The role of intersystem crossing in the deactivation of the singlet excited aminofluorenones

Solvent and substituent effects on the competition between internal conversion and triplet formation were studied systematically for aminofluorenones and their N-methylated derivatives. Intersystem crossing (ISC) was found to be the dominant process for the singlet excited 1-amino- and 1-methylaminofluorenone in all solvents. The short fluorescence decay time of these compounds does not originate from intramolecular hydrogen bonding induced internal conversion but it is due to the fast triplet formation. Rather slow (kISC⩽4.8 × 107 s−1) and solvent insensitive intersystem crossing characterizes the photophysical behavior of 2-, 3- and 4-aminofluorenones but their internal conversion rate strongly increases with solvent polarity. The change of the internal conversion rate constants with molecular structure and solvent can be rationalized in terms of the energy gap law.