Photoinduced ω-bond dissociation of m-halomethylbenzophenones studied by laser photolysis techniques and DFT calculations. Substituted position effects

Photochemical profiles of ω-cleavage of carbon–X (X = Br and Cl) bonds in m-bromo- and m-chloromethylbenzophenones (m-BMBP and m-CMBP) were investigated by laser photolysis techniques and DFT calculations. m-BMBP and m-CMBP were found to undergo ω-bond cleavage to yield the m-benzoylbenzyl radical (m-BBR) at 295 K, and the quantum yields were determined. No CIDEP signal was detected upon 308 nm laser photolysis of both the compounds. From these observations, it was inferred that the ω-bond of these m-halomethylbenzophenones (m-HMBP) cleaves in the lowest excited singlet state (S1(n,π*)) upon direct excitation. Upon triplet sensitization of acetone (Ac), the m-BBR formation was observed in transient absorption for an Ac–m-BMBP system, and an efficiency of the C–Br bond cleavage in the lowest triplet state (T1(n,π*)) of m-BMBP was determined. In contrast, formation of triplet m-CMBP was seen for an Ac–m-CMBP system. Absence of C–Cl bond cleavage in the triplet state of m-CMBP indicated the reactive state of m-CMBP for ω-cleavage is only the S1(n,π*) state. Based on the efficiencies and DFT calculations for excited state energies, photoinduced ω-bond dissociation of m- and p-HMBPs was characterized.