Effect of beta-alkylthioethyl substitution in 1,3-dithianes: quasianchimeric assistance in photoinduced electron transfer?

Nanosecond laser flash photolysis (LFP) experiments show that the rates of ET quenching of triplet benzophenone by 2-alkyldithianes significantly decrease with bulkier substitution. Introduction of sulfur at the beta-position of the flexible alkyl chain reverses this trend, whereas such substitution at the alpha-position has negligible effect. This is rationalized in terms of the three electron two center bonds, favorable due to the formation of five-membered cyclic radical cations in the case of beta-substitution, which is supported by DFT computations.