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Excited-State Dynamics and Photophysical Properties of para-Aminobenzophenone

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journal contribution
posted on 07.07.2000, 00:00 by Ajay K. Singh, A. C. Bhasikuttan, Dipak K. Palit, Jai P. Mittal
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.