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

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 (Φrad) 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 (αdis) of the CS bond fission in the triplet state is determined to be 0.51±0.03. The agreement between the Φrad and αdis 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.