Photoreactions of pi-Electron-System-Substituted Disilanes and their Application to Photoactive Polymers

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Alternating polymers composed of a disilanylene group and pi-electron system, poly[(disilanylene)phenylenes], poly[(disilanylene)naphthylenes], poly[(disilanylene)thienylenes], poly[(disilanylene)dithienylenes], poly[(disilanylene)butenyne-1, 4-diyls], and poly[(disilanylene)ethynylenes] were synthesized and their photochemical properties were investigated.These polymers are soluble in common organic solvents and can be casted to thin solid films by spin-coating. UV-irradiation of the films in air caused the cleavage of the Si-Si bonds leading to the formation of Si-O bonds. Photolysis of the polymers in solution was also investigated. Thus, when these polymers were photolyzed in the presence or absence of a trapping agent in benzene, products which can be best explained by assuming the silyl radicals formed by the homolytic scission of an Si-Si bond were obtained. The silyl radicals thus formed undergo two different reactions under the conditions used. One involves the disproportionation reaction leading to the formation of a silene and hydrosilane, while the other comprises a cross-linking addition reaction across the pi-electron system. In these photoreactions, it was found that introduction of an alkyl group on the silicon atom in the polymer, main chain is favorable for the cross-linking reaction, while replacement of the alkyl group by an aryl group facilitates the disproportionation reaction. It was also found that the extension of the pi-electron system in the polymer chain results in the decrease of the photoactivities of the present polymers.In order to learn more about the photochemical properties of these polymers, we prepared disilanylene-phenylene oligomers as model compounds and investigated their photochemical behavior.