The photophysical properties of the singlet (S-1) and tripler (T-1) excited
states of p-aminobenzophenone (p-ABP) have been investigated in various or
ganic solvents using steady-state as well as absorption spectroscopy with p
icosecond 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) simila
r 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 muc
h longer-lived (>100 ps). The tripler yield is much higher in nonpolar solv
ents than in polar solvents but the Lifetime shows the reverse trend. In no
npolar solvents, the T-1 state is an equilibrium mixture of the states havi
ng n pi* and pi pi* configurations because of their close prsximity in ener
gy, and it is photochemically reactive toward hydrogen-atom-abstraction rea
ctions. 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
.