Order of Quencher Participation in Photochemistry. I. Proton Transfer from the Excited p-Hydroxybenzophenone in Mixed Solvents of Cyclohexane and Alcohols

Abstract

Abstract The change in the absorption spectrum of p-hydroxybenzophenone (HOBP) and relative quantum yields Φrel for the photoreduction of HOBP have been studied in mixed solvents consisting of cyclohexane and four kinds of alcohol. The change in absorption spectrum occurs in two steps with methanol, ethanol, and 2-propanol, suggesting the following equilibria, HOBP+S\leftrightharpoonsA, and A+S\leftrightharpoonsB (S, alcohol molecule). Φrel changes in two steps with methanol and ethanol and in one step with 2-propanol and amyl alcohol roughly in line with the spectral change. The changes in Φrel have been interpreted to be due to the deprotonation of HOBP. Φrel=1⁄(1+K[S]n) holds for them with n=5.5 for 2-propanol and n=1 for other alcohols. A linear relation holds between 1⁄(n−1)logK and Hammett’s constant of alkyl group of alcohols, suggesting that the deprotonation tendency depends upon the specific interaction between alcohol molecules. The reactivity of A-species is considered to depend largely on the dielectric nature of the solvent.