Excited-State Dynamics and Photophysical Properties of para-Aminobenzophenone

The photophysical properties of the singlet (S₁) and triplet (T₁) excited states of p-aminobenzophenone (p-ABP) have been investigated in various organic solvents using steady-state as well as transient absorption spectroscopy with picosecond and subpicosecond time resolution. p-ABP is weakly fluorescent in benzene as well as in polar aprotic solvents, acetonitrile, DMSO, and DMF, but nearly nonfluorescent in cyclohexane and methanol. In cyclohexane, the S₁ state has the nπ* configuration and is short-lived [τ(S₁) ∼ 12 ps]. In methanol, a polar and protic solvent, the S₁ state is much shorter-lived [τ (S₁) < 1ps, and hence, we have not been able to detect any transient, even in subpicosecond time scale] because of the formation of an intermolecular hydrogen-bonded complex with the solvent. In all other solvents, the S₁ state has a ππ* or CT configuration and, hence, is much longer-lived (>100 ps). The triplet yield is much higher in nonpolar solvents than in polar solvents but the lifetime shows the reverse trend. In nonpolar solvents, the T₁ state is an equilibrium mixture of the states having nπ* and ππ* configurations because of their close proximity in energy, and it is photochemically reactive toward hydrogen-atom-abstraction reactions. In polar solvents, the T₁ state is unreactive because of its ππ* or CT character. A comparison has also been made among the photophysical properties of benzophenone (BP), p-hydroxybenzophenone (p-HOBP), and p-ABP.