True quantum yields and adsorption constants as tools for a mechanistic study of the TiO2-sensitized photooxidation of benzylic derivatives

The quantum yields (Φ) of the colloidal TiO2-sensitized photooxidation of 4- (1) and 3-methoxybenzyl alcohol (2) together with 4-methoxybenzyltrimethyl- (3) and 4-methoxybenzyltriisopropylsilane (4) were determined in CH3CN, in the presence of HClO4 for 3 and 4. The true quantum yields (Φ0) of 1, 2 and 3, obtained from a Langmuir–Hinshelwood isotherm treatment of Φ at different substrate concentrations, are linearly correlated with IA−1/2, where IA is the light intensity. According to the previously suggested mechanisms for alcohols and silanes, a kinetic scheme that justifies this correlation is suggested. It is shown that the ratio of the slopes (from the Φ0 versus IA−1/2 plots) for 1 and 2 is equal to the Φ01/Φ02 ratio at any IA; this ratio depends on the rate constants in the kinetic scheme, in this case principally on the electron transfer constant, ket. On the contrary, the Φ03/Φ04 ratio depends on kp, the cation radical desilylation rate constant, confirming a steric hindrance to nucleophylic assistance in the CSi fragmentation by the bulky isopropyl group in the SiR3 moiety. Differently from Φ0, the adsorption constants on the semiconductor under irradiation (KD, obtained from the above isotherm treatment) are independent of IA. Moreover, as KD1=KD2 and KD3=KD4, the structural modifications within the two alcohols and within the two silanes should be far enough away from the adsorption site. For all the substrates, K (the dark adsorption constant) is five times greater than KD, showing that this change does not depend on the substrate structure but is the result of different experimental conditions.