Photophysics and excited state relaxation dynamics of p-hydroxy and p-amino-substituted benzophenones:: a review

Very short lifetimes (a few picoseconds) of the lowest excited singlet (SI) state of benzophenone (BP) and p-hydroxybenzophenone (HOBP) suggest that they have pi* character in all kinds of solvents. However, in the case of the p-amino-substituted benzophenones (ABPs), the nature of the S-1 state, whether it is n-pi* or pi-pi*, depends on the nature of the solvent and the identity of the substituent groups. The lowest excited triplet (T-1) state of BP has n pi* character in all kinds of solvents and hence phosphorescence decay is short (a few ms). The T-1 state character of HOBP and ABP's are solvent dependent. In a few cases dual exponential decay of phosphorescence indicates that n-pi* and pi-pi* triplet states lie close to each other. The pi-pi* type of T-1 state has longer phosphorescence lifetime (a few tens of ms). Investigations of the ultrafast relaxation dynamics of 4-N,N-dimethylaminobenzophenoene (DMABP) in different kinds of solvents of varying polarities and viscosities reveal that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state by twisting of the dimethylanilino group is the major process, which is responsible for the efficient non-radiative relaxation mechanism of the S-1 state of this molecule and solvation dynamics plays a minor role in it. In self-quenching interaction between the triplet state and its ground state, the n-pi* kind of T-1 state plays the major role and no triplet exciplex is involved as an intermediate. The T-1 state of BP is equally reactive towards hydrogen atom abstraction reaction with the ground state of each of the ABPs either by direct hydrogen atom transfer or by a charge-transfer-coupled proton-transfer mechanism. Exciplex formation has been observed only in the case of the reaction between the T-1 state of BP and DMABP.