Structural and solvent effects on the C-S bond cleavage in aryl triphenylmethyl sulfide radical cations

Steady-state and laser flash photolysis (LFP) studies of a series of aryl triphenylmethyl sulfides [1, 3,4- (CH3O)2-C6H3SC(C6H5)3; 2, 4-CH3O-C6H4SC(C6H5)3; 3, 4- CH3-C6H4SC(C6H5)3; 4, C6H5SC(C6H5)3; and 5, 4-Br- C6H4SC(C6H5)3] has been carried out in the presence of Nmethoxyphenanthridinium hexafluorophosphate in CH3CN, CH2Cl2, CH2Cl2/CH3CN, and CH2Cl2/CH3OH mixtures. Products deriving from the C-S bond cleavage in the radical cations 1o+-5o+ have been observed in the steady-state photolysis experiments. Time-resolved LFP showed firstorder decay of the radical cations accompanied by formation of the triphenylmethyl cation. A significant decrease of the C- S bond cleavage rate constants was observed by increasing the electron-donating power of the arylsulfenyl substituent, that is, by increasing the stability of the radical cations. DFT calculations showed that, in 2o+ and 3o+, charge and spin densities are mainly localized in the ArS group. In the TS of the C-S bond cleavage an increase of the positive charge in the trityl moiety and of the spin density on the ArS group is observed. The higher delocalization of the charge in the TS as compared to the initial state is probably at the origin of the observation that the C-S bond cleavage rates decrease by increasing the polarity of the solvent.