Photoinduced hydrogen- and electron-transfer processes between chloranil and aryl alkyl sulfides in organic solvents. Steady-state and time-resolved studies

The photochemical behavior of three aryl alkyl sulfides, thioanisole (TA), benzyl phenyl sulfide (BPS) and 4-methoxybenzyl phenyl sulfide (MBPS), sens itized by triplet chloranil (CA), was investigated by nanosecond laser flas h photolysis and steady-state irradiation in CH2Cl2 and MeCN. The nature of the transients detected upon 355-nm laser excitation was independent of th e molecular structure of the aryl alkyl sulfides but strongly affected by t he solvent polarity. In particular, in CH2Cl2 the quenching process of trip let CA by aryl alkyl sulfides was accompanied by H-transfer, with formation of the CAH(.) and TA(-H)(.)/BPS(-H)(.)/MBPS(-H)(.) radicals. In contrast, a charge transfer process between triplet CA and aryl alkyl sulfides, with formation of the radical anion CA(.-) and radical cations of aryl alkyl sul fides, occurred in MeCN. In this solvent, a transient detected at long dela y time was tentatively assigned to the anion CAH(-) formed by H-transfer be tween radical ions. In all experiments, transient species were characterize d in terms of second-order decay rate constants and quantum yields of forma tion. Steady-state irradiation of the CA/TA system led to the stable photoa dduct C6H5SCH2OC6Cl4OH in both CH2Cl2 and MeCN with quantum yields of 0.033 and 0.27, respectively. In contrast, aldehydes, thioacetals, and disulfide s were the main products obtained upon irradiation of the CA/BPS and CA/MBP S systems. The photoaddition products were not observed, probably owing to their low stability. The nature of the photoproducts formed by irradiation of CA/aryl alkyl sulfides was independent of solvent properties, even thoug h the reactivity was higher in MeCN than in CH2Cl2.