Photophysics and ultrafast relaxation dynamics of the excited states of dimethylaminobenzophenone

Abstract

Steady-state as well as time-resolved absorption and emission studies in the femto-, pico-, and nanosecond time-domains have been performed to investigate the relaxation dynamics and the photophysical properties of dimethylaminobenzophenone (DMABP) by characterizing its excited singlet (S₁) and triplet (T₁) state properties in different kinds of solvents. Large bathochromic shifts of the steady-state absorption maximum in more polar solvents suggest an intramolecular charge transfer (ICT) character of the S₁ state. DMABP is weakly fluorescent but shows the features of "dual-fluorescence". Dynamics of very efficient nonradiative relaxation process in the S₁ state of DMABP has been described by invoking a "three state model", in which the involvement of the ground state and two adiabatically coupled S₁ states, a locally excited (LE) ππ^∗ or ICT state and a conformationally relaxed or twisted intramolecular charge transfer (TICT) state, have been considered. In aprotic solvents, the fluorescence maximum arises due to emission from the TICT state. On the other hand, in alcohols, the fluorescence maximum arises due to emission from the LE state, since the TICT state is weakly fluorescent due to strong intermolecular hydrogen-bonding interaction with the solvent. The excited-state relaxation process follows multiexponential dynamics. An ultrafast component, having lifetime τ₁, which varies in the range 0.2-0.5 ps in different polar solvents, may possibly originate from either the relaxation of the solvent-perturbed low-frequency intramolecular modes or the inertial solvation of the ICT state. The lifetime of another slower component (τ₂ ~0.2-5 ps) increases linearly with an increase in the viscosity of the solvent. This component has been attributed to the diffusive twisting motion of the dimethylaminophenyl group with respect to the benzoyl group. This twisting process with a rate, which is comparable to or faster than the average solvation time of the solvent, is associated with the crossing of a low barrier (or quasi-barrierless) and responsible for the population relaxation in the S₁ state. The third component (τ₃~5-900 ps), which is sensitive to the hydrogen bonding ability of the solvent, represents the decay of the S₁ state to the ground state. The spectroscopic nature, energy, and yield of the T₁ state are very sensitive to the characteristics of the solvent or medium.