Patent Application
20060208252
The present invention relates to molecules exhibiting rectifying properties. In particular, the present invention relates to a molecular rectifying assembly, comprising the general structure METAL1-CON1-BRIDGE-CON2-METAL2 in which CON1 and CON2 are a connecting group or connecting part and independently of each other are molecular groups bound to METAL1,2 in such a way that an adsorbate state with an energy close to the Fermi energy of the metal is formed at one or both of the METAL1,2/CON1,2 interfaces METAL1,2 is selected from metals and/or alloys of metals, and BRIDGE is the core of the molecule, consisting of pi-conjugated and non-conjugated parts. Furthermore, the present invention relates to uses of said molecular rectifying assembly or the production of electronic devices where molecules are placed between at least two electrodes.
All references as cited herein are incorporated by reference in their entireties for the purposes of the present invention.
The trends of miniaturization of electronic circuit elements has progressed continuously since the invention of the integrated circuit. The number of transistors per chip has steadily increased as the feature sizes continued to decrease. When the dimensions of the electronic devices decrease below 10 nm, the device performance will suffer because of quantum limitations and cross talk. Thus at these dimensions new technologies may evolve using individual or groups of molecules as electronic devices performing computations based on novel architectures.
Since the seminal introduction of the first molecular rectifier by Aviram and Ratner [1] based on a Donor-Insulator-Acceptor mechanism, selected molecular rectifiers based on different concepts have been proposed and experimentally realized. In addition, molecular wires and molecular switches have been fabricated, electrically characterized and partially also used for the realization of molecular memories using crossbar structures, consisting of aligned electrodes, bars, or wires that are organized in two planes forming a two-dimensional grid, wherein molecules are placed at the intersecting points between the bars, electrodes, or wires (data lines).
Molecules exhibiting rectifying properties are key for the realization of molecular electronic logic devices based on crossbar structures. It is therefore an object of the present invention, to provide novel molecules exhibiting rectifying properties that exhibit advantageous properties. The main problem in designing functional molecules, such as rectifiers, is to find a metal binding group that provides an excellent electrical coupling to the metal, so that the applied potential drops across the molecule (
In one aspect of the present invention, the object is solved by providing general structures and component parts for fabricating molecular rectifiers having a small potential drop at the interface at one or both metal/molecule interfaces. The present invention therefore relates to a molecular rectifying assembly, comprising the general structure
METAL1-CON1-BRIDGE-CON2-METAL2
in which CON1 and CON2 are a connecting group or connecting part and independently of each other are molecular groups bound to METAL1,2 in such a way that an adsorbate state with an energy close to or in resonance with the Fermi energy of the metal is formed at one or both of the METAL1,2/CON1,2 interfaces, METAL1,2 is selected from metals and/or alloys of metals, and BRIDGE is the core of the molecule, consisting of pi-conjugated and non-conjugated parts.
This invention concerns the realization of asymmetric molecules of the general structure CON1-BRIDGE-CON2 where one or both ends of the molecule that bind to the metal in the molecular rectifying assembly is a dithiocarbamate or dithiocarbamate derivative, which will be generally abbreviated herein as DTC. In this generalized notation of the molecular structure, it is recognized that certain atoms or ions, or groups or combinations thereof, may leave the asymmetric molecule when the rectifying assembly METAL1-CON1-BRIDGE-CON2-METAL2 is produced. For example, if the CON1 end of the rectifying assembly has the form METAL1-S-BRIDGE, the group at that end of the asymmetric molecule, before assembly, could be, for example, a thiol group (HS—), a protected thiol group (e.g., CH3C(O)S—), or a disulfide group (—SS—). In the case of that the CON1 end of the rectifying assembly has the form METAL1-DTC-BRIDGE, the group at that end of the asymmetric molecule, before assembly, could be, for example, a dithiocarbamate salt group (M+ −S2CNH— or M+ −S2CNR′—), a dithiocarbamate ester group (RS(S)CNH— or RS(S)CNR′—), a bis-dithiocarbonimidate group ((RS)2CN—), or a thiram group (—NHC(S)S—S(S)CNH— or —NR′C(S)S—S(S)CNR′—), wherein M+ represents a cationic species such as, for example, Na+ or NH4+, and the groups R and R′ independently of each other represent small organic residues such as, for example, methyl (CH3), ethyl (CH2CH3), or phenyl (C6H5).
One aspect of the present invention further proposes to utilise dithiocarbamate derivatives as metal-binding groups for the realization of the metal/molecule interface in molecular rectifiers. The dithiocarbamate (DTC) group exhibits good pi-delocalisation in the NCS2 plane, which is formed by the overlap between two p orbitals from the sulfur atoms, one p orbital from the nitrogen and one from the carbon atom [2]. Both S atoms bind equivalently to Au and the free electron is delocalised between the two sulfur atoms. According to Ariafard et al. a charge deficiency on the sulfur atoms leads to a charge deficiency in the pz orbital of the carbon atom, forcing the lone pair of the nitrogen atom to shift to the carbon atom and hence strengthening the π character of the C—N bond [3].
Upon attaching a dithiocarbamate group to a Au cluster, the electron density of molecular orbitals that are not solely associated with the gold cluster extends over the cluster and the conjugated part of the molecule. Thus if a benzene ring is directly attached via a dithiocarbamate group to a metal electrode, the electron density of the molecular orbitals that are not solely associated with the cluster extend all the way from the cluster to the benzene ring.
The general structures of the molecular rectifiers that are proposed here consist of a) mono-dithiocarbamate derivatives coupled to metal electrodes, and b) bis-dithiocarbamate derivatives coupled to metal electrodes having the general structure:
METAL1-CON1-BRIDGE-CON2-METAL2
where BRIDGE represents the core of the molecule, CON1 and CON2 denotes the connecting (coupling or metal-binding) groups, and METAL1 and METAL2 represent the metal electrodes, which may be, independently of each other, in the form of, for example, a data line, a scanning probe tip, or a metal particle. At least one of the connecting groups (CON1 and CON2) is a dithiocarbamate or derivative of a dithiocarbamate, and the metal comprising the electrodes (METAL1 and METAL2) may be the same or different. In one aspect of the present invention, CON1 and CON2 independently can be identical or different.
Theoretical investigations using a first-principles atomistic approach revealed that an increase in the atomic number of the anchoring group provides a better contact to the electrode [4]. Thus, Se and Te provide a superior contact to Au than S. In the cases of S and Se contacts, the Fermi level lies between the HOMO-LUMO gaps, while for Te the HOMO is in resonance with the Fermi level.
The effect of the metal-molecule contact on the I-V characteristics of molecules was demonstrated using rigid π-conjugated oligo(phenylene ethynylene) having an α-thio and a ω-thio, a ω-methyl, a ω-pyridine or a ω-nitro functionalities [5]. The large voltage drop at the methyl/metal interface leads to strong current rectification in the molecule having a α-thio and an ω-methyl functionality. This is also the case for the oligo(phenylene ethynylene) having an co-nitro functionality. This suggests that the nitro functionality does not provide a good contact to the metal. In contrast, both the oligo(phenylene ethynylene) with an α-thio and an co-thio or an co-pyridine group show symmetric I-V characteristics, indicating that electronic coupling of the pyridine group to Au is comparable with that of thiol to Au [5]. Rigid monophenyl, bi-phenyl and terphenyl rings have been used in order to investigate the difference between isocyanide and thiol binding groups experimentally [6] and theoretically [7]. This investigation showed that in case of a thiol (SH) binding group, the conductance gap decreased as the number of phenyl rings increased, while in case of the isocyanide (—NC) binding group, the conductance gap increases with increasing number of phenyl rings. This is probably due to the fact that the π-orbitals of the isocyanides strongly interact with the π-orbitals of the phenyl rings and the energy of their HOMO level increases with increasing number of phenyl rings enhancing the electron donating capabilities of the isocyanide group. Thus the isocyanide group becomes more positive upon binding to Au as the number of phenyl rings increases, leading to a lowering of the isocyanide orbital energy away from the Fermi energy.
So far, mainly thiols and disulfides have been used for the attachment of molecules to metals. However, in both cases the electronic coupling between the sulfur atom and the Au atom is weak because the Au—S (thiol) bond has a low density of states. The Au—S bond has a very low density of states. The Au atoms contribute s states at the Fermi level while the sulfur atom contributes p states. By symmetry, only the p states that are perpendicular to the electrode surface can couple to the s states of the Au forming σ bonds. The p states of the S atom that are parallel to the metal surface do not couple to the Au s states [8], thereby constituting a barrier for charge transport.
The metal is furthermore preferably selected from the group comprising the metals of the Group 12 elements, the Group 13 elements, the Group 14 elements, and the Transition elements, where the latter are broadly defined as elements that have partly filled d or f shells in any of their commonly occurring oxidation states. More preferably, the metal is independently selected from the group comprising Au, Ag, Pd, Pt, Cr, Cu, Ti, Ni and combinations of these metals and alloys thereof. Thus, yet another aspect of the present invention relates to a molecular rectifying assembly, wherein the metal is independently selected from the group comprising Au, Ag, Pd, Pt, Cr, Cu, Ti, Ni, and combinations of these metals, and alloys thereof. The metals can be the same or different.
Another preferred aspect of the present invention relates to a molecular rectifying assembly, wherein in BRIDGE-CON-METAL the group —CON— is selected from —NHCS2— (dithiocarbamate); —NHC(SR)S— (dithiocarbamate ester); —NC(SR)2— (bis-dithiocarbonimidate); —NR′CS2— (dithiocarbamate, N-substituted); —NR′C(SR)S— (dithiocarbamate ester, N-substituted); —NH2— (amine); —NC— (isonitrile); —NR′PS3— (trithiophosphoroamidate); —S— (thiolate); —S(SR)— (disulfide); —SCS2— (trithiocarbonate); —SC(SR)S— (trithiocarbonate ester); —CN— (nitrile); —CS2— (dithiocarboxylate); —C(SR)S— (dithiocarboxylate ester); —COS— (thiocarboxylate); —COSR— (thiocarboxylate ester); —C5H4N—(pyridine); —C3H3N2— (imidazole); —COO— (carboxylate); —OCS2— (xanthate); —OC(SR)S— (xanthate ester); —OP(OR)S2— (dithiophosphate); —OP(OR)(SR′)S— (dithiophosphate ester); —PR2— (phosphine); —PS3— (trithiophosphonate); —P(SR)2S— (trithiophosphonate ester); —B(SR)S— (thioborate ester); —BS2— (thioborate); —Te— (tellurate) and —Se— (selenate); wherein the group R and R′ independently of each other represent small organic residues, such as, for example, methyl (CH3), ethyl (CH2CH3), and phenyl (C6H5).
Another preferred aspect of the present invention relates to a molecular rectifying assembly, wherein the group BRIDGE is selected from pi-conjugated and non-conjugated parts, having the general structure (pi-conjugated)(non-conjugated) or (non-conjugated)(pi-conjugated)(non-conjugated) or (non-conjugated)x(pi-conjugated)(non-conjugated)y. More preferred is a molecular rectifying assembly according to the present invention wherein (non-conjugated)x and (non-conjugated)y independently are different in length.
Another preferred aspect of the present invention relates to a molecular rectifying assembly, wherein the pi-conjugated part is selected from the group of
wherein k is selected from integers between and including 1 to 6, preferably between and including 1 to 4, and further functionalized derivatives thereof. Also preferred is a molecular rectifying assembly according to the present invention, wherein the non-conjugated part is selected from the group of straight alkane chains, cyclic or polycyclic aliphatic carbon skeletons,
wherein n is selected from integers between and including 1 to 18, preferably between and including 1 to 14, and in is selected from integers between and including 1 to 5, and further functionalized derivatives thereof.
It should be evident to anyone skilled in the art that one or more of the hydrogen atom (H) substituents of these particular pi-conjugated parts may be replaced by other atoms or groups of atoms for connecting the pi-conjugated parts to other pi-conjugated parts or to non-conjugated parts of the molecule, or to modify the electronic properties of the molecular rectifying assembly. For example, an alkyl chain (non-conjugated part) may be attached to an aromatic ring (pi-conjugated part) directly, i.e. through a methylene (CH2) group, or indirectly, e.g. through an ether (O) group. Also by way of example, replacement of the hydrogen atoms by halogen atoms (F, Cl, Br, I), organic groups containing halogen atoms, such as CF3 and COCF3, or organic groups containing double or triple bonds, such as CHO, COCH3, C≡N, and NO2 tends to lower the chemical potential of the pi-conjugated part, while replacement of the hydrogen atoms by organic groups such as CH3, OCH3, and N(CH3)2 tend to increase the chemical potential of the pi-conjugated part. It should also be evident to anyone skilled in the art that one or more of the methylene (CH2) groups within the skeleton of these particular non-conjugated parts may be replaced by other atoms or groups of atoms for connecting the non-conjugated parts to other non-conjugated parts or to pi-conjugated parts of the molecule, or to modify the structural properties of the molecular rectifying assembly. For example, replacement of the hydrogen atoms by bulkier atoms or groups, such as I or CH3, will affect the way that the molecules pack within the assembly. Hydrogen atoms may also be replaced by groups such as OH or CONH2 to provide hydrogen-bonding capabilities between the molecules. It should also be evident to anyone skilled in the art that one or more of the carbon atoms within the skeleton of these particular non-conjugated parts may be replaced by other atoms or groups of atoms to modify the structural properties of the molecular rectifying assembly. For example, a (CH2)3 unit within a hydrocarbon chain can be replaced by (CH2)2O or (CH2)2NH, to introduce a greater degree of flexibility or to provide hydrogen-bonding capabilities between the molecules. Molecules that contain such substituents as described above will be understood as “functionalized derivatives” in the context of the present invention.
In the context of the present invention, a “tunnel barrier” is a region in space characterized by a potential energy, which is higher than the kinetic energy of an approaching electron. Due to the wavelike behavior the electron has a finitie probability of crossing the potential barrier. This process is called tunneling.
In the context of the present invention, an “adsorbate state” is a state that is formed upon the formation of a bond between a molecule and a metal as a result of the mixing between molecular orbitals and metal states. The energy levels of the adsorbate states are broadened. The electron density of the adsorbate states is delocalized over the metal and the metal binding group and hence provides an excellent electronic coupling between metal and molecule.
Another preferred aspect of the present invention relates to a molecular rectifying molecule, wherein one of the metal coupling groups is a dithiocarbamate derivative selected from the general formula BRIDGE-NHCS2-METAL, and BRIDGE-NR′CS2-METAL, wherein BRIDGE and METAL are as indicated above and wherein the group R′ represents a small organic residue, such as methyl (CH3), ethyl (CH2CH3), and phenyl (C6H5).
Yet another preferred aspect of the present invention relates to a molecular rectifying molecule, wherein said molecule is selected from the group of
wherein a, n and b, m are selected from integers between and including 1 and 18, and wherein the group R represents a small organic residue, such as methyl (CH3), ethyl (CH2CH3), and phenyl (C6H5), and functionalized derivatives thereof.
In an even more preferred molecular rectifying assembly according to the present invention, said non-conjugated parts independently have a length of between and including 1-23 Å, preferably between and including 1-20 Å, most preferred between and including 1-16 Å.
Directed assembly of molecules—Critical for the fabrication of devices comprising several asymmetric molecules is to control the molecular orientation in the assembled film. Langmuir-Blodgett deposition can be used to orient amphiphilic molecules on surfaces [9]. Alternatively, molecules with only one metal binding group have been used to construct molecular rectifiers that assemble with a controlled orientation on the surface [10]. Thus, another general aspect of the present invention is related to a method for producing a molecular rectifying assembly as described above, wherein said method comprises the steps of providing different protecting groups and sequential deprotection of the metal binding groups. More preferred, in the method according to the present invention the molecule contains a dithiocarbamate ester group or a derivative thereof, and said method comprises the steps of protecting the thiol via trimethylsilylethyl (TMSE), and removing the TMSE by tetrabutylammonium fluoride in THF after the assembly of molecules with the dithiocarbamate ester facing to the metal. Most preferred is a method according to the present invention, comprising the step of providing a particular orientation to the molecule during adsorption onto the metal surface by introducing interacting side groups into the molecule and/or chemically modifying the molecule after formation of the self-assembled monolayer (SAM).
In both cases, however, the requirement for the control over the molecular orientation limits the possibilities to the design the molecular backbone in order to optimise e.g. rectification properties of molecules.
Pollack et al. demonstrated that sequential deprotection of aryl dithiols allows to position α,ω-dithio functionalized molecules with an asymmetric central core such as
with a preferred polar order [11]. Deprotection of the thioacetate was achieved under acidic conditions (concentrated sulfuric acid) in the presence of gold, while the trimethylsilylethyl (TMSE) protecting group was removed by immersion into a THF solution of tetrabutylammonium fluoride.
A further possibility for introducing orientation of molecules in SAMs through the assembly process is to implement hydrogen bonding moieties at one side of the molecule. The formation of hydrogen bonds within the SAM is the driving force for controlling the orientation of the molecules in the SAM.
In yet another aspect, the method comprises the step of activating terminal carboxylic acid groups of SAMs of 11-mercaptoundecanoic acid and subsequently reacting them with amines or alcohols to form amide or ester linkages or converting of the amine group to the dithiocarbamate group, a salt or ester derivative thereof in the solution phase.
Finally, the present invention relates to the use of a molecular rectifying assembly as above for the production of electronic devices where molecules are placed between at least two electrodes.
Rectifying molecules—Molecular rectifiers are potentially one of the key enabling electronic components for the realization of molecular memories and molecular logic. These applications require molecular rectifiers with high rectification ratios and sharp voltage thresholds. So far, molecular rectifiers based on six different concepts have been introduced.
1. Donor-σ-Acceptor
Aviram and Ratner [1] introduced a Donor-Insulator-Acceptor mechanism as a molecular rectifier. In this concept D is a strong electron donor and A is a strong electron acceptor. An insulating bridge consisting of non-conjugated σ bonds separates D and A. If such a molecule is placed between two electrodes, the I-V characteristics should be asymmetric if the zwitterionic state D+-σ-A− is relatively easily accessible from the neutral ground state and the opposite zwitterionic state D−-σ-A+ lies significantly higher in energy. In the forward direction the electrons tunnel inelastically from the LUMO of D via the insulating σ bridge to the low-lying HOMO of A when the Fermi level of the electrodes line up with the LUMO of A and the HOMO of D.
2. Energy Mismatch Between Two Localized Energy Levels
A slight variation of the Donor-σ-Acceptor mechanism was introduced by Ellenbogen and Love [12]. This mechanism is based on molecules consisting of two conducting levels that are isolated from each other by an insulating bridge consisting of non-conjugated σ-bonds. In the forward direction both conducting levels align at a certain applied bias facilitating resonant tunneling through the molecule. In the reverse direction, the applied bias leads to a further separation of the conducting levels and hence to a suppression of the tunnel current.
Both cases require a very good electrical isolation between the two molecular systems that are responsible for the rectification and the potential drop across the electrode/molecule interface and the insulating barrier as well as the location of the LUMO and HOMO relative to the Fermi level under zero bias and under applied bias are key for the design of molecular rectifiers and determine the rectification current and the threshold voltage. These systems have not been experimentally realized.
3. Asymmetric Position of HOMO and LUMO Levels on Molecule
A Langmuir Blodgett monolayer of a hexadecylquinolinium tricyanoquinodimethanide between Mg and Pt electrodes [13-15] as well as between Al electrodes [16, 17] has been used in order to provide experimental verification of the Donor-σ-Acceptor mechanism. However, theoretical modeling revealed that hexadecylquinolinium tricyanoqinodimethanide is a Donor-π-Acceptor molecule [18]. The rectification ratios (RR=(current at V0)/(current at −V0)) obtained from multilayers of hexadecylquinolinium tricyanoquinodimethanide sandwiched between Al electrodes varied between 2.4 and 26.4. The rectification ratio decreased continuously with each repeated cycle and disappeared after 4-6 cycles. Theoretical modelling showed that the asymmetry in the I-V characteristics of D-π A systems is due to asymmetric positions of the LUMO and HOMO with respect to the Fermi level and to the asymmetric profile of the electrostatic potential across the molecule [18]. Furthermore the calculations showed that the presence of tunnel barriers like alkyl chains at the D or A side have a pronounced influence on the charge transport characteristics.
The D-σ-A and D-π-A rectifiers are complex molecular systems. Application of a bias potential leads to large inhomogeneous electric fields resulting from the molecular dipoles and the screening induced by the molecules themselves and the electrodes. Such systems are very difficult to describe theoretically and to simulate the expected I-V characteristics. Large discrepancies between theoretical modeling and experimental results have been described for hexadecylquinolinium tricyanoquinodimethanide [18].
4. Asymmetry of HOMO and LUMO Levels with Respect to EF and Asymmetrically Positioned Electroactive Unit
A general and simple concept for molecular rectification has been introduced by Kornilovitch et al. [19]. In this concept a single electroactive unit is positioned asymmetrically between two electrodes and the HOMO and LUMO are placed asymmetrically with respect to the Fermi energy EF. Thus it is sufficient to have one conducting molecular level located between two different tunnel barriers between electrodes. Based on the assumption that most of the potential drops across the larger tunnel barrier, the conditions for resonant tunneling are achieved at very different voltages for both directions [19]. The molecules have the general structure:
They consist of a central conjugated island located between two insulating barriers IL and IR. The island provides an energy level, which is not too far from the Fermi energies of the electrodes. The end groups can for example be thiols or they may be absent altogether if the application does not require a strong contact. One molecular structure that resembles such molecular rectifiers is HS—(CH2)a—C6H4—(CH2)b—SH. The two main parameters determining the rectification ratio are the energy difference between the LUMO and the Fermi energy of the electrodes and the potential drop across the two insulating barriers. Thus a pronounced increase in conductivity is expected when the Fermi level of the electrode to which the smaller tunnel barrier is attached aligns with the LUMO of the isolated conductive part of the rectifier. The onset in current is due to resonant tunneling through the LUMO of the conjugated unit of the molecule. Theoretical calculations have shown that the highest rectification ratio of RR˜500 is obtained for molecules where a=2 and b=10 [19]. The optimization of the rectification ratio is limited by the low conductivity of the non-conjugated molecular bonds that are required for the realization of the two insulating parts, since even alkane chains as short as C12H25 are fairly resistive [20].
Additional possible structures for molecular rectifiers disclosed by Kornilovitch et al. are shown below:
According to the theoretical calculations of Kornilovitch et al., the results should be independent of the nature of the connections to the electrodes at both ends. They suggest that the end- or binding groups can be adjusted to the desired experimental set-up or that they can be absent [19].
5. Asymmetric Coupling to Electrodes
A further possibility for realizing rectification in a metal-molecule-metal junction is to couple a molecule to two metals that have different work functions. Asymmetric contacts can induce asymmetric I-V characteristics in symmetric molecules through differential charging. The characteristics of the asymmetry depend on whether the occupied or unoccupied molecular orbitals contribute to the charge transport through the molecule. Thus the asymmetry can be switched by altering the applied potential to induce either conduction through the occupied or unoccupied molecular orbitals. This effect has been experimentally observed, however, the rectification ratio was negligibly small [21].
6. Asymmetric Central Unit
An asymmetric I-V characteristic was also observed for a molecule that had an asymmetric central benzene ring. The molecule was coupled at both ends via SH to Au electrodes. The asymmetry was introduced by substituting the benzene ring on the 2 and 5 positions with an electron donating amide group and with an electron withdrawing NO2 group, respectively [22]. However, the observed asymmetry was very small and the rectification was insignificant.
The present invention shall now be further described by the following examples with reference to the accompanying figures, without being limited to said examples. All theoretical calculations were performed using Density Functional Theory (DFT) at the generalized gradient approximation level with either the PW91 functional (
The Ultraviolet Photoelectron Spectroscopy (UPS) spectra were recorded with an Axis Ultra from Kratos. The photoelectrons were excited with a He lamp (He II) at the Energy of 40.8 eV and the spectra detected with a 165 mm radius hemispherical analyzer.
In the figures,
), phenylenebisdithiocarbamate (
) and of thiram (
) on Au (111).
A detailed study has been performed in order to investigate the influence of the metal binding group and the degree of conjugation on the charge transport through symmetrical bifunctional linker molecules in 3-dimensional assemblies of interlinked gold nanoparticles [23, 24]. The observed optical and experimental results indicate that the bond formation between Au and the dithiocarbamate group leads to the formation of an adsorbate state with an energy that is close to the energy of the Fermi level of Au.
Theoretical modeling of various molecules with dithiocarbamate and thiol binding groups attached to Au13-clusters using DFT generalized gradient approximation (PW91 functional) further supported the experimental findings. The calculated orbitals of the molecule-cluster complexes reveal notable difference between the dithiocarbamate-Au— and the thiol-Au coupling.
This situation is different if the molecule is attached via a dithiocarbamate group to the Au-cluster.
These findings are further supported by UPS spectra of self-assembled monolayers of thiol and dithiocarbamate derivatives on Au(111) (
XPS studies have shown that both S-atoms of the dithiocarbamate group bind equivalently to Au. The pronounced electron withdrawing capabilities of the C atom in the dithiocarbamate group probably facilitates the formation of the adsorbate state, providing the excellent electronic coupling between Au and the molecule. It is likely that this is also the case when the dithiocarbamate group binds to other metals such as Pd, Pt, Cu, Ni, and Ag.
The molecular rectifiers that are proposed in the present invention preferably consist of a) mono-dithiocarbamate derivatives coupled to metal electrodes, and b) bis-dithiocarbamate derivatives coupled to metal electrodes having the general structure:
METAL1-CON1-BRIDGE-CON2-METAL2
where BRIDGE represents the core of the molecule, CON1 and CON2 denotes the connecting (coupling) groups, and METAL1 and METAL2 represent the metal electrodes. At least one of the connecting groups (CON1 and CON2) is a dithiocarbamate or derivative of a dithiocarbamate, and the metal comprising the electrodes (METAL1 and METAL2) may be the same or different.
The central BRIDGE itself can be further classified with respect to the position and number of pi-conjugated and non-conjugated parts:
(pi-conjugated)(non-conjugated)
(non-conjugated)(pi-conjugated)(non-conjugated)
Classification of the building blocks of the molecular rectifier proposed:
The combination BRIDGE-CON2-METAL2 in the general structure METAL1-CON1-BRIDGE-CON2-METAL2 may be chosen from the following list:
where discrete charges have been omitted, and the groups R or R′ independently represent small organic residues such as methyl (CH3), ethyl (CH2CH3), and phenyl (C6H5), and the like.
Conjugated building blocks: The following list of pi-conjugated systems comprises potential structural elements to be part of a molecular rectifier. The pi-conjugated systems may be further functionalized as described above. The structure is meant to be part of the molecular rectifier with the free sites attached to the molecular scaffold as indicated. Preferred examples are:
Non-conjugated building blocks: The non-conjugated moieties comprise aside from simple alkane chains [—(CH2)—] cyclic and polycyclic aliphatic carbon skeletons which may be further functionalized as described above. The structure is meant to be part of the molecular rectifier with the free sites attached to the molecular scaffold as indicated.
Mono-dithiocarbamate Derivatives
a1) One of the metal coupling groups is a dithiocarbamate derivative BRIDGE-NHCS2-METAL or BRIDGE-NRCS2-METAL attached to the conjugated moiety (BRIDGE). The other moieties of the molecule may be chosen from the said building block lists. The molecular rectifier has the following general structure:
The rectifiers proposed possess a constitutionally inherent asymmetry due to the combination of pi-conjugated and non-conjugated bridge and different metal binding groups.
a2) One of the metal coupling groups is a dithiocarbamate derivative BRIDGE-NHCS2-METAL or BRIDGE-NRCS2-METAL attached to the non-conjugated moiety (BRIDGE). The other moieties of the molecule may be chosen from the said building block lists. The molecular rectifier has the following general structure:
a3) One of the metal coupling groups is a dithiocarbamate derivative BRIDGE-NHCS2-METAL or BRIDGE-NRCS2-METAL attached to the non-conjugated moiety (BRIDGE). The other moieties of the molecule may be chosen from the said building block lists. The pi-conjugated part is placed in between two non-conjugated, resistive parts. The molecular rectifier has the following general structure:
Bis-dithiocarbamate Derivatives
b1) Both of the metal coupling groups are a dithiocarbamate derivative BRIDGE-NHCS2-METAL or BRIDGE-NRCS2-METAL attached to the conjugated moiety (BRIDGE). The other moieties of the molecule (pi-conjugated and non-conjugated part) may be chosen from the said building block lists. The molecular rectifier has the following general structure:
b2) Both of the metal coupling groups are a dithiocarbamate derivative BRIDGE-NHCS2-METAL or BRIDGE-NRCS2-METAL attached to the non-conjugated moiety (BRIDGE).
A difference in length between the non-conjugated parts will be sufficient for a rectifying behavior. For the following selected molecular rectifiers the orbital of the molecules and/or molecule-cluster conjugates have been calculated.
1) Molecular rectifiers having the general structure
consisting of one dithiocarbamate binding group and one amine binding group,
and 2) one dithiocarbamate binding group and one thiol binding group,
a and b show the electron density of the first five occupied and five unoccupied molecular orbital energy levels, respectively, for molecule 1 coupled with the dithiocarbamate end to a Au13 cluster. The occupied orbitals are mainly delocalised over the Au cluster and the S atoms of the dithiocarbamate binding group. Roughly 0.7 eV above the Fermi level the orbitals are delocalised over the Au-cluster and the pi-conjugated part of the molecule including the O atom attached in para position to the benzene ring. Similarly, in case of the unoccupied orbitals the orbital are mainly delocalised over the Au-cluster and the whole dithiocarbamate group. The LUMO+3 and LUMO+4 orbitals also show a small contribution on the benzene ring and the O atom.
Although one of the metal binding group and the length of the alkane chain is different for these two molecules, these examples indicate that the O atom attached in para position to a benzene ring that is substituted in the other para position with a dithiocarbamate group has a pronounced effect on the orbitals that are delocalised over the Au cluster, the dithiocarbamate binding group and the benzene ring. The electron density on the benzene ring is in case of molecule 1 not as pronounced as in the case of molecule 2.
3) Molecular Rectifiers Having the General Structure
consisting of one dithiocarbamate binding group and one thiol binding group,
and consisting of two dithiocarbamate binding groups.
Oriented assembly via sequential deprotection of metal binding groups. An oriented assembly of the devised structures can be achieved by employing the technique of orthogonal protecting groups for the metal binding groups, i.e. the use of different protecting groups for both ends of the rectifying molecule. Employing orthogonal protecting groups for the metal binding groups means that one metal binding group can be selectively deprotected after each other and exposed to the metal by using different deprotection conditions.
An example is the assembly of molecules containing a dithiocarbamate ester or derivative and a thiol protected via the trimethylsilylethyl group (TMSE). After the assembly of molecules facing with the dithiocarbamate ester to the metal the TMSE can be removed by exposition to tetrabutylammonium fluoride in THF.
With respect to the vast number of protecting groups available for thiols and other metal binding groups it is referred to the literature [25].
This section specifically addresses the fabrication of molecular rectifiers in which the connecting groups CON1 and CON2 have similar metal-binding affinities, such as when both are dithiocarbamate derivatives. It can be anticipated that molecules having the general structure CON1-BRIDGE-CON2 will assemble onto a metal surface with no preferred orientation, since the binding affinities of the two ends are essentially the same, thereby negating the intrinsic rectifying properties of the individual molecules.
One way to circumvent this problem is to introduce interacting side groups into the molecules to promote a particular orientation during adsorption onto the metal surface. However, this approach has several drawbacks, including increased synthetic requirements and potentially complicated electronic effects.
An alternative approach to circumventing this problem is chemical modification after SAM formation. Specifically, molecules having the general structure DTC-BRIDGE-NH2, in which DTC represents a dithiocarbamate derivative, are expected to assemble preferentially onto metal surfaces with the orientation METAL1-DTC-BRIDGE-NH2 instead of METAL1-NH2-BRIDGE-DTC due to the significantly greater metal-binding energy of dithiocarbamates compared to amines.
Previous work has demonstrated that SAMs of thiols on gold surfaces are robust enough to sustain chemical modification after assembly. Examples include the activation of terminal carboxylic acid groups of SAMs of 11-mercaptoundecanoic acid and subsequent reaction with amines or alcohols to form amide or ester linkages and involve exposing the SAMs to various kinds of organic solvents and reagents. The conversion of the amine group to the dithiocarbamate group (salt or ester derivative), which proceeds in a straightforward manner in the solution phase, is likewise expected to occur in the METAL1-DTC-BRIDGE-NH2 assembly by exposing it to the appropriate reagents, making it possible to fabricate METAL1-DTC-BRIDGE-DTC-METAL2 assemblies in which the BRIDGE units have a single, preferred orientation.
This procedure is based on one that was described by Rosink et al. [26]. They demonstrated the self-assembly of pi-conjugated azomethine oligomers by sequential deposition of monomers from solution, and found that the azomethine molecules order approximately perpendicular to the gold surface. The process starts with the growth of a monolayer of 4-aminothiophenol (4-ATP) from solution phase onto a gold substrate. The thiol group of 4-ATP binds covalently to the gold surface, so that the molecules in the resulting SAM are oriented with the amine groups free for subsequent chemical reaction. In the second step of the process, the SAM is treated with a solution of 1,4-diformylbenzene (terephthaldicarboxaldehyde, TDA). One of the aldehyde groups of TDA condenses with the amine group of 4-ATP in the SAM to form a pi-conjugated imine bond between the two aromatic rings, which is terminated by the unreacted aldehyde group of the TDA moiety. The third step of the process involves the condensation reaction of one of the amine groups of 1,4-diaminobenzene (DAB) with the unreacted aldehyde group of the TDA moiety, extending the degree of pi-conjugation by forming another imine bond.
The procedure of Rosink et al. is modified in this example to provide a molecular rectifying assembly. The first two steps of the process are the same as they described, i.e., the formation of a SAM of 4-ATP on a gold surface and reaction of the SAM with TDA. The structure 6 of the resulting SAM after these two steps is shown in
Similar segment by segment growth strategies can be used to provide examples of METAL1CON1-(non-conjugated)(pi-conjugated)(non-conjugated)-CON2-METAL2 and METAL1-CON1-(non-conjugated)(pi-conjugated)-CON2-METAL2 rectifying assemblies, as indicated in
A rectifier with a dithiocarbamate ester group as connecting group 1 (CON1) and an amino group as connecting group 2 (CON2) has been synthesized according to the following scheme:
1H-NMR (400 MHz, CD2Cl2) spectrum of intermediate isolated after reaction step c). The spectrum matches the devised structure (inset) which is further supported by the elemental analysis (C, 52.69; H, 6.59; N, 8.26; 0, 14.3; S, 18.4/calcd. C, 52.61; H, 6.47; N, 8.18; 0, 14.02; S, 18.73) [29].
The synthesis of the target compound as hydrochloride proceeds smoothly with 3M HCl in ethyl acetate at room temperature. Yield is about 200 mg (99%) without the need of any further purification. The free base can be formed by treating the hydrochloride with 1N NaOH and extracting the amine into chloroform or ether.
1H-NMR (400 MHz, MeOD) spectrum of the target compound methyl 4-(2-aminoethoxy)phenylcarbamodithioate hydrochloride isolated after reaction step d). The spectrum matches the devised structure (inset).
The procedure allows easily for the attachment of longer alkyl chains to the phenoxy-subunit. Optionally the attachment of an alkyl chain can be carried out via a Mitsunobu-type reaction according to Quéléver et. al. [30].
The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realizing the invention in diverse forms thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05001448.9 | Jan 2005 | EP | regional |
