| Abstract
| - The excitation behaviors for 4‘-N,N-diethylamino-3-hydroxyflavone (Ia) have been investigated via femtosecondfluorescence upconversion approaches to gain detailed insights into the mechanism of the proton/charge-transfer coupling reaction. In polar solvents such as CH2Cl2 and CH3CN, in addition to a slow, solvent-polarity-dependent rate (a few tens of picoseconds-1) of excited-state intramolecular proton transfer (ESIPT)reported previously, early femtosecond relaxation dynamics clearly reveal that the proton-transfer tautomeremission consists of a rise component of a few hundred femtoseconds. The temporal spectral evolution at thetime domain of zero to a few hundred femtoseconds further resolves two distinct emission bands consistingof a proton-transfer tautomer emission and a time-dependent Stokes shifted emission. The results, in combinationwith ab initio calculations on the dipolar vectors for normal and tautomer species, lead us to unveil theimportance of the relationship of the dipolar vectors among various states, and hence the corresponding solvationenergetics in the overall ESIPT reaction. We conclude a similar dipolar character between ground-state normal(N) and excited proton-transfer tautomer (T*) species, whereas due to the excited-state intramolecular chargetransfer (ESICT), the normal excited state (N*) possesses a large dipolar change with respect to N and T*.ESIPT is thus energetically favorable at the Franck−Condon excited N*, and its rate is competitive withrespect to the solvation relaxation process. After reaching the solvent equilibration, there exists an equilibriumbetween N* and T* states in, for example, CH3CN. Due to the greatly different equilibrium polarizationbetween N* and T*, both forward and reversed ESIPT dynamics are associated with a solvent-induced barrier.The latter viewpoint of the equilibrium type of ESIPT in Ia is in agreement with the previous reports basedon steady-state,8 picosecond,9,13 and femtosecond14,15 dynamic approaches.
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