Photoinduced electron transfer reactions in the 10-methylacridinium cation–benzyltrimethylsilane system: steady-state and flash photolysis studies

Abstract

The mechanism of the photoinduced reaction of the lowest excited singlet state of the 10-methylacridinium (AcrMe ? ) cation with benzyltrimethylsilane (BTMSi) in acetonitrile has been investigated by means of steady-state and time-resolved methods. A variety of stable products was found after irradiation (365 nm) of the reaction mixture under aerobic and oxygen-free conditions. The stable products were identified and analyzed using UV-Vis spectrophotometry, high performance liquid chromatography (HPLC), and mass spectrometry (MS). Based on Stern-Volmer plots of the AcrMe ? fluorescence quenching by BTMSi (using fluorescence intensity and lifetime measurements), the rate constants were determined to be k q = 1.24 (± 0.02) 9 10 10 M -1 s -1 and k q = 1.23 (± 0.02) 9 10 10 M -1 s -1 , i.e., close to the diffusion-controlled limit in acetonitrile, indicating the dynamic quenching mechanism. The quenching process was shown to occur via an electron-transfer reaction leading to the formation of acridinyl radicals (AcrMe • ) and C 6 H 5 CH 2 Si(CH 3 ) 3•? radical cations. Based on stationary and flash photolysis experiments, a detailed mechanism of the secondary reactions is proposed and discussed. The AcrMe • radical was shown to decay by two processes. The fast decay, observed on the nanosecond timescale, was attributed to the back-electron transfer occurring within the initial radical ion pair. The slow decay on the microsecond timescale was explained by recombination reactions of radicals which escaped from the radical pair, including benzyl radicals formed via C-Si bond cleavage in the C 6 H 5 CH 2 Si(CH 3 ) 3•? radical cation.