Syntheses of new functionalized azobenzenes for potential molecular electronic devices
Graphical abstract
New non-symmetrical azobenzene derivatives have been synthesized as potential molecular electronics switching device candidates.
Introduction
Molecular electronics is a promising area of research that focuses on a bottom-up strategy to fabricate nanoscopic electronic devices based on self-assembled monolayers (SAMs) of organic molecules on surfaces.1 One of the classes of compounds we have concentrated on is a series of oligo(phenylene ethynylene) (OPE) derivatives that have enabled the study of molecular switching processes in electronic devices.1, 2, 2(a), 2(b), 2(c), 2(d), 2(e), 2(f), 2(g), 2(h)
Alternatively, the self-assembly of photo-responsive molecules onto the substrates creates the possibility of using switching devices as photo-responsive components in optical storage and molecular recognition applications.3 Azobenzene derivatives are interesting molecular device candidates that have been widely investigated due to their non-degradative reversible trans–cis photoisomerization during repeated switching cycles; azobenzenes also possess useful electrochemical activity.4, 4(a), 4(b), 4(c), 4(d), 4(e), 4(f), 4(g), 4(h), 4(i) Accordingly, the isomerization of azobenzene derivatives has been observed under the influence of an external electric field at the single molecule level.5
Previously we have shown that symmetrical azobenzene OPEs bearing protected thiol end groups form SAMs on metal surfaces.2e We envisioned that the synthesis of non-symmetrical azobenzene derivatives could be realized by changing the substituents (R1 and R2 in A, Fig. 1) of a disubstituted azobenzene. The resulting products would lead to an azobenzene OPE thiol/thioacetate that could form SAMs on many metal surfaces through compound B1, 2 or alternatively to an azobenzene diazonium salt C for grafting of molecular layers on semiconductor surfaces (Fig. 2).6
Section snippets
Syntheses of azobenzene derivatives of type B
Functionalized azobenzene OPEs bearing free thiol or protected thiol end groups are preferred for formation of SAMs on gold substrates. Therefore the azobenzene OPEs 1–5 were prepared. After some preliminary experiments, the Oxone® mediated oxidation of anilines 9b,c to provide nitrosoarenes 10a–c,7 followed by condensation with iodoaniline 11, was found to be an efficient path to prepare a series of azobenzene derivatives. For example, the nitrosobenzene 10a (Scheme 1, prepared from aniline7)
Summary
In summary, the methodology detailed here provides versatile synthetic pathways to azobenzene derivatives that could be used as potential molecular electronic or photo-responsive nanoscale devices. Work is currently underway to evaluate these photo-responsive azobenzene derivatives for their effectiveness as molecular switches integrated in nanoscale devices.
General synthetic methods
Unless stated otherwise, reactions were performed in oven-dried, nitrogen flushed glassware equipped with a magnetic stir bar using freshly distilled solvents. Reagent grade tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl. Triethylamine (TEA) was distilled from calcium hydride. Trimethylsilylacetylene (TMSA) was donated by FAR Research Inc. and Petra Chemical. Compound 13 was prepared as described previously.1 All other commercially available reagents were used as received.
Acknowledgements
We thank DARPA and the AFOSR for funding this research. B.-C.Y. thanks Mokwon University for funding during sabbatical leave in 2005.
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While on sabbatical leave to Rice University.