Ac. Bhasikuttan et al., LASER FLASH-PHOTOLYSIS STUDIES ON THE MONOHYDROXY DERIVATIVES OF BENZOPHENONE, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(20), 1998, pp. 3470-3480
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.