True quantum yields and adsorption constants as tools for a mechanistic study of the TiO 2-sensitized photooxidation of benzylic derivatives

Abstract

The quantum yields ( Φ) of the colloidal TiO 2-sensitized photooxidation of 4- ( 1) and 3-methoxybenzyl alcohol ( 2) together with 4-methoxybenzyltrimethyl- ( 3) and 4-methoxybenzyltriisopropylsilane ( 4) were determined in CH 3CN, in the presence of HClO 4 for 3 and 4. The true quantum yields ( Φ 0) of 1, 2 and 3, obtained from a Langmuir–Hinshelwood isotherm treatment of Φ at different substrate concentrations, are linearly correlated with I A −1/2, where I A 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 Φ 0 versus I A −1/2 plots) for 1 and 2 is equal to the Φ 0 1 /Φ 0 2 ratio at any I A; this ratio depends on the rate constants in the kinetic scheme, in this case principally on the electron transfer constant, k et. On the contrary, the Φ 0 3 /Φ 0 4 ratio depends on k p, 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 SiR 3 moiety. Differently from Φ 0, the adsorption constants on the semiconductor under irradiation ( K D, obtained from the above isotherm treatment) are independent of I A. Moreover, as K D 1 =K D 2 and K D 3 =K D 4 , 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 K D, showing that this change does not depend on the substrate structure but is the result of different experimental conditions.