Laser flash photolysis studies on the monohydroxy derivatives of benzophenone

Abstract

Time-resolved studies in the pico- and nanosecond time domain have been performed to characterize the triplet states of monohydroxy-substituted benzophenones, namely, para- (p-), meta- (m-), and ortho- (o-) hydroxybenzophenones (HOBP). Due to a very fast intersystem crossing (ISC) process, only the triplet states have been detected in the subnanosecond time domain. Spectral characteristics and lifetimes of the triplet states of HOBP have been seen to be extremely sensitive to the position of the OH group in the phenyl ring as well as the solvent characteristics. In case of m-HOBP and p-HOBP, the excited triplet state in non-hydrogen-bond-forming solvents has an nπ^∗ configuration and is capable of abstracting a hydrogen atom from another unexcited molecule to form ketyl and phenoxy type radicals. But in hydrogen-bond-forming solvents, the triplet state, which is strongly associated with the solvent molecules as a hydrogen-bonded complex, is very short-lived due to fast nonradiative relaxation via hydrogen-stretching vibrations in intermolecular hydrogen bonds with the solvents and is capable of abstracting a hydrogen atom neither from the solvent molecule nor from another unexcited HOBP molecule. In the case of o-HOBP, due to strong intramolecular hydrogen bonding, the internal conversion in the excited singlet state is very efficient and hence the yield of the triplet state is low (<15%) and also the triplet state is very short-lived. However, in methanol and DMSO, due to disruption of the intramolecular hydrogen bond, the triplet yield is higher and also the generation of the phenolate ion via excited-state proton transfer is a significant process. Deprotonation reactions probably taking place from both the excited singlet state as well as the triplet state of the other two derivatives also have been found to be significant in polar solvents.