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), <i>N,N</i>-diethyl-<i>N</i>-methyl-<i>N</i>-(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

Extension of Light-Harvesting Ability of Photosynthetic Light-Harvesting Complex 2 (LH2) through Ultrafast Energy Transfer from Covalently Attached Artificial Chromophores

Introducing appropriate artificial components into natural biological systems could enrich the original functionality. To expand the available wavelength range of photosynthetic bacterial light-harvesting complex 2 (LH2 from Rhodopseudomonas acidophila 10050), artificial fluorescent dye (Alexa Fluor 647: A647) was covalently attached to N- and C-terminal Lys residues in LH2 α-polypeptides with a molar ratio of A647/LH2 ≃ 9/1. Fluorescence and transient absorption spectroscopies revealed that intracomplex energy transfer from A647 to intrinsic chromophores of LH2 (B850) occurs in a multiexponential manner, with time constants varying from 440 fs to 23 ps through direct and B800-mediated indirect pathways. Kinetic analyses suggested that B800 chromophores mediate faster energy transfer, and the mechanism was interpretable in terms of Förster theory. This study demonstrates that a simple attachment of external chromophores with a flexible linkage can enhance the light harvesting activity of LH2 without affecting inherent functions of energy transfer, and can achieve energy transfer in the subpicosecond range. Addition of external chromophores, thus, represents a useful methodology for construction of advanced hybrid light-harvesting systems that afford solar energy in the broad spectrum