The role of hydrogen-bonding interactions in the ultrafast relaxation dynamics of the excited states of 3- and 4-aminofluoren-9-ones

Abstract

The dynamics of the excited states of 3- and 4-aminofluoren-9-ones (3AF and 4AF, respectively) are investigated in different kinds of solvents by using a subpicosecond time-resolved absorption spectroscopic technique. They undergo hydrogen-bonding interaction with protic solvents in both the ground and excited states. However, this interaction is more significant in the lowest excited singlet (S₁) state because of its substantial intramolecular charge-transfer character. Significant differences in the spectroscopic characteristics and temporal dynamics of the S₁ states of 3AF and 4AF in aprotic and protic solvents reveal that the intermolecular hydrogen-bonding interaction between the S₁ state and protic solvents plays an important role in its relaxation process. Perfect linear correlation between the relaxation times of the S₁ state and the longitudinal relaxation times (t_L) of alcoholic solvents confirms the prediction regarding the solvation process via hydrogen-bond reorganization. In the case of weakly interacting systems, the relaxation process can be well described by a dipolar solvation-like process involving rotation of the OH groups of the alcoholic solvents, whereas in solvents having a strong hydrogen-bond-donating ability, for example, methanol and trifluoroethanol, it involves the conversion of the non-hydrogen-bonded form to the hydrogen-bonded complex of the S₁ state. Efficient radiationless deactivation of the S₁ state of the aminofluorenones by protic solvents is successfully explained by the energy-gap law, by using the energy of the fully solvated S₁ state determined from the time-resolved spectroscopic data.