Intramolecular End-to-End Reactions of Photoactive Terminal Groups Linked by Poly(oxyethylene) Chains

Abstract

Abstract The triplet-sensitized photochemical reaction using a series of poly(oxyethylene) chains with a pair of photoactive terminal groups, dibenz[b,f]azepine (DBA) chromophores (DBA–COCH2CH2(OCH2CH2)nOCH2CH2CO–DBA, n=0–10) was examined. The photoirradiation of bichromophoric compounds caused either intra- or intermolecular reactions. These reactions were kinetically analyzed by two different methods: the measurement of deactivation processes of the reaction intermediates (excited triplet state of DBA) by nanosecond laser photolysis and the quantitative analysis of the reaction products by GPC. The intramolecular deactivation rate constant, kintra, showed a remarkable chain-length dependence; the maximum kintra value appeared at n=5 and it was found to be 5.9×104 s−1. On the other hand, the intramolecular cyclization rate also depends on the chain length; the maximum quantum yield, φdintra, was given at n=7 (φdintra=0.51). The chain length for the maximum cyclization yield shifted slightly to the longer region than that for the maximum kintra value due to the restriction of the terminal structure (anti-configuration). The results obtained for this reaction system are compared with those obtained for the previously reported polymethylene system and the effect of chain flexibility on the intramolecular ring-closure reaction is discussed.