In search of D-σ-A molecular rectifiers: the D-σ-A system derived from triptycenequinone and tetrathiafulvalene
Introduction
The field of unimolecular electronics was initiated in 1974 by Aviram and Ratner [1], who proposed a `Gedankenmolekül' D-σ-A that consists of a donor (D) and an acceptor (A) connected via a saturated σ-bond system, where D=TTF-derivative, A=TCNQ-derivative, and σ=a bicylooctane. Alignment of that D-σ-A molecule between two appropriate metal electrodes and application of a suitable potential should lead to an electron flow through that molecule, resulting in a `molecular rectifier'. This `Gedankenmolekül' was never synthesized, and only a few examples of true σ-bond connected donor-acceptor systems are known [2]. Unimolecular `through-bond' rectification in such D-σ-A systems has not yet been observed.
We were interested in devising a σ-bond linked donor–acceptor system (D-σ-A) that could be suitable for the formation of Langmuir–Blodgett (LB) films. Therefore, long aliphatic chains should be present at one end. Since the presence of a donor and an acceptor could lead to the formation of an intermolecular charge transfer complex, and suppress the proper alignment of the molecules in an LB-film, a bulky group, placed close to the acceptor or the donor end, would therefore sterically hinder the formation of charge-transfer complexes. Theoretical estimates proposed that the difference between the redox potentials of donor and acceptor should be smaller than ΔE≈0.6 V, and that the length of the connecting chain should be between 2 and 7 atoms 3, 4, 5.
We embarked upon the synthesis of a system, in which a quinone acceptor is incorporated in a bulky triptycene unit, and connected via a σ-butylene chain to a TTF-derivative donor. The quinone moiety could then be converted into a DCNQI derivative, in order to increase the acceptor strength. Two different kinds of TTF-derivative were chosen, in order to vary the donor strength.
We report here the synthesis of four σ-bond linked quinone–TTF derivatives, their electrochemical and spectroscopic properties, and their film-forming behavior. A full account is being presented elsewhere [6].
Section snippets
Synthesis
Since many synthetic operations are incompatible with the simultaneous presence of a quinone and a TTF moiety, we chose to connect the TTF unit to a quinone precursor, which could later be converted into a quinone. The bromo-functionalized methoxymethylether (MOM)-protected hydroquinone 2 was prepared in the following way. MOM-triptycene 1 was prepared in 67% yield by deprotonation of a known hydroquinone 7, 8with sodium hydride in THF, followed by reaction of the phenolate with chloromethyl
Conclusion
The intent of preventing intermolecular association by LB film-forming D-σ-A molecules 4b and 6b was accomplished. The conversion of 6b into a D-σ-A system with A=strong acceptor along with D=strong donor failed. The quinone is a weak acceptor, and is too protected by the triptycene moiety to make 4b or 6b truly amphiphilic. The electron depletion of the donor unit of 6b is probably due to a direct withdrawal through the σ bridge. Molecules 4b and 6b make Pockels–Langmuir monolayers, but these
Acknowledgements
We thank the National Science Foundation (DMR 94–20699) for supporting this work.
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