Molecular mechanism for the radiationless deactivation of the intramolecular charge-transfer excited singlet state of aminofluorenones through hydrogen bonds with alcohols

The fluorescence quenchings of 12 aminofluorenone (AF) derivatives by ethanol were examined, and the quenching dynamics of the intramolecular charge-transfer excited singlet state of 3-amino-9-fluorenone, as a model molecule, induced by alcohol were studied by steady-state, picosecond fluorescence measurements and quenching experiments. The AF in benzene solution showed substantial red-shiftings of its fluorescence following addition of alcohol. Fluorescence decay analysis revealed that two different relaxed hydrogen-bonded states exist: an emissive state and a nonemissive state. These different states are due to the microscopic anisotropy of the hydrogen bond around the carbonyl oxygen. In the emissive state, the hydroxyl group of the alcohol interacts through an in-plane mode with the carbonyl oxygen of AF, while the alcohol interacts through an out-of-plane mode in the nonemissive state. The out-of-plane mode was shown to be more common with cumene hydroperoxide and diols than with ethanol owing to their steric hindrance in the in-plane mode. Because of its simple structure and charge-transfer nature, AF was found to be the most suitable molecule for examining the microscopic mechanism of radiationless deactivation of the intramolecular charge-transfer excited singlet state through a hydrogen bond.