Spectroscopy and Femtosecond Dynamics of 7-N,N-Diethylamino-3-hydroxyflavone. The Correlation of Dipole Moments among Various States To Rationalize the Excited-State Proton Transfer Reaction

Abstract

Comprehensive excitation behaviors of 7-N,N-diethylamino-3-hydroxyflavone (I) have been investigated via steady state, temperature-dependent emission, and fluorescence upconversion to probe the excited-state intramolecular proton transfer (PT) reaction. Upon excitation, I undergoes ultrafast (≪120 fs), adiabatic type of charge transfer (CT), so that the dipolar vector in the Franck−Condon excited state is much different from that in the ground state. In polar solvents such as CH₂Cl₂ and CH₃CN, early relaxation dynamics clearly reveals the competitive rates between solvent relaxation and PT dynamics. After reaching thermal equilibrium, a relatively slow, solvent-polarity-dependent rate (a few tens of picoseconds^-1) of PT takes places. Firm support of the early relaxation dynamics is rendered by the spectral temporal evolution, which resolves two distinct bands ascribed to CT and PT emission. The results, in combination with ab initio calculations on the dipolar vectors for various corresponding states, led us to conclude that excited-state normal (N*) and excited proton-transfer tautomer (T*) possesses very different dipole orientation, whereas the dipole orientation of the normal ground state (N) is between that of N* and T*. PT is thus energetically favorable at the Franck−Condon excited N*, and its rate is competitive with respect to the solvent relaxation dynamics induced by CT. Unlike the well-known PT system, 4‘-N,N-diethylamino-3-hydroxyflavone,^10-19 in which equilibrium exists between solvent-equilibrated N_eq* and T_eq*, N_eq* → T_eq* PT for I is a highly exergonic, irreversible process in all solvents studied. Further temperature-dependent studies deduce a solvent-polarity-perturbed energy barrier of 3.6 kcal/mol for the N_eq* → T_eq* PT in CH₃CN. The proposed dipole-moment-tuning PT mechanism with the associated relaxation dynamics is believed to apply to many PT molecules in polar, aprotic solvents.