Effects of molecular structure and hydrogen bonding on the radiationless deactivation of singlet excited fluorenone derivatives

Substituent effects on intramolecular radiationless deactivation and hydrogen-bonding-induced quenching have been examined for various fluorenone derivatives. In toluene, triplet formation is the dominant process from the singlet excited state when an electron-withdrawing group is attached to the fluorenone moiety, whereas an electron-donating substituent promotes internal conversion. There is a clear correlation between the internal conversion rate constant and the lowest excited singlet state energy, which can be explained in terms of the energy gap law. It is shown that both the electron-donating character of the substituent in the excited fluorenone and the hydrogen-bonding power of alcohol play important roles in determining the rate of dynamic quenching by alcohols. The intermolecular hydrogen bonding with alcohols in the singlet excited state acts as an effective accepting mode of radiationless deactivation for fluorenones substituted with an electron-donating group. However, 2-NO2- and 2-COOCH3- derivatives are poorly quenched by alcohols. The parallel change of the hydrogen-bonding-induced quenching rate constants and the dipole moment difference between the ground and the singlet excited states suggests that the electron density around the carbonyl oxygen controls the quenching rate in the series of 2-substituted fluorenones.