Excited-State Dynamics and Photophysical Properties of para-Aminobenzophenone

The photophysical properties of the singlet (S 1 ) and triplet (T 1 ) 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 S 1 state has the nπ ∗ configuration and is short-lived [τ(S 1 ) ~12 ps]. In methanol, a polar and protic solvent, the S 1 state is much shorter-lived [τ (S 1 ) 1 state has a π π ∗ 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 T 1 state is an equilibrium mixture of the states having n960; ∗ and π π ∗ configurations because of their close proximity in energy, and it is photochemically reactive toward hydrogen-atom-abstraction reactions. In polar solvents, the T 1 state is unreactive because of its pp or CT character. A comparison has also been made among the photophysical properties of benzophenone (BP), p-hydroxybenzophenone (p-HOBP), and p-ABP.