Generation of Polyphenylene Radical Cations and Their Cosensitization Ability in the 9,10-Dicyanoanthracene-Sensitized Photochemical Chain Reactions of 1,2-Bis(4-methoxyphenyl)cyclopropane

Article abstract of DOI:10.1021/jo971649y

Cosensitization effects of polyphenylene compounds (PP) such as biphenyl (BP), terphenyls (o-, m-, p-TP), and phenanthrene (Phen) in photoinduced electron-transfer reactions were examined. The 9,10-dicyanoanthracene (DCA)-sensitized cis-trans photoisomerization of 1,2-bis(4-methoxyphenyl)-cyclopropane (CP), which proceeds in a chain reaction via free radical cation of CP (CP^.+) as a chain carrier, was accelerated by adding PP, particularly by TP. A similar accelerating effect was observed in the DCA-sensitized photooxygenation of CP as another example. BP and TP were more stable under the oxygenation condition than phenanthrene and naphthalene, which also accelerate the photooxygenation CP. CP^.+ is generated by the direct electron transfer from CP to the excited singlet state of DCA (¹DCA*) and also by the secondary electron transfer from CP to PP^.+, which is generated by the primary electron transfer from PP to ¹DCA*. The laser flash photolysis study revealed that the quantum yield for the formation of free CP^.+ in the direct electron transfer from CP to ¹DCA* (Φ_CP.+ ≈ 0.1) was smaller than that in the presence of PP. This is due to the high yield of free PP^.+ generation by the primary electron transfer and the efficient secondary electron transfer from CP to PP^.+. The secondary electron transfers were found to take place in nearly diffusion-controlled rates (0.9-1.5 × 10¹⁰ M^-1 s^-1). The high yield of PP^.+ as free radical ions does not seem to be the sole factor of the cosensitization of PP for the DCA-sensitized photoreactions of CP. The ratio of the quantum yields of the photoreactions to that of the initial CP^.+ formation (turnover) also increased by the addition of PP from 3 (isomerization) and 15 (oxygenation) to 32-90 for both reactions. The second-order rate constant for the decay of CP^.+ in aerated acetonitrile was decreased by a factor of 0.5-0.8 by the addition of PP. We concluded that the cosensitization effect in the photoreaction involves a π-complex formation between CP^.+ and PP assisting the chain reaction as well as initial CP^.+ formation.