Picosecond kinetic studies of charge separation in 9,9'-bianthryl as a function of solvent viscosity and comparisons with electron transfer in bacterial photosynthesis

Abstract

Transient absorption spectra of 9,9'-bianthryl (BA) in the picosecond time range have been recorded in nonpolar cyclohexane, in polar acetonitrile and in the highly viscous solvent glycerol triacetate (GTA). High pressure (0.1-300 MPa) is employed to vary the solvent properties of GTA over an unusually wide range. Transient spectra indicate an anthracene-like band corresponding to the locally excited state (LE) and a longer wavelength band corresponding to the twisted intramolecular charge transfer state (TICT). The electron transfer (ET) in BA was found to be solvent controlled. By applying pressure we can vary the viscosity of the solvent without changing its chemical nature. The viscosity has a large effect on the ET dynamics. The ET kinetics in GTA is highly nonexponential and could be fit best with a stretched exponential function. One-half of the overall TICT concentration is formed in a time shorter than our time-resolution and this occurs almost independently of the pressure (viscosity). We attribute this to a subset of solvent molecules favorably preoriented to support ET. Comparisons are made between ET in BA/GTA and ET in the photosynthetic bacterial reaction center. Several minimum overlap situations, reminiscent of BA, that involve aromatic protein residues, occur in the reaction center. They suggest that the microscopic structure of the protein in which the chromophores are embedded not only induces the asymmetric charge separation, but also provides a polar solvent environment optimized for fast activationless ET and preformed to stabilize the charge separated chromophores.