A new-type photoreaction of a carbonyl compound - Part 2. Photoinduced omega-bond dissociation in p-halomethylbenzophenone studied by time-resolved EPR technique and laser flash photolysis

摘要

Photodissociation of the carbon-X (X = Br and Cl) bonds in p-bromo- and p-chloromethylbenzophenone (BMBP and CMBP) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BMBP and CMBP were found to undergo W-bond cleavage to yield the p-benzoylbenzyl radical (BBR) at 295 K, and the quantum yields (Phi(BBR)) were determined. The CIDEP signal originated from BBR formed upon decomposition of CMBP was obtained while that for BMBP was absent. By using triplet sensitization of acetone, the efficiencies (alpha(BBR)) of the C-X bond fission in the triplet states of BMBP and CMBP were determined. The agreement between the Phi(BBR) and alpha(BBR) values for CMBP indicates that the C-Cl bond dissociation occurs only in the triplet state. In contrast to CMBP, the cleavage of the C-Br bond in BMBP upon direct excitation was concluded to be the event only in the excited singlet state without triplet formation, whereas the triplet state was also reactive for omega-bond dissociation. The rate of C-Br bond dissociation seemed to be greater than that of intersystem crossing from the S-1 to the T-1 state. Schematic energy diagrams of the excited states of BMBP and CMBP were shown, and the reaction profiles were discussed from the viewpoint of the C-X bond enthalpies. (c) 2004 Elsevier B.V. All rights reserved.