A new-type photoreaction of a carbonyl compound: Part 1. Photoinduced ω-bond dissociation in p-mercaptomethylbenzophenone studied by time-resolved EPR technique, steady-state and laser flash photolyses in solution

Abstract

Photodissociation of the carbonsulfur bond in p-mercaptomethylbenzophenone (MMBP) in acetonitrile has been investigated by means of steady-state photolysis, time-resolved EPR and laser photolysis techniques. MMBP undergoes photodecomposition to yield p-methylbenzophenone in acetonitrile at 295 K. The initial intermediate due to the photodecomposition of MMBP is revealed to be the p-benzoylbenzyl radical (BBR) from the transient absorption and CIDEP measurements. Based on the molar absorption coefficient of BBR, the quantum yield ( Φ rad) of the BBR formation upon direct photoexcitation was determined to be 0.49±0.03. Triplet sensitization of MMBP by acetone is performed to study the CS bond dissociation in the triplet state of MMBP. Based on the quantum yields and rates of the BBR formation upon sensitization of MMBP, the efficiency ( α dis) of the CS bond fission in the triplet state is determined to be 0.51±0.03. The agreement between the Φ rad and α dis values indicates that the CS bond dissociation occurs only in the triplet state which is produced with a triplet yield of unity due the rapid intersystem crossing from the lowest singlet state to the triplet state. The lifetime of triplet MMBP at 295 K was determined to be 1.8 ns by using triplet energy transfer from triplet MMBP to 1-methylnaphthalene. The apparent activation energy for the photodecomposition of MMBP was determined to be 0.4 kcal mol −1 whereas the enthalpy of the CS bond of MMBP was estimated to be 60.9 kcal mol −1 that is smaller than the triplet energy (68.5 kcal mol −1) of MMBP. The energy diagram of the excited states of MMBP is shown including the thermodynamic mechanism for the CS bond dissociation of MMBP.