Electronically excited diacetylenes

Photochemistry. The Hg(3P,) sensitized photolysis ofC2H2 has produced benzene, hydrogen, and polymer [Shida et al. (872)]. Since the Hg(3Fi) state does not have sufficient energy required to dissociate the H—C2H bond, the products must be produced by reactions of an electronically excited C2H2. The photolysis of acetylene at 1849 A has produced hydrogen, ethylene, vinylacetylene, diacetylene, benzene, and solid polymers [Tsukada and Shida (979), Zelikoff and Aschenbrand (1082)]. 

In summary, it is well established that the species that initiates exothermic growth of a polydiacetylene chain is a diradical dimer. It can be generated thermally, the activation energy being determined by the energy of the librational motion required to temporarily shorten the C1-C4 reaction distance of a molecular pair to about 2 A, or by electronic excitation of the diacetylene moiety via UV- or y-irradiation. Upon UV-excitation the number of chains initiated is of order 10 per absorbed photon. The active precursor state is likely to be of triplet character. Even in case of optical... 

Vila, F. Borowski, P. Jordan, K.D. Theoretical study of the low-lying electronically excited states of diacetylene. J. Phys. Chem. A 2000, 104, 9009 9016. 

In course of subsequent work Bubeck, Tieke, and Wegner discovered that the action spectrum for photopolymerization of undoped diacetylene multilayers extends into the visible provided some polymer formed in course of previous UV-irradiation is present. Since obviously excitation of the polymer can sensitize the reaction this effect has been termed self-sensitization. Checking the absorption spectrum of the polymer produced via self-sensitization assured that the final product is identical with the product obtained under UV excitation of the monomer. Later work by Braunschweig and Bassler demonstrated, that the effect is not confined to multilayer systems but is also present in partially polymerized single crystalline TS-6, albeit with lower efficiency. Interestingly, the action spectrum of self-sensitization follows the action spectrum for excitation of an electron from the valence band of the polymer backbone to the conduction band rather than the excitonic absorption spectrum of the polymer which is the dominant spectral feature in the visible (see Fig. 21). The quantum yield is independent of the electric field, whereas in a onedimensional system the yield of free carriers, determined by thermal dissociation of optically produced, weakly bound geminate electron-hole pairs, is an linear function of an applied electric field 29.30,32,129) Apparently, the sensitizing action does not... 

In summary, the results on sensitized photopolymerization of diacetylenes are of relevance for potential application and, in addition, augment understanding of the polymerization process in general. In conjunction with what is known about chain propagation and self-sensitization, they indicate that the basic requirements for both chain initiation and propagation are presence of an unpaired electron — in form of either a radical or a charge carrier — on the reactive center and excitation of a librational motion of the reactant(s) in course of which the reaction distance is temporarily reduced to a critical value. 

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