Ultrafast Time-Resolved Broadband Fluorescence Studies of the Benzene-Tetracyanoethylene Complex: Solvation, Vibrational Relaxation, and Charge Recombination Dynamics

Abstract

The charge-transfer (CT) state relaxation dynamics of the benzene-tetracyanoethylene (BZ-TCNE) complex was studied with broadband ultrafast time-resolved fluorescence spectroscopy implemented by optical Kerr gating in three solvents of different polarities. The CT state of the BZ-TCNE complex is reached via femtosecond laser excitation, and the subsequent temporal evolutions of the fluorescence spectra were measured. Analyses of various time-dependent spectral properties revealed rapid relaxations along solvent and vibrational coordinates in competition with charge recombination (CR). By comparing the results in solvents of different polarities, we partially separated solvation and vibrational relaxation dynamics and explored the solvent-dependent CR dynamics. Time-dependent dynamic fluorescence Stokes shift (TDFSS) measurements unveiled the solvation and vibrational relaxation contributions to the observed spectral relaxation. The biphasic and slow time scales of the vibrational contributions identified in TDFSS suggested nonstatistical and hindered intramolecular vibrational-energy redistribution that can be attributed to the unique structural properties of EDA complexes. The slowest spectral relaxation of 10-15 ps identified in TDFSS was ascribed to relaxation of the BZ(+)-TCNE(-) intermolecular vibrations, which is equivalent to a structural relaxation from the initial Franck-Condon configuration to the equilibrium CT-state structure. The time scales of vibrational relaxation indicate that a fraction of the CT-state population undergoes CR reactions before complete vibrational/structural equilibrium is achieved. In carbon tetrachloride, a nonexponential temporal profile was observed and attributed to vibrational nonequilibrium CR. In dichloromethane, polar solvation greatly accelerates CR reactions, and a slower reaction-field-induced structural relaxation gives rise to a pronounced biexponential decay. The equilibrium CR time constants of the BZ-TCNE CT state are 29 ps, 150 ps, and 68 ps in dichloromethane, carbon tetrachloride, and cyclohexane, respectively.