Internal conversion of o-aminoacetophenone in solution

The photophysical properties of o-aminoacetophenone (o-AAP) in solution have been studied by using a femtosecond laser-single photon counting system and time-resolved thermal lensing (TRTL) method. The fluorescence quantum yield (Phi (f)) and lifetime (tau (f)) of o-AAP depend strongly on the nature of the solvent. In nonpolar solvents, o-AAP gives very small Phi (f) values (Phi (f) = 2.4 x 10(-4) in n-hexane) and remarkably short fluorescence lifetimes (tau (f) = 9.4 ps in n-hexane), suggesting the presence of very fast nonradiative deactivation processes. The measurements of the quantum yield (Phi (isc)) of intersystem crossing based on the energy transfer and TRTL methods clearly show that the fast radiationless processes in nonpolar solvents are due to internal conversion. In aprotic solvents, the rate (k(ic)) of internal conversion for o-AAP decreases significantly with increasing solvent polarity (k(ic) = 1.0 x 10(11) s(-1) in n-hexane, k(ic) = 2.4 x 10(9) s(-1) in acetonitrile). In protic solvents, the value tends to increase with an increase of hydrogen-bonding donor ability of the solvent. The internal conversion rate in aprotic solvents is scarcely affected by deuterium substitution of the NH2 group in o-AAP, while a large isotope effect is found for o-AAP in deuteriated protic solvents. It is concluded that the efficient S-1 --> S-0 internal conversion in nonpolar aprotic solvents arises from vibronic interactions between close-lying (1)(pi,pi*) and (1)(n,pi*) states (the proximity effect), and in protic solvents intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative process.