Competition between back electron transfer and bond fission in photoinduced ether cleavage

Abstract

The dependence of the quantum yields of the electron-transfer-sensitized photolysis of substituted phenyl(trimethylsilyl)methyl ethers on the substituent is explained by the dependence of the rate of back electron transfer on the exothermicity of the reaction (Marcus-inverted region). In order to confirm the mechanism, the ion yields of the forward electron transfer were determined using photoconductivity measurements with dicyanoanthracene as acceptor. The ion yields correspond to the quantum yields of ether cleavage. For N-methylquinolinium perchlorate as acceptor, the quantum yields of ether cleavage were used to calculate the rate constants of back electron transfer. The rate constants showed a marked decrease with increased Δ G −et values. Semiquantum mechanical theory of electron transfer was used to analyse the data and to compare with results in the literature. The data agree with those in the literature obtained from photoconductivity measurements. Deprotonation of the ether radical cation within the radical pair (formed after electron transfer to the quinolinium salt) plays the role of an energy wasting process with ethers of low donor capacity.