Polydiacetylene, description

On the other hand in the exciton description occasionnally adopted by some authors to interpret the main absorption peak in the polydiacetylenes one finds x " negative and its values two orders of magnitude lower than expression 6 since electron correlation (28) is essential in the exciton model, the calculation of even the simplest optical properties becomes prohibitively complicated and the physical insight is obscured. 

Here we outline a dynamical description (42) of the polymerisation of the polydiacetylenes. The approach relies much on the one used (43,44) in the theory of non radiative transitions in crystals and the soliton description of the defects in the lD-or-ganic semiconductors. 

Although interesting within the framework of polymer physics and material science this would not be sufficient to attract so many workers from areas outside of conventional polymer research. Additional interest arouse because of the unusual structure of the polymers obtained via solid-state polymerization of diacetylenes and because of the mechanistic features related to its formation. Polydiacetylenes exhibit a fully conjugated and planar backbone in the crystalline state and are thus considered the prototype study object as far as the nature and physical behavior of polyconjugated macromolecules are concerned Theoretical discussions of the electronic structure of these polymers (2) lead to a description in terms of a wide band one-dimensional semiconductor... 

Fig. 2 Polyenes and the 2 state, (a) Schematic representation of the singlet 2Ag state in a polyene, showing its equivalent description either as a triplet-triplet or as soliton-antisoliton pair. Adapted from ref. 33. (b) Polyene-type structures discussed in the text. 1 Polydiacetylene [34,35], 2 Poly(diethyldipropargylmalonate) 136], 3 Poly(3-dodecylthienyl-enevinylene) [37], 4 Polyibenzodithiophene thiophene dioxide) [38]. R-groups denote solubilizing chains, (c) Models of singlet fission in polyenes, mediated by formation of 2Ag (left) or directly from IBu (right).

In the intermediate-coupling regime Mott-Wannier excitons are the more appropriate description for large dimerization (J = 0.2), while for the undimerized chain Mott-Hubbard excitons are the correct description. For dimerizations relevant to polyacetylene and polydiacetylene (that is, S ... 

It is instructive to apply these exciton theories to actual conjugated polymers. Calculations on single poly(para-phenylene) chains (see Section 11.2.3) predict the l Sj (n = 1, J = 1) exciton at 3.7 eV, the (n = 2, j = 1) exciton at 5.1 eV and the l A triplet close in energy to the 2 A+ state, at 5.5 eV. This progression indicates a Mott-Wannier series of excitons. An equivalent description applies to poly(para-phenylene-vinylene). In contrast, polyacetylene and polydiacetylene have predominately Mott-Hubbard excitons. In polyacetylene the vertical energies of the and 2M+ states are virtually degenerate... 

The following is a brief description of a by no means exhaustive list of possible applications of polydiacetylene monolayers. 

In this chapter, a detailed description is given of these intriguing optical properties of certain polythiophene derivatives, and their analogy with chromic effects found in other 7r-conjugated (polydiacetylenes) and or-conju-gated polymers (polysilanes) is discussed. In order to explain these transitions, various molecular mechanisms are described. Finally, potential applications of these interesting optical effects are presented. 

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