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1 Wiadomości Chemiczne

1 2001

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Wilk K. A., Domagalska B.W.

Wiadomości Chemiczne

2001

[Z] 55, 5-6

439-471

The design of optoelectronic components with molecular dimensions is an area of active research and theoretical calculations. Unsaturated organic molecules containing p-conjugated bonds have been explored in connection with "molecular wires", transfer of electrons across lipid membranes, molecular mimicry of photosynthetic energy and electron transfer. In this paper we report the most recent achievements related to synthesis and properties (i.e., spectroscopic data, intramolecular charge transfer (ICT) and electron transfer processes, thermal stability) as well as potential applications of well defined oligomeric conjugated structures derived from oligomers of acetylene; in the literature they are named conjugated polyenes. These compounds are characterized by the presence of polar end groups: the donor (D - e.g., derived from anilines, thiophenes, ferrocenes, 1,4-dithiafulvenes, julolidine moiety) with electron donating capabilities and the acceptor (A - e.g., nitro, formyl, dicyanomethylen, derived from thiobarbituric acid) able to withdraw electrons. The D and A groups are connected through a conjugated p - system (e.g., polyenic, polyaromatic or polyheteroaromatic chain). Unsymmetrical substitution of the polyene chain defines a class of push-pull polyenes and symmetrical one creates push - push or pull - pull entities. Their structural properties are the results of a subtle balance between different factors, among which of particular importance are the chain length and the donor acceptor strength. The synthesis of polyene chains has been widely explored in literature. The most commonly employed procedure for the synthesis of conjugated spacers involves a sequence of Wittig-type reactions. The alternative, iterative building block method applies oligosililoxy ethers and tempaltes (e.g. 5,5-dietoxypenta-2-enal) to involve the polyene chain increase. According to the literature all conjugated polyenes reveal satisfactorily high decomposition temperatures (above 230 oC) which makes them good candidates for optoelectronic devices. The conjugated polyenes are of particular interest as model systems of one dimensional conjugated chromophores. It is now generally recognized that bipolaronic charge states dominate in most oxidatively doped p-electron conjugated structures with non-degenerate ground states. In general, all such molecules undergo a Peierls distortion, making charge transport between chain ends dependent upon thermal processes and limiting the rate of such charge transport. The use of solitonic moieties as charge carriers in molecular wires is among the initial proposals for molecular electronics. Solitons have also been suggested for use in molecular gates or switches. The UV-VIS spectra of the D-p-A system in solution display a low energy absorption band, which is attributed to the ICT from the donor to the acceptor unit, and the efficiency of such a process modulates the physical properties of the system.