PRESSURE-DEPENDENCE OF THE KINETICS OF PHOTOINDUCED INTRAMOLECULAR CHARGE SEPARATION IN 9,9'-BIANTHRYL MONITORED BY PICOSECOND TRANSIENT ABSORPTION - COMPARISON WITH ELECTRON-TRANSFER IN PHOTOSYNTHESIS

Transient absorption spectra of 9,9′-bianthryl (BA) in the picosecond time range have been recorded in nonpolar cyclohexane (CH), in polar acetonitrile (ACN), 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. To our knowledge, this is the first time that picosecond absorption spectra at high pressures have been reported. Transient spectra (25-ps resolution) in GTA can be resolved into 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). In addition, the LE spectra in GTA contain a small but distinct band at 640 nm, which is discussed in terms of a precursor in the charge-separation process. The electron transfer (ET) in BA was found to be solvent controlled. By applying pressure, we can vary the dielectric constant and the viscosity of the solvent without changing its chemical nature. The dielectric constant affects the TICT/LE equilibrium. 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. The degree of nonexponentiality, described by the stretching parameter β, varies from β = 0.35 (0.1 MPa) to β = 0.12 (300 MPa). The latter value especially represents an extremely broad distribution of relaxation times. At all pressures the LE decay matches the TICT rise. 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. 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. © 1990 American Chemical Society.