Internal conversion of o-aminoacetophenone in solution

The photophysical properties of o-aminoacetophenone (o-AAP) in solution have been studied by using a femtosecond laserÈsingle photon counting system and time-resolved thermal lensing (TRTL) method. The Ñuorescence quantum yield and lifetime of o-AAP depend strongly on the nature of the solvent. In f (qf) nonpolar solvents, o-AAP gives very small values in n-hexane) and remarkably short f (Uf\ 2.4 ] 10~4 Ñuorescence lifetimes ps in n-hexane), suggesting the presence of very fast nonradiative deactivation (qf\ 9.4 processes. The measurements of the quantum yield of intersystem crossing based on the energy transfer isc and TRTL methods clearly show that the fast radiationless processes in nonpolar solvents are due to internal conversion. In aprotic solvents, the rate of internal conversion for o-AAP decreases signiÐcantly with ic increasing solvent polarity s~1 in n-hexane, s~1 in acetonitrile). In protic ic \ 1.0] 1011 ic \ 2.4] 109 solvents, the value tends to increase with an increase of hydrogen-bonding donor ability of the solvent. The ic internal conversion rate in aprotic solvents is scarcely a†ected by deuterium substitution of the group in 2 o-AAP, while a large isotope e†ect is found for o-AAP in deuteriated protic solvents. It is concluded that the efficient internal conversion in nonpolar aprotic solvents arises from vibronic interactions between 1 ] 0 close-lying 1(p,p*) and 1(n,p*) states (the proximity e†ect), and in protic solvents intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative process.