 mol/L for 4 and 5.
gIn CH3CN.
hIn toluene.
In comparison to the UV-vis absorption spectrum of the corresponding platinum chloride complex 3, the low-energy absorption band of 4 and 5 is 23 nm and 17 nm red-shifted, respectively. This is the result of the electron donating PTZ acetylide ligand, which not only causes the red-shift of the 1MLCT band due to the reduced energy gap between the destabilized platinum based HOMO and the bipyridine based LUMO, but also admixes 1LLCT character into the lowest excited state. In addition, the molar extinction coefficients of 4 and 5 are somewhat larger than those of 3, reflecting the influence of the electron-donating acetylide ligand. In contrast, introducing the PTZ substituent on the acetylide ligand exhibits minor effect on the low-energy charge transfer bands of 4 and 5 in comparison to those in their acetylide analogues F-3 and F-4. However, the molar extinction coefficients of the UV absorption bands in 4 and 5 are larger than those in F-3 and F-4.
Complexes 4 and 5 are emissive at room temperature. As shown in
Another piece of evidence that supports the admixture of the 3ILCT/3π,π* character into the emitting state arises from the minor solvent effect on the emission energy, as listed in Table 2 for 4 and 5. This reflects the delocalization of the emitting state and is consistent with that observed in the platinum ĈN̂N complexes with alkoxyl substituent. Although the solvatochromic effect on the emission energy is insignificant, it is still evident that less polar solvents, such as CH2Cl2, cause a slight bathochromic shift of the emission band compared to polar solvents, such as CH3CN, which implies the participation of the charge transfer character in the emitting state
The emission quantum yields of 4 and 5 are measured to be 0.18 and 0.053 in toluene, respectively, which are much higher than platinum(II) terpyridyl complexes with PTZ acetylide ligands (Φem less than 0.00033). See Chakraborty, S.; Wadas, J. T.; Hester, H.; Schmehl, R.; Eisenberg, R. Inorg. Chem. 2005, 44, 6865 (incorporated by reference herein). The emission quantum yield of 4 is approximately 4 times and 2.4 times as large as those of its corresponding platinum chloride precursor 3 and its acetylide analogues F-3 and F-4, respectively, and is 1.6 times larger than that of the “alkoxyl free” ĈN̂N platinum phenylacetylide complexes without the fluorenyl substituent (Φem=0.07). The stronger emission of 4 and 5 than that of their corresponding platinum terpyridyl PTZ acetylide dyads and triads is attributed to the nature of the emitting states for 4 and 5 as discussed in the previous two paragraphs, which admixes 3MLCT/3ILCT/3π,π* characters and thus is much less affected by the acetylide ligand. In addition, stronger emission is commonly seen from Pt(ĈN̂N) complexes than from platinum terpyridyl complexes.
Emission measured in butyronitrile glassy solution at 77 K (
The triplet transient difference absorption spectra of 4 and 5 in toluene at zero time delay after excitation and the time-resolved triplet transient difference absorption spectra of 5 are illustrated in
The broad absorption band at ca. 640 nm for complex 4 locates at the similar region to those observed in the precursor complex 3 and the acetylide analogues F-3 and F-4, suggesting that this band emanates from the 3MLCT/3ILCT states that are independent on the monodentate ligand. The lifetimes deduced from the decay of both bands are essentially the same (Table 1), indicating that the absorption originates from the same transient species. Additionally, these lifetimes are comparable to those obtained from the decay of the emission. Therefore, the transient absorption likely arises from the same excited state that emits or a state that is in equilibrium with the emitting state. Therefore, the state that gives rise to the transient absorption is tentatively assigned as the 3MLCT/3ILCT state. At the same excitation condition (Å=0.4 at 355 nm), the ΔOD value at 532 nm for 5 is approximately 2 times as large as that for 4. Therefore it is expected that the reverse saturable absorption for 5 at 532 nm would be stronger than that for 4, which is demonstrated by the nonlinear transmission measurement.
Complexes 4 and 5 exhibit broad positive triplet transient absorption in the visible to the near-IR region, and both complexes possess long-lived triplet excited state. Therefore, reverse saturable absorption (RSA) is expected to occur in the visible to the near-IR region. To demonstrate this, nonlinear transmission measurements were performed at 532 nm using nanosecond laser pulses.
Platinum(II) ĈN̂N complexes with hydrophilic substituents can form LB films due to their amphiphilic property. To explore whether complexes 4 and 5 can form LB films, surface pressure-mean molecular area of these two complexes have been measured and the results are presented in
The UV-vis absorption spectra of the LB films of 4 and 5 are shown in
The LB films of both complexes are emissive at room temperature. The emission spectra for the 5-layer and 11-layer LB films are shown in
Introducing PTZ acetylide ligands to platinum (II) complexes causes a mixture of 1LLCT character into the 1MLCT/1ILCT state due to the electron-donating ability of the PTZ acetylide ligand, which results in a broadening and slight red-shift of the lowest-energy charge-transfer band of 4 and 5 compared to that of their corresponding platinum chloride precursor 3. Both complexes are emissive at room temperature in fluid solutions and at 77 K in glassy matrix; with the emission quantum yield of 4 (Φem=0.18) at room temperature in toluene is larger than that of 5 (Φem=0.053). Both complexes exhibit broadband triplet excited-state absorption, therefore considerable reverse saturable absorption (RSA) was observed at 532 nm for ns laser pulses. Due to the smaller ground-state absorption cross-section but larger triplet excited-state absorption at 532 nm for 5 compared to those of 4, 5 exhibits stronger RSA than 4 does. The presence of the alkyl chain on the ĈN̂N ligand makes it possible to fabricate LB films for 4 and 5. Although aggregation is evident in the LB films of these two complexes via AFM images, the electronic absorption energy and the emission energy are comparable to those in solutions, indicating that the intermolecular distance in the LB films is not close enough for intermolecular Pt—Pt interaction to occur.
In this example, two new ĈN̂N ligands containing the fluorene unit at the 4-position of the central pyridine ring and their platinum complexes with different monodentate co-ligands were synthesized and characterized. Structures of these complexes and their synthetic routes are displayed in Scheme 2. The photophysics of these complexes and their reverse saturable absorption for ns laser pulses at 532 nm were systematically investigated and analyzed.
The compounds pyridacylpyridinium iodide (Neve, F.; Crispini, A.; Campagna, S.; Serroni, S. Inorg. Chem. 1999, 38, 2250 (incorporated by reference herein)), 5-6 (Koizumi, Y.; Seki, S.; Tsukuda, S.; Sakamoto, S.; Tagawa, S. J. Am. Chem. Soc. 2006, 128, 9036 (incorporated by reference herein)), 7 (He, F.; Xia, H.; Tang, S.; Duan, Y.; Zeng, M.; Liu, L.; Li, M.; Zhang, H.; Yang, B.; Ma, Y.; Liu, S.; Shen J. J. Mater. Chem. 2004, 14, 2735 (incorporated by reference herein)), 10 (Shao, P.; Li, Y.; Azenkeng, A.; Hoffmann, M.; Sun, W., supra) and 13-16 (Lee, S. H.; Nakamura, T.; Tsutsui, T. Org. Lett. 2001, 3, 2005 (incorporated by reference herein)) were prepared according to the literature procedures. All the solvents and reagents were purchased from Alfa Aesar and used as received unless otherwise stated. Column chromatography was carried out on silica gel (Sorbent Technologies, 60 Å, 230×450 mesh) or neutral aluminum oxide (Sigma-Aldrich, 58 Å, ˜150 mesh).
1H NMR spectra were measured on a Varian 300 or 400 MHz VNMR spectrometer. High-resolution mass spectrometry was carried out on a Bruker BioTOF III mass spectrometer. Elemental analyses were carried out by NuMega Resonance Laboratories, Inc. in San Diego, Calif.
8. To a mixture of 7 (1.43 g, 4 mmol) and 2-acetylpyridine (0.48 g, 4 mmol) in 20 mL ethanol, 4 mL of 1.5 M aqueous NaOH solution was added. After stirring at room temperature for 4 hrs, the solid formed was collected by filtration. The crude product was purified by recrystallization from methanol to afford 0.21 g yellow solid as the pure product (yield: 11%). 1H NMR (CDCl3): δ 8.79 (d, J=4.5 Hz, 1H), 8.32 (d, J=16.2 Hz, 1H), 8.23 (d, J=8.1 Hz, 1H), 8.06 (d, J=16.2 Hz, 1H), 7.88-7.93 (m, 1H), 7.72 (d, J=5.1 Hz, 4H), 7.50-7.54 (m, 1H), 7.35 (d, J=3.3 Hz, 3H), 1.98-2.04 (m, 4H), 1.04-1.14 (m, 12H), 0.75 (t, J=6.6 Hz, 6H), 0.59-0.63 (m, 4H) ppm. ESI-HRMS: m/z calcd for [C33H39NO+H]+: 466.3104; found, 466.3124. Anal. Calcd (%) for C33H39NO: C, 85.11; H, 8.44; N, 3.01. Found: C, 84.75; H, 8.90; N, 3.05.
9. A mixture of 8 (1.42 g, 3 mmol), N-(benzoylmethyl)pyridinium bromide (0.85 g, 3 mmol) and NH4OAc (3.00 g, 39 mmol) in 70 mL methanol was refluxed for 6 hrs. Methanol was removed and 40 mL water was added to the residue. The resultant mixture was extracted with CH2Cl2. The organic layer was washed with brine (40 mL×2) and dried over Na2SO4. The crude product was purified using a silica gel column eluted with hexane/ethyl acetate (v/v=20/1) to yield 0.75 g colorless viscous oil (yield: 44%). 1H NMR (CDCl3): δ 8.77-8.80 (m, 1H), 8.73-8.76 (m, 2H), 8.29 (dd, J=1.5, 8.4 Hz, 2H), 8.09 (d, J=1.5 Hz, 1H), 7.91 (dd, J=2.1, 7.5 Hz, 1H), 7.86-7.89 (m, 2H), 7.83 (d, J=1.2 Hz, 1H), 7.78-7.81 (m, 1H), 7.59 (td, J=1.5, 6.6 Hz, 2H), 7.47-7.53 (m, 1H), 7.35-7.44 (m, 4H), 2.02-2.13 (m, 4H), 1.11-1.19 (m, 12H), 0.73-0.82 (m, 10H) ppm. ESI-HRMS: m/z calcd for [C41H44N2+H]+: 565.3577; found, 565.3572. Anal. Calcd (%) for C41H44N2: C, 87.19; H, 7.85; N, 4.96. Found: C, 86.91; H, 8.05; N, 4.97.
11. A mixture of 7 (1.17 g, 3.23 mmol), 10 (0.80 g, 3.23 mmol) and KOH (0.84 g, 15 mmol) in dry MeOH (80 mL) was refluxed for 24 hrs. The solvent was removed and 40 mL water was added to the residue. The resultant mixture was extracted with ether. The organic layer was washed with brine and dried over Na2SO4. The crude product was purified by a silica gel column eluted with CH2Cl2/hexane (v/v=1/1) to afford 0.71 g yellow oil (yield: 37%). 1H NMR (CDCl3): δ 8.09 (d, J=8.4 Hz, 2H), 7.93 (d, J=15.6 Hz, 1H), 7.71-7.73 (m, 2H), 7.59-7.66 (m, 3H), 7.34 (d, J=2.4 Hz, 3H), 7.00 (d, J=8.4 Hz, 2H), 3.93 (d, J=5.4 Hz, 2H), 1.98-2.04 (m, 4H), 1.75-1.77 (m, 1H), 1.33-1.54 (m, 8H), 1.05-1.14 (m, 12H), 0.86-0.97 (m, 6H), 0.73-0.78 (m, 6H), 0.63 (s, 4H) ppm. ESI-HRMS: m/z calcd for [C42H56O2+H]+: 593.4353; found, 593.4363. Anal. Calcd (%) for C42H56O2.C6H14: C, 84.89; H, 10.40. Found: C, 85.35; H, 10.70.
12. A mixture of 11 (1.46 g, 2.5 mmol), pyridacylpyridinium iodide (0.80 g, 2.5 mmol) and NH4OAc (2.00 g, 26 mmol) was refluxed in 50 mL MeOH for overnight. The solvent was removed and 40 mL ether was added to the residue. The resultant mixture was washed with brine and dried over Na2SO4. The crude product was purified by a silica gel column eluted with CH2Cl2/hexane (v/v=1/1) to give 0.22 g colorless viscous oil (yield: 13%). 1H NMR (CDCl3): δ 8.73 (d, J=4.8 Hz, 1H), 8.68 (d, J=7.8 Hz, 1H), 8.61 (d, J=1.2 Hz, 1H), 8.17 (d, J=8.7 Hz, 2H), 7.97 (d, J=1.2 Hz, 1H), 7.84-7.90 (m, 1H), 7.80 (t, J=1.2 Hz, 2H), 7.73-7.76 (m, 2H), 7.32-7.37 (m, 4H), 7.05 (d, J=9.0 Hz, 2H), 3.93 (d, J=5.7 Hz, 2H), 2.00-2.05 (m, 4H), 1.73-1.80 (m, 1H), 1.33-1.55 (m, 8H), 1.05-1.14 (m, 12H), 0.91-0.97 (m, 6H), 0.75 (t, J=6.3 Hz, 6H), 0.64-0.66 (m, 4H) ppm. ESI-HRMS: m/z calcd for [C49H60N2O+H]+: 693.4778; found, 693.4809. Anal. Calcd (%) for C49H60ON2.⅓CH2Cl2: C, 82.14; H, 8.48; N, 3.88. Found: C, 82.23; H, 8.27; N, 3.56.
F-5. A mixture of ligand 9 (0.20 g, 0.35 mmol) and K2PtCl4 (0.15 g, 0.35 mmol) was refluxed in 60 mL AcOH for 24 hrs. After cooled to room temperature, the yellow precipitant was collected by filtration. The crude product was purified by a neutral alumina gel column eluted with CH2Cl2, and then further purified by recrystallization from CH2Cl2/ether. Orange solid was obtained as the pure product (0.23 g, yield: 83%). 1H NMR (CDCl3): δ 8.73 (d, J=5.4 Hz, 1H), 7.92 (d, J=3.6 Hz, 2H), 7.85 (d, J=8.4 Hz, 1H), 7.78-7.81 (m, 1H), 7.72-7.74 (m, 2H), 7.60 (d, J=1.8 Hz, 1H), 7.44 (dd, J=7.2, 1.2 Hz, 1H), 7.37-7.41 (m, 3H), 7.31 (t, J=4.2 Hz, 2H), 7.22-7.24 (m, 1H), 6.96 (td, J=1.2, 7.8 Hz, 1H), 6.88 (td, J=1.2, 7.5 Hz, 1H), 2.07-2.16 (m, 4H), 1.09-1.14 (m, 12H), 0.76 (t, J=6.6 Hz, 6H), 0.69 (br. s, 4H) ppm. ESI-MS: m/z calcd for [C41H43N2195Pt+CH3CN]+, 799.3338; found, 799.3396. Anal. Calcd (%) for C41H43ClN2Pt: C, 61.99; H, 5.46; N, 3.53. Found: C, 62.30; H, 5.10; N, 3.64.
F-6. A mixture of 12 (0.25 g, 0.36 mmol) and K2PtCl4 (0.15 g, 0.36 mmol) was refluxed in 60 mL AcOH for 24 hrs. After cooled down to room temperature, the solid formed was collected by filtration. The crude product was purified by a neutral alumina gel column eluted by CH2Cl2, and then recrystallized from CH2Cl2/hexane. 0.25 g orange solid was obtained as the pure product (yield: 76%). 1H NMR (CDCl3): δ 8.72 (d, J=5.1 Hz, 1H), 7.77-7.82 (m, 5H), 7.71 (d, J=8.1 Hz, 1H), 7.46 (s, 1H), 7.36-7.39 (m, 3H), 7.18-7.23 (m, 2H), 7.15 (d, J=8.4 Hz, 1H), 6.98 (d, J=2.4 Hz, 1H), 6.49 (d, J=8.4 Hz, 1H), 3.72 (d, J=5.7 Hz, 2H), 2.06-2.12 (m, 4H), 1.67-1.72 (m, 1H), 1.26-1.51 (m, 8H), 1.05-1.15 (m, 12H), 0.85-0.90 (m, 6H), 0.72-0.79 (m, 10H) ppm. ESI-MS: m/z calcd for [C49H59N2O195Pt+CH3CN]+, 927.4539; found, 927.4551. Anal. Calcd (%) for C49H59ClN2OPt: C, 63.79; H, 6.45; N, 3.04. Found: C, 63.82; H, 6.22; N, 3.35.
F-7. A mixture of F-5 (0.13 g, 0.16 mmol), 1-pentyne (40 μL, 0.41 mmol), CuI (3.0 mg, 0.01 mmol) and KOH (60 mg, 1 mmol) in degassed CH2Cl2/CH3OH (40 mL/20 mL) was stirred at room temperature under argon for 18 hrs. After the reaction, the solvent was removed and the crude product was purified by a neutral alumina gel column eluted by CH2Cl2/hexane (v/v=3/1), and then recrystallized from CH3OH. Red solid was obtained as the pure product (89 mg, yield: 67%). 1H NMR (CDCl3): δ 9.05 (d, J=5.4 Hz, 1H), 7.92-7.98 (m, 2H), 7.69-7.86 (m, 5H), 7.60 (s, 1H), 7.53 (s, 1H), 7.34-7.38 (m, 5H), 6.96-7.05 (m, 2H), 2.68 (t, J=7.2 Hz, 2H), 2.02-2.08 (m, 4H), 1.67-1.77 (m, 2H), 1.07-1.17 (m, 15H), 0.67-0.77 (m, 10H) ppm. ESI-MS: m/z calcd for [C46H50N2195Pt+H]+, 826.3698; found, 826.3695. Anal. Calcd (%) for C46H50N2Pt: C, 66.89; H, 6.10; N, 3.39. Found: C, 66.51; H, 6.37; N, 3.54.
F-8. A mixture of F-5 (0.13 g, 0.16 mmol), phenylacetylene (24 μL, 0.24 mmol), CuI (3.0 mg, 0.01 mmol) and KOH (60 mg, 1.00 mmol) in degassed CH2Cl2/CH3OH (50 mL/25 mL) was stirred at room temperature under argon for 24 hrs. The solvent was removed and the crude product was purified by a neutral alumina gel column with CH2Cl2/hexane (v/v=3/1) used as the eluent. Recrystallization from CH2Cl2/ether yields 82 mg red solid as the pure product (yield: 59%). 1H NMR (CDCl3): δ 9.10 (d, J=4.8 Hz, 1H), 8.01 (d, J=4.4 Hz, 2H), 7.75-7.81 (m, 4H), 7.71 (d, J=8.0 Hz, 1H), 7.64 (s, 1H), 7.58 (d, J=8.0 Hz, 3H), 7.37-7.43 (m, 6H), 7.29 (t, J=8.0 Hz, 2H), 7.18-7.20 (m, 1H), 7.03 (dd, J=3.2, 5.6 Hz, 1H), 2.01-2.05 (m, 4H), 1.07-1.14 (m, 12H), 0.76 (t, J=7.2 Hz, 6H), 0.66 (br. s, 4H) ppm. ESI-MS: m/z calcd for [C49H48N2195Pt+Na]+, 882.3362; found, 882.3343. Anal. Calcd (%) for C49H48N2Pt.0.1CH2Cl2: C, 67.90; H, 5.59; N, 3.23. Found: C, 67.70; H, 5.26; N, 3.38.
F-9. A mixture of F-6 (0.48 g, 0.52 mmol), 16 (0.10 g, 0.26 mmol), CuI (6.0 mg, 0.02 mmol) and KOH (56 mg, 1.0 mmol) in degassed CH2Cl2/CH3OH (50 mL/40 mL) was stirred at room temperature under argon for 18 hrs. The solvent was removed and the crude product was purified by a neutral alumina gel column (CH2Cl2/hexane (v/v=3/1) was used as the eluent), and then recrystallized from CH2Cl2/ether. Red solid was obtained as the pure product (0.34 g, yield: 30%). 1H NMR (CDCl3): δ 9.38 (d, J=5.7 Hz, 2H), 8.05 (s, 4H), 7.57-7.82 (m, 22H), 7.45 (d, J=8.4 Hz, 2H), 7.39 (br. s, 6H), 6.65 (d, J=9.0 Hz, 2H), 4.03 (br. s, 4H), 2.02 (br. s, 12H), 1.78 (br. s, 2H), 1.35-1.52 (m, 16H), 1.08-1.23 (m, 36H), 0.91-0.98 (m, 12H), 0.75-0.78 (m, 30H) ppm. ESI-MS: m/z calcd for [C127H150N4O2Pt2+2Na]2+, 1100.0432; found, 1100.0466. Anal. Calcd (%) for C127H150N4O2Pt2: C, 70.79; H, 7.02; N, 2.60. Found: C, 70.73; H, 6.69; N, 2.64.
The UV-vis absorption spectra were measured using an Agilent 8453 spectrophotometer in a 1-cm or 1-mm quartz cuvette. The steady-state emission spectra were obtained on a SPEX fluorolog-3 fluorometer/phosphorometer. The emission quantum yields were determined by the optical dilute method (Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991 (incorporated by reference herein)) in degassed solutions, and a degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem=0.042, λex=436 nm) (Van Houten, J.; Watts, R. J. Am. Chem. Soc. 1976, 98, 4853 (incorporated by reference herein)) was used as the reference. The excited-state lifetimes, the triplet excited-state quantum yields, and the triplet transient difference absorption spectra were measured in degassed solutions on an Edinburgh LP920 laser flash photolysis spectrometer. The third harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant, pulsewidth ˜4.1 ns, repetition rate was set at 1 Hz) was used as the excitation source. Each sample was purged with Ar for 30 minutes before measurement.
The self-quenching rate constants (kQ) in CH2Cl2 were deduced following the Stern-Volmer equation:
k
obs
=k
Q
[C]+k
0 (1)
where kobs is the decay rate constant of the emission (kobs=1/τem), kQ is the self-quenching rate constant, [C] is the concentration of the complex in mol/L, and k0 (k0=1/τ0) is the decay rate constant of the excited-state at infinite dilute solution. A plot of the observed decay rate constant versus concentration should give rise to a straight line. The slope of the straight line corresponds to kQ and the intercept corresponds to k0.
The triplet excited-state molar extinction coefficient and triplet quantum yield were determined by the partial saturation method. Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref Data 1986, 15,1 (incorporated by reference herein). The optical density at 585 nm was monitored when the excitation energy at 355 nm was gradually increased. Saturation was observed when the excitation energy was higher than 10 mJ. The following equation was then used to fit the experimental data to obtain the εT and ΦT. Id.
ΔOD=a(1−exp(−bIp)) (2)
where ΔOD is the optical density at 585 nm, Ip is the pump intensity in Einstein·cm−2, a=(εT−ε0)dl, and b=2303 ε0ex T/Å. εT and ε0 are the absorption coefficients of the excited state and the ground state at 585 nm, ε0ex is the ground state absorption coefficient at the excitation wavelength of 355 nm, d is the concentration of the sample (mol·L−1), l is the thickness of the sample, and A is the area of the sample irradiated by the excitation beam.
The nonlinear transmission measurement experimental setup was similar to that described previously (F. Guo, W. Sun, Y. Liu, K. Schanze, Inorg. Chem. 2005, 44, 4055 (incorporated by reference herein)), with a 20-cm focal length lens used to focus the beam to the 2-mm thick sample cuvette. A Q-switched Quantel Brilliant Nd:YAG laser operated at the second-harmonic wavelength (532 nm) was used as the light source. The repetition rate of the laser was 10 Hz. Energies of the incident laser beam were attenuated by a combination of a half-wave plate and a polarizer. The beam was then split by a wedged beamsplitter. One of the reflected beams was used to monitor the incident energy. The diameter of the transmitted beam was reduced to half of the original size by a telescope and was focused by a 20-cm plano-convex lens (f/52.5) to the center of a 2-mm sample cell. The radius of the beam waist was approximately 17.8 μm. The incident energy and the output energy were monitored by two Molectron J4-09 pyroelectric joule meters.
The open-aperture Z-scan measurements experimental setup is similar to the one reported previously. Li, Y.; Pritchett, T. M.; Huang, J.; Ke, M.; Shao, P.; Sun, W. J. Phys. Chem. A 2008, 112, 7200 (incorporated by reference herein). For ns Z-scan measurements, the second harmonic output (532 nm) from a Quantel Brilliant Nd:YAG laser with a pulsewidth of 4.1 ns and a repetition rate of 10 Hz was used as the light source. The laser beam was focused by a 30-cm focal-length plano-convex lens to a beam waist of 30 μm at the focal point, which corresponds to a Rayleigh length (
where ω0 is the radius at the beam waist) of 5.31 mm. For ps Z scans, the light source was the second harmonic output of an EKSPLA PL2143A passively mode-locked, Q-switched Nd:YAG laser (pulsewidth=21 ps, repetition rate=10 Hz). A 15-cm plano-convex lens was used to focus the beam to a beam waist of 34 μm at the focal point, which gave rise to a Rayleigh length of 6.82 mm. Therefore, the sample solution placed in a 1-mm thick quartz cuvette for ns measurements and in a 2-mm cuvette for ps measurements could be considered as thin samples. In both nanosecond and picosecond Z-scan measurements, the laser beam was split by a wedged beamsplitter. One of the reflected beams was used to monitor the incident energy, while the transmitted beam was focused by the lens to the sample cell. The sample was mounted on a translation stage (Newport M-UTM100) and moved through the vicinity of the focal plane. A computer was used to control the translation stage movement and the data acquisition. A 50-cm plano-convex lens was placed at approximately 30 cm after the linear focal plane to collect all of the transmitted light into the Molectron J4-09 joulemeter probe.
A five-band model that consists of a ground state, two singlet excited states and two triplet excited states was used to fit the Z-scan experimental data. The detailed descriptions of the model and the fitting procedure were reported previously. Li, Y.; Pritchett, T. M.; Huang, J.; Ke, M.; Shao, P.; Sun, W. J. Phys. Chem. A 2008, 112, 7200 (incorporated by reference herein).
The UV-vis absorption spectra of F-5-F-9 were investigated in CH2Cl2 at different concentrations (5×10−6 to 5×10−4 mol/L), and the results are presented in
The influence of the 4-fluorenyl substituent on the UV-Vis absorption spectra of F-5-F-9 is notable. The transition energies in these complexes are quite similar to those of their corresponding platinum ĈN̂N complexes without the 4-fluorenyl substituent, especially for the lowest-energy charge transfer bands, indicating that the fluorenyl substituent has a negligible effect on the energy level of the bipyridine based LUMO (π*(N̂N)) for the Pt(ĈN̂N)X complexes. However, the presence of the fluorenyl substituent significantly increases the molar extinction coefficients for the bands above 300 nm. This phenomenon likely arises from the extended π-system in the fluorenyl-substituted complexes, which could increase the oscillator strength for these transitions by increasing the transition dipoles.
The extinction coefficients for the dinuclear complex F-9 are much larger than those of the mononuclear complexes F-5-F-8. The extinction coefficients of the bands at ca. 360 nm and 490 nm are more than double of the respective bands in the mononuclear complexes, and the ε value at the low-energy absorption band apex is approximately 2.4×104 M−1 cm−1.
The charge transfer nature of the low-energy absorption bands in F-5-F-9 is supported by the solvent-dependency studies. As exemplified in
F-5-F-9 are emissive in solutions at room temperature and in glassy solutions at 77 K.
The high-energy emission band observed in the solution of F-9 are not attributed to a trace amount of uncoordinated 4-fluorenyl-ĈN̂N ligand or fluorenyl impurity because the free 4-fluorenyl-ĈN̂N ligand exhibits a structureless fluorescence at 378 nm in CH2Cl2 upon excitation at 325 nm and the free fluorene shows a slightly structured fluorescence at 312 nm in CH2Cl2 upon excitation at 265 nm. Neither the emission energy nor the spectral feature from the free 4-fluorenyl-ĈN̂N ligand and free fluorene is consistent with the high-energy band observed in F-9 solution. However, this high-energy fluorescence band is substantially the same as the fluorescence spectrum of 2,7-diethynyl-9,9-dihexylfluorene. Though not wishing to be bound by a particular theory, the fact that this band is only observed from F-9 solution but not from the solutions of the other four complexes suggests that this band is associated with the unique structural feature of F-9, i.e. the presence of the 2,7-diethynyl-9,9-dihexylfluorenyl bridging ligand. This high-energy emission band in F-9 solution may emanate from the coordinated bridging ligand, rather than the free bridging ligand impurity because the excitation spectrum.
The possibility of the low-energy red/orange emission band arising from fluorescence resonance energy transfer (FRET) from the fluorenyl component to the Pt(ĈN̂N) component could be excluded based on the fact that no low-energy red/orange emission band appears in the emission spectrum of the free 4-fluorenyl-ĈN̂N ligand.
The concentration-dependent emission was investigated for all five complexes. When the concentration is increased from 5×10−6 mol/L to 5×10−4 mol/L, the emission energies and the shapes of the spectra of F-5-F-9 remain essentially the same, however, the emission intensity decreases at high concentrations and the emission lifetimes decrease with increased solution concentration. Because these complexes have almost no ground-state absorption at the emission band apex and the shapes of the spectra are the same, self absorption of emission could be excluded. As described above, the UV-vis absorption obeys the Lambert-Beer's law in the concentration range of 5×10−6 mol/L to 5×10−4 mol/L, suggesting that no ground-state aggregation occurs in this concentration range. Therefore, the intensity decrease at high concentrations cannot be attributed to the ground-state aggregation. In view of the decreased lifetime at high concentrations, it appears that self-quenching occurs at high concentrations. The self-quenching rate constants (kQ) were measured according to the Stern-Volmer relation and the results are listed in Table 3. The kQ for F-5-F-9 varies between 1.12×109 and 1.63×109 L mol−1 s−1, which falls in the same order of values reported for the terpyridine platinum phenylacetylide complexes (1.45×109−1.74×109 L mol−1 s−1). The self-quenching constant for F-5 was much higher than that for F-6. Though not wishing to be bound by a particular theory, this could be explained by the presence of the branched alkoxyl chain in F-6, which could reduce the π,π stacking and thus the formation of excimer.
aMeasured at room temperature.
bIn butyronitrile glassy solutions at 77 K.
cAt a concentration of 5 × 10−5 mol/L.
dSelf-quenching rate constant.
eAt a concentration of 5 × 10−6 mol/L.
gAt a concentration of 5 × 10−6 mol/L in acetone.
hIn acetone solution.
jToo weak to be measured.
Complex F-9 exhibits a slightly different concentration-dependent behavior from the other four complexes. As shown in
With respect to the emission spectra shown in
F-5-F-8 are emissive in butyronitrile at 77 K. Compared to the emission at room temperature in CH2Cl2, the emission spectra shift to higher-energy and exhibit clear vibronic structures, as exemplified in
The lifetimes measured from the decay of the emission of F-5-F-9 suggest that the triplet excited state is much long-lived than the laser pulse width. Therefore, triplet excited-state absorption is expected to be observed for these complexes.
F-5 and F-6 exhibit broad positive absorption from 380 nm to 830 nm. The profiles of the TA spectra of F-7 and F-8 are similar to that of F-5, with the exception of that some bleaching occurred in the region of 420-480 nm and a flatter and stronger absorption in the NIR region of 770-850 nm. Though not wishing to be bound by a particular theory, the charge transfer band in the UV-vis absorption spectra and the different features in the NIR region indicate that the TA could possibly originate from the 3MLCT/3ILCT/3LLCT state for F-7 and F-8. For F-5 and F-6, the excited-state that gives rise to the transient absorption could be 3MLCT/3ILCT. All the TA decays monoexponentially throughout the whole spectral range monitored, indicating that the transient absorption arises from the same excited state or excited states in close proximity that are in equilibrium. In addition, the lifetime measured from the decay of the transient absorption and from the decay of the emission are essentially consistent, suggesting that and the TA may arise from the same excited state that emits, or a state that is in equilibrium with the emitting state.
Complexes F-5-F-8 exhibit broad positive TA in most of the visible to the near-IR region, which implies that their excited-state absorption cross-sections are larger than the ground-state absorption cross-sections. In addition, the TA decay times are hundreds of nanoseconds. Therefore, reverse saturable absorption (RSA) could occur for these platinum complexes under the irradiation of ns laser pulses. To demonstrate this, the nonlinear transmission experiments were carried out for F-5-F-9 in CH2Cl2 and the results are illustrated in
When the incident fluence is increased, the transmittance of F-5-F-8 decreases significantly. The RSA threshold, defined as the incident fluence at which the transmittance decreases to 70% of the linear transmittance, is 0.29 J/cm2 for F-5 and F-6, 0.48 J/cm2 for F-7, and 0.65 J/cm2 for F-8. When the incident fluence is increased to 1.8 J/cm2, the transmittance drops to 0.28 for F-5 and F-6, 0.37 for F-7, and 0.42 for F-8. For F-9, the transmittance almost keeps constant even at high fluences, indicating that almost no RSA occurs, which is consistent with the TA results discussed in the previous section. The strength of the RSA for these five complexes obviously follows this trend F-5=F-6>F-7>F-8>F-9.
The degree of RSA is determined at least in part by the ratio of the excited-state absorption cross-section to that of the ground-state. The ground-state absorption cross-sections at 532 nm for these complexes are 7.7×10−19 cm2 for F-5, 5.4×10−19 cm2 for F-6, 3.3×10−18 cm2 for F-7, and 4.2×10−18 cm2 for F-8 by using the E values obtained from their UV-Vis absorption spectra and the conversion equation σ=2303ε/NA, where NA is the Avogadro constant. To obtain the excited-state absorption cross-sections, open-aperture Z-scan experiments that measure the transmission changes due to nonlinear absorption were carried out at 532 nm using both ns and ps laser pulses for F-5-F-8. In the open-aperture Z-scan experiments, the laser beam at 532 nm is split by a wedge beamsplitter. One of the reflected beams is used to monitor the incident energy; while the transmitted beam is focused to the cuvette that contains the sample solution. The cuvette is placed on a translation stage that moves in the vicinity of the linear focal plane. The transmitted energy is monitored at each position where the translation stage stops. Then the transmission of the sample is plotted vs. the position of the translation stage. The resultant curve is fitted using the five-energy level model described previously to obtained the excited-state absorption cross sections. Li, Y.; Pritchett, T. M.; Huang, J.; Ke, M.; Shao, P.; Sun, W. J. Phys. Chem. A 2008, 112, 7200 (incorporated by reference herein).
aGround-state absorption cross-section.
bSinglet excited-state absorption cross-section.
cTriplet excited-state absorption cross-section.
The excited-state absorption cross-section values (both σs and σT) at 532 nm for F-5-F-8 are much larger than those for (ĈN̂N)PtC5H7 complex, which is in line with the trend observed in the UV-Vis absorption, reflecting the influence of fluorenyl unit on the ĈN̂N complexes. The ratios of σS/σ0 and σT/σ0 for F-5 and F-6 are among the largest values reported in the literature. When fitting the Z-scan results, the population fraction on related excited states are calculated versus time. For ns excitation, the triplet excited states (T1 and T2) are much more populated than the singlet excited states (S1 and S2). Therefore, the excited-state absorption for ns laser pulses is dominated by the triplet excited-state absorption. In such a case, the RSA is not only predominantly determined by the ratio of σT/σ0, but also affected by the triplet excited state quantum yield. Taking these factors into account, the ratio of ΦσT/σ0 (listed in Table 4) was found to correlate very well with the observed RSA trend shown in
In sum, introducing the fluorenyl substituent on the ĈN̂N ligand causes a pronounced effect on the photophysics of mononuclear and dinuclear platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine complexes F-5-F-9, which is reflected by their much larger molar extinction coefficients for their low-energy charge transfer absorption band, longer triplet excited-state lifetimes, higher emission quantum yields, and significantly increased ratios of the excited-state absorption cross-section to that of the ground-state compared to those of their corresponding Pt complexes without the fluorenyl substituent. Additionally, F-6 shows much improved solubility in CH2Cl2 (˜150 mg/mL), making it possible to prepare high-concentration solutions for two-photon absorption study in the near-IR region. Therefore, two-photon induced excited-state absorption was observed in the near-IR region for this complex.
All reagents and solvents (analytical grade) were purchased from VWR Scientific Company and used without further purification unless otherwise stated. The silica gel (230-400 mesh) was purchased from Alfa Aesar Company. Neutral Al2O3 (standard grade, 150 mesh) was purchased from Aldrich Company. All products were characterized by 1H NMR, elemental analysis, and high resolution mass spectrometry (HRMS) 1H NMR spectra were obtained using a Varian 400 MHz or 500 MHz VNMR spectrometer. HRMS was conducted on a Bruker Daltonics BioTOF system with electrospray ionization (ESI) source. Elemental analyses were conducted by NuMega Resonance Labs, Inc. in San Diego, Calif. High resolution MS data were obtained using Bruker BioT of III.
The precursors 2-iodofluorene (21), 2-bromo-7-iodofluorene (22), 2-bromo-9,9-di(2-ethylhexyl)-7-iodofluorene (23), 2-bromo-9,9-di(2-ethylhexyl)-7-diphenylaminofluorene (24), 4′-bromo-2,2′:6′,2″-terpyridine, 4′-vinyl-2,2′:6′,2″-terpyridine (28), 4′-(trifluoromethyl)sulfonyloxy-2,2′:6′,2″-terpyridine (OTf-tpy), were synthesized according to the procedures reported in the literature.
2-Bromo-9,9-di(2-ethylhexyl)-7-diphenylaminofluorene (24): 2-Bromo-9,9-di(2-ethylhexyl)-7-iodofluorene (23) (7.00 g, 0.012 mol), diphenylamine (2.58 g, 0.015 mol), 18-crown-6 ether (0.26 g, 0.001 mol), Cu (1.50 g, 0.023 mol), and K2CO3 (2.60 g, 0.019 mol) were added in 40 mL mesitylene. The mixture was heated to reflux under argon overnight. After the solvent was removed, the residue was dissolved in Et2O. The organic phase was washed with water, and dried over MgSO4. After removal of the solvent, the crude product was purified by a silica gel column using hexane as the eluent to give 4.50 g colorless oil (Yield: 42%). 1H NMR (400 MHz, CDCl3): δ 0.56 (8H, m), 0.75-1.00 (22H, m), 1.73-1.92 (4H, m), 7.00-7.12 (8H, m), 7.25 (4H, m), 7.46 (3H, m), 7.55 (1H, m) ppm.
7-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-di(2-ethylhexyl)fluoren-2-yl-diphenylamine (25): Compound 24 (1.0 g, 1.12 mmol) was dissolved in 50 mL dried THF at −78° C. 1.6 M BuLi/hexane solution (2.0 mL, 3.20 mmol) was added slowly. The mixture was stirred at −78° C. for 1 hr. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.42 mL, 1.12 mmol) was added dropwise. The mixture was allowed to reach room temperature and stirred at r.t. overnight. After that 50 mL brine was added to terminate the reaction. The aqueous layer was extracted with Et2O for three times (30 mL each time). The combined organic layer was dried over Na2SO4. After removal of the solvent, the crude product was purified by a silica gel column using hexane:toluene (v/v=5:1) as the eluent to give 735 mg colorless oil (Yield: 70%). 1H NMR (400 MHz, CDCl3): δ 0.50 (8H, m), 0.6-0.9 (22H, m), 1.34 (12H, s), 1.7-2.0 (4H, m) 6.9-7.1 (8H, m), 7.21 (4H, m), 7.58 (2H, m) 7.76 (2H, m) ppm.
Ligand 17: Compound 25 (0.75 g, 0.80 mmol) and 4′-bromo-2,2′:6′,2″-terpyridine (311 mg, 1.00 mmol) were added to 50 mL toluene. 2 M K2CO3 aqueous solution (2.5 mL) was added. The mixture was degassed in argon for 20 mins. Pd(PPh3)4 (46 mg, 0.04 mmol) was added. The mixture was heated to reflux under argon overnight. The organic layer was washed with water, dried over MgSO4 and filtered. After the solvent was removed, the crude product was purified on a silica gel column eluted by CH2Cl2 to give 422 mg pale yellow solid (Yield: 67%). 1H NMR (500 MHz, CDCl3): δ 0.6 (8H, m), 0.7-1.0 (22H, m), 1.86-2.10 (4H, m), 7.03 (2H, m), 7.11 (6H, m), 7.16 (4H, m), 7.38 (2H, t, J=6.5 Hz), 7.66 (1H, m), 7.76 (1H, m), 7.86 (2H, t, J=7.0 Hz), 7.91 (2H, t, J=7.0 Hz), 8.70 (2H, d, J=8.0 Hz), 8.78 (4H, m) ppm. HRMS: calcd for [C56H61N4]+, 789.4891; found, 789.4875. (100%).
Complex F-10: Ligand 17 (560 mg, 0.71 mmol) and Pt(DMSO)2Cl2 (300 mg, 0.73 mmol) were added to 80 mL CHCl3. The mixture was heated to reflux under argon for 24 hrs. After the solvent was removed, the residue was purified on an Al2O3 column eluted by CH2Cl2, followed by a mixture of CH2Cl2 and MeOH (1:1). The crude product was purified by recrystallization from CH2Cl2/Hexane/Et2O to give 486 mg red solid (Yield: 65%). 1H NMR (500 MHz, CDCl3): δ 0.6 (8H), 0.7-1.2 (22H), 2.0 (2H, m), 2.35 (2H, m), 7.13 (8H, m), 7.29 (4H, m), 7.48 (2H, m), 7.70 (1H, d, J=8.0 Hz), 7.97 (1H, d, J=8.0 Hz), 8.32 (3H, m), 8.53 (3H, m), 8.77 (2H, t, J=8.0 Hz), 9.11 (2H, d, J=7.5 Hz) ppm. HRMS: calcd for [C56H60N4PtCl]+, 1019.4156; found, 1019.4159. (100%). Anal. Calcd. for C56H60N4PtCl2.2.5CH2Cl2: C, 55.30; H, 5.12; N, 4.41. Found: C, 55.75; H, 5.30; N, 4.57.
Compound 26: Compound 24 (1.10 g, 1.70 mmol) and 2-methyl-3-butyn-2-ol (0.33 mL, 3.40 mmol) was added to 30 mL triethylamine. CuI (7.00 mg, 0.04 mmol), PPh3 (18.00 mg, 0.07 mmol), and Pd(PPh3)4 (50.0 mg, 0.04 mmol) were added. The mixture was heated to reflux under argon overnight. The solvent was removed, and the residue was dissolved in CH2Cl2. The solution was washed with water, and dried over Na2SO4. After the solvent was removed, the crude product was purified on a silica gel column eluted by a mixture of hexane and CH2Cl2 (v/v=1:1) to give 435 mg colorless oil (Yield: 40%). 1H NMR (400 MHz, CDCl3): δ 0.46-0.53 (8H, m), 0.68-0.86 (22H, m), 1.62 (6H, t, J=2.8 Hz), 1.75-1.89 (4H, m), 1.99 (1H, t, J=3.2 Hz), 7.01 (8H, m), 7.22 (4H, m), 7.34 (2H, m), 7.52 (2H, m) ppm.
Compound 27: Compound 26 (0.41 g, 0.64 mmol) and KOH (0.30 g, 5.36 mmol) were added to 10 mL 2-propanol. The mixture was heated to reflux under argon for 3 hrs. After the solvent was removed, the residue was purified on a silica gel column eluted by hexane to give 268 mg colorless oil (Yield: 72%). 1H NMR (400 MHz, CDCl3): δ 0.51 (8H, m), 0.68-0.90 (22H, m), 1.55-1.99 (4H), 3.06 (1H, s), 6.96-7.06 (8H, m), 7.23 (4H, m), 7.42 (2H, m), 7.53 (2H, m) ppm. HRMS: calcd for [C43H51N]+, 581.4016; found, 581.4019 (100%).
Ligand 18: Compound 27 (323 mg, 0.56 mmol) and OTf-tpy (212 mg, 0.56 mmol) were added to a mixture of benzene (50 mL) and iso-propylamine (20 mL). Pd(PPh3)4 (30 mg, 0.02 mmol) was then added. The mixture was heated to reflux under argon for overnight. After the solvent was removed, the residue was dissolved in CH2Cl2. The solution was washed with water, and dried over Na2SO4. After the solvent was removed, the crude product was purified on a neutral Al2O3 column eluted by a mixture of hexane and CH2Cl2 (v/v=10/1) to remove the unreacted reagent, and then eluted by a mixture of hexane and CH2Cl2 (1:1) to give 341 mg pale yellow solid (Yield: 75%). 1H NMR (400 MHz, CDCl3): δ 0.58 (8H, m), 0.7-1.0 (22H), 1.8-2.0 (4H, m), 7.0 (8H, m), 7.23 (4H, t, J=8.1 Hz), 7.35 (2H, m), 7.57 (4H, m), 7.87 (2H, dt, J=7.5, 1.5 Hz), 8.63 (4H, m), 8.74 (2H, dt, J=4.8 Hz) ppm. HRMS: calcd for [C58H61N4]+, 813.4891; found, 813.4917 (100%).
Complex F-11: Ligand 18 (340 mg, 0.42 mmol) and Pt(DMSO)2Cl2 (212 mg, 0.52 mmol) were added to 80 mL CHCl3. The mixture was heated to reflux under argon for 24 hrs. After the solvent was removed, the residue was purified by an Al2O3 column eluted first by a mixture of CH2Cl2 and CH3CN (v/v=5/2), and then by a mixture of CH2Cl2 and MeOH (10:1). The crude product was further purified by recrystallization from CH2Cl2/Hexane/Et2O to give 258 mg red solid (Yield: 57%). 1H NMR (400 MHz, CDCl3): δ 0.6 (8H), 0.7-1.0 (22H), 1.8-2.0 (4H), 7.10 (8H, m), 7.26 (4H, m), 7.52 (1H, m), 7.60 (2H, m), 7.70 (2H, m), 8.20 (2H, m), 8.40 (2H, t, J=8.0 Hz), 8.87 (2H, d, J=8.0 Hz), 9.01 (2H, d, J=4.8 Hz), 9.29 (2H, s) ppm. HRMS: calcd for [C58H60N4PtCl+C2H6O+H]+, 1089.4576; found, 1089.4543 (100%). Anal. Calcd. for C58H60N4PtCl2.0.8CH2Cl2.CH3CH2OH: C, 61.22; H, 6.04; N, 4.97. Found: C, 61.21; H, 5.71; N, 4.70.
Ligand 19: Compound 24 (580 g, 0.90 mmol), 28 (233 mg, 0.90 mmol) and P(o-tolyl)3 (100 mg, 0.33 mmol) were added to 30 mL triethylamine. Pd(OAc)2 (10 mg, 0.04 mmol) was then added. The mixture was heated to reflux under argon overnight. The mixture was filtered out and washed with Et2O. The filtrate was washed with water. The organic phase was dried over Na2SO4. The crude product was purified by a neutral Al2O3 column eluted by CH2Cl2 to give 571 mg orange viscous oil (Yield: 78%). 1H NMR (500 MHz, CDCl3): δ 0.6 (8H, m), 0.7-1.0 (22H), 1.8-2.1 (4H), 7.15 (9H, m), 7.25 (4H, m), 7.40 (2H, dd, J=6.5, 1.5 Hz), 7.59 (2H, m), 7.67 (2H, d, J=7.5 Hz), 7.93 (2H, d, J=16 Hz), 7.92 (2H, t, J=7.5 Hz), 8.57 (2H, s), 8.72 (2H, d, J=7.5 Hz), 8.80 (2H, d, J=5.0 Hz) ppm. HRMS: calcd for [C58H63N4]+, 815.5047; found, 815.5015. (100%) Anal. Calcd. for C58H62N4.toluene.1.5H2O: C, 83.56; H, 7.88; N, 6.00. Found: C, 83.11; H, 7.84; N, 6.02.
Complex F-12: Compound 19 (91.8 mg, 0.11 mmol) and Pt(DMSO)2Cl2 (47.6 mg, 0.12 mmol) were added to 80 mL CHCl3. The mixture was heated to reflux under argon for 24 hrs. After the solvent was removed, the residue was purified by an Al2O3 column eluted by CH2Cl2, and then by 1:1 CH2Cl2 and MeOH. The crude product was purified by recrystallization from CH2Cl2/Hexane/Et2O to give 55.8 mg red solid (Yield: 47%). 1H NMR (400 MHz, CDCl3): δ 0.6 (8H, m), 0.7-1.0 (22H), 1.8-2.2 (4H), 7.10 (8H, m), 7.26 (4H, m), 7.49 (2H, m), 7.61 (2H, m), 7.85 (2H, m), 8.20 (2H, m), 8.50 (5H, m), 8.92 (2H, m) ppm. HRMS: calcd for [C58H62N4PtCl]+, 1045.4313; found, 1045.4339 (100%). Anal. Calcd. for C58H62N4PtCl2.2.5CH2Cl2: C, 56.18; H, 5.22; N, 4.33. Found: C, 56.09; H, 5.32; N, 4.66.
The electronic absorption spectra were recorded on a SHIMADZU 2501 PC UV-vis spectrophotometer. The emission spectra at room temperature were recorded on a SPEX Fluorolog-3 fluorometer/phosphorometer. Complexes F-10, F-11, and F-12 were dissolved in CH3CN, and ligands 17, 18, and 19 were dissolved in CH2Cl2. The solutions were degassed via bubbling Ar gas for 30 min prior to each measurement. The emission lifetimes of ligand 17, 18, and 19 were measured on an Edinburgh LP920 laser flash photolysis spectrometer. The excitation beam was the third-harmonic output (355 nm) of a Quantel Brilliant Q-switched Nd:YAG laser (FWHM pulsewidth was 4.1 ns and the repetition rate was set at 1 Hz). The sample solutions were degassed for 30 min. before each measurement. The emission lifetime of complexes F-10, F-11, and F-12 were measured by time correlated single photon counting (TCSPC) technique (λex=375 nm). The sample solutions were prepared to have an absorbance at 375 nm of approximately 0.1-0.2. The emission quantum yields of the ligands 17, 18, and 19, and the complexes F-10, F-11, and F-12 were determined by the comparative method, in which 9,10-dipenylanthracence in ethanol (φem=0.9, excited at 350 nm) was used as the reference for the ligands, and a degassed aqueous solution of [Ru(bpy)3]Cl2 (φem=0.042, excited at 436 nm) was used as the reference for the complexes. The nanosecond triplet transient difference absorption spectra were measured on the Edinburgh LP920 laser flash photolysis spectrometer. The excitation was provided by the third-harmonic output (355 nm) of the Quantel Brilliant Q-switched Nd:YAG laser. The solutions were degassed via bubbling Ar gas for 30 min. before each measurement. The absorbance of the solution was adjusted to A=0.4 at 355 nm in a 1-cm quartz cuvette. The femtosecond transient difference absorption spectra were measured on a femtosecond pump-probe UV-vis spectrometer (HELIOS) manufactured by Ultrafast Systems LLC. The sample was excited at 400 nm with a 260 fs Ti:sapphire laser pulse, and the absorption was probed from 440 to 800 nm with white light continuum.
The 2PA spectra of the ligands 17-19 in toluene (toluene was chosen as the solvent instead of CH2Cl2 because the toluene solutions of 17-19 were much stable than the CH2Cl2 solutions upon laser irradiation) were measured by modified fluorescent method. The measurements were done by monitoring the wavelength-dependent two-photon-excited fluorescence, which allowed for direct measurement of 2PA in a broad variety of compounds with the fluorescence (or phosphorescence) quantum yield Φ>0.005. The relative spectrum was measured using coumarin 485 in methanol as a reference with a 2-nm interval for both the sample and the reference. Then the spectrum was normalized to a correct cross section measured at a single wavelength relative to 9,10-dichloroanthracene in dichloromethane.
The 2PA experimental setup and the detailed description of the experimental method were reported previously. N. S. Makarov, M. Drobizhev, A. Rebane, Opt. Expr. 2008, 16, 4029-4047 (incorporated by reference herein). The laser system comprises Ti:Sapphire femtosecond oscillator (Coherent Mira 900) pumped with a CW frequency-doubled Nd:YAG laser (Coherent Verdi). The oscillator is used to seed a 1-kHz repetition rate Ti:Sapphire femtosecond regenerative amplifier (Coherent Legend-HE). The output pulses from the amplifier are down-converted with an optical parametric amplifier (OPA) (Quantronix TOPAS-C). The output of the OPA (signal and idler) can be continuously tuned from 1100 to 2200 nm. For two-photon excitation second harmonic of either idler (790-1100 nm) or signal (550-790 nm) beam was used. A Glan-prism polarizer was placed before the second harmonic generation (SHG) crystal to select either vertical (signal) or horizontal (idler) polarization. The residual fundamental beam (signal or idler) was cut with color filters, placed after the SHG crystal. For one-photon excitation, the second harmonic of the Ti:Sapphire amplifier output (397 nm) was used. The polarization of the excitation laser beam was vertical for both 1PA and 2PA. In the case of second harmonic of signal, a λ/2 plate was used after the reference detector to rotate polarization by 90°.
The fluorescence was collected at 90° to the laser beam direction with a spherical mirror (f=50 cm, diameter d=10 cm), which focused the horizontally-elongated image of fluorescence track with the magnification ratio ˜1:1 on the entrance plane of the fluorescence grating spectrometer (Jobin Yvon Triax 550). The height of the vertical spectrometer slit was much larger than the height of the fluorescence image. The spectral dispersion on a two-dimensional CCD detector (Jobin Yvon Spectrum One) occurred in the horizontal direction, while the signal in the vertical direction was integrated over the whole slit height. The slit width was much smaller than the horizontal dimension of the fluorescence image and was kept the same in both 1PA and 2PA signal measurements. While recording the fluorescence spectrum, special care was taken to eliminate any spurious signals, such as scattered laser light, fluorescence of impurities, etc. The fluorescence spectra of the sample excited via 1PA and 2PA always had the same shape. The fluorescence intensity was measured by integrating the CCD output over 0.5-5 seconds and over 40-60 nm spectral region around the emission peak wavelength. Each data point was obtained by averaging of 2-5 acquisitions.
The raw spectra were obtained by measuring 2PA-excited fluorescence normalized to a square of the excitation laser power in a range of interest of excitation wavelengths. The absolute spectra were obtained by calibrating the unknown efficiency of fluorescence registration and fluorescence quantum yield and by correcting the raw spectra for the wavelength-dependent spatial and temporal laser profile.
An optical parametric generator (EKSPLA PG401) pumped by the third harmonic output of an EKSPLA PL2143A passively mode-locked, Q-switched Nd:YAG laser (pulsewidth=21 ps, repetition rate=10 Hz) was used as the light source. A 25-cm or 30-cm plano-convex lens was used to focus the beam to a beam waist of ˜30 μm at the focal point, which gave rise to a Rayleigh length (
where ω0 is the radius at the beam waist) of approximately 3.4-4.9 mm at the spectral range used for the Z-scan study. Therefore, the sample solution placed in a 2-mm cuvette could be considered as thin samples. A 50-cm plano-convex lens was placed at approximately 30 cm after the linear focal plane to collect all of the transmitted light into the Molectron J4-09 joulemeter probe.
The experimental Z-scan data were fitted using the five-band model, in which each chromophore molecule is assumed to lie in the vibration-rotation manifold of one of five electronic states: the ground state, S0, a singlet; one of two singlet excited states, S1 or S2; or one of two triplet states, T1 or T2. The following rate equations specify the time evolution of n0, nS, nT, nS2, and nT2, the number densities of molecules in, respectively, the S0-, S1-, T1-, S2—, and T2-bands.
Here, vis the frequency of the laser radiation, h is the Planck constant, I is the irradiance, and N is the overall number density of chromophore molecules. The constants kS, kS2, kT, kT2, and kisc are, respectively, the rate constants for the decays S1→S0, S2→S1, T1→S0, T2→T1, and S1→T1. Equations (1) and (2), which reflect the effects of radiative transitions from the ground state, include terms for both single-photon and two-photon S0→S1 transitions: σ0n0I[hv]−1 and σ2n0I2[2hv]−1, respectively. At wavelengths at which the linear absorption of the material is non-negligible (here, for wavelengths less than 680 nm), the former process dominates and σ2(λ) is set to zero. Conversely, at wavelengths longer than 740 nm, S1 is assumed to be populated from the ground state primarily by two-photon absorption and σ0(λ) is set to zero in the fitting.
Completing the five-band model is the following extinction law, which describes the decrease in intensity of the propagating beam as a result of single-photon absorption from S1 and T1 and either single- or two-photon absorption from S0, depending on the wavelength.
Electronic absorption: The electronic absorption spectra of 17-19 in CH2Cl2 at a concentration of 1×10−5 mol/L are shown in
[a]Electronic absorption band maxima and molar extinction coefficients in CH2Cl2 for 17-19, and in CH3CN for F-10-F-12, c ≈ 1 × 10−5 mol/L.
[b]Room temperature emission band maxima and decay lifetimes measured at a concentration of 1 × 10−5 mol/L.
[c]The emission band maxima and decay lifetimes measured in CH3CN solutions. λex = 375 nm.
[d]Emission quantum yield measured in CH2Cl2 solutions with A350 = 0.1.
[e]Emission quantum yield measured at λex = 436 nm with A436 = 0.1 in CH3CN solution. [Ru(bpy)3]Cl2 was used as the reference.
[f]The emission band maxima at 77 K measured with 10 equivalents of CH3I in 4:1 CH3CH2OH/CH3OH glassy solution, c ≈ 1 × 10−5 mol/L.
[g]The emission band maxima and decay lifetimes at 77 K measured in BuCN glassy solution, c ≈ 1 × 10−5 mol/L. λex = 355 nm.
Because diphenylamino group is an electron-donating group and terpyridine motif would become a stronger electron-withdrawing group upon protonation, therefore, stronger and red-shifted intramolecular charge transfer (ICT) is anticipated to occur in acidic solution. To verify this, titration of 17-19 with p-TsOH was carried out. As exemplified in
The electronic absorption spectra of platinum complexes F-10, F-11, and F-12 were measured in CH3CN solutions. As shown in
Photoluminescence: All of the ligands and the platinum complexes are emissive at room temperature in solutions and at 77 K in glassy matrix. The room temperature emission spectra of 17, 18, and 19 in CH2Cl2 are shown in
The assignment of the emitting state of the ligands to mixed 1ICT/1π,π* states can be partially supported by the emission lifetime measurement using the 355 nm laser beam as excitation source. The emission from all ligands exhibits bi-exponential decays by deconvolution of the decay curve, with a short lifetime of tens of ns being attributed to the 1ICT emission and a longer lifetime of approximately 100 ns being assigned to the 1π,π* emission. In addition, the emission lifetime and intensity remain the same in argon-saturated solution compared to those in air-saturated solution. These results clearly indicate that the observed emission from the ligands is fluorescence from singlet excited state that admixes 1π,π* and 1ICT characters.
The emission spectra of platinum complexes F-10-F-12 at room temperature upon excitation at wavelengths shorter than 400 nm are shown in
Upon excitation at the low-energy 1ILCT/1π,π*/1MLCT band, a very weak, structureless emission appears at approximately 570 nm for F-10, 590 nm for F-11, and 620 nm for F-12, as exemplified in
To verify the involvement of 1ICT or 1ILCT character into the emitting states of the ligands and the platinum complexes, the emission of the protonated ligands has been investigated.
As described above, the emission of F-10-F-12 is assigned as the mixture of 1π,π*/1ILCT characters, similar to those of their corresponding ligands, although the emission of F-10-F-12 also possibly admixes some 1MLCT character. However, the emission quantum yields and the emission lifetimes are much lower or shorter than those of the respective ligand. Though not wishing to be bound by a particular theory, these differences could be rationalized by the following three possible reasons. First, the coordination of platinum(II) ion with the terpyridine ligand decreases the electron-density on the terpyridine ligand, which increases the electron-withdrawing ability of the terpyridine component and enhances the charge transfer character of the complexes. Consequently, the emission from the complexes is quenched, which has been demonstrated by the ligand titration experiment. Secondly, the heavy-metal effect of the platinum increases the intersystem crossing from the singlet excited state to the triplet excited state. However, the triplet excited state is weakly-emissive due to the low-lying excited state that facilitates the decays through nonradiative relaxation to ground state; alternatively, it may decay through thermally accessible low-lying non-emissive 3d,d state. Thirdly, the decreased emission quantum yield may be also caused by the re-absorption due to the partial overlap of the low-energy absorption band and the emission spectrum.
The assignment of the emission of F-10, F-11 and F-12 at room temperature as fluorescence is further supported by the emission measurement at 77 K, which generally measures the emission from the triplet excited state. As shown in
Nanosecond transient absorption: The triplet excited-state absorption of 17, 18, and 19 were studied by excitation with 4.1 ns 355 nm laser pulse. The time-resolved spectra are exemplified in
Femtosecond transient absorption: F-10-F-12 exhibit fluorescence at room temperature and the transient absorption from the triplet excited state was not detected. To further investigate the singlet excited-state properties of F-10-F-12, the measurements of singlet excited-state absorption of these complexes were carried out using ultrafast femtosecond laser excitation (260 fs) at 400 nm. The transient difference absorption spectra at different decay time along with the ground-state absorption spectrum are exemplified in
Two photon absorption: Because of the conjugated structure and the charge-transfer nature of the ligands and the complexes, it is expected that all of the ligands and the complexes exhibit at least a moderately strong two-photon absorption (2PA) (σ2>100 GM, 1 GM=10−50·cm4·s.photon−l.molecule−1) upon NIR excitation. The 2PA spectra of 17, 18, and 19 in toluene were measured by two-photon excited fluorescence method and the results are shown in
Even though the peak σ2 values for the ligands are not particularly large, they are comparable to the values in similar structures such as BDPAS (4,4′-bis(diphenylamino)stilbene), in which σ2=230 GM at 650 nm. After coordination with the platinum ion, the intraligand charge transfer is enhanced, which is reflected by the red-shift of the low-energy absorption band in the UV-vis absorption spectra of the complexes. Wavelength dependent open-aperture Z-scan measurements were performed using 21 ps duration NIR pulses. To properly account for possible stepwise absorption from the excited states, the experimental data was fitted using a five-band model, which includes both the excited-state absorption and the two-photon absorption. In order to disambiguate the relative contributions of two-photon and excited-state absorption at those wavelengths at which the two-photon process represents the dominant mechanism for populating the excited states, the singlet excited-state absorption cross section σS(λ) was estimated from the fs transient absorption spectrum at zero time delay and only σ2(λ) was used as a fitting parameter. Nonlinear absorption was observed from 575 nm to 740 nm for F-10, 550 nm to 825 nm for F-11, and 575 nm to 670 nm for F-12.
43 ± 5[d]
[a]Ground-state absorption cross-section.
[b]Effective singlet excited-state absorption cross-section with the assumption of σS2 = σS.
[c]Estimated from the fs TA data at zero time delay.
[d]σS2 = (12 ± 7) × 10−18 cm2.
All solvents and reagents were purchased from Aldrich or Alfa Aesar and used as is unless otherwise stated. Compounds 33, 29 and F-14 were characterized by 1H-NMR and elemental analyses. Additional characterization by high-resolution electrospray ionization mass spectrometry (ESI-HRMS) was carried out on 29 and F-14. 1H-NMR spectra were measured on a Varian Oxford-400 VNMR spectrometer or a Varian Oxford-500 VNMR spectrometer; and ESI-HRMS analyses were conducted on a Bruker Daltonics BioTOF III mass spectrometer. Elemental analyses were performed by NuMega Resonance Labs, Inc. in San Diego, Calif.
6-Bromo-2,2′-bipyridine (34), 2,7-dibromo-9,9-diethyl-9H-fluorene (30), 7-bromo-9,9-diethyl-9H-fluorene-2-carbaldehyde (31), and 2-(7-bromo-9,9-diethyl-9H-fluoren-2-yl)-benzothiazole (32) were prepared according to the procedures published in the literature.
33. Compound 32 (3.05 g, 7.00 mmol) was dissolved in degassed dry THF (40 mL) and the solution was cooled down to −78° C. in a dry ice-heptane bath. 5.1 mL n-butyl lithium in hexane (1.60 M, 8.20 mmol) was then added dropwise under argon. After stirring for 30 mins, isopropyl pinacolyl borate (1.70 mL, 1.52 g, 8.20 mmol,) was added using a syringe. The reaction mixture was stirred overnight, first at −78° C. and then slowly warmed up to room temperature. After reaction, the mixture was again cooled down to 5° C., and treated with a hydrochloric acid solution (15 mL, 6.00 M). Then THF was removed by distillation, and the aqueous phase was extracted three times with diethyl ether (3×50 mL). The organic layer was washed with brine, dried with Na2SO4 and the solvent was removed. The residual solid was recrystallized from toluene to give 2.66 g yellow crystal (yield: 70%). 1H-NMR (CDCl3): δ 8.13 (s, 1H), 8.11 (d, 1H, J=8.0 Hz), 8.04 (d, 1H, J=8.0 Hz), 7.92 (d, 1H, J=8.0 Hz), 7.80-7.86 (m, 2H), 7.74-7.80 (m, 2H), 7.51 (t, 1H, J=9.0 Hz), 7.40 (t, 1H, J=9.0 Hz), 8.04 (m, 4H), 7.99 (t, 2H, J=8.0 Hz), 7.51 (t, 2H, J=8.0 Hz), 2.10-2.22 (m, 4H), 1.40 (s, 12H), 0.28-0.33 (m, 6H). Anal. Calc. for C30H32BNO2S: C, 74.8; H, 6.7; N, 2.9; Found: C, 74.8; H, 7.1; N, 3.2.
29. Compounds 33 (0.48 g, 1.00 mmol), 34 (0.35 g, 1.50 mmol), and K2CO3 (5.50 g, 0.04 mol) were dissolved in a mixed solvent of dioxane (40 mL), toluene (40 mL) and water (20 mL), and the solution was degassed with argon for 30 mins. Pd(PPh3)4 (33 mg, 0.03 mmol) and PPh3 (16 mg, 0.06 mmol) were then added. After refluxing for 72 hrs. under argon, the aqueous phase was extracted with diethyl ether (3×50 mL). The organic layer was washed with brine, dried with Na2SO4 and the solvent was removed. The residual solid was purified by chromatography on silica gel (Sorbent Technologies, 60 Å, 230˜450 mesh) column. The byproduct was removed first by toluene, then the desired product was obtained by using dichloromethane or ether as the eluent. The crude product was purified by recrystallization from dichloromethane and heptane to give colorless crystal (0.35 g, yield: 70%) suitable for X-ray diffraction analysis. 1H-NMR (CDCl3): δ 8.72 (d, 1H, J=4.0 Hz), 8.70 (d, 1H, J=8.0 Hz), 8.44 (dd, 1H, J=8.0 Hz and 1.2 Hz), 8.24 (d, 1H, J=1.6 Hz), 8.21 (dd, 1H, J=8.0 Hz and 1.6 Hz), 8.20 (s, 1H), 8.14 (d, 1H, J=8.0 Hz), 8.05 (dd, 1H, J=8.0 Hz and 1.6 Hz), 7.79-7.90 (m, 6H), 7.47 (dt, 1H, J=8.0 Hz and 1.2 Hz), 7.36 (dt, 1H, J=8.0 Hz and 1.2 Hz), 7.26-7.32 (m, 1H), 2.20-2.28 (m, 4H), 0.46 (t, 6H, J=7.6 Hz). ESI-HRMS: m/z calcd for [C34H27N3S+H]+: 510.1998; found, 510.1980. Anal. Calc. for C34H27N3S: C, 80.12; H, 5.34; N, 8.24; Found: C, 79.85; H, 5.66; N, 8.40.
F-13. Ligand 29 (130 mg, 0.26 mmol) and Pt(DMSO)2Cl2 (126 mg, 0.30 mmol) were dissolved in DMF (5 mL) with a few drops of water. The solution was stirred at 80° C. for 30 hrs. under argon. The formed yellow solid was collected by filtration, washed with water, methanol and ether, and dried in vacuum. 190 mg crude product of 7 was obtained in quantitative yield. Due to the poor solubility of 7 (insoluble in ethanol, ether, hexane and toluene, slightly soluble in DMSO, DMF and CH2Cl2), it could not be further purified by column chromatography or recrystallization. Therefore, it was used directly in the following step for preparation of F-14 without further purification and/or characterization.
F-14. To a degassed suspension of F-13 (128 mg, 0.16 mmol) and 1-ethynyl-4-methylbenzene (23 mg, 0.20 mmol, 25 μL) in DMF (70 mL), powder KOH (14 mg, 0.25 mmol) and catalytic amount of CuI were added. The reaction mixture was heated and stirred at 80° C. for 48 hrs. under argon. After removing the solvent, the residue was washed with water and ether, dried in vacuum, and purified by column chromatography on silica gel (Sorbent Technologies, 60 Å, 230˜450 mesh) column Dichloromethane with 3% methanol (V/V) was used as the eluent. The pure product of complex 1 was obtained as orange solid (44 mg, yield: 31%). 1H-NMR (500 MHz, CDCl3): δ 9.24 (s, 1H), 8.44 (t, 1H, J=42 Hz), 8.15 (d, 1H, J=1.5 Hz), 8.12 (d, 1H, J=8.0 Hz), 8.03 (dd, 1H, J=8.0 Hz and 1.5 Hz), 8.01 (d, 1H, J=10 Hz), 8.88-7.96 (m, 3H), 7.82 (t, 1H, J=8.0 Hz), 7.63 (br, 2H), 7.57 (d, 2H, J=8.0 Hz), 7.52 (dt, 2H, J=8.0 Hz and 1.0 Hz), 7.41 (dt, 2H, J=8.0 Hz and 1.0 Hz), 7.36 (s, 1H), 7.18 (d, 2H, J=8.0 Hz), 2.06-2.26 (m, 4H), 0.43 (t, 6H, J=7.5 Hz). ESI-HRMS: m/z calcd for [C43H33N3PtS+H]+: 819.2119; found, 819.2105. Anal. Calc. for C43H33N3PtS.CH2Cl2: C, 58.47; H, 3.90; N, 4.65; Found: C, 58.33; H, 3.93; N, 4.91.
The UV-vis spectra of 29 and F-14 in different solvents (spectrophotometric grade) were acquired using a UV-2501 spectrophotometer. The steady state emission spectra in different solvents were obtained on a SPEX fluorolog-3 fluorometer/phosphorometer. The emission quantum yields were measured by the relative actinometry method in degassed solutions. A degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem=0.042, λex=436 nm) was used as the reference for complex F-14 and an aqueous solution of quinine sulfate (Φf=0.546, λex=347.5 nm) was used as the reference for ligand 29. The excited-state lifetimes, the triplet transient difference absorption spectra, and the triplet excited-state quantum yields were measured on an Edinburgh LP920 laser flash photolysis spectrometer. The third harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant, pulsewidth ˜4.1 ns, repetition rate is set to 1 Hz) was used as the excitation source. Sample solutions were purged with Ar for 30 mins. prior to each measurement. The femtosecond transient difference absorption spectra and the singlet excited-state lifetime were measured using a femtosecond pump-probe UV-vis spectrometer (HELIOS) manufactured by Ultrafast Systems LLC. The sample solution in a 2-mm cuvette was excited at 400 nm using a 150-fs Ti:Sapphire laser (Spectra Physics Hurricane, 1 kHz repetition rate, 1 mJ/pulse at 800 nm) and the absorption was probed from 425 to 800 nm with sapphire generated white-light continuum.
The triplet excited-state molar extinction coefficients (εT) at the TA band maximum were determined by the singlet depletion method, in which the following equation was used to calculate the εT.
where ΔODS and ΔODT are the optical density changes at the minimum of the bleaching band and at the maximum of the positive band in the TA spectrum, respectively, and εS is the ground-state molar extinction coefficient at the wavelength of the bleaching band minimum. After obtaining the εT value, the triplet excited-state quantum yield was measured by relative actinometry, in which SiNc in benzene was used as the reference (ε590=70,000 M−1cm−1, ΦT=0.20).
The nonlinear absorption of complex F-14 was characterized by open-aperture Z-scan experiment using ns laser at 532 nm and ps laser from 450 nm to 900 nm, and by nonlinear transmission experiment at 532 nm using ns laser. The experimental data were fitted using a five-level model to extract the excited-state absorption cross sections and the two-photon absorption cross sections.
The nonlinear transmission experiment for complex F-14 was conducted in CH2Cl2 in a 2-mm cuvette using 4.1 ns laser pulses at 532 nm. The light source was a Quantel Brilliant ns laser with a repetition rate of 10 Hz. The experimental setup and details are the same as previously described. A 20-cm plano-convex lens was used to focus the beam to the 2-mm thick sample cuvette. The linear transmission of the solution was adjusted to 80% at 532 nm.
Electronic Absorption. The electronic absorption of 29 and F-14 obeys Beer-Lambert law in the concentration range used in herein (5×10−6 mol/L-5×10−3 mol/L). The UV-Vis absorption spectra of 29 and F-14 in CH2Cl2 are shown in
aUV-vis absorption band maxima and molar extinction coefficients in CH2Cl2.
bEmission band maximum, quantum yield, and lifetime in CH2Cl2 at a concentration of 5 × 10−6 mol/L.
cEmission band maxima and lifetime in butyronitrile matrix at 77 K at a concentration of 5 × 10−6 mol/L.
dfs TA band maximum and singlet excited-state lifetime in CH2Cl2.
ens TA band maximum, triplet extinction coefficient, triplet excited-state lifetime and quantum yield.
fMeasured in butyronitrile.
gMeasured in CH2Cl2•SiNc in C6H6 was used as the reference. (ε590 = 70,000 L · mol−1 · cm−1, ΦT = 0.20)
The absorption of 29 is dominated by structured bands in the UV region, which emanate from the 1π,π* transitions. The polarity of solvent exhibits minor effect on the UV-Vis spectrum of 29, which is consistent with the 1π,π* assignment. For complex F-14, the dominant absorption also appears in the UV region, however, the bands are red-shifted compared to those in 29, indicating the delocalization of the ligand centered molecular orbitals through interactions with the platinum dπ orbitals. Considering the similarity in energy of these bands for F-14 and 29 and the large molar extinction coefficients of these bands in F-14, these bands can be assigned to 1π,π* transitions within the 6-(7-benzothiazol-2′-yl-9,9-diethyl-9H-fluoren-2-yl)-2,2′-bipyridine ligand (predominantly within the benzothiazolylfluorene component) as well. In addition, a broad, structureless tail between 430 nm and 530 nm is observed in complex F-14, but not in ligand 29. With reference to other Pt(II) ĈN̂N and terpyridyl acetylide complexes, this tail could be attributed to the 1MLCT/1LLCT (metal-to-ligand/ligand-to-ligand charge transfer) transitions. The assignment of this low-energy absorption band is bolstered by DFT calculations, in which the HOMO is dominated by the tolylacetylide ligand and the Pt components, and the LUMO has major contribution from the bipyridine component. Another piece of evidence that supports the charge transfer nature of the low-energy absorption band is the negative solvatochromic effect. This band shifts to a longer wavelength in less polar solvents, such as toluene and hexane, in comparison to those in more polar solvents (CH3CN, CH2Cl2 and DMSO). This is indicative of the charge transfer character of the ground state.
Emission. 29 and F-14 are both emissive in solutions at room temperature and in glassy matrix at 77 K. As shown in
For complex F-14, upon excitation at 388 nm, a broad, somewhat structured emission appears at 591 nm with a shoulder at ca. 634 nm. The vibronic spacing between the peak and the shoulder is approximately 1150 cm−1, corresponding to the ring breathing mode of the aromatic rings in the ligands. The lifetime of the emission is approximately 1.43 μs. In view of the large Stokes shift of the emission and the long lifetime, the emission from F-14 at room temperature should originate from a triplet excited state. The vibronic structure in the emission spectrum suggests that the 3π,π state should be involved in the emission, possibly mixed with some 3MLCT character, which is partially supported by the negative solvatochromic effect of the emission. In less polar solvents, such as in toluene and hexane, the emission spectra are red-shifted and become less structured in contrast to those in more polar solvents, such as ethanol, acetonitrile, CH2Cl2 and acetone. This probably is indicative of different degrees of mixing 3MLCT character into 3π,π state in solvents with different polarities. In polar solvents, the less polar 3MLCT state (in comparison to the more polar MLCT ground state) is less stabilized than the ground state, which would cause the blue-shift of the 3MLCT emission. In contrast, the influence of the solvent polarity on the 3π,π excited state is not as significant as that on the 3MLCT state. This would allow for the 3MLCT state and the 3π,π state to be energetically more close to each other in polar solvents, resulting in more configurational mixing of 3π,π and 3MLCT characters in the emission in polar solvents. On the other hand, in less polar solvents, the 3MLCT state would be more stabilized and its energy level would be lowered, while the energy of the 3π,π state is less affected. Consequently, the energy gap between these two states becomes larger and the contribution from the 3π,π state is reduced. This is reflected by the less structured emission spectra and shorter lifetime in less polar solvents. When the concentration of the CH2Cl2 solution increases from 1×10−6 mol/L to 1×10−4 mol/L, the intensity of the emission keeps increasing and the lifetime remains the same, suggesting that no self-quenching occurs in the concentration range used for our study.
The emission spectra of 29 and F-14 in butyronitrile matrix at 77 K are given in
Transient Absorption Spectroscopy. Transient difference absorption spectroscopy measures the difference between the excited-state absorption and the ground-state absorption. Thus, it can provide information on the spectral region where the excited-state absorption is stronger than that of the ground-state and predict the wavelength region where reverse saturable absorption could occur. From the decay of the transient absorption, the lifetimes of the excited state giving rise to the excited-state absorption are obtained. This is especially important for measuring the singlet excited-state lifetime of the Pt(II) complexes that cannot be obtained from the decay of fluorescence because of the lack of fluorescence at room temperature in many cases. By estimating the triplet excited-state molar extinction coefficient at the triplet excited-state absorption band maximum using the singlet depletion method, and using the relative actinometry, with SiNc in benzene as the reference, the triplet excited-state quantum yield can be obtained. Both the singlet and triplet transient difference absorption spectra of 29 and F-14 were measured using fs and ns pump-probe UV-Vis spectrometers, respectively.
The time-resolved triplet transient difference absorption spectra of 29 in butyronitrile and F-14 in CH3CN are presented in
Z-scan Study and Nonlinear Absorption Cross Sections. Z-scan is a simple nonlinear optical characterization technique that is used to separately measure the contributions of nonlinear absorption and nonlinear refraction to the observed optical nonlinearity of a material. To obtain both the singlet and triplet excited-state absorption cross sections and to separate the contribution of two-photon absorption from excited-state absorption in the near-IR region, open-aperture Z scans were performed in CH2Cl2 solution at 532 nm using both ns and ps laser pulses and at a variety of visible and near-IR wavelengths using ps pulses. The experimental data were then fitted using the five-level model with the input parameters (σ0, τs, τT, ΦT) obtained from the photophysical studies described above. In this way, it was possible to obtain values for the triplet and singlet excited-state absorption cross sections at various wavelengths. Representative Z-scan data and fitting curves are provided in
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†σT(532 nm) = 1.03 × 10−16 cm2 was determined from combined fitting of nanosecond and picosecond Z-scan data. For other wavelengths, σT(λ) is determined from the value of σT(532 nm) and the fs TA spectrum at 5.8-ns time delay.
‡Effective two-photon absorption cross section for the Z scan of lowest energy (0.5 J/cm2). Z scans at a progression of higher energies (0.7 and 1.1 J/cm2) yield effective σ2 values of 1900 GM and 2000 GM, respectively, clear evidence for two-photon-initiated excited-state absorption.
£Effective two-photon absorption cross section for the Z scan of 0.3 J/cm2 fluence on axis.
The data in Table 9 show that the excited-state absorption cross sections at all of the wavelengths studied are in the range of 10−17 cm2 to 10−16 cm2, which is comparable to or even larger than those reported in the literature for other reverse saturable absorbers. The combination of strong excited-state absorption and weak ground-state absorption in the visible and near-IR leads to large ratios of the excited-state to ground-state absorption cross sections. The ratio becomes extremely large at longer wavelengths, which places it among the largest ratios reported to date for reverse saturable absorbers. The ratios of F-14 are larger than most of the reported reverse saturable absorbers. Although the ratios of F-14 are smaller than those for platinum 2,2′-bipyridine complex bearing 2-(benzothiazol-2′-yl)-9,9-diethyl-7-ethynylfluorene ligands at multiple wavelengths, the reverse saturable absorption spectral region for F-14 (430-680 nm) is broader than that (450-600 nm) for the platinum 2,2′-bipyridine complex. This feature along with the large absorption cross-section ratios, as well as the long triplet excited-state lifetime makes complex F-14 a very promising broadband reverse saturable absorber.
In the near-IR region, where the ground-state absorption is extremely weak, the excited state is populated by two-photon absorption. The observed nonlinear absorption is thus two-photon initiated excited-state absorption. The singlet and triplet excited-state absorption cross sections at 740 nm and 800 nm were estimated from the values at 532 nm and the femtosecond transient difference absorption spectra at zero and 5.8 ns time delays, respectively, and these estimated values were used as parameters in the model, which allowed the two-photon absorption cross sections at these wavelengths to be obtained by fitting the Z-scan data. At wavelengths of 850 nm and above, femtosecond transient difference absorption data were not available, so this method could not be used to provide an estimate of the excited-state absorption cross sections. As the relative contributions of two-photon and excited-state absorption cannot be unambiguously deconvolved for wavelengths of 850 nm, 875 nm, and 910 nm, the two-photon absorption cross sections given in Table 9 for these wavelengths represent effective values. The two-photon absorption cross sections of F-14 are among the largest values reported for platinum complexes.
Reverse Saturable Absorption. Both the fs and ns transient absorption measurements suggest that complex F-14 exhibits stronger excited-state absorption than ground-state absorption in the visible to the near-IR region. Therefore, reverse saturable absorption in this spectral region should occur. To demonstrate this, a nonlinear transmission experiment was carried out at 532 nm using 4.1 ns laser pulses. The result is shown in
All of the reagents were purchased from Aldrich Chemical Company or Alfa Aesar and used as is. The ligands 4,4′-di(tert-butyl)-2,2′-bipyridine (tBu2 bpy) and 2-(benzothiazol-2′-yl)-9,9-diethyl-7-ethynylfluorene (35), and the precursor (tBu2 bpy)PtCl2 were prepared according to the literature procedures. All of the solvents purchased from VWR International are HPLC grade and used without further purification unless otherwise stated. The precursors and complex F-15 were characterized by 1H NMR, electrospray ionization mass spectrometry (ESI-MS) and elemental analyses. 1H NMR spectra were obtained on a Varian Oxford-400 VNMR spectrometer or a Varian Oxford-500 VNMR spectrometer. ESI-MS analyses were performed with a Bruker BioTOF III mass spectrometer. Elemental analyses were carried out by NuMega Resonance Laboratories, Inc. in San Diego, Calif.
The ligand 35 (200 mg, 0.53 mmol) and (tBu2bpy) PtCl2 (134 mg, 0.25 mmol) were dissolved in degassed dry CH2Cl2 (50 mL) and diisopropyl amine (5 mL). The catalyst CuI (˜5-10 mg) was then added. The reaction mixture was refluxed under argon for 24 hrs. After cooling to room temperature, the reaction solution was washed with brine, dried with Na2SO4 and the solvent was removed. The residual solid was purified by column chromatography on silica gel using CH2Cl2 as the eluent. The product was further purified by recrystallization from dichloromethane and hexane to yield yellow needle crystal 0.21 g (yield: 64%). 1H NMR (400 MHz, CDCl3): δ 9.75 (d, 2H, J=6.0 Hz), 8.08 (s, 2H), 8.06 (d, 2H, J=8.4 Hz), 8.00 (dd, 2H, J=8.0 Hz, 1.6 Hz), 7.96 (s, 2H), 7.88 (d, 2H, J=8.0 Hz), 7.72 (d, 2H, J=8.0 Hz), 7.64 (d, 2H, J=8.0 Hz), 7.55-7.63 (m, 6H), 7.46 (dt, 2H, J=8.0 Hz, 1.2 Hz), 7.34 (dt, 2H, J=8.0 Hz, 1.2 Hz), 7.26-7.32 (m, 1H), 2.03-2.15 (m, 4H), 1.43 (s, 18H), 0.36 (t, 6H, J=7.6 Hz). ESI-HRMS: m/z calcd for [C70H64N4PtS2+H+Na]+: 1243.4130; found, 1243.4123. Anal. Calc. for. C70H64N4PtS2.C6H14, C, 69.86; H, 6.02; N, 4.29; found: C, 69.67; H, 6.31; N, 4.57.
The UV-vis absorption spectra were acquired on an Agilent 8453 spectrophotometer in different HPLC grade solvents. The steady-state emission spectra were recorded on a SPEX fluorolog-3 fluorometer/phosphorometer in different solvents. The emission quantum yields were determined by the relative actinometry method in degassed solutions, in which a degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem=0.042, λex=436 nm) was used as the reference. The femtosecond transient absorption measurements were performed using a femtosecond pump-probe UV-vis spectrometer (HELIOS) manufactured by Ultrafast Systems LLC. The sample solution in a 2-mm cuvette was excited at 400 nm using a 150-fs Ti:Sapphire laser (Spectra Physics Hurricane, 1 kHz repetition rate, 1 mJ/pulse at 800 nm) and the absorption was probed from 425 to 800 nm with sapphire generated white-light continuum. The emission lifetime and the triplet transient difference absorption (TA) spectrum and the decay time were measured in degassed solutions on an Edinburgh LP920 laser flash photolysis spectrometer. The third harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant, pulsewidth ˜4.1 ns, repetition rate was set at 1 Hz) was used as the excitation source. Each sample was purged with Ar for 30 minutes before each measurement.
The triplet excited-state absorption coefficient (εT) at the TA band maximum was determined by the singlet depletion method. The following equation was used to calculate the εT.
where ΔODS and ΔODT are the optical density changes at the minimum of the bleaching band and the maximum of the positive band in the TA spectrum, and εS is the ground-state molar extinction coefficient at the wavelength of the bleaching band minimum. After obtaining the εT value, the triplet excited-state quantum yield could be obtained by the relative actinometry, in which SiNc in benzene was used as the reference (ε590=53,400 M−1cm−1, ΦT=0.20).
The open aperture Z-scan measurements were carried out using ns laser at 532 nm and ps laser from 450 nm to 900 nm. The experimental setup and experimental details were similar to those reported previously by our group. The experimental data were fitted using a five-level model.
The nonlinear transmission experiment was carried out using 4.1 ns laser pulses at 532 nm. A Quantel Brilliant ns laser with a repetition rate of 10 Hz was used as the light source. The focal length of the plano-convex lens used to focus the beam to the 2-mm thick sample cuvette was 20 cm.
Electronic absorption. The UV-vis absorption of F-15 obeys Lambert-Beer's law in the concentration range used in our study (2×10−6 mol/L-1×10−4 mol/L), indicating that no ground-state aggregation occurs in this concentration range.
aUV-vis absorption band maxima and molar extinction coefficients in CH2Cl2.
bEmission band maximum, quantum yield, intrinsic lifetime, and self-quenching rate constant in CH2Cl2.
cEmission band maxima and lifetimes in BuCN matrix at 77 K.
dfs TA band maximum and singlet excited-state lifetime in CH2Cl2.
ens TA band maximum, triplet extinction coefficient, triplet excited-state lifetime and quantum yield in CH2Cl2.
In addition to the major band at 374 nm, a low-energy tail is observed between 410 nm and 500 nm in the spectrum of F-15. This band is red-shifted in less polar solvents, such as hexane and toluene, indicative of the charge-transfer nature of this band. With reference to that reported for other diimine platinum acetylides complexes, this band can be attributed to the 1MLCT transition.
To understand the singlet excited-state absorption and obtain the lifetime of the singlet excited state, which will be used as an input parameter for fitting the Z-scan data, fs transient absorption measurements were carried out. The time-resolved TA spectrum of F-15 in CH2Cl2 is given in
Z scan. Both the ns and fs transient absorption spectra indicate that complex F-15 exhibits relatively strong singlet and triplet excited-state absorption from 450 nm to 800 nm. However, the observed transient difference absorption is likely a combination of the excited-state absorption from the first excited state to the second excited state (S1→S2 or T1→T2) and from S2 or T2 to a higher excited state Sn or Tn (S indicates the singlet excited state and T refers to the triplet excited state). It is not possible to deconvolve these contributions using the transient absorption measurement alone. To obtain the absorption cross sections of the singlet and triplet excited states and separate the contributions from S1 and S2 states, open-aperture Z scans were carried out at 532 nm using both ns and ps laser pulses and at a variety of visible and near-IR wavelengths using ps laser pulses having a series of different energies. It is expected that at lower pulse energies only the lowest lying excited states will be populated, and the singlet excited-state absorption would be dominated by the absorption of S1→S2. At higher excitation energies, however, the S2 state could acquire a significant population so that the absorption driving the transition S2→Sn would make a non-negligible contribution to the Z-scan signal, which would have to be taken into account. At longer wavelengths where the ground-state absorption of F-15 is negligible, it is still possible to populate the excited state via two-photon absorption (TPA), so at these wavelengths two-photon absorption from the ground state in combination with subsequent excited-state absorption should be considered. To obtain values for the various absorption cross sections, the Z-scan experimental data were fitted by a five-level model that tracks the relative populations of the ground state S0, and of the S1, S2, T1, and Tm excited states, where Tm denotes a triplet excited state lying above T1. The various photophysical parameters that appear in the model were determined from independent measurements: values of the ground-state absorption cross section σ0(λ) were obtained from the UV-vis absorption spectrum; the singlet excited-state lifetime, from the decay of the fs TA; the triplet excited state lifetime, from the decay of ns TA; the triplet quantum yield, from the relative actinometry; and the effective triplet-triplet excited-state absorption cross section σT(λ) was deduced from the fs TA curve at 5.8-ns time delay in combination with the value of σT at 532 nm, 4.6×10−16 cm2, which itself was determined from combined fitting of ns and ps Z-scan data.
70g
40g
34g
28g
28g
600i
300i
300i
aDetermined by fitting Z-scan data except where otherwise indicated.
bThe ground-state absorption cross section.
cThe first singlet excited-state absorption cross section.
dThe effective triplet excited-state absorption.
eThe second singlet excited-state absorption cross section.
fThe two-photon absorption cross section.
gDetermined from the value σS(532 nm) = 6 × 10−17 cm2 and the fs transient difference absorption spectrum at 0 time delay.
hσT(532 nm) = 4.6 × 10−16 cm2 determined from combined fitting of ns and ps Z-scan data. For other wavelength, σT(λ) is determined from the value of σT(532 nm) and the fs transient difference absorption spectrum at 5.8-ns time delay.
iEffective cross-section for excited-state-assisted two-photon absorption.
The excited-state absorption cross sections shown in Table 11 are all in the range of 10−17 cm2 to 10−15 cm2. These values are comparable to or even larger than those reported in the literature for other reverse saturable absorbers. Most importantly, due to the weak ground-state absorption of F-15 in the visible to the near-IR region, the ratios of the excited-state absorption cross section to that of the ground state become extremely large when the wavelength becomes longer. In addition to these large ratios, the two-photon absorption cross sections at the near-IR region deduced for F-15 are also the largest values reported for platinum complexes. It is also worth noting that the TPA band maximum of F-15 almost coincides with its corresponding one-photon absorption band maxima (
Reverse saturable absorption. The Z scan experiments and fitting results discussed above imply that complex F-15 could exhibit strong reverse saturable absorption in the visible spectral region due to the extremely large ratio of the excited-state absorption to that of the ground state. To demonstrate this, a nonlinear transmission experiment was carried out at 532 nm using 4.1 ns laser pulses. The result is shown in
The platinum 2,2′-bipyridine complex bearing 2-(benzothiazol-2′-yl)-9,9-diethyl-7-ethynylfluorene ligands exhibits a strong absorption band at 374 nm in CH2Cl2 solution, which is attributed to the somewhat delocalized 1π,π* transition of the acetylide ligands. A broad, weak 1MLCT band appears between 410 and 500 nm. It is emissive at room temperature and at 77 K. The emitting state at room temperature can be switched from the acetylide ligand localized 3π,π* state in polar solvents, such as CH3CN and CH2Cl2, to the 3MLCT state in less polar solvents such as hexane and toluene. The modulation of the order of the excited states by solvent polarity is supported by a transient absorption study using solvents of differing polarity. The most striking feature of complex F-15 is its very broad and strong nonlinear absorption in the visible to the near-IR region, which manifests itself in extremely large ratios of the excited-state absorption cross section to that of the ground-state in the visible spectral region and in the largest two-photon absorption cross sections in the near-IR region compared to the other reported platinum complexes. This feature, along with its weak ground-state absorption in the visible to the near-IR region, makes complex F-15 a very promising candidate for photonic devices that require large and broadband nonlinear absorption.
Synthesis and Characterization. All of the reagents and solvents for synthesis were purchased from Aldrich Chemical Co. or Alfa Aesar and used as is unless otherwise stated. Silica gel is from sorbent technology in standard grade (60 Å, 230-400 mesh, 500-600 m2/g, pH: 6.5-7.5). The complexes F-16-F-21 were characterized by 1H NMR, electrospray ionization mass spectrometry (ESI-MS), and elemental analyses. The ligands 38-41 were characterized by 1H NMR and elemental analyses. Every intermediate is characterized by 1H NMR. 1H NMR was obtained on Varian Oxford-VNMR spectrometers at the frequencies of 300 M, 400 M, or 500 M. ESI-MS analyses were performed at a Bruker BioTOF III mass spectrometer. Elemental analyses were carried out by NuMega Resonance Laboratories, Inc. in San Diego, Calif.
4,4′-Di(5,9-diethyl-7-tridecanyl)-2,2′-bipyridine (56) was synthesized following literature procedure. Domínguez-Gutíerrez, D.; De Paoli, G.; Guerrero-Martínez, A.; Ginocchietti, G.; Ebeling, D.; Eiser, E.; De Cola, L.; Cornelis J.; Elsevier, C. J. J. Mater. Chem. 2008, 18, 2762 (incorporated herein by reference). 57 was synthesized by the reaction of K2PtCl4 with 56 in refluxing aqueous HCl solution. 42, 44, 46, 48 and 50 were synthesized by Heck reaction. 53 was synthesized by Wittig reaction. 4 was synthesized by Ullmann reaction from 53. Sonogashira coupling reaction of 42, 44, 46, 48, 50, and 54 with ethynyltrimethylsilane or 2-methyl-3-buytn-2-ol followed by hydrolysis by treating with K2CO3 or KOH in i-PrOH afforded ligands 36-41.
56. Colorless oil. 1.13 g, yield: 89%. 1H NMR (400 M Hz, CDCl3): 8.55 (d, J=4.8 Hz, 2H), 8.23 (s, 2H), 7.08 (dd, J=5.2, 1.6 Hz), 2.80 (m, 2H), 1.42-1.62 (m, 8H), 0.98-1.40 (m, 36H), 0.70-0.90 (m, 24H).
57. Yellow solid. 1.03 g, yield: 64%.1H NMR (400 M Hz, CDCl3): 0.72-0.87 (m, 24 H), 0.92-1.37 (m, 36H), 1.55 (m, 8H), 7.32 (dd, J=6, 1.6 Hz, 2H), 7.60 (s, 2H), 9.64 (d, J=6 Hz, 2H).
General procedure for synthesis of complexes F-16-F-21. The mixture of 57 (0.1 mmol), stilbenzylacetylide ligand (0.24 mmol), CuI (5 mg), CH2Cl2 (15 mL), and diisopropylamine (5 mL) was refluxed under argon for 24 hrs. After reaction, the excess ligands were removed by flash silica gel column eluting with dichloromthane. The collected yellow or red solid was recrystallized from dichloromethane and hexane.
F-16. Red powder, 60 mg, yield: 49%. 1H NMR (CDCl3, 400 MHz): 9.57 (s, 2H), 8.17-8.20 (m, 4H), 7.72 (s, 2H), 7.59-7.61 (m, 8H), 7.39-7.46 (m, 6H), 7.23 (d, J=16.4 Hz, 2H), 7.08 (d, J=16.4 Hz, 2H), 2.92 (m, 2H), 1.56 (m, 8H), 1.05-1.38 (m, 36H), 0.75-0.89 (m, 24H). ESI-HRMS: m/z Calc. for [C76H96N4O4Pt+Na]+: 1347.6994; Found: 1347.6966. Anal. Calc. for C76H96N4O4Pt.2H2O.CH2Cl2: C, 63.97; H, 7.11; N, 3.88; Found: C, 63.96; H, 7.61; N, 4.20.
F-17. Red powder, 72 mg, yield: 55%. 1H NMR (CDCl3, 400 MHz): 9.96 (s, 2H), 9.65 (s, 2H), 7.84 (d, J=8.4 Hz, 4H), 7.71 (s, 2H), 7.62 (d, J=8.4 Hz, 4H), 7.55 (d, J=8.0 Hz, 2H), 7.43 (d, J=8.0 Hz, 2H), 7.39 (d, J=5.6 Hz, 2H), 7.23 (d, J=16.4 Hz, 2H), 7.08 (d, J=16.4 Hz, 2H), 2.91 (m, 2H), 1.57 (m, 8H), 1.04-1.40 (m, 36H), 0.75-0.89 (m, 24H). ESI-HRMS: m/z Calc. for [C76H98N2Pt+Na]+: 1312.7174; Found: 1312.7166. Anal. Calc. for C78H98N2O2Pt.2H2O: C, 70.40; H, 8.03; N, 2.11; Found: C, 70.29; H, 8.22; N, 2.48.
F-18. Red powder, 57 mg, yield: 41%. 1H NMR (CDCl3, 400 MHz): 9.69 (d, J=5.6 Hz, 2H), 7.69 (s, 2H), 7.31-7.53 (m, 18H), 7.05 (d, J=16.4 Hz, 2H), 6.96 (d, J=16.4 Hz, 2 H), 2.90 (m, 2H), 1.57 (m, 8H), 1.05-1.36 (m, 36H), 0.75-0.89 (m, 24H). ESI-HRMS: m/z Calc. for [C76H96N2Br2Pt+Na]+: 1415.5478; Found: 1415.5461. Anal. Calc. for C76H96Br2N2Pt: C, 65.55; H, 6.95; N, 2.01; Found: C, 65.59; H, 7.17; N, 2.34.
F-19. Yellow powder, 26 mg, yield: 21%. 1H NMR (CDCl3, 400 MHz): 9.71 (d, J=5.6 Hz, 2H), 7.69 (s, 2H), 7.48-7.54 (m, 8H), 7.31-7.42 (m, 10H), 7.19-7.24 (m, 2H), 7.02 (d, J=16.4 Hz, 2H), 7.09 (d, J=16.4 Hz, 2H), 2.91 (m, 2H), 1.53-1.58 (m, 8H), 1.05-1.38 (m, 36H), 0.75-0.90 (m, 24H). ESI-HRMS: m/z Calc. for [C76H98N2Pt+Na]+: 1256.7276; Found: 1256.7227. Anal. Calc. for C76H98N2Pt.0.5C6H14: C, 74.26; H, 8.28; N, 2.19; Found: C, 74.53; H, 8.40; N, 2.37.
F-20. Yellow powder, 78 mg, yield: 60%. 1H NMR (CDCl3, 400 MHz): 9.76 (d, J=6.0 Hz, 2H), 7.72 (m, 2H), 7.41-7.56 (m, 12H), 6.98 (d, J=16 Hz, 2H), 7.05 (d, J=16 Hz, 2 H), 6.92 (d, J=9.0 Hz), 3.86 (s, 6H), 2.95 (m, 2H), 1.50-1.73 (m, 8H), 1.09-1.46 (m, 36H), 0.70-0.99 (m, 24H). ESI-HRMS: m/z Calc. for [C78H102N2O2Pt+Na]+: 1316.7487; Found: 1316.7515. Anal. Calc. for C78H102N2O2Pt: C, 72.36; H, 7.94; N, 2.16; Found: C, 71.87; H, 8.25; N, 2.29.
F-21. Red powder, 71 mg, yield: 45%. 1H NMR (CDCl3, 400 MHz): 9.69 (m, 2H), 7.69 (s, 2H), 7.51 (d, J=8.4 Hz, 2H), 7.35-7.38 (m, 10H), 7.21-7.26 (m, 8H), 7.07-7.10 (m, 8H), 6.97-7.04 (m, 12H), 2.91 (m, 2H), 1.58 (m, 8H), 1.05-1.41 (m, 36H), 0.75-0.90 (m, 24H). Anal. Calc. for C100H116N4Pt.0.5CH2Cl2: C, 74.90; H, 7.32; N, 3.48; Found: C, 75.20; H, 7.72; N, 3.74.
The solvents for photophysical experiments that are purchased from VWR International are spectroscopic grade and used as is without further purification. An Agilent 8453 spectrophotometer was used to record the UV-vis absorption spectra in different solvents. A SPEX fluorolog-3 fluorometer/phosphorometer was used to record the steady-state emission spectra in different solvents. The emission quantum yields were determined by the relative actinometry method14 in degassed solutions, in which a degassed 1 N sulfuric acid solution of quinine bisulfate (Φem=0.546, λex=347.5 nm) was used as the reference. The femtosecond transient absorption measurements were performed using a femtosecond pump-probe UV-vis spectrometer (HELIOS) manufactured by Ultrafast Systems LLC. The sample solution in a 2 mm cuvette was excited at 400 nm using a 150 fsTi:Sapphire laser (Spectra Physics Hurricane, 1 kHz repetition rate, 1 mJ/pulse at 800 nm), and the absorption was probed from 425 to 800 nm with sapphire generated white-light continuum. The emission lifetime and the triplet transient difference absorption (TA) spectrum and the decay time were measured in degassed solutions on an Edinburgh LP920 laser flash photolysis spectrometer. The third harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant, pulsewidth 4.1 ns, repetition rate was set at 1 Hz) was used as the excitation source. Each sample was purged with argon for 30 min before each measurement. The triplet excited-state absorption coefficient (εT) at the TA band maximum was determined by the singlet depletion method.16 The following equation was used to calculate the εT.
where ΔODS is minimum of the bleaching band and ΔODT is the maximum of the absorption band in the TA spectrum, and εs is the ground-state molar extinction coefficient at the wavelength of the bleaching band minimum. After the εT value is obtained, the ΦT could be obtained by the relativeactinometry, in which SiNc in benzene was used as the reference (ε590=70000 M−1 cm−1, ΦT=0.20).
The nonlinear absorption of complex F-15-F-21 was characterized by nonlinear transmission experiment at 532 nm using a ns laser. The nonlinear transmission experiment was conducted in CH2Cl2 in a 2-mm cuvette using 4.1 ns laser pulses at 532 nm. The light source was a Quantel Brilliant ns laser with a repetition rate of 10 Hz. The experimental setup and details are the same as previously described. Guo, F.; Sun, W.; Liu, Y.; Schanze, K. S. Inorg. Chem. 2005, 44, 4055 (incorporated herein by reference). A 40-cm plano-convex lens was used to focus the beam to the 2-mm thick sample cuvette. The linear transmission of the solution was adjusted to 80% at 532 nm.
Electronic absorption. The UV-Vis spectra of complexes F-16-F-21 are shown in
The absorption spectra of complexes F-16, F-19, F-20, and F-21 (
Theorλabs/nm (ex. state)
Theorλabs—calculated wavelength corresponding to the transition between the ground and excited state of interest (number of excited state is shown in parentheses),
Photoluminescence. All of the complexes exhibit weak emission both at room temperature in dichloromethane solution and at 77 K in butyronitrile glassy matrix. The lifetime of the emission could not be detected by our spectrometer due to either too short lifetime or too weak signal. The emission spectra of the complexes F-16-F-21 at room temperature are shown in
To elucidate the nature of emission of complexes F-16-F-21, their spectra are compared to the photoluminescence of the corresponding ligands, 36, 39, 40, and 41 (
TD-DFT calculations of the singlet and triplet emission support this assignment and show that in complexes 1 and 6 the singlet emission stems from the dissymmetric 1π,π* transition in one of the single stilbenzylacetylide ligands, which become slightly twisted relative to the Pt coordination plane. In all other complexes this character is strongly mixed with the 1MLCT from Pt to bPy moiety. Triplet state emission follows the same scheme as the singlet emission, but involves a more symmetric 3π,π* excitation delocalized over both stylbenilacetylide ligands. Overall, calculated trends in singlet and triplet emission are very close to the trends observed by experiments (Table 14).
The nature of red-shift in the photoluminescence spectra observed for the complexes with the stronger electron donating/accepting groups (F-16 and F-21) can be explained by the interplay in the electronic levels of the stylbenzyl ligands and the MLCT states, similar to the picture discussed in the absorption part. Strong electron donating/accepting groups lower the energies of the stylbenilacetylide π,π* transitions relative to the MLCT states, so that the lowest excited states bear more of the π,π* character. This process is highly sensitive to the solvent polarity, as solvent can partially stabilize the dipole moment induced by the substituent group and raise the intraligand transition energies. In other words, in a less polar solvent, π,π* transitions have more admixture of MLCT character.
theorλfluo/
(theor
λphos/
indicates data missing or illegible when filed
Transient difference absorption. The nanosecond and femtosecond transient difference absorption (TA) for complexes F-16-F-21 and nanosecond TA for ligand 36, 39, 40 and 41 were all measured. Through this TA experiment, the scope of the reverse saturate absorption (RSA) can be determined. The nanosecond TAs of complexes F-16-F-21 in acetonitile solution at zero time decay were shown in
Nonlinear transmission. The nonlinear transmission of the complexes F-16-F-21 for ns 532 nm laser pulses was studied in dichloromethane solution at a linear transmittance of 80%. The results are shown in
The metal complexes synthesized in the above examples will be used in optical-switching devices. In forming the optical-switching device, the metal complexes will be dissolved in a solvent, and the resulting solution will be substantially filling the cavity between the transparent substrates of the optical-switching device.
The metal complexes and ligands synthesized in the above examples will be used as light-emitting materials in an organic light-emitting diode (OLED). Layers of indium tin oxide, the organic light-emitting materials, and aluminum will be continuously deposited in a vacuum chamber on a glass substrate. The indium tin oxide will be used as an anode, and the aluminum layers will be used as a cathode. The organic compound layer of light-emitting materials is thus interposed between the anode and the cathode. A DC voltage will be applied to the OLED with the anode as a positive electrode and the cathode as a negative electrode, as result of which light will be emitted.
The metal complexes and ligands synthesized in the above Examples will be used as chemical sensors. Filter paper strips will be impregnated with a coating solution that includes the metal complexes. The coated filter paper strips will be dried. The dried filter paper strips will be contacted with organic vapors, and the color of the filter paper strips before the exposure to the organic vapors will be compared to that of the filter paper strips after the exposure.
The metal complexes and ligands synthesized in the above examples will be used as anion sensors. They are sensitive to basic anions such as F−, H2PO4− and OAc−. Upon addition of F−, H2PO4−, or OAC− to the DMSO solutions of the ligand or complexes, the color of the solutions will change drastically from yellow to purple or blue; whereas addition of NO3−, Cl−, Br− and I− (all as tetra-n-butylammonium salts, TBA salts) has no effect on the solution color. Addition of F−, H2PO4− or OAC− into the DMSO solutions of the ligands or complexes also induces substantial changes in their respective emission spectrum.
This application claims the benefit of U.S. Provisional Application Ser. No. 61/405,387, filed Oct. 21, 2010 which is incorporated by reference herein.
This invention was made with government support under grant CHE-0449598 awarded by the National Science Foundation (NSF), and grants W911NF-06-2-0032 and W911NF-10-2-0055 awarded by the Army Research Lab. The Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61405387 | Oct 2010 | US |
