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| - Radiationless Deactivation Process of 1-Dimethylamino-9-fluorenone Induced byConformational Relaxation in the Excited State: A New Model Molecule for the TICTProcess
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| - The deactivation process of excited 1-(dimethylamino)-9-fluorenone (1-DMAF) was investigated by meansof steady-state and time-resolved fluorescence spectroscopy. Fluorescence decay profiles for 1-DMAF, whichhas a relatively short lifetime (several tens of picoseconds in low viscosity solvents at ambient temperature)are much affected by the fluidity of the surrounding solvent. The lifetimes increase in glassy solvents at 77K as well as with increasing viscosity. These results indicate that conformational relaxation by thedimethylamino group plays an important role in the deactivation process of 1-DMAF. Detailed analysis ofthe temperature dependence of the fluorescence lifetime revealed that the conformational relaxation processhas a very small intramolecular activation barrier (5.4 kJ/mol). Results of MO calculations suggest that thedimethylamino moiety of 1-DMAF is pretwisted in the ground state and that the most stable structure in theexcited state is a twisted intramolecular charge transfer (TICT) state. 1-DMAF could be a new model moleculethat exhibits the TICT phenomena.
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