Excited-state dynamics and photophysical properties of para-aminobenzophenone

The photophysical properties of the singlet (S-1) and tripler (T-1) excited states of p-aminobenzophenone (p-ABP) have been investigated in various organic solvents using steady-state as well as 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 n pi* configuration and is short-lived [tau (S-1) similar to 12 ps]. In methanol, a polar and protic solvent, the S-1 state is much shorter-lived [tau (S-1) < 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 SI state has a pi pi* or CT configuration and, hence, is much longer-lived (>100 ps). The tripler 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 n pi* and pi pi* configurations because of their close prsximity in energy, and it is photochemically reactive toward hydrogen-atom-abstraction reactions. In polar solvents, the T-1 state is unreactive because of its pi pi*: Or CT character. A comparison has also been made among the photophysical properties of benzophenone (BP), p-hydroxybenzophenone (p-HOBP), and p-ABP.