Sub-100 fs Charge Separation and Subsequent Diffusive Solvation Observed for Asymmetric Bianthryl Derivative in Ionic Liquid

Femtosecond transient absorption (TA) and picosecond time-resolved fluorescence (TRF) spectroscopies were applied to the charge separation (CS) dynamics of 10-cyano-9,9′-bianthryl (CBA) in a normal polar solvent, acetonitrile (Acn), and in a highly viscous room temperature ionic liquid (IL), N,N-diethyl-N-methyl-N-(methoxyethyl)­ammonium tetrafluoroborate (DemeBF4). The primary CS took place within the ultrafast sub-100 fs time range in both solvents, which was completely independent of diffusive solvation. Subsequent viscosity-dependent spectral evolution was observed by the TA measurement in the picosecond range which was ascribed to the structural relaxation. A red shift of the TRF spectrum in the picosecond to nanosecond range was observed in DemeBF4 which was due to the diffusive solvation in the CS state. Interestingly, integrated fluorescence intensity decayed more rapidly than TA in the IL, while they decayed simultaneously in Acn. It was concluded that diffusive solvation decreases the radiative transition rate of the CS state through the temporal evolution of the CS state electronic structure.