Solvent dynamics and twisted intramolecular charge transfer in bis(4-aminophenyl) sulfone

Picosecond time-resolved fluorescence spectroscopy is used to probe the formation kinetics of the twisted intramolecular charge-transfer state of bis(4-aminophenyl) sulfone in acetonitrile and alcohol solvents. Emission kinetics were measured at 20-nm intervals from 400 to 560 nm, covering the entire visible region of the emission spectrum. Wavelength-independent rise times were observed in methanol and acetonitrile. In all the longer chain alcohol solvents examined (ethanol to hexanol), wavelength-dependent rise times were observed; longer rise times were observed with decreasing emission energy. In addition, for wavelengths higher in energy than the maximum of the TICT emission maximum, λ_max ^TICT, multiple decay components were observed in all solvents. Only the red edge of the emission (λ > λ_max ^TICT) revealed rises and decays that could be described by single exponentials. The observed kinetics are interpreted in terms of solvent restructuring to stabilize the charge-separated state. Excellent agreement between the fitted rise times of the emission at 500, 540, and 560 nm and the solvent longitudinal relaxation time, τ_L, is observed.