Patent Application
20150303390
The present invention relates to a device having an electrically conductive surface and carrying a molecular assembly, preferably composed of two or more redox-active based molecular components arranged in a specific order or sequence, such that the sequence of the components and their thickness dictate the assembly properties and consequently the uses of the device.
Abbreviations: AFM, atomic force microscopy; BPEB, 1,4-bis[2-(4-pyridyl)ethenyl]benzene; CV, cyclic voltammogram; DCM, dichloromethane; DMF, dimethylformamide; FTIR, fourier-transform infrared; ITO, indium tin oxide; MLCT, metal-to-ligand charge-transfer; RT, room temperature; SDA, sequence dependent assembly; SPMA, self-propagating molecule-based assembly; TBAPF6, tetrabutylammonium hexafluorophosphate; THF, tetrahydrofuran; XPS, X-ray photoelectron spectroscopy; XRR, X-ray reflectivity.
Multi-component materials might display synergistic effects and possess functions not attainable with single-component systems. The composition, structure, and phase segregation of multi-component materials is difficult to control. The controlled layer-by-layer assembly of metal complexes can induce systematic changes in the physicochemical properties of the materials. However, the use of a layer-by-layer assembly technique inherently brings about a certain assembly sequence. For instance, for mono-metallic molecular assemblies, the sequence follows a simple order where each deposition of a metal complex is followed by the deposition of the cross linker. In fact most systems follow such straightforward deposition sequence. However, what sequence does one follow if multiple metal and/or functionalities are incorporated into a single molecular assembly, and how does the assembly sequence affect the molecular properties. These are critical questions, with important implications in the field of multistate memory, electrochromic windows, smart windows, binary memory, electrochromic displays, bulk-hetero-junction solar cells, inverted type solar cells, dye sensitized solar cells, molecular diodes, charge storage devices, capacitors, or transistors. There is thus a great need to answer these questions and to study the effect of the assembly sequence on the molecular properties.
International Publication No. WO 2011/141913 discloses a solid-state, multi-valued, molecular random access memory device, comprising an electrically, optically and/or magnetically addressable unit, a memory reader, and a memory writer. The addressable unit comprises a conductive substrate; one or more layers of electrochromic, magnetic, redox-active, and/or photochromic materials deposited on the conductive substrate; and a conductive top layer deposited on top the one or more layers. The memory writer applies a plurality of predetermined values of potential biases or optical signals or magnetic fields to the unit, wherein each predetermined value applied results in a uniquely distinguishable optical, magnetic and/or electrical state of the unit, thus corresponding to a unique logical value. The memory reader reads the optical, magnetic and/or electrical state of the unit.
International Application No. PCT/IL2013/050584 discloses a logic circuit for performing a logic operation comprising a plurality of predetermined solid-state molecular chips, each molecular chip having multiple states obtained after application of a corresponding input. After applying predetermined inputs on the molecular chips, reading the states of the molecular chips produces a logical output according to the logic operation.
The aforesaid patent publications are herewith incorporated by reference in their entirety as if fully disclosed herein.
In one aspect, the present invention provides a device comprising a substrate having an electrically conductive surface and carrying an assembly of one or more molecular components, each molecular component having a thickness and an oxidative or reductive peak potential, and comprising one or more entities each independently is a redox-active compound,
provided that:
wherein exposure of said device, when comprising one molecular component, to a potential change, causes electron transfer, which results in an electrochemical signature which can be read out electrically, optically, magnetically, or by conductivity measurements; and exposure of said device, when comprising more than one molecular components, to a potential change, causes (a) reversible electron transfer; (b) oxidative catalytic electron transfer with charge trapping; (c) reductive catalytic electron transfer; or (d) blocking of the electron transfer, dependent on the order of said components and the thickness of each one of said components, which results in an electrochemical signature which can be read out electrically, optically, magnetically, or by conductivity measurements.
In certain embodiments, the redox-active compounds composing the molecular components of the device of the present invention each independently is a metal, preferably a transition metal, complex, e.g., a tris-bipyridyl complex of said transition metal. Particular such tris-bipyridyl complexes exemplified herein are those of the general formula I:
wherein
M is said transition metal;
X is a counter anion selected from Br−, Cl−, F−, I−, PF6−, BF4−, OH−, ClO4−, SO3−, SO4−, CF3COO−, CN−, alkylCOO−, arylCOO−, or a combination thereof;
R2 to R25 each independently is selected from hydrogen, halogen, hydroxyl, azido, nitro, cyano, amino, substituted amino, thiol, C1-C10 alkyl, cycloalkyl, heterocycloalkyl, haloalkyl, aryl, heteroaryl, alkoxy, alkenyl, alkynyl, carboxamido, substituted carboxamido, carboxyl, protected carboxyl, protected amino, sulfonyl, substituted aryl, substituted cycloalkyl, substituted heterocycloalkyl, or group A, wherein at least two, preferably three, of said R2 to R25 each independently is a group A:
wherein A is linked to the ring structure of the compound of general formula II via R1; and R1 is selected from cis/trans C═C, C≡C, N═N, C═N, N═C, C—N, N—C, alkylene, arylene or a combination thereof; and any two vicinal R2-R25 substituents, together with the carbon atoms to which they are attached, may form a fused ring system selected from cycloalkyl, heterocycloalkyl, heteroaryl or aryl, wherein said fused system may be substituted by one or more groups selected from C1-C10 alkyl, aryl, azido, cycloalkyl, halogen, heterocycloalkyl, alkoxy, hydroxyl, haloalkyl, heteroaryl, alkenyl, alkynyl, nitro, cyano, amino, substituted amino, carboxamido, substituted carboxamido, carboxyl, protected carboxyl, protected amino, thiol, sulfonyl or substituted aryl; and said fused ring system may also contain at least one heteroatom selected from N, O or S.
In other embodiments, the redox-active compounds composing the molecular components of the device of the present invention each independently is an organic molecule. Particular such organic molecules exemplified herein are 1,3,5-tris(4-ethenyl pyridyl)benzene, 1,3,5-tris(2-(pyridin-4-yl)ethyl)benzene, and 1,4-bis[2-(4-pyridyl)ethenyl]benzene.
In certain embodiments, the device of the present invention comprises a substrate having an electrically conductive surface and carrying an assembly of one molecular component, e.g., such devices wherein the molecular component comprises two or more, preferably two, entities. Such devices can be used in fabrication of a multistate memory, electrochromic window, smart window, electrochromic display, or binary memory.
In other embodiments, the device of the present invention comprises a substrate having an electrically conductive surface and carrying an assembly of more than one molecular component, e.g., two molecular components wherein each component preferably comprises one entity and the components are preferably assembled in any alternate or successive order; or three or more molecular components wherein each component preferably comprises one entity and the components are preferably assembled in any random, alternate or successive order. Particular such devices, when comprising an assembly of more than one molecular component assembled in an alternate order, can be used in fabrication of a multistate memory, electrochromic window, smart window, binary memory, electrochromic display, bulk-hetero-junction solar cell, inverted type solar cell, dye sensitized solar cell, molecular diode, charge storage device, capacitor, or transistor. Other such devices, when comprising an assembly of more than one molecular component assembled in a successive order, can be used in fabrication of a smart window, electrochromic display, bulk-hetero-junction solar cell, inverted type solar cell, dye sensitized solar cell, molecular diode, charge storage devices capacitor, or transistor.
The device of the present invention can be described as a molecular assembly composed of two or more molecular components, e.g., the molecular components A, B and C, each composed of one or more redox active entities such as metal complexes, inorganics, organics, polymers etc., wherein the molecular components are arranged in a specific order or sequence, i.e., in a SDA. Together with the surface-interface thickness, i.e., the thickness of each layer (or molecular component) during the deposition process (usually consisting of one redox active entity), the SDA dictates the multi-component material, i.e., the overall assembly, properties, which in turn dictates the functionality of the device (solar cell, memory, battery, diode, electrochromic window etc.).
The material properties result from the SDA of molecular components A, B, and C, wherein A, B, and C are chosen from a family of redox active entities such that the separation of the oxidative peak potential between any of the molecular entities in molecular components A, B, or C, e.g., EoxA−EoxB or EoxB−EoxC, is larger than 100 mV, i.e. for any of the molecular components DEox≧100 mV. This separation simultaneously applies for the separation of the reductive peak potentials so DEred≧100 mV. The total requirement therefore for a successful device is that: DEox and DEred≧100 mV, wherein EoxA>EoxB>EoxC> . . . EoxZ and EredA<EredB<EredC< . . . EredZ, so that E1/2A>E1/2B>E1/2C> . . . E1/2Z (the labels A, B, C etc. are, of course, arbitrarily assigned to fulfill those conditions, so that A always has the highest oxidation potential and Z has the lowest oxidation potential).
However, upon assembly of two molecular components, comprising of one or more entities, for instance, there are different possibilities in which the components can be arranged (see, e.g.,
In cases wherein molecular components A and B are arranged in an alternating fashion (I), wherein the electrochemical differences of said entities in molecular components A and B is EoxA−EoxB≧100 mV so that EoxA>EoxB as defined above, the electron transfer of each one of the individual entities is not affected by the presence of the other entity, and the oxidation/reduction waves of both entities are thus visible in the CV. The order in which components A and B are alternating (ABABAB or BABABA) is not important, and can also include a third component (C) or fourth component (D) until the amount of desirable components, as long as the abovementioned requirements (alternating order; electrochemical requirements for said entities) are met. It is important to note that the thickness of the components (layer thickness) in the alternating assembly cannot exceed a certain thickness, i.e., the thickness of the molecular components once assembled in the molecular assembly cannot exceed a threshold limit, so that they become insulating (e.g., 8 nm in the case of Os and Ru system exemplified herein). The electrochemical properties in such this specific kind of assembly order (alternating; I) allow for individual addressing of the molecular component and therefore direct towards the fabrication of multi-state memory and electrochromic windows (as discussed in Study 2 hereinafter). The mechanism of electron transfer described above is shown in the
In contrast, in cases wherein molecular components A and B are arranged in a sequential order (II or III); A followed by B or alternatively B followed by A, wherein the electrochemical differences of said entities in molecular components A and B is EoxA−EoxB≧100 mV so that EoxA≧EoxB as defined above, a different electrochemical behavior is observed. If component A is assembled first followed by component B, where the thickness of component A exceeds a certain threshold (8.0 nm in case A is Ru), the electrochemical behavior is controlled solely by component A. In particular, since the entity comprising component A has an oxidation potential higher than the entity that comprises component B, and the thickness is such that component A is insulating component B from the surface, the molecular entity in component B is insulated from the surface such that no oxidation occurs when a potential of EoxB is applied. However, when EoxA is approached, small amounts of the entity in component A are oxidized, which in turn are able to catalytically oxidize the entire entities of component B. In such a way, the molecular entities in component A behaves as a catalytic gate for the oxidation of the entities in component B and the electron transfer occurs unidirectional. Moreover, when applying a reducing potential, since now entities in component A is reduced first, the catalytic gate is closed, such that there is no way for the entities in component B to be reduced (note: component A insulates component B from the surface). This results in charge trapping of component B on the outside. This type of behavior is of course preserved if a component C is added, as long as the entities in component C has a lower oxidation potential than that of those component A, and the assembly follows the order ABC or ACB. In short, any additional component can be added as long as the oxidation potential of the entities in the component added is lower than that of A, and the assembly order after component A has been deposited is irrelevant (e.g., ABCD or ACDB or ADBC etc. . . . should all give identical electrochemical behavior). This electrochemical behavior is specific for SDA II, results in uni-molecular current flow with charge trapping, and is good for molecular diodes, solar cells, and battery technology. The mechanism is shown in
However, in cases molecular component B is first assembled followed by molecular component A, two distinct electron-transfer pathways (i and ii) are observed depending on the surface-interface thickness of molecular component B. When the thickness of component B is sufficiently low (e.g., <2.6 nm in the case of Os and Ru), the electron transfer occurs exactly as described for an alternating assembly, and direct oxidation of the entities in components A and B by the electrode is possible. At intermediate thickness of component B (e.g., 3.6-6.1 nm in the case of Os and Ru), the oxidation of the molecular component A is more difficult due to the interference (insulating nature) of molecular component B and is directly attributed to the fact that electron transfer from the entities in components A and B; Ared to Box is thermodynamically unfavourable. The thermodynamic and kinetic effects of electron transfer at the interface of molecular components A and B is even more pronounced, when the molecular assembly is reduced. Scanning in the negative direction, two distinct pathways (i and ii) are observed, in which the electrode is able to reduce the molecular entities in component A. For pathway (i), at low scan rates (<100 mVs−1) the electron transfer occurs similarly in assembly sequence I. When the scan rate is increased, a second pathway (ii) is preferred. A typical characteristic of pathway (ii) is that at the onset of the reduction of the molecular entities in component B, the reduction from Box→*Bred starts to occur, which forms a conductive path to catalytically reduce the remaining entities in component A; Aox→Ared, i.e., the Aox that has not yet been reduced by means of pathway (i). Since the reduction by pathway (i) occurs at a higher potential than that of pathway (ii), there is a temporary charge trapping. In the last stage, the surface-interface thickness of component B exceeds a certain threshold (e.g., 11 nm in case of Os and Ru), and at this thickness, molecular component A is completely isolated from the surface, and its electrochemical oxidation/reduction wave are completely absent in the CV. These electrochemical properties are specific for SDA III, and might be useful for electrochromic materials and battery technology. Although more than two molecular components can be used, it is predicted that similar results are obtained as long as molecular entities in component C or D have a higher oxidation potential than B, although the exact behavior of such multi-component films is difficult to estimate for this specific assembly technique. The mechanism underlying electron transfer in SDA III is shown in
In the last assembly technique (SDA IV), molecular components A and B are homogeneously mixed in a solution (50:50), and deposited from this solution. In this case there is a random distribution throughout the assembly of the entities that comprises components A and B, and not unlike previous examples more distinct “layers”. The electrochemical behavior is such that both molecular entities in components A and B are electrochemically addressable in the assembly. The behavior is identical to that as described in
As shown in the various studies described herein, the SDA of the device of the present invention can be addressed optically, magnetically, electrochemically, etc.
In one aspect, the present invention thus provides a device comprising a substrate having an electrically conductive surface and carrying an assembly of one or more molecular components, each molecular component having a thickness and an oxidative or reductive peak potential, and comprising one or more entities each independently is a redox-active compound,
provided that:
wherein exposure of said device, when comprising one molecular component, to a potential change, causes electron transfer, which results in an electrochemical signature which can be read out electrically, optically, magnetically, or by conductivity measurements; and exposure of said device, when comprising more than one molecular components, to a potential change, causes (a) reversible electron transfer; (b) oxidative catalytic electron transfer with charge trapping; (c) reductive catalytic electron transfer; or (d) blocking of the electron transfer, dependent on the order of said components and the thickness of each one of said components, which results in an electrochemical signature which can be read out electrically, optically, magnetically, or by conductivity measurements.
As defined above, in devices according to the present invention, when comprising one molecular component comprising more than one, e.g., two, redox-active compounds, i.e., entities, the difference between the oxidative- and/or reductive peak potentials of each one of said entities is larger than 100 mV. It should also be understood that in devices according to the present invention, when comprising more than one molecular components each comprising a sole entity, i.e., redox-active compound, a difference as defined above between the oxidative- and/or reductive peak potentials of two of said redox-active compounds, in fact, reflects the difference between the oxidative- and/or reductive peak potentials of two of the molecular components. Similarly, in such devices when comprising more than one molecular components each comprising more than one redox-active compounds, the redox-active compounds whose oxidative- and/or reductive peak potentials are compared can be any couple of redox-active compounds no matter whether both of these compounds are comprised within the same molecular component or one of them is comprised within one of the molecular components and the other one is comprised within another one of the molecular components, and the difference between the oxidative- and/or reductive peak potentials of those redox-active compounds causes a difference between the oxidative- and/or reductive peak potentials of two of the molecular components.
In certain embodiments, the substrate comprised within the device of the invention is hydrophilic, hydrophobic or a combination thereof.
In particular such embodiments, the substrate includes a material selected from glass, a doped glass, ITO-coated glass, silicon, a doped silicon, Si(100), Si(111), SiO2, SiH, silicon carbide mirror, quartz, a metal, metal oxide, a mixture of metal and metal oxide, group IV elements, mica, a polymer such as polyacrylamide and polystyrene, a plastic, a zeolite, a clay, wood, a membrane, an optical fiber, a ceramic, a metalized ceramic, an alumina, an electrically-conductive material, a semiconductor, steel or a stainless steel. In more particular such embodiments, the substrate is in the form of beads, microparticles, nanoparticles, quantum dots or nanotubes, preferably wherein the substrate is optically transparent to the ultraviolet (UV), infrared (IR), near-IR (NIR) and/or visible spectral ranges.
In certain embodiments, the redox-active compounds composing the molecular components of the device of the present invention each independently is a metal, modified nanoparticle or quantum dot, organometallic compound, metal-organic, organic or polymeric material, inorganic material, metal complex, organic molecule, or a mixture thereof.
Specific examples of such metals include, without being limited to, transition metals such as Os, Ru, Fe, Pt, Pd, Ni, Ir, Rh, Co, Cu, Re, Tc, Mn, V, Nb, Ta, Hf, Zr, Cr, Mo, W, Ti, Sc, Ag, Au or Y; lanthanides such as La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu; actinides such as Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No or Lr; or main group element metals such as Zn, Ga, Ge, Al, Cd, In, Sn, Sb, Hg, Tl or Pb.
In certain particular such embodiments, the redox-active compounds composing the molecular components of the device each independently is a tris-bipyridyl complex or terpyridyl complex of said transition metal, e.g., a tris-bipyridyl complex or terpyridyl complex of ruthenium, osmium, iron or cobalt, a complex of a porphyrin, corrole, or chlorophyll with said transition metal. The term “pyridyl complex”, as used, herein, refers to a metal having one or more, e.g., two, three, or four, pyridyl ligands coordinated therewith.
More particular such embodiments are those wherein the redox-active compounds composing the molecular components of the device each independently is a tris-bipyridyl complex of the general formula I:
wherein
M is a transition metal as defined above;
n is the formal oxidation state of the transition metal, wherein n is 0-4;
X is a counter anion selected from Br−, Cl−, F−, I−, PF6−, BF4−, OH−, ClO4−, SO3−, SO4−, CF3COO−, CN−, alkylCOO−, arylCOO−, or a combination thereof;
R2 to R25 each independently is selected from hydrogen, halogen, hydroxyl, azido, nitro, cyano, amino, substituted amino, thiol, C1-C10 alkyl, cycloalkyl, heterocycloalkyl, haloalkyl, aryl, heteroaryl, alkoxy, alkenyl, alkynyl, carboxamido, substituted carboxamido, carboxyl, protected carboxyl, protected amino, sulfonyl, substituted aryl, substituted cycloalkyl, substituted heterocycloalkyl, or group A, wherein at least two, i.e., two, three, four, five or six, preferably three, of said R2 to R25 each independently is a group A:
wherein A is linked to the ring structure of the compound of general formula II via R1; and R1 is selected from cis/trans C═C, C≡C, N═N, C═N, N═C, C—N, N—C, alkylene, arylene or a combination thereof; and any two vicinal R2-R25 substituents, together with the carbon atoms to which they are attached, may form a fused ring system selected from cycloalkyl, heterocycloalkyl, heteroaryl or aryl, wherein said fused system may be substituted by one or more groups selected from C1-C10 alkyl, aryl, azido, cycloalkyl, halogen, heterocycloalkyl, alkoxy, hydroxyl, haloalkyl, heteroaryl, alkenyl, alkynyl, nitro, cyano, amino, substituted amino, carboxamido, substituted carboxamido, carboxyl, protected carboxyl, protected amino, thiol, sulfonyl or substituted aryl; and said fused ring system may also contain at least one heteroatom selected from N, O or S.
The term “oxidation state”, as used herein, refers to the electrically neutral state or to the state produced by the gain or loss of electrons to an element, compound or chemical substituent/subunit. In a preferred embodiment, this term refers to states including the neutral state and any state other than a neutral state caused by the gain or loss of electrons (reduction or oxidation).
The term “alkyl”, as used herein, typically means a straight or branched hydrocarbon radical having preferably 1-10 carbon atoms, and includes, e.g., methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, 2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl and the like. The alkyl may further be substituted. The term “alkylene” refers to a linear divalent hydrocarbon chain having preferably 1-10 carbon atoms and includes, e.g., methylene, ethylene, propylene, butylene, pentylene, hexylene, octylene and the like.
The terms “alkenyl” and “alkynyl” refer to a straight or branched hydrocarbon radical having preferably 2-10 carbon atoms and containing one or more double or triple bond, respectively. Non-limiting examples of such alkenyls are ethenyl, 3-buten-1-yl, 2-ethenylbutyl, 3-octen-1-yl, and the like.
The term “cycloalkyl” typically means a saturated aliphatic hydrocarbon in a cyclic form (ring) having preferably 3-10 carbon atoms. Non-limiting examples of such cycloalkyl ring systems include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclodecyl and the like. The cycloalkyl may be fused to other cycloalkyls, such in the case of cis/trans decalin. The term “heterocycloalkyl” refers to a cycloalkyl, in which at least one of the carbon atoms of the ring is replaced by a heteroatom selected from N, O or S.
The term “alkylCOO” refers to an alkyl group substituted by a carboxyl group (—COO—) on any one of its carbon atoms. Preferably, the alkyl has 1-10 carbon atoms, more preferably CH3COO−.
The term “aryl” typically means any aromatic group, preferably having 6-14 carbon atoms such as phenyl and naphtyl. The aryl group may be substituted by any known substituents. The term “arylCOO” refers to such a substituted aryl, in this case being substituted by a carboxylate group.
The term “heteroaryl” refers to an aromatic ring system in which at least one of the carbon atoms is replaced by a heteroatom selected from N, O or S. Non-limiting examples of heteroaryl include pyrrolyl, furyl, thienyl, pyrazolyl, imidazolyl, oxazolyl, isoxazolyl thiazolyl, isothiazolyl, pyridyl, 1,3-benzodioxinyl, pyrazinyl, pyrimidinyl, 1,3,4-triazinyl, 1,2,3-triazinyl, 1,3,5-triazinyl, thiazinyl, quinolinyl, isoquinolinyl, benzofuryl, isobenzofuryl, indolyl, imidazo[1,2-a]pyridyl, pyrido[1,2-a]pyrimidinyl, benz-imidazolyl, benzthiazolyl and benzoxazolyl.
The term “halogen” includes fluoro, chloro, bromo, and iodo. The term “haloalkyl” refers to an alkyl substituted by at least one halogen.
The term “alkoxy” refers to the group —OR, wherein R is an alkyl group. The term “azido” refers to —N3. The term “nitro” refers to —NO2 and the term “cyano” refers to —CN. The term “amino” refers to the group —NH2 or to substituted amino including secondary, tertiary and quaternary substitutions wherein the substituents are alkyl or aryl. The term “protected amino” refers to such groups which may be converted to the amino group. The term “carboxamido” refers to the group —CONH2 or to such a group substituted, in which one or both of the hydrogen atoms is/are replaced by a group independently selected from an alkyl or aryl.
The term “carboxyl” refers to the group —COOH. The term “protected carboxyl” refers to such groups which may be converted into the carboxyl group, e.g., esters such as —COOR, wherein R is an alkyl group or an equivalent thereof, and others which may be known to a person skilled in the art of organic chemistry.
The expression “any two vicinal R2-R25 substituents” refers to any two substituents on the pyridine rings, being ortho to one another. The expression “fused ring system” refers to at least two rings sharing one bond, such as in the case of quinolone, isoquinoline, 5,6,7,8-tetrahydroisoquinoline, 6,7-dihydro-5H-cyclopenta[c]pyridine, 1,3-dihydrothieno[3,4-c]pyridine, 1,3-dihydro furo[3,4-c] pyridine, and others. The fused ring system contains at least one pyridine ring, being the ring of the compound of general formula I and another ring being formed by the ring closure of said any two vicinal R2-R25 substituents. The said another ring may be saturated or unsaturated, substituted or unsubstituted and may be heterocylic.
Specific examples of tris-bipyridyl complexes of the general formula I are those wherein n is 2; X is a counter anion as defined above, i.e., Br−, Cl−, F−, I−, PF6−, BF4−, OH−, ClO4−, SO3−, SO4−, CF3COO−, CN−, alkylCOO−, arylCOO−, or a combination thereof; R2, R4 to R7, R9, R10, R12 to R15, R17, R18, R20 to R23 and R25 each is hydrogen; R3, R11 and R19 each is methyl; and R8, R16 and R24 each is A, wherein R1 is C═C. Particular such complexes exemplified herein are those wherein M is Ru, Os or Co; and X is PF6−, i.e., tris[4′-methyl-4-(2-(4-pyridyl)ethenyl)-2,2′-bipyridine]ruthenium(II)[bis(hexafluorophosphate)], tris[4′-methyl-4-(2-(4-pyridyl)ethenyl)-2,2′-bipyridine]osmium(II)[bis(hexafluorophosphate)], tris[4′-methyl-4-(2-(4-pyridyl)ethenyl)-2,2′-bipyridine]cobalt(II)[bis(hexafluorophosphate)], herein identified compounds (or complexes) 1, 2 and 4, respectively, of the formulas:
The various tris-bipyridyl complexes of the general formula I described herein can be prepared by any suitable method or technique known in the art, e.g., as described in Study 1 hereinafter (additional data may be found in Mentes and Singh, 2013).
In other particular such embodiments, the redox-active compounds composing the molecular components of the device each independently is an organic molecule, and said organic molecule is a thiophene, quinone, porphyrin such as those described in detail in International Patent Application No. PCT/IL2013/050584, corrole, chlorophyll, a vinylpyridine derivative such as 1,3,5-tris(4-ethenylpyridyl)benzene (herein identified compound 3) and 1,4-bis[2-(4-pyridyl)ethenyl]benzene (herein identified BPEB or compound 6), a pyridylethylbenzene derivative such as 1,3,5-tris(2-(pyridin-4-yl)ethyl)benzene (herein identified compound 5), or a combination thereof.
Compounds such as compounds 3, 5 and 6 can be prepared by any suitable method or technique known in the art, e.g., as described in Studies 1 and 4 hereinafter.
In further particular such embodiments, the redox-active compounds composing the molecular components of the device each independently is an organic or metal-organic material, and said organic or metal-organic material is selected from (i) viologen (4,4′-bipyridylium salts) or its derivatives such as, without being limited to, methyl viologen (MV); (ii) azol compounds such as, without limiting, 4,4′-(1E,1′E)-4,4′-sulfonylbis(4,1-phenylene)bis(diazene-2,1-diyl)-bis(N,N-dimethylaniline); (iii) aromatic amines; (iv) carbazoles; (v) cyanines; (vi) methoxybiphenyls; (vii) quinones; (viii) thiazines; (ix) pyrazolines; (x) tetracyanoquinodimethanes (TCNQs); (xi) tetrathiafulvalene (TTF); (xii) metal coordination complex wherein said complex is [MII(2,2′-bipyridine)3]2+ or [MII(2,2′-bipyridine)2(4-methyl-2,2′-bipyridine-pyridine]2+, wherein said M is iron, ruthenium, osmium, nickel, chromium, copper, rhodium, iridium or cobalt; or a polypyridyl metal complex selected from tris(4-[2-(4-pyridyl)ethenyl]-4′-methyl-2,2′-bipyridine osmium(II) bis(hexafluorophosphate), tris 4-[2-(4-pyridyl)ethenyl]-4′-methyl-2,2′-bipyridine cobalt(II) bis(hexafluorophosphate), tris(4-[2-(4-pyridyl)ethenyl]-4′-methyl-2,2′-bipyridine)ruthenium(II)bis-(hexafluorophosphate), bis(2,2′-bipyridine)[4′-methyl-4-(2-(4-pyridyl)ethenyl)-2,2′-bipyridine]osmium(II) [bis(hexafluorophosphate)/di-iodide], bis(2,2′-bipyridine)[4′-methyl-4-(2-(4-pyridyl)ethenyl)-2,2′-bipyridine]ruthenium(II) [bis(hexafluorophosphate)/di-iodide], bis(2,2′-bipyridine) [4′-methyl-4-(2-(4-(3-propyl trimethoxysilane)pyridinium)ethenyl)-2,2′-bipyridine]osmium(II) [tris(hexafluorophosphate)/tri-iodide], or bis(2,2′-bipyridine) [4′-methyl-4-(2-(4-(3-propyl trimethoxysilane)pyridinium)ethenyl)-2,2′-bipyridine]ruthenium(II)[tris(hexafluorophosphate)/tri-iodide]; (xiii) metallophthalocyanines or porphyrins in mono, sandwich or polymeric forms; (xiv) metal hexacyanometallates; (xv) dithiolene complexes of nickel, palladium or platinum; (xvi) dioxylene complexes of osmium or ruthenium; (xvii) mixed-valence complexes of ruthenium, osmium or iron; or (xviii) derivatives thereof.
In still further particular such embodiments, the redox-active compounds composing the molecular components of the device each independently is an inorganic material, and said inorganic material is tungsten oxide, iridium oxide, vanadium oxide, nickel oxide, molybdenum oxide, titanium oxide, manganese oxide, niobium oxide, copper oxide, tantalum oxide, rhenium oxide, rhodium oxide, ruthenium oxide, iron oxide, chromium oxide, cobalt oxide, cerium oxide, bismuth oxide, tin oxide, praseodymium, bismuth, lead, silver, lanthanide hydrides (LaH2/LaH3), nickel doped SrTiO3, indium nitride, ruthenium dithiolene, phosphotungstic acid, ferrocene-naphthalimides dyads, organic ruthenium complexes, or any mixture thereof.
In yet further particular such embodiments, the redox-active compounds composing the molecular components of the device each independently is a polymeric material, and said polymeric material is a conducting polymer such as a polypyrrole, a polydioxypyrrole, a polythiophene, a polyselenophene, a polyfuran, poly(3,4-ethylenedioxythiophene), a polyaniline, a poly(acetylene), a poly(p-phenylene sulfide), a poly(p-phenylene vinylene) (PPV), a polyindole, a polypyrene, a polycarbazole, a polyazulene, a polyazepine, a poly(fluorene), a polynaphthalene, a polyfuran, a metallopolymeric film based on a polypyridyl complex or polymeric viologen system comprising pyrrole-substituted viologen pyrrole, a disubstituted viologen, N,N′-bis(3-pyrrol-1-ylpropyl)-4,4′-bipyridilium, or a derivative thereof.
In still further particular such embodiments, the redox-active compounds composing the molecular components of the device each independently is an electrochromic compound.
The molecular components of the device of the present invention may be formed, e.g., deposited, on the electrically conductive surface by any suitable technique known in the art, e.g., by the layer-by-layer deposition technique exemplified herein, which enables incorporation of multiple components in one assembly by depositing different type of molecules in each deposition step. Other suitable techniques may include, without being limited to, physical/chemical vapor deposition (PVD/CVD), halogen bonding, spin coating, dip coating, and spray coating (Shirman et al., 2008; Decher, Gero, 2012, Multilayer thin films—sequential assembly of nanocomposite materials, vol 2. Weinheim, Germany: Wiley-VCH).
According to the present invention, exposure of a device as defined above, when comprising one molecular component, to a potential change, causes electron transfer, which results in an electrochemical signature which can be read out electrically, optically, magnetically, or by conductivity measurements; and exposure of a device as defined above, when comprising more than one molecular components, to a potential change, causes (a) reversible electron transfer; (b) oxidative catalytic electron transfer with charge trapping; (c) reductive catalytic electron transfer; or (d) blocking of the electron transfer, dependent on the order of said components and the thickness of each one of said components, which results in an electrochemical signature which can be read out electrically, optically, magnetically, or by conductivity measurements.
In certain embodiments, said electrical read-out is carried out by an electrochemical technique such as cyclic voltammetry (CV), differential pulse voltammetry (DPV), current-voltage changes, and conductivity changes; and said optical read-out is carried out in the UV, IR, NIR, or visible region or by fluorescence spectroscopy.
In Study 1 hereinafter, four different types of interfaces were demonstrated with two molecular entities, 1 and 2. As a result of the applied SDA, different electrochemical behavior was observed for all four SPMAs. Successive deposition of the molecular entities 1 and 2, resulted in the occurrence of catalytic pre-waves that oxidized/reduced the outer layer of the SPMA, depending on which entity was deposited first. If Ru was deposited first (SPMA II | Rux—Osy), catalytic oxidation of the outer Os layer was observed, provided that the thickness of the Ru layer exceeded 8.0 nm. However, instead of this thermodynamic effect, a kinetic effect was observed when the Os was deposited first (SPMA III | Osx—Ruy). The two observed pathways for electron transfer to the outer Ru layer were strongly dependent on the scan rate and the thickness of the Os layer. Assembling the molecular entities in an alternating fashion (SPMA I | Rux—Osy), or from a mixture of 1 and 2 (SPMA IV | Ru—Os)x+y), however, resulted in a reversible oxidation/reduction process of both metal centers independent of the SPMA thickness. This study unequivocally demonstrates that upon changing the SDA strategy and assembly thickness, the electrochemical properties of SPMAs can be controlled. To this end, the SDA concept is unlikely to be limited only to interfaces; it might also be applied in multi-component systems in solution, including self-sorting assemblies and molecular networking (Campbell et al., 2010; Deng et al., 2010; Northrop et al., 2009; Sknepnek et al., 2008; Lehn, 2002).
In Study 2, functional SPMAs incorporating two different, yet very similar, entities 1 and 2 were constructed using a bi-molecular assembly protocol. The well-separated half-wave potentials between the Os and Ru complexes allowed three well-defined oxidation states of the SPMA. The optical properties of the SPMA can be controlled by applying different potential biases and allowed us to address these states for the formation of binary and ternary memory. Since three physical distinguishable states are demonstrated, our ternary memory set-up is not dependent on the assembly thickness, as similar switching behavior is demonstrated for various thicknesses. In addition, two different types of memory can be read-out in a dual way; resulting in the simultaneous operation of binary and ternary memory. Moreover, these materials can also find applications in related areas, especially in the field of molecular logic (Avellini et al., 2012; Remón et al., 2011; Andréasson et al., 2011; de Ruiter and van der Boom, 2011a; de Ruiter and van der Boom, 2011b; de Silva, 2011; Amelia et al., 2010; Andreasson and Pischel, 2010). With retention times of several minutes, the SPMAs are within the needed requirements for mimicking the output behavior of flip-flops and related logic circuits operating on base 3 (e.g., flip-flap-flops) (Lee et al., 2011). Therefore, this molecular approach, based on the separate addressing of molecular entities in a SPMA, unequivocally demonstrates the exciting possibilities of information processing and storage in a ternary platform.
In Study 3, three different SPMAs were obtained according to the SDA shown in
In Study 4, sandwich-like multi-component assemblies were generated. The lengthwise increasing intermediate component containing the BPEB entity displayed a linear growth in its optical properties and thickness during formation. XRR analysis provided an insight about the internal structure and sequence, which confirmed the sandwich-like structure with a low electron density organic chromophore component confined by two high electron density redox-active components containing the Os or Ru entity. Additionally, gradual transitions between the different components at the Ru|BPEB and BPEB|Os interfaces were observed. The electrochemical properties of the assemblies are governed by a number of variables. The primary route to control these properties was by changing the thickness of the component containing the BPEB entity. Since each BPEB deposition cycle contributes 1.1 nm on average, a delicate tuning of the electrochemical profile was achieved. At low thickness of the BPEB containing component thicknesses both the Os and Ru entities could be addressed individually by the ITO electrode, which is applicable for multi-state memory devices. Upon increasing the thickness, the 2-based top domain became less and less electrochemically accessible due to its distance from the electrode. At the same time, an alternative two-step pathway for electron transfer from the top Os containing component was generated. In this pathway, catalytic amounts of the surface-adjacent Ru entities play an active role in the electron transfer process. This metal-mediated electron transfer restricts the current flow directionality, which results in current rectification. And finally, above a certain threshold thickness, an isolation of the Os entities was achieved. An additional degree of control over the electrochemical properties of our assemblies was demonstrated by subjecting the already-formed assemblies to different environmental conditions. Electrochemical reversibility could be partially to fully restored by heating the assemblies and by increasing the supporting electrolyte concentration. The importance of the internal structure in determining the electrochemical properties and the dynamic nature of the assemblies was demonstrated by two individual methods. First, a prolonged heating of the assemblies resulted in structural changes that have led to a more electrochemically reversible system, and second, a prolonged UV irradiation of the assemblies resulted in a photochemical reaction of the BPEB entities, producing substantially different assemblies in terms of the molecular structure, which had a pronounced effect on their electrochemical properties. Such photoreactions in monolayers have been studied extensively. The ability to carry out this type of a reaction in our multilayered architectures implies on a high degree of internal order since a proper alignment and specific distances between the reacting species are of mandatory importance.
Study 5 demonstrates that molecular composition of binary assemblies consisting of polypyridyl entities having the same ligands can be significantly different from the equimolar mixture solution ratio by constructing the assemblies on pre-modified surfaces. The bare surfaces were modified with a template layer composed of organic or organometallic molecules. The assemblies were constructed by alternate binding of PdCl2 and mixture of the Os and Ru entities. It is known that pyridine-derivatives bind to PdCl2 in a trans-configuration. The binary assemblies were composed of different combination of Os and Ru polypyridyl entities, which are both redox-active and therefore allow the determination of the molecular assembly composition using electrochemistry. The ratio of the entities in the in each assembly was varied depending on the constructed template layer. Assemblies generated on template layer consisted of organometallic complexes or non-planar organic molecules displayed a constant ratio of the entities upon increasing the film thickness. In contrast, a unique behavior of the entity ratio was observed when the assemblies were constructed on a template layer composed of planar organic molecules. These assemblies exhibited an increase of Os/Ru ratio upon increasing the thickness of the assembly. The assemblies presented in this work have an advantage over other multicomponent assemblies as they composed of redox-active entities. As a result, the binding behavior of the molecular entities can be followed using a simple method such as electrochemistry. In general, assemblies with multiple entities are good candidate systems for studying the self-assembly process of molecules on surfaces due to the molecules binding competition. The competition between the entities enables us to understand better which parameters control the self-assembly process of molecules on surfaces.
In certain embodiments, the device of the present invention, in any one of the configurations defined above, comprises a substrate having an electrically conductive surface and carrying an assembly of one molecular component.
Particular such devices are those wherein said molecular component comprises two or more, preferably two, entities each independently as defined above. Specific such devices are those wherein each one of said entities independently is selected from the herein identified compounds 1, 2, 3, 4, 5 or 6, preferably wherein one of said entities is compound 1, and another one of said entities is compound 2, 3, 4, 5 or 6; one of said entities is compound 2, and another of said entities is compound 3, 4, 5 or 6; one of said entities is compound 3, and another of said entities is compound 4, 5 or 6; one of said entities is compound 4, and another of said entities is compound 5 or 6; or one of said entities is compound 5, and another of said entities is compound 6. More particular such devices are those wherein the molar ratio between said entities is in a range of 1:1 to 1:10.
In other embodiments, the device of the present invention, in any one of the configurations defined above, comprises a substrate having an electrically conductive surface and carrying an assembly of more than one molecular component.
In certain particular such embodiments, the device of the present invention, in any one of the configurations defined above, comprises a substrate having an electrically conductive surface and carrying an assembly of two molecular components.
Particular such devices are those comprising a substrate having an electrically conductive surface and carrying an assembly of two molecular components, wherein each one of said molecular components comprises one entity as defined above. Specific such devices are those wherein each one of said entities independently is selected from the herein identified compounds 1, 2, 3, 4, 5 or 6, i.e., one of said entities is compound 1, and another one of said entities is compound 2, 3, 4, 5 or 6; one of said entities is compound 2, and another of said entities is compound 3, 4, 5 or 6; one of said entities is compound 3, and another of said entities is compound 4, 5 or 6; one of said entities is compound 4, and another of said entities is compound 5 or 6; or one of said entities is compound 5, and another of said entities is compound 6.
More particular such devices are those comprising a substrate having an electrically conductive surface and carrying an assembly of two molecular components each comprising one entity as defined above, wherein the two molecular components are assembled in an alternate or successive order. In certain specific such devices, each one of said entities independently is selected from the herein identified compounds 1, 2, 3, 4, 5 or 6, i.e., one of said entities is compound 1, and another one of said entities is compound 2, 3, 4, 5 or 6; one of said entities is compound 2, and another of said entities is compound 3, 4, 5 or 6; one of said entities is compound 3, and another of said entities is compound 4, 5 or 6; one of said entities is compound 4, and another of said entities is compound 5 or 6; or one of said entities is compound 5, and another of said entities is compound 6, and said two molecular components are assembled in any alternate order.
In other particular such embodiments, the device of the present invention, in any one of the configurations defined above, comprises a substrate having an electrically conductive surface and carrying an assembly of three or more molecular components.
Particular such devices are those comprising a substrate having an electrically conductive surface and carrying an assembly of three or more molecular components, wherein each one of said molecular components comprises one entity as defined above. Specific such devices are those wherein each one of said entities independently is selected from the herein identified compounds 1, 2, 3, 4, 5 or 6.
More particular such devices are those comprising a substrate having an electrically conductive surface and carrying an assembly of three or more molecular components each comprising one entity as defined above, wherein the three or more molecular components are assembled in any random, alternate or successive order.
Devices according to the present invention, when comprising a substrate having an electrically conductive surface and carrying an assembly of one molecular component, can be used in fabrication of a multistate memory, electrochromic window, smart window, electrochromic display, or binary memory.
Certain devices according to the present invention, when comprising a substrate having an electrically conductive surface and carrying an assembly of more than one molecular component assembled in an alternate order, can be used in fabrication of a multistate memory, electrochromic window, smart window, binary memory, electrochromic display, bulk-hetero-junction solar cell, inverted type solar cell, dye sensitized solar cell, molecular diode, charge storage device, capacitor, or transistor. Particular examples of such devices, without limiting, are those comprising a substrate having an electrically conductive surface and carrying an assembly of two molecular components assembled in an alternate order, wherein each one of the two molecular components comprises a compound independently selected from the herein identified compounds 1, 2, 3, 4, 5 or 6, and the thickness of each one of said molecular components is less than 8 nm.
Other devices according to the present invention, when comprising a substrate having an electrically conductive surface and carrying an assembly of more than one molecular component assembled in a successive order, can be used in fabrication of a smart window, electrochromic display, bulk-hetero-junction solar cell, inverted type solar cell, dye sensitized solar cell, molecular diode, charge storage devices capacitor, or transistor.
In certain embodiments, the device of the present invention, in any one of the configurations defined above, is fabricated as a solid state device and further comprises an electrolyte and an electrical conductive electrode, wherein said electrical conductive electrode is fabricated on top of said assembly of one or more molecular components. In particular such embodiments, the electrolyte is a conductive polymer, gel electrolyte, or liquid electrolyte.
The invention will now be illustrated by the following non-limiting Examples.
Materials and Methods.
Complexes 1, 2 and 1,3,5-tris(4-ethenylpyridyl)benzene (3) were prepared as previously described (Motiei et al., 2008; Choudhury et al., 2010; Amoroso et al., 1995). p-Chloromethyl-phenyltrichlorosilane and dry propylene carbonate (<10 ppm H2O) were purchased from Gelest Inc. and Aldrich, respectively, and used as received. Solvents (AR grade) were purchased from Bio-Lab (Jerusalem), Frutarom (Haifa) or Mallinckrodt Baker (Phillipsburg, N.J.). Toluene was dried and purified using an M. Braun solvent purification system. Single-crystal silicon (100) substrates (2.0×1.0 cm) were purchased from Wafernet (San Jose, Calif.) and ITO-coated glass substrates (7.5×0.8 cm) were purchased from Delta Technologies (Loveland, Colo.). The ITO and silicon substrates were cleaned by sonication in DCM followed by toluene, acetone, and ethanol, and subsequently dried under an N2 stream, after which they were cleaned for 30 min with a UVOCS cleaning system (Montgomery, Pa.). Quartz substrates (2.0×1.0 cm; Chemglass Inc.) were cleaned by immersion in a “piranha” solution (7:3 (v/v) H2SO4/30% H2O2) for 1 h. Caution: piranha solution is an extremely dangerous oxidizing agent and should be handled with care using appropriate personal protection. Subsequently, the substrates were rinsed with deionized (DI) water followed by the Radio Corporation of America (RCA) cleaning protocol: 1:5:1 (v/v) NH4OH/H2O/30% H2O2 at 80° C. for 45 min. The substrates were washed with DI water and dried under an N2 stream. All substrates were then dried in an oven for 2 h at 130° C. The siloxane-based chemistry and the formation of the 3-based template layer were carried out in a glovebox or by using standard schlenk-cannula techniques (Kaminker et al., 2010; Yerushalmi et al., 2004; Li et al., 1993). These template layers were stored in toluene and used within 24 h. UV/vis spectra were recorded on a Cary 100 spectrophotometer. Spectroscopic ellipsometry was recorded on an M 2000V (J. A. Wollam Co. Inc.) instrument with VASE32 software. Electrochemical measurements (cyclic voltammetry, differential pulse voltammetry and chronoamperometry) were performed using a potentiostat (CHI660A). The electrochemical measurements were performed in a three-electrode cell configuration consisting of (i) a self-propagating molecule-based assembly (SPMA)-functionalized ITO substrate as the working electrode; (ii) Pt wire as the counter electrode; and (iii) Ag-wire as the reference electrode with ferrocene as the internal standard, using 0.1 M solutions of TBAPF6 in CH3CN as the supporting electrolyte. For spectroelectrochemistry, 0.1 M solutions of TBAPF6 in dry propylene carbonate (to avoid evaporation of the solvent) were used. All experiments were carried out at RT, unless stated otherwise. The thicknesses of the SPMAs on ITO were estimated by spectroscopic ellipsometry measurements of SPMAs grown simultaneously on silicon substrates. One deposition step is defined as the deposition of one type of metal complex (1 or 2) and the palladium salt Pd(PhCN)2Cl2.
Sequence-Dependent Assembly I: Formation of Multi-Component SPMAs by Alternating Assembly of Complexes 1, 2 and PdCl2(PhCN)2.
Substrates functionalized with the 3-based template layer (Kaminker et al., 2010; Yerushalmi et al., 2004; Li et al., 1993) were loaded onto a Teflon holder and immersed for 15 min in a 1.0 mM solution of PdCl2(PhCN)2 in THF. The samples were then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed for 15 min in a 0.2 mM solution of compound 1 in THF/DMF (9:1, v/v). The samples were then sonicated twice in THF and once in acetone for 5 min each (=deposition step 1). Next, the samples were immersed for 15 min in a 1.0 mM solution of PdCl2(PhCN)2 in THF. The samples were then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed for 15 min in a 0.2 mM solution of compound 2 in THF/DMF (9:1, v/v). Finally, the samples were sonicated twice in THF and once in acetone for 3 min each (=deposition step 2). This procedure was repeated until eight deposition steps were obtained, i.e., four for each metal. Then, the samples were rinsed in ethanol and dried under a stream of N2. All steps of this procedure were carried out at RT. Two solutions of PdCl2(PhCN)2 were used with identical concentrations to rigorously exclude cross-contamination between the polypyridyl complexes 1 and 2 (
Sequence-Dependent Assembly II: Formation of Multi-Component SPMAs by Successive Assembly of Complexes 1, 2 and PdCl2(PhCN)2.
Substrates functionalized with the 3-based template layer (Kaminker et al., 2010; Yerushalmi et al., 2004; Li et al., 1993) were loaded onto a Teflon holder and immersed for 15 min in a 1.0 mM solution of PdCl2(PhCN)2 in THF. The samples were then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed for 15 min in a 0.2 mM solution of compound 1 in THF/DMF (9:1, v/v). The samples were sonicated twice in THF and once in acetone for 5 min each (=deposition step 1). This cycle (a) was repeated 1, 2, 3 or 4 times, depending on the nature of the formed molecular assembly. Hereafter, the samples were immersed for 15 min in a 1.0 mM solution of PdCl2(PhCN)2 in THF. The samples were then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed for 15 min in a 0.2 mM solution of compound 2 in THF/DMF (9:1, v/v). Finally, the samples were then sonicated twice in THF and once in acetone for 3 min each. This cycle (b) was repeated 1, 2, 3 or 4 times, depending on the nature of the formed SPMA. Then, the samples were rinsed in ethanol and dried under a stream of N2. All steps of this procedure were carried out at RT. Two solutions of PdCl2(PhCN)2 were used with identical concentrations to rigorously exclude crosscontamination between polypyridyl complexes 1 and 2 (
Sequence-Dependent Assembly III: Formation of Multi-Component SPMAs by Successive Assembly of Complexes 1, 2 and PdCl2(PhCN)2.
Substrates functionalized with the 3-based template layer (Kaminker et al., 2010; Yerushalmi et al., 2004; Li et al., 1993) were loaded onto a Teflon holder and immersed for 15 min in a 1.0 mM solution of PdCl2(PhCN)2 in THF. The samples were then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed for 15 min in a 0.2 mM solution of compound 2 in THF/DMF (9:1, v/v). The samples were then sonicated twice in THF and once in acetone for 5 min each (=deposition step 1). This cycle (a) was repeated 1, 2, 3 or 4 times, depending on the nature of the formed molecular assembly. Hereafter, the samples were immersed for 15 min. in a 1.0 mM solution of PdCl2(PhCN)2 in THF. The samples were then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed for 15 min in a 0.2 mM solution of compound 1 in THF/DMF (9:1, v/v). Finally, the samples were sonicated twice in THF and once in acetone for 3 min each. This cycle (b) was repeated 1, 2, 3 or 4 times, depending on the nature of the formed SPMA. Then, the samples were rinsed in ethanol and dried under a stream of N2. All steps of this procedure were carried out at RT. Two solutions of PdCl2(PhCN)2 were used with identical concentrations to rigorously exclude cross-contamination between polypyridyl complexes 1 and 2 (
Sequence-Dependent Assembly IV: Formation of Multi-Component SPMAs by Assembly from a Mixture of Complexes 1, 2 with PdCl2(PhCN)2.
Substrates functionalized with the 3-based template layer (Kaminker et al., 2010; Yerushalmi et al., 2004; Li et al., 1993) were loaded onto a Teflon holder and immersed for 15 min, at RT, in a 1.0 mM solution of PdCl2(PhCN)2 in THF. The samples were then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed for 15 min in a 0.2 mM solution (total concentration of metal complexes) of compound 1 and 2 (50:50, 0.1 mM each) in THF/DMF (9:1, v/v). The samples were then sonicated twice in THF and once in acetone for 5 min each (=deposition step 1). This procedure was repeated until eight deposition steps were obtained. Then, the samples were rinsed in ethanol and dried under a stream of N2. All steps of this procedure were carried out at RT (
Functional molecular materials have been obtained by liquid/vapor-phase epitaxy or layer-by-layer assembly with (i) electro-optic responses sufficiently high to build high-speed electro-optical modulators (Frattarelli et al., 2009; Rashid et al., 2003); (ii) high-k dielectrics for fabricating organic field effect transistors (OFETs) (Ortiz et al., 2010; Klauk et al., 2007); and (iii) ultra-low-β materials to generate molecular wires (Terada et al., 2012; Sedghi et al., 2011; Motiei et al., 2010a; Kurita et al., 2010; Tuccitto et al., 2009; Sedghi et al., 2008). Moreover, combining metal-ligand coordination chemistry with stepwise solution-based deposition resulted in the formation of crystalline assemblies, including highly porous metal-organic frameworks (MOFs) on inorganic surfaces (Ariga et al., 2012; Makiura et al., 2010; Shekhah et al., 2009; Kanaizuka et al., 2008). The key for fabricating these and other molecular materials is frequently found in a highly conserved assembly sequence that directs them towards their unique properties and desired function. Similarly, nature dictates the function of enzymes and the genetic information encoded in DNA/RNA by means of the sequence in which the amino acids and nucleotides are arranged. Yet nature is able to create diverse functionalities with the same molecular building blocks. An intriguing question thus remains; can we harvest new and useful material properties by only changing the assembly sequence of the molecular components?
To address this challenge, the present study introduces a SDA of molecular interfaces and shows how this strategy—specific to a set of given building blocks—can be fully exploited to form SPMAs with diverse functionalities. Each SPMA (I-IV) was formed with the same molecular complexes (1, 2) that subdivides our SDA into four branches: (I) alternating assembly of 1 and 2; (II) successive assembly of molecular component 1, then complex 2; (III) successive assembly of molecular component 2, then 1; and (IV) assembly of the molecular components from a mixture of 1 and 2, i.e., SPMA I | Rux—Osy; SPMA II | Rux—Osy; SPMA III | Osy—Rux; and SPMA IV | (Ru—Os)x+y, where x and y denote the number of deposition steps in which complex 1 and 2 was deposited, respectively. The difference between each branch of the SDA is undoubtedly reflected in the multi-faceted electrochemical properties of the corresponding SPMAs. Furthermore, for SDA II and III, we can control the pathway by which electron transfer occurs by tuning the surface-interface thickness of the molecular components (1, 2). The delicate interplay between the SDA and the surface-interface thickness resulted in four distinctly observable electrochemical signatures: 1) reversible electron transfer; 2) oxidative catalytic electron transfer with charge trapping; 3) reductive catalytic electron transfer; and 4) blocking of the electron transfer. The importance of the appropriate SDA strategy is not only paramount in forming surface-confined molecular interfaces; it might also be applied in self-sorting assemblies, molecular networking, and multi-component MOFs, in which instances of sequential order can be identified (Campbell et al., 2010; Deng et al., 2010; Northrop et al., 2009; Sknepnek et al., 2008; Lehn, 2002).
For construction of the SPMAs by our SDA strategy we relied on our recent examples of molecule-based materials that are active participants in their continuing self-propagating assembly (Motiei et al., 2012). These materials have already been applied in electrochromic materials, solar cells, and molecular data storage (de Ruiter et al., 2010a; Motiei et al., 2010b; Motiei et al., 2009). The exponential growth processes observed in these assemblies involves absorption of an excess of a palladium salt into a unimolecular network consisting of complex 2 linked by palladium dichloride (Motiei et al., 2008). For our SPMAs I-IV; numbers coincide with the SDA strategy I-IV employed in their preparation, composed of complexes 1 and 2, similar growth processes and identical optical properties have been observed (
In SDA I, the molecular components (1, 2) are arranged in an alternating manner to give a SPMA that is 11.4 nm thick (SPMA I | Ru2—Os2). The CV of this SPMA exhibits reversible electrochemical waves for both the Os2+/3+ and Ru2+/3+ redox couples (
The half-wave potentials for 1 and 2 in SPMA I | Ru2—Os2 are similar to the ones measured in solution (2: 0.758 V and 1: 1.180 V (SPMA) vs. 2: 0.770 V and 1: 1.200 V (solution;
In SDA II, complexes 1 and 2 are deposited successively, the electrochemical properties are markedly affected, by the presence of the inner ruthenium layer. For a SPMA with a Ru thickness of 8.0 nm and an Os thickness of 4.1 nm (SPMA II | Ru3—Os1), the electrochemical behavior exhibits a sharp catalytic oxidative pre-wave at approximately 1.08 V (
Moreover, in the negative scan direction, reduction of the Os layer from Os3+→Os2+ is absent. At the half-wave potential of the Os2+/3+ redox couple, all the Ru3+ centers have been reduced, and there is no pathway available to reduce the Os3+ in the outer layer, and consequently charge trapping occurs. This charge trapping is further manifested by a decrease in the intensity of the oxidative pre-wave in the 2nd scan-cycle (
In SDA III, two electron-transfer pathways (A and B) were observed depending on the surface-interface thickness of the osmium layer and the scan rate of the electrochemical experiments (
In SDA IV, the multi-component SPMAs were obtained by deposition from a solution containing an equimolar amount of complexes 1 and 2. These SPMAs exhibit reversible behavior for redox couples Os2+/3+ and Ru2+/3+ up to a thickness of 30.0 nm (
The results presented herein demonstrate the importance of the assembly sequence and the surface-interface thickness of the molecular components 1 and 2 on the physicochemical properties, which are important for device fabrication. For instance SPMAs suitable for ternary memory devices in high-density data storage (HDDS) can be constructed by SDA I (de Ruiter et al., 2010a; de Ruiter et al., 2010b). This SDA allows the independent addressing of each type of metal-center that displays reversible, reliable, and stable electrochemical properties. The individual addressability of both molecular components in SPMA I may also be ideal for applications in three-dimensional integrated circuits (3D-ICs). Other SDA strategies result in the formation of molecular rectifiers, among others. The observed unidirectional current flow and the diverse electrochemical properties (SDA II and III) are of particular interest for fabricating solar-cells, where charge trapping and unidirectional current flows are important (Wurfel, 2009). Along with the photo-activity of Ru-polypyridyl complexes in solar cells (Reynal and Palomares, 2011), it is important to consider how to assemble those complexes in binary systems, e.g., blended or separated (McGehee and Topinka, 2006).
The electrochemical rectification of redox-active polymers in a bilayer fashion has been known since the seminal work of Murray and Wrighton (Abruna et al., 1981; Denisevich et al., 1981; Leidner and Murray, 1985; Chidsey and Murray, 1986; Smith et al., 1986). Unidirectional current flows have been subsequently reported between redox-active organic (mono/multi)-layers and ferrocyanide solutions (Berchmans et al., 2002; Oh et al., 2002), or in redox-active (ionic) polymers that might contain metal complexes (Alvarado et al., 2005; Hjelm et al., 2005; DeLongchamp et al., 2003; Cameron and Pickup, 1999; Araki et al., 1995). However, the versatility of the SDA and the resulting properties of the demonstrated interfaces are unprecedented. These films not only exhibit different electrochemical behavior upon changing the assembly sequence, they also dramatically change their behavior as a function of a controllable surface-interface thickness. This thickness in turn controls the electron transfer at the metal/metal interface. Together they determine the overall material properties in each SDA.
Materials and Methods.
See Study 1 above.
Formation of Multi-Component SPMAs with Complexes 1, 2 and PdCl2(PhCN)2.
The procedure was identical to that described in Study 1, SDA I, except for that it was repeated until twelve deposition steps were obtained (Note: two solutions of PdCl2(PhCN)2 were used with identical concentrations to exclude cross-contamination between the polypyridyl complexes 1 and 2).
The fabrication of molecular memory devices for high density data storage (HDDS) is essential due to ever increasing technological demands (Lieber, 2001; Ball, 2000). For instance, 0.4-1.4 zettabytes were generated in 2010, and this is expected to grow to 35 zettabytes by 2020 (Hilbert and Lopez, 2011; Gantz et al., 2010). Moreover, since 2007 more digital information is created that can be stored (Gantz et al., 2010). These facts leave many opportunities for the development of future information storage technologies. Ternary memory is especially attractive as the data is efficiently stored in trits (3n) (Knuth, 1997). In order to store multiple states one might use: (i) a combination of two, or more, redox-active molecules in a single assembly; or (ii) multiple redox-states in a single molecule (Lindsey and Bocian, 2011). However, formation of ternary memory with redox-active molecules on surfaces is rare (Lindsey and Bocian, 2011; Simao et al., 2011; Lee et al., 2011; de Ruiter et al., 2010a; de Ruiter et al., 2010b; Li et al., 2010; Fioravanti et al., 2008; Yu et al., 2008; Lauters et al., 2006; Li et al., 2004). To illustrate, porphyrin-derivatives covalently attached to silicon were used to generate electrochemically addressable and readable ternary memory (Lindsey and Bocian, 2011). More recently, Rovira and Torrent used the redox-chemistry of organic radicals for the formation of ternary memory that is readable in a dual way (Simao et al., 2011). Nevertheless, the formation of molecular platforms that exhibits several well-separated redox processes on the surface, for the formation of ternary memory is a challenging task (Lindsey and Bocian, 2011; Nishimori et al., 2009; Palomaki and Dinolfo, 2010). The use of metal complexes herein is desirable, as their redox properties might allow for such data storage (Lindsey and Bocian, 2011; Terada et al., 2011; de Ruiter et al., 2010c; Fabre, 2010).
The present study introduces a multi-component SPMA with complexes of Ru and Os (1, 2), cross-linked with a palladium salt, for multi-state data storage (for other multicomponent assemblies see: Motiei et al., 2011a; Mondal et al., 2011; Nair et al., 2011; Palomaki and Dinolfo, 2010; Gauthier et al., 2008; Miyashita and Kurth, 2008; Schiitte et al., 1998; Liang and Schmehl, 1995). The self-propagating nature of these assemblies results from the storage of excess of palladium within the assembly, which allows for the exponential increase of the SPMA after each chromophore deposition (Motiei et al., 2008). The nature of complexes 1 and 2 ensures that the geometry, size, symmetry and coordination chemistry is nearly identical, while the electrochemical properties are dissimilar. This dissimilarity is reflected in the two characteristic oxidation/reduction processes for both the Os and Ru centers in the resulting SPMAs. The separate addressability of these metal centers in a single assembly results in a solid-state platform that ensures the physical separation of the memory states. A dual optical read-out at λ=495 and 700 nm resulted in the construction of binary and ternary memory respectively, where at λ=495 nm three different states can be distinguished based on the absorbance of complex 1 or 2. In this regard our SPMAs are suitable for HDDS, under ambient conditions, in a dynamic/static random access memory (DRAM/SRAM) like fashion.
The SPMAs were generated by alternate and iterative immersion of a pyridine-terminated template layer, on silicon, ITO or quartz (Kaminker et al., 2010), in a 1.0 mM solution of Pd(PhCN)2Cl2 in THF, followed by immersion in 0.2 mM solutions of complexes 1 or 2 in THF/DMF, 9:1 v/v (
CVs and differential pulse voltammograms (DPVs) were recorded for SPMA with thicknesses up to 54 nm (
Upon increasing the assembly thickness, the peak-to-peak separation increases from 10 to 79 and from 17 to 76 mV for the Os2+/3+ and Ru2+/3+ redox-couples, respectively. The increase in the peak-to-peak separation is indicative of a decrease in the kinetics of the electron transfer, with increasing SPMA thicknesses (
Characterization of the SPMAs by UV/Vis spectroscopy revealed that the SPMAs grow exponentially. The exponential growth results from the storage of excess palladium in the forming SPMA which is porous (Motiei et al., 2011b; Motiei et al., 2010a). Each deposition step of 1 or 2 exhibits the characteristic MLCT band of the corresponding metal center. The alternating deposition of the metal centers on the surface is evident from the variation of the λmax of the SPMA that varies between 495 and 510 nm, which corresponds to the λmax of the MLCT bands of the Ru and Os complexes (
The electrochemical properties of the SPMAs permit the formation of three distinct states (
Discrimination between the Os2+/3+- and Ru2+/3+-based redox processes is optically possible since the Ru-based complex 1 lacks a 3MLCT band at λ≈700 nm (Campagna et al., 2007; Juris et al., 1988). As a consequence, a decrease of the 3MLCT band is only observed when a potential of 0.95 V (Os2+→Os3+) is applied, whereas such a decrease is absent when a potential of 1.60 V (Ru2+→Ru3+) is used (
Based on the abovementioned three-state switching, the ternary memory was constructed, where the presence or absence of the applied potentials is defined as the input and the optical response of the 1MLCT at λ=495 nm is used as the read-out (output) of the memory, e.g., applying a potential of 1.60 V is defined as write state III. Initially, the reversible separate addressing of the Ru and Os metal complexes was demonstrated for ternary applications (
In order to assess the electrochromic properties of the SPMAs for ternary data storage in detail, the optical responses of the SPMAs were measured as a function of the potential. For instance, gradually increasing the switching potential between 0.5 and 0.5+n0.05 V, with n=0-22, in the chronoamerometric mode, results clearly in a double-step sigmoidal shape that is associated with the characteristic electrochemical properties of the redox-couples Os2+/3+ and Ru2+/3+ (
The thermal and electrochemical stability of the SPMAs were tested by cycling the potential for at least 1000 times between 0.40 and 1.60 V with 5 s intervals, and heating the SPMA to 130° C. in air for several hours. Both experiments confirmed the robust nature of the SPMA as there is no significant signal loss in the optical absorption or in the peak current of the Os2+/3+ and Ru2+/3+ redox-couples in the SPMA (
General Procedures.
See Study 1 above.
XRR.
Synchrotron XRR studies were performed at beamline X6B of the National Synchrotron Light Source (NSLS; Brookhaven National Laboratory, USA), using a Huber four-circle diffractometer in the specular reflection mode (the incident angle is equal to the exit angle θ). The reflected intensity was measured as a function of the scattering vector component qz=(4π/λ) sin θ, perpendicular to the reflecting surface. X-rays of energy E=10 keV (λ=1.240 Å) were used with a beam size of 0.3 mm vertically and 0.5 mm horizontally. The resolution was 3×10−3 Å−1. The samples were placed under a slight overpressure of helium during the measurements to reduce the background scattering from the ambient gas and radiation damage. The off-specular background was measured and subtracted from the specular counts. Details of the data acquisition and analysis are given elsewhere (Evmenenko et al., 2001; Evmenenko et al., 2011). The XRR measurements were performed at 20-25° C.
XPS.
Angle-resolved (AR)-XPS were made at different takeoff angles with a PHI 5600 Multi Technique System (base pressure of the main chamber 2×10−1° Torr). Resolution, corrections for satellite contributions, procedures to account for steady-state charging effects, and background removal have been described elsewhere. Experimental uncertainty in binding energies lies within ±0.4 eV.
Electrochemical measurements.
Cyclicvoltammetry and chronoamperometry were performed in a three-electrode cell configuration on a CHI 660A potentiostat. ITO electrodes functionalized with our SPMAs were used as the working electrode, whereas Pt- and Ag-wires were used as counter and references electrode, respectively. Solutions of Bu4NPF6 (0.1 M) in dry acetonitrile were used as the electrolyte. The Fc/Fc+ redox-couple, used as internal standard, was set at 0.40 V vs. SCE under these conditions (Connelly and Geiger, 1996). All electrochemical measurements were performed at RT in air.
Spectroelectrochemistry.
Spectroelectrochemical measurements were performed in a 3 ml quartz cuvette fitted in a Varian Cary 100 spectrophotometer operating in the double-beam transmission mode (200-800 nm). The potential was modulated with a CHI 660 A potentiostat operating in a three-electrode cell configuration consisting of (i) an SPMA-functionalized ITO substrate as the working electrode; (ii) a Pt wire as the counter electrode; and (iii) an Ag-wire as the reference electrode. Dry propylene carbonate containing 0.1 M Bu4NPF6 was used as the electrolyte solution. The UV-vis spectra were recorded in the dark, as soon as the electrochemical potential was applied. All spectroelectrochemical measurements were performed in the chronoamperometry mode at RT.
Understanding the many variables involved in forming supramolecular structures using metal-ligand coordination is often challenging. Factors like coordination number and geometry together with the nature of the ligand and the metal salt are but a few examples that are important in the complex niche of coordination chemistry (Ribas, 2008). Variation of the above-mentioned parameters has led to numerous fascinating structures (Ribas, 2008; Alexeev et al., 2010). Nitschke et al. demonstrated the formation of copper and zinc helicates in solution, whose stability not only depends on the ratio of the ligands, but also on the addition sequence (Campbell et al., 2010; de Hatten et al., 2012). The delicate interplay between those parameters resulted in dynamic self-assembly processes, able to cascade chemical transformations similar to signal transduction cascades in biology (Campbell et al., 2010). Stang et al., reported various well-defined shapes such as triangles, squares, rectangles, and three-dimensional structures such as cubes, by considering the geometrical constraints implied by the ligands and metal salts (Cook et al., 2009; Northrop et al., 2009; Zheng et al., 2010). In the last decade, these principles have also been extended to surface-chemistry by others (Altman et al., 2008; Doron-Mor et al., 2000; Hoertz and Mallouk, 2005; Kanaizuka et al., 2008; Katz et al., 1991; Kurita et al., 2010; Mondal et al., 2011; Motiei et al., 2008; Shekhah et al., 2009; Terada et al., 2012; Tuccitto et al., 2009; Zacher et al., 2011). The chemical modification of inorganic surfaces is an important development in the ongoing research towards hybrid functional materials. Diverse materials have been obtained that have found applications in sensors (de Ruiter et al., 2008; Gupta and van der Boom, 2006), electro-optics (Frattarelli et al., 2009; Rashid et al., 2003), photovoltaics (Motiei et al., 2010b), catalysis (Gao et al., 2010), and organic field effect transistors (OFETs) (Klauk et al., 2007; Ortiz et al., 2010) amongst others. Although there are established techniques available for surface modification (Shirman et al., 2008; Cerclier et al., 2010; Perl et al., 2009; Xia and Whitesides, 1998; Kumar et al., 1995; Piner et al., 1999; Andres and Kotov, 2010; Scheres et al., 2010), layer-by-layer assembly from solution is attractive as it offers many advantages. For instance, multiple molecular building blocks can be incorporated in a highly ordered and structured manner by utilizing directional inter-molecular forces such as hydrogen bonding, π-π stacking, and electrostatic, dipole-dipole or van der Waals interactions (Desiraju, 2007; Loi et al., 2005; Cragg, 2005; Lehn, 1995; Schneider, 1991). The information that is encoded in the molecular building blocks—by means of their geometry and inter-molecular interactions—govern the resulting supramolecular structures (Northrop et al., 2009). To demonstrate control over the sequence in which the molecules are arranged in an assembly is of critical importance for governing their material properties (de Ruiter et al., 2013). Such a molecular control can be implemented by a using SDA. Biology makes extensive use of this principle, for instance in cis-regulatory elements in DNA (Wittkopp and Kalay, 2012).
In the present study, we show how the internal composition and properties of the SPMAs (I-III) can be controlled by a SDA. For our SDA, we use polypyridyl complexes 1 and 2. These ruthenium (1) and osmium (2) complexes are highly stable, and are known to exhibit reversible electrochromic behavior by electrochemically changing their oxidation state from M2+→M3+ (M=Os, Ru) (de Ruiter et al., 2013; Motiei et al., 2009). These type of iso-structural and iso-electronic complexes are used in dye-sensitized solar cells (Wu et al., 2012; Yin et al., 2012; Freys et al., 2012) and electroluminescent devices (Buda et al., 2002; Welter et al., 2003). The SDA follows an iterative deposition procedure illustrated in
Molecular Assembly Formation.
The SPMAs were formed by immersing pyridine-terminated template layers in a 1.0 mM THF solution of Pd(PhCN)2Cl2 to allow for the coordination of PdCl2 (Kaminker et al., 2010). This enables the first deposition of one of the metal complexes (1, 2) on ITO, quartz, or silicon. Iterative immersion in a THF solution of Pd(PhCN)2Cl2, followed by immersion in a THF/DMF (9:1) solution containing the metal polypyridyl complex 1 or 2 (0.2 mM) resulted in formation of SPMAs with various compositions. In this study, three possible assembly sequences were used: (i) alternating deposition of 1 and 2; (ii) successive deposition of 1, followed by 2; and (iii) successive deposition of 2, followed by 1 (de Ruiter et al., 2013). As a result, the SPMAs only differ in the internal ordering of the used metal complexes. In accordance with the assembly strategy the names of the SPMAs coincide. SPMA I | Rux—Osy, SPMA II | Rux—Osy, and SPMA III | Osx—Ruy, refer to SDA I, II and III, where x and y denote the number of depositions steps in which complexes 1(Ru) or 2 (Os) were deposited.
UV-Vis Spectroscopy and Spectroscopic Ellipsometry.
The growth of the SPMAs was followed by UV-vis spectroscopy with SPMAs formed on quartz substrates. The absorption spectra of complexes 1 and 2 are nearly identical (
Upon formation of SPMA I | Ru3—Os3, SPMA II | Ru3—Os3, and SPMA III | Os3—Ru3, the λmax of the 1MLCT either alternates (SPMA I) or exhibits a bathochromic (SPMA II) or hypsochromic (SPMA III) shift (
Monitoring the 1MLCT and π-π* bands centered at λ=500 nm and λ=317 nm, respectively, revealed an exponential growth behavior for all three types of SPMAs (
Synchrotron XRR. The XRR data demonstrates the uniformity of the SPMAs (
However, due to negligible changes in electron density between osmium and ruthenium layers, the small fluctuations in the Patterson functions—which usually indicate slight non-uniformity of the electron density profiles inside the SPMAs—are not reflected in the electron density profiles (
aXRR-derived film thicknesses.
bEllipsometry-derived thicknesses for the XRR samples.
The XRR-derived thickness corresponds well with those derived from spectroscopic ellipsometry, and demonstrates and exponential growth behavior (
XPS (for a review of XPS on self-assembled architectures on surfaces see: Gulino, 2013). The internal composition of the SPMAs was analyzed by AR-XPS. For fully formed networks, with two pyridine groups coordinated to a palladium center, the following ratios are expected: Pd/N=0.17; Pd/M=1.5; and N/M=9 (M=Os or Ru) (Motiei et al., 2008). For all SPMAs, the XPS-derived elemental ratios are close to their expected values. However the palladium content is slightly higher than their predicted theoretical values. An higher palladium content is not uncommon, since our SPMAs are able to store excess palladium inside their porous network (Motiei et al., 2008). The ratios for SPMA I | Ru4—Os4, SPMA II | Ru4—Os4, and SPMA III | Os4—Ru4, are summarized in Table 2.
aXPS derived elemental ratios, where M = Os and Ru.
bAtomic concentration of Os or Ru.
For SPMAs I and II, significant atomic concentrations of ruthenium (1) are observed, although the film is terminated with a layer of the osmium complex 2 (Table 2). For example, in SPMA I, higher ruthenium concentrations are observed for entry Ru1—Os1 (5.4 nm) and Ru2—Os2 (11.4 nm), where the thickness of the combined osmium layers is 1.5 and 3.2 nm respectively. In SPMA II for entry Ru4—Os1 (15.5 nm) the underlying ruthenium layer is observed as well, after a deposition of a 5.0 nm thick osmium layer. This effect might be a result of the XPS probe depth of −6.0 nm at a 45° take-off angle (Merzlikin et al., 2008). Alternatively, the pronounced presence of the ruthenium can be explained by some Ru/Os inter-mixing at the internal interfaces of the SPMA.
For higher thicknesses in SPMA I, only one of the metals is observed; entry Ru3—Os3 (23.8 nm) and Ru4—Os4 (36.7 nm), depending on which metal complex was deposited last. These results indicate that clear and distinct layers are being formed inside the SPMA that are composed of only one type of metal complex. The same effects are observed for SPMA II and III (Table 2). This layering is a direct result of the SDA and is responsible for the spectroelectrochemical properties as discussed below.
Electrochemistry.
The SDA-dependent physicochemical properties (e.g., film thickness and interface formation) are expressed in the electrochemical properties of the SPMAs. For SDA I, the electron transfer is reversible for SPMA I at various thicknesses (
The thickness of the layers of metal complexes (1, 2) contributes to the observed reversible behavior. For SDA II similar behavior is observed for SPMA II | Ru1—Os1 (5.8 nm; blue trace) and SPMA II | Ru2—Os2 (12.4 nm; red trace), since for these SPMAs, the thickness of the ruthenium layer is below the threshold value of 8.0 nm (
For SDA III, the opposite behavior is observed, since the thermodynamic driving force of the electrochemical potential is now reversed. This effect is most pronounced in SPMA III | Os4—Ru4 (
Spectroelectrochemistry.
The different electrochemical behavior among the SPMAs, formed with the different SDAs I-III, is also expressed in their spectroelectrochemical properties.
When holding the potential between 0.95-1.10 V, all the osmium complexes (2) of the assembly are oxidized, while the Ru-based components are still in their reduced state (
In order to further investigate the oxidation/reduction of the individual type of metal complexes; i.e. ruthenium (1) or osmium (2), SPMAs constructed according to SDA I were selected. These SPMAs are preferable since there is no interference by catalytic electron transfer, as is the case in SDA II and III. In order to assess the electrochromic properties in detail, the optical response of SPMA I | Ru5—Os4 was measured as a function of the potential. For instance, gradually increasing the switching potential between 0.5 and 0.5+n0.05 V, with n=0-22, in the chronoamerometric mode, results clearly in a double-step sigmoidal shape associated with the characteristic electrochemical properties of the Ru2+/3+ and Os2+/3+ redox-couples (
The optical response of the 1MLCT at λ=495 nm for SDA I, II, and III was further used to read-out the electronic properties of the SPMAs by applying short potential biases. For instance, for SDA I the optical response of the 1MLCT of SPMA I | Ru4—Os3 is shown in
The difference between reversible and unidirectional current flow in SDA II is also manifested in the spectroelectrochemical behavior of the SPMAs. For SPMAs with a thickness of the ruthenium layer of 5.7 nm and a thickness of the osmium layer of 6.8 nm (SPMA II | Ru2—Os2), reversible behavior in the electro-optical properties was observed. Applying potential biases of 0.40, 1.00, and 1.60 V for 5 s (
The spectroelectrochemical properties of SPMAs constructed according to SDA III are presented in
The effect of the reductive catalytic pre-wave only becomes apparent in the spectroelectrochemical properties upon increasing the thickness of the osmium layer to 6.1 nm (SPMA III | Os3—Ru3). At this thickness, the insulating nature of the osmium layer becomes apparent, so oxidation of the Ru metal centers is retarded. This hampered oxidation is clearly visible optically, since the transmission slowly increases upon applying a potential bias of 1.6 V (
3.8 nma
aThe appearance of the reductive pre-wave depends on the scan rate. Only for scan rate>300 mVs−1, the catalytic reductive pre-wave are clearly observed (FIG. 62).
Materials.
See study 1 above. BPEB and PdCl2(PhCN)2 were synthesized as previously described (Burdeniuk and Milstein, 1993; Anderson, 1990).
Multilayer Formation.
Substrates functionalized with a 1-based template layer were loaded onto a Teflon holder and immersed for 15 min in a 1.0 mM solution of PdCl2(PhCN)2 in THF. The samples were then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed in a 0.2 mM solution of compound 1 in THF/DMF (9:1, v/v) for 15 min. The samples were then sonicated twice in THF and once in acetone for 5 min each (=deposition step 1). Next, the samples were immersed for 10 min in a 1.0 mM solution of PdCl2(PhCN)2 in THF and then sonicated twice in THF and once in acetone for 3 min each. Subsequently, the samples were immersed for 10 min in a 1.0 mM solution of BPEB in THF and sonicated twice in THF and once in acetone for 3 min each (=deposition step 2). The 2nd deposition cycle procedure was repeated zero to twenty times to obtain assemblies with zero to twenty deposition cycles of BPEB (only slides with even number of BPEB deposition cycles were kept for subsequent depositions of complex 2). Then, the 1st deposition cycle procedure was repeated twice using a 0.2 mM solution of compound 2 in THF/DMF (9:1, v/v). Finally, the samples were rinsed in ethanol and dried under a stream of N2. All steps were carried out at RT. Three separate PdCl2(PhCN)2 solutions with identical concentrations were used to rigorously exclude possible cross contaminations between compounds 1, 2, and BPEB (
Characterization Methods.
UV/vis spectroscopy was carried out using a Cary 100 spectrophotometer. Thicknesses were estimated by spectroscopic ellipsometry on an M-2000V variable angle instrument (J. A. Woollam Co., Inc.) with VASE32 software. Electrochemical measurements (i.e., cyclic voltammetry and spectroelectrochemistry) were performed using a potentiostat (CHI660A). The electrochemical measurements were performed in a three-electrode cell configuration consisting of the functionalized ITO substrate, Pt wire, and Ag wire as working, counter, and reference electrodes, respectively, using 0.1 M solutions (unless stated otherwise) of TBAPF6 in CH3CN as the supporting electrolyte. XRR measurements were performed at the 12-BM-B beamline of the Advanced Photon Source (APS), Argonne National Laboratory (Argonne, Ill., USA). A four-circle Huber diffractometer was used in the specular reflection mode (i.e., the incident angle was equal to the exit angle). An X-ray beam with an energy of E=10.0 keV (λ=1.24 Å) was used. The beam size was 0.40 mm vertically and 0.60 mm horizontally. The samples were held under a helium atmosphere during the measurements to reduce radiation damage and background scattering from the ambient gas. The off-specular background was measured and subtracted from the specular counts. AFM images were recorded using a Bruker multimode AFM operated in semicontact mode. Current-Voltage (I-V) measurements were performed using a Keithley 6430 subfemtoamp source meter. A thin homogeneous oxide layer was grown from an oxidizing solution on an etched surface of highly doped Si, which served as the bottom contact. The samples were contacted on the back by applying In—Ga eutectic, after scratching the surface with a diamond knife. Hanging Drop Mercury Electrode (HDME) served as the top contact (˜500 μm in diameter). Several scans from −1 to +1 V (applied to Hg) were measured for each junction with a scan rate of 20 mV/s. 4 junctions were made on each sample, and the results represent the average of the measurements. XPS measurements were carried out with Kratos AXIS ULTRA system using a monochromatized Al Kα X-ray source (hν=1486.6 eV) at 75 W and detection pass energies ranging between 20 and 80 eV. Angle-resolved spectra were recorded at 0=0° (normal) and 0=50°, where 0 is the takeoff angle with respect to the surface normal. Low-energy electron flood gun (eFG) was applied for charge neutralization. Attenuated total reflectance (ATR)-FTIR spectroscopy measurements were performed using a Bruker Equinox-55 spectrometer with a liquid N2 cooled mercury cadmium telluride (MCT) detector. Spectra were averaged over 128 scans and referenced to freshly cleaned silicon substrate. All measurements were carried out at RT, unless stated otherwise. Temperature-dependent measurements were performed using a Varian Cary Dual Cell Peltier accessory.
Organizing molecular building blocks on solid surfaces has been demonstrated to be a powerful strategy to generate diverse functional interfaces, which have been used to fabricate, inter alia, nanostructures, light emitting diodes (LEDs), electro-optic modulators, photovoltaic cells, and field-effect transistors (FETs). Numerous methods for surface modification have been reported in the recent decades (Yitzchaik and Marks, 1996; Ariga et al., 2007; Kumar et al., 1995; Piner et al., 1999; Shirman et al., 2008; Cerclier et al., 2010; Palomaki and Dinolfo, 2010), including Sagiv's layer-by-layer (LbL) deposition methodology (Netzer and Sagiv, 1983; Maoz et al., 1995; Ulman, 1996; Zeira et al., 2009), which is attractive for the possibility to create composite materials by incorporating multiple molecular building blocks in an ordered and well-defined fashion. The precise control over the structure and properties of such materials is demonstrated by the linear correlation between the physicochemical properties (e.g., thickness, absorption intensity and electrochemical response) and the number of deposition steps, commonly achieved using the LbL methodology (Yitzchaik and Marks, 1996; Wanunu et al., 2005; Katz et al., 1991; Palomaki and Dinolfo, 2010, Netzer and Sagiv, 1983; Maoz et al., 1995; Ulman, 1996; Zeira et al., 2009, van der Boom et al., 2001; Evmenenko et al., 2001; Lee et al., 1988; Altman et al., 2006; Altman et al., 2008; Altman et al., 2010; Choudhury et al., 2010; Kaminker et al., 2010; Zhao et al., 2010; DeLongchamp and Hammond, 2004). Multiple components can often be combined in a manner that allows synergistic interactions between the different species and the formation of an assembly possessing complex physicochemical properties as a result of the combination (DeLongchamp et al., 2003; DeLongchamp and Hammond, 2004; Cluster et al., 2002; Motiei et al., 2011a). In such multi component assemblies, the sequence by which the components are arranged is of a great importance in determining their properties, in particular the electron transfer characteristics for electrochemically active assemblies (de Ruiter et al., 2013). Gaining a deeper understanding and achieving nano-scale control over electron transfer at interfaces has become a most relevant topic in nanotechnology and electronics for the construction of functional molecular-level systems, which are able to duplicate the functions of bulk electronic devices (Motiei et al., 2010b; Lonergan, 1997; Willner and Katz, 2005; Balzani et al., 2008; Green et al., 2007; Lezama et al., 2012). In principle, molecular electronics is based on electron transfer processes between and through molecules. Such processes are well known in biology, where they have a key role in energy conversion (Blankenship, 2002). Similarly to biology, the directionality of the electron transfer has a significant meaning for applications in electronics. Control over the directionality enables the generation of functions like current rectification across an interface (Abruna et al., 1981; Denisevich et al., 1981; Mukherjee et al., 2006). A remaining challenge in material science is related to the design and formation of specific supramolecular architectures displaying tailor-made structure and function.
In the present study, hybrid surface-confined coordination-based assemblies were formed. A precise control over the composition and the internal arrangement of the assemblies was achieved using our iterative solution-based deposition methodology (Lee et al., 1988; Altman et al., 2006; Altman et al., 2010; Choudhury et al., 2010; Motiei et al., 2011b; Mukherjee and Mohanta, 2006). Redox-active ruthenium and osmium polypyridyl complexes (1 and 2, respectively) and the organic chromophore BPEB were used as building blocks to create a well-defined model structure for studying electron-transfer phenomena across interfaces. The surface chemistry of polypyridyl complexes as well as of organic chromophores has been studied extensively (Lee et al., 1988; Altman et al., 2006; Altman et al., 2008; Altman et al., 2010; Abruna et al., 19811 Denisevich et al., 1981; Mukherjee and Mohanta, 2006; Motiei et al., 2008; Hirao, 2006; Maeda et al., 2013; Chu and Yam, 2006). We have previously utilized the reversible electrochemical behavior of complexes 1 and 2 to fabricate electrochromic thin films (Motiei et al., 2009), solar cells (Motiei et al., 2010b), molecular sensors (de Ruiter and van der Boom, 2011a), and logic gates (de Ruiter and van der Boom, 2011a; de Ruiter et al., 2010c; de Ruiter et al., 2010a; de Ruiter and van der Boom, 2012). These components have been incorporated in the new assemblies in such a way that an appropriate potential gradient for vectorial electron transfer is created. The assemblies have been thoroughly characterized in terms of growth fashion, internal structure and dimensions, surface roughness, and electrochemical features. A gradual transition between reversible electrochemical behavior to oxidative catalytic electrochemical behavior of the osmium metal centers was observed as a function of their distance from the underlying electrode surface. As found, although the structural features of the assemblies play a vital role in determining their electrochemical properties, these properties can be tuned and adjusted after the assembly formation by various means, e.g., exposure to elevated temperatures and prolonged UV irradiation. The interplay between the structural characteristics and the post-assembly modifications allows us to understand and control the electron transfer processes across the assemblies, and thus, their resulting physicochemical properties.
For the formation of the multi-component assemblies, silicon, quartz, and ITO-coated glass substrates functionalized with 1-based template layer (Altman et al., 2010; Motiei et al., 2012) were repeatedly immersed in a solution of PdCl2(PhCN)2 followed by a solution of one of the molecular components (1, 2, or BPEB) according to
A detailed characterization of the new assemblies was carried out by UV/vis spectroscopy, ellipsometry, synchrotron XRR, AFM, electrochemistry, XPS, and FTIR spectroscopy. Electrical characterization was done using Hanging Drop Mercury Electrode (HDME). By combining such methods, we were able to gain a thorough insight about the structural features of the assemblies.
UV/vis measurements in the transmission mode of the functionalized quartz slides were performed during the film formation (
Ellipsometry-derived thickness of the silicon-bound assemblies was monitored as well during the film formation. The BPEB-domain exhibits a linear increase in thickness with every even deposition cycle (
Synchrotron XRR measurements were performed on four selected assemblies with increasing thickness of the BPEB-domain in order to obtain a representative structural characterization including thickness, roughness, and electron density (ED) profile. The XRR-derived thickness is in a good agreement with the ellipsometric data (
The thickness, surface roughness, and the ED profile of the assemblies were estimated from the Kiessig fringes in the specular reflectivity spectra (a representative spectrum of the Ru2-BPEB4-Os2 assembly is shown in
A representative AFM image of the Ru2-BPEB18-Os2 assembly on silicon is shown in
After obtaining a detailed structural characterization of the new multi-component assemblies, their electrical properties were examined. Solid-state semiconductor devices are typically characterized by current-voltage (I-V) curves in order to determine their diode characteristics. As a first step towards such devices we probed the conductivity of the multi-component assemblies. This is achieved by immobilizing the assemblies between two electrodes, similar to multilayer structures of redox polymers acting as electrochemical diodes (Zhao et al., 1994).
I-V measurements were carried out on a highly doped silicon substrate with a homogeneous, 8.6 Å-thick oxide layer. Highly doped p-Si electrode was chosen for its minimal semiconductor-related effects. Liquid Hg was used to form a soft, non-destructive top contact, following the roughness of the surface (Haag et al., 1999; Holmlin et al., 2001; Selzer et al., 2002; Nesher et al., 2007). Typical I-V curves of Hg/film/SiOx-p-Si junctions are shown in
We have reported that coordination-based surface-confined assemblies, based on one of the metal complexes (1 or 2) or a combination of both, are electrochemically active (de Ruiter et al., 2013; Motiei et al., 2009; Motiei et al., 2010a). Likewise, the new multi-component assemblies formed on ITO are redox-active, with changeable electrochemical characteristics as a function of the internal structure of the assemblies. By fine-tuning the distance between the 1- and 2-based redox-active domains, we were able to control the pathway by which electron transfer occurs during a redox cycle. CV measurements revealed that molecular-scale modifications in the thickness of the intermediating BPEB-domain result in a gradual transition between three distinctive electrochemical signatures, which correspond to two possible pathways for the oxidation of the outer Os metal centers.
When the BPEB-domain thickness is <2.4 nm, the assemblies exhibit reversible electrochemical waves for both Os2+/3+ and Ru2+/3+ redox couples at the half-wave potentials similar to the ones measured in solution (1: 1.194-1.212 V and 2: 0.742−0.753 V for the surface-confined assemblies vs. 1: 1.200 V and 2: 0.770 V in solution). A representative voltammogram of the Ru2-BPEB2-Os2 assembly, having a total thickness of 4.8 nm and a BPEB-domain thickness of 1.4 nm, is shown in
As the BPEB-domain thickness increases, the peak-to-peak separation (ΔEp) of the Os2+/3+ redox couple increases and its current magnitude decreases. The redox-inactive BPEB-domain partially insulates the outer Os metal centers from the electrode, interfering with the electron-transfer process under these conditions. At the same time, a catalytic oxidative pre-wave appears. As the BPEB-domain thickness increases from 2.4 nm up to 6.6 nm, the catalytic oxidative pre-wave appears at higher potentials, starting from approximately 1.03 V to 1.13 V (versus Ag/AgCl). The voltammograms of the 5.9 nm-thick Ru2-BPEB4-Os2 and the 7.0 nm-thick Ru2-BPEB6-Os2 assemblies, having 2.4 and 3.5 nm-thick BPEB-domains, respectively (
For the 7.0 nm-thick Ru2-BPEB6-Os2 assembly, the Os pre-wave current and the Ru cathodic wave current are proportional to the scan rate within the range of 50-700 mVs−1, indicating a surface-confined process that is not limited by diffusion (
Assemblies having BPEB-domain thicknesses in the range of 4.8-6.6 nm (see
Electrochemical isolation of the Os metal centers occurs at BPEB-domain thicknesses of >6.6 nm, in which the Os metal centers are not accessible both to the electrode and the Ru metal centers. This is demonstrated, for instance, by the 10.0 nm-thick Ru2-BPEB10-Os2 assembly, having a 6.6 nm-thick BPEB-domain (
The present study demonstrates a gradual transition between three distinct electrochemical states of the multi-component assemblies, which are characterized by (i) reversible electron transfer; (ii) catalytic electron transfer; and (iii) blockage of electron transfer. In the first state, the metal centers of 1 and 2 are independently addressable, whereas in the second state 1-2 metal centers communication is observed, resulting in unidirectional current flow accompanied by charge trapping.
To further examine the charge transfer properties of the new multi-component assemblies, the influence of the environmental temperature on the electrochemical behavior has been investigated. The structural stability of the assemblies is governed by a competition between the disordering effect of entropy and the ordering effect of the coordination-based interactions among the different molecular components. An increase in the environmental temperature enhances the role of entropy, making the structure looser. Looser structure and elevated temperatures permit an enhanced diffusion of the supporting electrolyte charge carriers through the assemblies in order to maintain electro-neutrality during electron-transfer between fixed sites. In addition, electron-transfer rate constants are temperature dependent according to Arrhenius law (Smalley et al., 1995; Boiko et al., 2013; Smalley et al., 2003; Park and Hong, 2006). The combination of enhanced mobility of the charge carriers and enhanced electron-transfer rate constant results in a more reversible electrochemical profile at elevated temperatures. This is expressed in the CV by decreased peak-to-peak separation values and increased peak currents due to the thermally facilitated interfacial electron-transfer processes.
A representative assembly, Ru2-BPEB6-Os2, exhibiting both reversible Os2+/3+ redox waves and oxidative catalytic pre-wave (
Interestingly, after a prolonged heating of the assemblies the electrochemical behavior changes significantly, displaying higher reversibility. As opposed to the heating-cooling cycles, the change in this case is irreversible, implying a possible structural reorganization of the assemblies.
In order to confirm our hypothesis regarding the penetration of Os complexes into the assemblies upon a prolonged heating, AR-XPS measurements were performed. The data for the Ru2-BPEB6-Os2 assembly is summarized in Table 7.
At the standard takeoff angle of θ=0°, the majority of the experimental elemental ratios are close to the theoretical ones. The slightly larger ratios observed between Pd and other elements are not uncommon and are due to storage of excess of the Pd precursor inside the assemblies. This phenomenon was observed previously for assemblies consisting of the metal complexes (1, 2) (Choudhury et al., 2010; Motiei et al., 2008). The Os/Ru ratio, on the other hand, is smaller than the theoretical value by a factor of 2. A combination of two factors is responsible for this result: 1) The excess of the Pd precursor being stored by the porous 1-based template layer is used for depositing more Ru complexes than expected at the first deposition cycle (which is also the reason for the non-linear growth fashion of the 1- and 2-based domains). This is confirmed by the smaller than expected Pd/Ru and N/Ru ratios. 2) Because of the highly branched nature of our assemblies and the bulkiness of the metal complexes, it is expected that the assemblies will not be fully formed. This effect will be more pronounced at the upper region of the assemblies, reducing the amount of the incorporated Os complexes. This explanation is confirmed by the larger than expected Pd/Os and N/Os ratios.
aElemental ratios.
bAtomic concentrations
In order to examine the effect of the heating treatment, additional measurements at a takeoff angle of θ=50° were performed. At this takeoff angle the XPS probing depth is lower than at θ=0° and the signal intensities of elements located at the outermost region of the assembly are higher than of the ones located at its depth (Merzlikin et al., 2008). As a consequence, the outcome of a diffusion of Os complexes inwards upon the heating treatment will be a decrease in the atomic concentration of Os at θ=50°. For a normalized comparison, we examined the ratio between the atomic concentration of Os at θ=50° and at θ=0°. This ratio before the heating treatment is 1.56 and after the treatment is 1.29. The same trend is apparent when comparing the elemental ratios of the Os/Ru, Pd/Os, and N/Os pairs at the two takeoff angles (i.e. the ratio between the elemental ratios) before and after the heating treatment, but is absent for all other elements and pairs.
The combination of the AR-XPS results, which demonstrate the penetration of Os complexes into the assembly upon heating, and the electrochemical findings, that show a more reversible behavior of the Os metal centers after heating, supports the dynamic nature of our new assemblies.
The thermally modified assembly was then electrochemically probed at 60° C. and afterwards again at 20° C. This was repeated twice and the results are shown in
Next, the influence of the supporting electrolyte concentration was examined. CVs of the representative Ru2-BPEB6-Os2 assembly in three different concentrations of TBAPF6 in acetonitrile are shown in
The present study demonstrates the importance of the internal molecular structure of our assemblies in determining their physicochemical properties. To further test this hypothesis, we have chosen to address the intermediate BPEB-domain and alter its molecular structure.
Conjugated olefins and in particular the BPEB molecule, are known to undergo a [2+2] photoreaction in the solid state, as crystalline materials or as monolayers bound to a solid surface, to produce cyclobutanes (Schmidt, 1971; MacGillivray et al., 2000; McMahon et al., 1985; Naciri et al., 2000; Fang et al., 2001; Yang et al., 2003; Li et al., 1997). We have irradiated functionalized ITO slides with UV light at 254 nm and followed the changes by UV/vis spectrometry and electrochemistry (
To further characterize the resulting product, FTIR was performed (
UV irradiation has a pronounced effect on the electron transfer ability and thus, on the electrochemical profile of the assemblies (insets of
The photoreaction couples each two BPEB molecules, leaving a larger unoccupied space, and in addition, their movement is even more restricted. This results in a higher porosity, which facilitates the electrolyte charge carriers movement throughout the assemblies.
These results also imply that the effect of conjugation in the BPEB molecules is of minor importance in the electron-transfer processes.
The results presented herein unequivocally demonstrate the ability to control and manipulate the electron transfer properties of surface-confined, multi-component assemblies. The extend of electrochemical reversibility, catalytic redox processes, unidirectional current flow, and electrochemical isolation are all fundamentally different states that can be achieved using the same molecular building block.
Self-assembly of molecules on surfaces provides highly ordered three dimensional systems. The ability to incorporate a wide range of molecules on different surfaces leads to generation of a variety of solid state constructions. Surface modification can be obtained using different techniques such as Langmuir-Blodgett and Layer-by-Layer deposition. Layer by layer deposition is an attractive technique as it can offer incorporation of multiple components in one assembly by depositing different type of molecules in each deposition step. Moreover, it can offer the generation of new materials by altering the components order within the assembly. Study 1 above demonstrates direct relationship between compositional sequence of surface-confined assemblies and their electrochemical behavior. These assemblies were consisted of ruthenium and osmium redox-active polypyridyl complexes. Changing the complexes order within these surface-confined assemblies lead to charge trapping and electrochemical isolation of one of the components. Assemblies consisting of electro-active complexes such as polypyridyl complexes are ideal candidates for molecular memory applications and electrooptic devices. Another way to incorporate multiple components into one assembly is by simultaneous adsorption of different kinds of molecules on the surface. Such systems are termed “mixed assemblies”. So far, the most frequently studied mixed assemblies are binary monolayers composed of alkanethiols deposited on gold surfaces. These monolayers were composed of alkanethiols with different chain lengths, different terminal groups or a mixture of aromatic and long chain mercaptans. Studies have been done to investigate binary monolayers properties such as phase separation, wettability and structural properties. Binary monolayers can also be composed of one or two redox-active components. Li et al. (2004) demonstrated the formation of redox-active two-component monolayers in order to achieve multibit functionality for hybrid memory devices. Incorporation of two redox-active components allows increasing memory density, particularly if one of the components exhibits multiple redox states. Their monolayers were consisted of ferrocene-based molecule and zinc porphyrin derivative molecule on silicon surfaces. Since both components were electro-active, they were able to determine the binary monolayer composition using electrochemistry. They revealed that although the molecules were deposited from a solution mixture of 1:1 ratio, surface coverage of ferrocene-based molecule was higher than that of zinc-porpherin derivative molecule. The difference in the coverage of both molecules was attributed to the smaller size of ferrocene-based molecule comparing to zinc-porpherin derivative molecule. Binary monolayers composed of similar sized complexes were demonstrated by Forster and Faulkner (1995). Their mixed monolayers were consisted of osmium and ruthenium complexes having the same ligands structure. The complexes were deposited on platinum surfaces from an equimolar mixture solution. The surface coverage ratio of the complexes was similar to the complexes ratio in the mixture solution.
Materials and Methods.
Solvents (AR grade) were purchased from Bio-lab (Jerusalem), Frutarom (Haifa) or Mallinckrodt Baker (Phillipsburg, N.J.). Anhydrous acetonitrile was purchased from Sigma Aldrich. Toluene was dried and purified using a M. Braun solvent purification system. ITO coated glass substrates (0.7×5 cm) were purchased from Delta Technologies. Single-crystal silicon (100) substrates were purchased from Wafernet (San Jose, Calif.). All glassware and Teflon holders for SMPA formation were cleaned by immersion in a piranha solution (7:3 v/v, H2SO4/30% H2O2) for 10 min and DI water. ITO and silicon substrates were cleaned by sonication in DCM, toluene, acetone and ethanol, successively, 8 min in each solvent. Subsequently, they were dried under a N2 stream and cleaned for 20 min using the UVOCS cleaning system (Montgomery, Pa.), then sonicated in ethanol and placed in the oven (130° C.) for 2 hours. Quarts (Chemglass, Inc.) substrates (2×1 cm) were rinsed several times with DI water and cleaned by immersion in a piranha solution for 1 h. The substrates were then rinsed with deionized water followed by sonication in RCA solution (1:5:1 (v/v) NH4.OH/H2O/30% H2O2) at 80° C. for 45 min. After RCA treatment, the substrates were washed with deionized water, sonicated in ethanol, dried under a N2 stream, and placed in the oven (130° C.) for 2 hours. UV/vis spectra were recorded using a Cary 100 spectrophotometer on quartz slides using double beam mode in a range of 200-800 nm. Baseline measurements were recorded using bare quartz slides. Thicknesses measurements were performed on silicon by using a J. A. Woollam (Lincoln, Neb.) model M-2000V spectroscopic ellipsometer with VASE32 software (for each 2 degree at a range of 65-75° over wavelengths of 399-1000 nm). Electrochemical measurements were carried out using a CHI660A potentiostat with platinum as the counter electrode, Ag/AgCl as the reference electrode, and ITO substrate (single or double side coated glass) as the working electrode in a solution of 0.1M TBAPF6 in CH3CN. Ferrocene was used as the internal standard. All measurements were carried out under inert atmosphere at 298 K unless stated otherwise. Multilayer formation was carried out under air at RT.
XRR measurements were performed on silicon (100) substrates, at the 12-BM-B beamline at the Advanced Photon Source (APS) in the Argonne National Laboratory (Argonne, Ill., USA). A four-circle Huber diffractometer was used in the specular reflection mode (i.e., the incident angle θin was equal to the exit angle θex and the wave vector transfer |q|=qz=(4π/λ) sin θ is along the surface normal). X-rays of energy of E=10.0 keV (λ=1.24 Å) were used for these measurements. The beam size was 0.40 mm vertically and 0.60 mm horizontally. The samples were held under a helium atmosphere during the measurements to reduce radiation damage and background scattering from the ambient gas. The off-specular background was measured and subtracted from the specular counts. XRR measurements were performed at ambient laboratory temperatures, which ranged from 20 to 25° C.
AR-XPS measurements were performed. Films on quartz substrates were measured at five different take-off angles, relative to the surface plane (5°, 15°, 30°, 45°, 80°) with a PHI 5600 MultiTechnique System (base pressure of the main chamber 2×10−10 Torr). The acceptance angle of the analyzer and the precision of the sample holder concerning the takeoff angle are ±3° and ±1°, respectively. The quartz slides were radiated using a monochromator that allowed a resolution of 0.25 eV. Samples were mounted on Mo stubs. Spectra were excited with Al Kα radiation. The structure due to satellite radiation has been subtracted from the spectra before the data processing. High-resolution spectra of C(1s), O(1s), Si(2p), N(1s), Pd(3d), Cl(2p), Os(4f) Ru(3p3) and Fe(2p) were collected with 5 eV pass energy and a resolution of 0.45 eV. The XPS peak intensities were obtained after Shirley background removal and Gaussian line shapes were used for the curve fitting in the data analysis. The C(1s) line at 285.0 eV was used for calibration.
1H NMR spectra was obtained using a Bruker AMX 400 NMR spectrometer or a Bruker DPX 250 NMR spectrometer. Mass spectrometry was performed using a Micromass Platform LCZ 4000 mass spectrometer.
Synthesis.
Compounds 1-5 were prepared according to literature procedures.
Coupling Layer Formation.
Under inert conditions, ITO, silicon and quartz substrates were loaded onto a holder and immersed in a beaker with dry toluene and para-chloromethyl-phenyl trichlorosilane (0.5 mM) for 45 min; afterwards the holder was immersed into three toluene beakers, iteratively. The substrates were then sonicated for 8 min in toluene (×2) and in hexane, and dried under a stream of N2.
Organic Template Layer (TL) Formation.
Under inert conditions, 3 or 5 (0.5 mM) were dissolved in a solution of dry toluene in a reactor. A holder with ITO, silicon and quartz substrates coated with coupling layer was immersed in the solution and the reactor was sealed. The sealed reactor was kept at 95° C. for 3 days. The slides were then sonicated in DCM (×2) and in THF for 8 min in each solvent, and were subsequently dried under a stream of N2 The substrates were stored under ambient conditions with the exclusion of light.
Organometallic Template Layer (TL) Formation.
Under inert conditions, 1, 2 or 4 (0.2 mM) were dissolved in a solution of dry toluene/acetonitrile (1:1 v/v) in a reactor. A holder with ITO, silicon and quartz substrates coated with coupling layer was immersed in the solution and the reactor was sealed. The sealed reactor was kept at 95° C. for 4 days. The slides were then sonicated in acetonitrile (×2) and in acetone for 8 min in each solvent, and were subsequently dried under a stream of N2. The substrates were stored under ambient conditions with the exclusion of light.
Formation of Binary SPMAs TL-[Os/Ru] Consisting of Complexes 1 and 2 and PdCl2.
Quartz, ITO and silicon substrates, functionalized with an organic or organometallic template layer, were immersed for 15 min in a THF solution of PdCl2(PhCN)2 (1 mM) at RT. The samples were then sonicated in THF (×2) and in acetone for 3 min each. Subsequently, the substrates were immersed in an equimolar solution consisting of 1 and 2 (0.1 mM each, THF:DMF=9:1, v/v) for 15 min at RT. The samples were then sonicated in THF (×2) and in acetone for 5 min each. This procedure was repeated eight times. Deposition step is defined as the deposition of palladium salt and a mixture of complexes. The slides dried under a stream of N2 prior to UV/Vis, ellipsometric and electrochemistry analyses.
Formation of monolayers consisting of complexes 1 or 2 (TL-[M], M=Fe, Os or Ru) and PdCl2.
ITO substrates functionalized with organic or organometallic template layer were immersed for 15 min in a THF solution (2 ml) of PdCl2(PhCN)2 (1 mM) at RT. The samples were then sonicated in THF (×2) and in acetone for 3 min each. Subsequently, the substrates were immersed in a solution consisting of 1 or 2 (0.2 mM, THF:DMF=9:1, v/v) for 15 min at RT. The samples were then sonicated in THF (×2) and in acetone for 5 min each. The slides were dried under a stream of N2 prior to electrochemistry analysis.
Blocking Experiments.
ITO substrates functionalized with 3 were immersed for 15 min in a THF solution of PdCl2(PhCN)2 (0.1 mM) at RT. The samples were then sonicated in THF (×2) and in acetone for 3 min each. Subsequently, the substrates were immersed in a solution consists of 1, 2 or 4 (0.2 mM, THF: DMF=9:1, v/v) for 15 min at RT. The samples were then sonicated in THF (×2) and in acetone for 5 min each. Next, the slides were reacted again with PdCl2 and treated as mentioned above. After PdCl2 treatment, the slides were immersed in an equimolar solution consists of 1 and 2 (0.1 mM each, THF:DMF=9:1, v/v) for 15 min at RT. The samples were then sonicated in THF (×2) and in acetone for 5 min each. The slides were dried under a stream of N2 prior to electrochemistry analysis.
Compounds Preparation.
Compounds 1-5 were synthesized according to literature procedures and were characterized by 1H NMR, mass spectrometry and UV/Vis spectroscopy. Complexes 1 and 2 were also characterized by CVs.
Formation of Coupling Layer.
Cleaned quartz, silicon (100) and ITO substrates were functionalized with (p-chloromethyl)phenyl-trichlorosilane. Coupling layer formation was confirmed by spectroscopic ellipsometry and aqueous contact angle measurements which showed an average thickness of 8 Å and aqueous contact angle of 70°, respectively.
Formation of Organic Template Layer.
Silicon, quartz and ITO surfaces functionalized with coupling layer were reacted with chromophore 3 or 5 to generate template layer TL3 or TL5, respectively (Scheme 1). Formation of the template layer was confirmed by UV/Vis spectroscopy and spectroscopic ellipsometry measurements. UV/Vis absorption measurements on quartz modified with TL3 revealed a band at λmax=325 nm corresponding to the 3-based template layer. UV/Vis spectrum of TL5 showed a band at λmax=356 nm (
Formation of Organometallic Template Layer.
Silicon, quartz and ITO surfaces functionalized with coupling layer were reacted with complex 1, 2 or 4 to generate template layer TL1, TL2 or TL4, respectively (Scheme 1). Formation of the template layer was confirmed by UV/Vis spectroscopy and spectroscopic ellipsometry measurements. UV/Vis absorption measurements on quartz of all three template layers showed a band at λmax=317 nm which corresponds to π-π* band of the ligand. TL1 and TL2 UV/Vis spectrum showed additional characteristic MLCT bands of complex 1 and 2 respectively. Complex 1 has MLCT band at λmax=490 nm, and complex 2 has singlet and triplet MLCT bands at λmax=510 nm and 680-700 nm, respectively (
Formation of SPMA TL-[Os/Ru].
SPMA TL-[Os/Ru] containing both ruthenium and osmium redox centers was formed by iterative binding of PdCl2 and a polypyridyl complex (1 or 2). Surfaces modified with organic or organometallic template layer were immersed in PdCl2 (1 mM, THF) solution for 15 min, followed by sonication in THF and acetone solvents. Subsequently, the substrates were immersed in an equimolar solution consisting of complexes 1 and 2 for 15 min (0.1 mM each, THF:DMF=9:1, v/v), sonicated in THF and acetone solvents. This procedure was repeated 8 times. The slides were dried under N2 stream. One deposition step is the deposition of PdCl2 and a mixture of complexes on the surface.
The growth of SPMA TL-[Os/Ru] on different template layers was monitored by UV/Vis spectroscopy and ellipsometry measurements on quartz and silicon substrates, respectively, after each deposition step. UV/Vis spectrum showed three bands: (i) λmax=317 nm which corresponds to π-π* band of the ligands; (ii) λmax=500 nm corresponds to MLCT bands of complexes 1 and 2 mixture; and (iii) λmax=700 nm, an additional MLCT band of complex 2 (
Film thicknesses of SPMA 1-[Os/Ru], SPMA 2-[Os/Ru], SPMA 3-[Os/Ru] and SPMA 5-[Os/Ru] were measured on silicon (100) after each deposition step using ellipsometry (
CVs of SPMA TL-[Os/Ru] measured on ITO slides showed a reversible redox process characteristic of both couples Os2+/3+ and Ru2+/3+ with a half-wave redox potential, E1/2, of 0.76 V and 1.21 V (vs. Ag/AgCl), respectively (
The composition of the assemblies can be determined according to the total oxidation charge value, Q, of complexes 1 and 2. Q is estimated by integration of the voltammetric oxidation peaks. The ratio between the number of osmium and ruthenium molecules on the surface is derived from Q values of 1 and 2 at a scan rate of 100 mV in accumulative manner. For example: Os:Ru ratio of deposition step number 3 is related to the number of osmium and ruthenium molecules in deposition steps 1-3. Q values for individual deposition steps were calculated by subtracting Q value of previous deposition step from the Q value of each step (Qn=Qn−Qn−1).
Surprisingly, the amount of 2 in SPMA 3-[Os/Ru] is significantly lower than the amount of 1 although the assembly was constructed from an equimolar solution. In the first deposition step, the ratio between 2 and 1 was about 1:10. The ratio increased with the film thickness up to about 1:2 due to growing number of bonded molecules of 2 on the surface (
The observed increase in Os:Ru ratio is also shown for individual deposition steps suggesting that this phenomenon is not a result of the accumulative nature of the deposition steps (
In contrast to the dynamic ratios of SPMA 3-[Os/Ru], compositional analysis of SPMA 1-[Os/Ru], SPMA 4-[Os/Ru] and SPMA 5-[Os/Ru] showed significant different trend of Os:Ru ratios as a function of the deposition steps. These assemblies displayed an increased Os:Ru ratio which leveled off at a certain value and becomes constant (
Reactivity Evaluation of Complexes 1 and 2 on Different Template Layers.
Complexes 1 and 2 were deposited individually on ITO surfaces modified with organic or organometallic template layer to form TL-[M] monolayers. Modified ITO were immersed for 15 min in a THF solution of PdCl2(PhCN)2 (1 mM) at RT. The samples were then sonicated in different solvents. Subsequently, the substrates were immersed in a solution consists of 1 and 2 (0.2 mM) for 15 min at RT. The samples were then sonicated in different solvents for 5 min each.
The quantity of the individual complexes on the surface indicates their reactivity towards the surface. The amount of molecules deposited on the surface was estimated by electrochemistry according to the total oxidation charge value, Q. Q values for each complex on various template layers are summarize in Table 8. Electrochemistry measurements of 2 on TL2 gave a total Q value of both TL2 and complex 2 deposited upon TL2. Therefore, in order to derive the Q value of 2-based monolayer, TL2 was measured individually and its Q value was subtracted from the total Q value. Q value of 2 on TL2 shown in Table 8 is after subtraction.
Complexes 1 and 2 have different reactivity towards TL2, TL3, TL4 and TL5. Upon these template layers, ruthenium was deposited in higher quantity than osmium. Similar behavior of the complexes was observed in the first deposition step of SPMA 3-[Os/Ru], SPMA 4-[Os/Ru] and SPMA 5-[Os/Ru] where complex 1 was more dominant than 2 (
Blocking the Effect of TL3.
Osmium and ruthenium complexes deposited on TL3 displayed an unexpected binding behavior. The complexes are not attached to the surface equally. There is a strong preference for ruthenium molecules to be deposited on TL3. The complexes binding behavior is controlled by the template layer (TL3 in this case) specific organization and orientation on the surface. To support this assumption, a blocking experiment was performed by depositing a layer of molecules (1, 2 or 4) on TL3 before the surface was reacted with the mixture of 1 and 2. As a result, the effect of TL3 will be isolated due to the alteration in the molecules packing upon TL3. The procedure to modify the surface upon TL3 is done similarly as generating surfaces for reactivity experiment mentioned above. Complexes 1, 2 and 4 have similar structure that is different from the structure of 3. Upon TL3, Osmium-ruthenium ratio was about 0.15. Addition of the 4-based isolating layer resulted in osmium-ruthenium ratio of about 0.75 (
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IL2013/050834 | 10/16/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61715041 | Oct 2012 | US |
