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

Abstract

Very short lifetimes (a few picoseconds) of the lowest excited singlet (S₁) state of benzophenone (BP) and p-hydroxybenzophenone (HOBP) suggest that they have π^∗ character in all kinds of solvents. However, in the case of the p-amino-substituted benzophenones (ABPs), the nature of the S₁ state, whether it is n–π^∗ or π–π^∗ , depends on the nature of the solvent and the identity of the substituent groups. The lowest excited triplet (T₁) state of BP has nπ^∗ character in all kinds of solvents and hence phosphorescence decay is short (a few ms). The T₁ state character of HOBP and ABP's are solvent dependent. In a few cases dual exponential decay of phosphorescence indicates that n–π^∗ and π–π^∗ triplet states lie close to each other. The π–π^∗ type of T₁ 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 S₁ 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–π^∗ kind of T₁ state plays the major role and no triplet exciplex is involved as an intermediate. The T₁ 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 T₁ state of BP and DMABP.