LASER FLASH-PHOTOLYSIS STUDIES ON THE MONOHYDROXY DERIVATIVES OF BENZOPHENONE

Time-resolved studies in the pico-and nanosecond time domain have been performed to characterize the tripler states of monohydroxy-substitut ed benzophenones, namely, para-(p-), meta-(m-), and ortho-(o-) hydroxy benzophenones (HOBP). Due to a very fast intersystem crossing (ISC) pr ocess, only the triplet states have been detected in the subnanosecond time domain. Spectral characteristics and lifetimes of the triplet st ates 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 characterist ics. In case of m-HOBP and p-HOBP, the excited triplet state in nonhyd rogen-bond-forming solvents has an n pi: configuration and is capable of abstracting a hydrogen atom from another unexcited molecule to for m ketyl and phenoxy type radicals. But in hydrogen-bond-forming solven ts, the tripler state, which is strongly associated with the solvent m olecules as a hydrogen-bonded complex, is very short-lived due to fast nonradiative relaxation via hydrogen-stretching vibrations in intermo lecular hydrogen bonds with the solvents and is capable of abstracting a hydrogen atom neither from the solvent molecule nor from another un excited HOBP molecule. In the case of o-HOBP, due to strong intramolec ular hydrogen bonding, the internal conversion in the excited singlet state is very efficient and hence the yield of the triplet state is lo w (<15%) and also the triplet state is very short-lived. However, in m ethanol and DMSO, due to disruption of the intramolecular hydrogen bon d, the triplet yield is higher and also the generation of the phenolat e ion via excited-state proton transfer is a significant process. Depr otonation reactions probably taking place from both the excited single t state as well as the triplet state of the other two derivatives also have been found to be significant in polar solvents.