Internal conversion of o-aminoacetophenone in solution

The photophysical properties of o-aminoacetophenone (o-AAP) in solution hav e been studied by using a femtosecond laser-single photon counting system a nd time-resolved thermal lensing (TRTL) method. The fluorescence quantum yi eld (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) value s (Phi (f) = 2.4 x 10(-4) in n-hexane) and remarkably short fluorescence li fetimes (tau (f) = 9.4 ps in n-hexane), suggesting the presence of very fas t nonradiative deactivation processes. The measurements of the quantum yiel d (Phi (isc)) of intersystem crossing based on the energy transfer and TRTL methods clearly show that the fast radiationless processes in nonpolar sol vents are due to internal conversion. In aprotic solvents, the rate (k(ic)) of internal conversion for o-AAP decreases significantly with increasing s olvent 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 w ith an increase of hydrogen-bonding donor ability of the solvent. The inter nal conversion rate in aprotic solvents is scarcely affected by deuterium s ubstitution of the NH2 group in o-AAP, while a large isotope effect is foun d for o-AAP in deuteriated protic solvents. It is concluded that the effici ent S-1 --> S-0 internal conversion in nonpolar aprotic solvents arises fro m vibronic interactions between close-lying (1)(pi,pi*) and (1)(n,pi*) stat es (the proximity effect), and in protic solvents intermolecular hydrogen-b onding interactions with solvent molecules also facilitate the nonradiative process.