Synthesis and testing of new end-functionalized oligomers for molecular electronics
The synthesis and testing of a resetable molecular switch is described.
Introduction
Due to physical and economic constraints, silicon based semiconductor technology is rapidly nearing a production brick wall.1 As the miniaturization of solid-state silicon circuitry continues in order to increase speed, capacity and computing power, a point will be reached at which processors can no longer be made smaller, faster and cheaper. It has been proposed that by incorporating organic molecules into functioning molecular circuits, one may overcome many of the challenges that complimentary metal-oxide semiconductor (CMOS) technology is facing.2 Work performed in the molecular electronics field has demonstrated that single molecules exhibit reversible switching behavior, which may lead researchers to molecular memory and logic devices.3., 4., 5. Our research has centered around oligo(phenylene ethynylene)s (OPEs) which, with the redox active nitro group, have demonstrated negative differential resistance (NDR) at variable temperatures.2
Several new classes of potential molecular electronics molecules have been synthesized in our laboratory in order to develop further understanding of the switching process.6., 7., 8., 9. In this paper, we will discuss our synthetic work on azobenzene derivatives, pyridine systems, and oligo(phenylene vinylene)s (OPVs) that have been synthesized as possible device candidates and that all bear protected thiol end groups for self-assembled attachment to metallic probes. Due to the additional redox center of azobenzenes, the electron deficient nature of pyridyl oligomers, and the high electrical transport seen in OPVs,10 these molecules are good candidates to study device behavior.
In addition to these syntheses, we performed electrochemical testing of selected compounds, a method found to be useful for qualitative comparisons of molecular electronic devices.11 We also include some results obtained from planar test devices using the bipyridyl compounds which show a resetable on-off state and NDR behavior.
Section snippets
Azobenzenes
It has been shown that OPEs containing a redox active aromatic nitro functionality exhibit NDR at various temperatures.3 The proposed mechanism is that the redox center contributes to the switching behavior of the mononitro OPE.12 However, other theories have recently been put forth for NDR behavior including molecule/metal-based contact variations that could result in NDR-like performance.13., 14. By incorporating an azo functionality into an OPE, an additional redox center is created where
Pyridyl devices
By replacing the phenyl8 or biphenyl9 core of the OPE molecule with one or two pyridine moieties, we surmise that the LUMO will be lowered, producing a better match with the Fermi level of the metal contact and higher current through the device.12., 19. In addition, due to the absence of the 2- and 2′-steric interactions, less inter-ring twisting in the bipyridyl system would be present. Molecular modeling (AM1 theory) indicates a reduction of the dihedral angle from 45.5° for the
Oligo(phenylene vinylene)s
In an attempt to design more efficient molecular devices (lower impedance, larger ON:OFF ratios and longer electronic hold times), several features need to be optimized. In order to achieve the highest efficiency in terms of energy used, transport needs to be maximized across a molecular device. To date, most of our research has focused on OPE-based devices.1., 2. Recent work by Chidsey et al.10 has shown that electrical transport is higher through OPVs than through OPEs. Similar results, both
Planar testbed results
Compounds 8 and 9 have been tested for device behavior using the planar device testbed wherein a SAM of the compound is made on a lithographically patterned Au substrate followed by Au evaporation atop the SAM structure.3., 4., 11. The device size can range from 1 to 5 μm2. As shown in Figure 7, compound 8 exhibited reproducible NDR in both sweep directions at room temperature. Previously, NDR had been reported only in the positive direction (in the negative sweep direction, the NDR was far
Summary
Several new classes of oligomers have been synthesized bearing functionalities to interface between proximal electronic probes for molecular electronics studies. The compounds synthesized were series of azobenzenes, bipyridines and oligo(phenylene vinylene)s. Some initial electrochemical results point to electrical similarities between these oligomers and previously prepared OPEs. Initial solid-state test results are also reported for the new class of bipyridines which show encouraging
General
All reactions were performed under an atmosphere of N2 unless otherwise stated. Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl. Hexanes were distilled. Triethylamine (TEA), diisopropylethylamine (DIEA) and CH2Cl2 were distilled from CaH2 under N2. Silica gel plates were 250 μm thick, 40 F254 grade obtained from EM Science. Silica gel was grade 60 (230–400 mesh) from EM Science. Mass spectrometry was performed at Rice University's Mass Spectrometry lab. All new compounds were
Acknowledgements
This work was supported by the Defense Advanced Research Projects Agency, the Office of Naval Research, and the National Institute for Standards and Technology. The National Science Foundation, CHEM 0075728, provided partial funds for the 400 MHz NMR. We thank Dr I. Chester at FAR Research Inc. for supplying trimethylsilylacetylene.
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