Solvent-induced charge separation

Zijlstra, R. W.J., Grozema F. C., Swart M., Feringa B. L. and Duijnen R Th. van, Solvent Induced Charge Separation in the Excited States of Symmetrical Ethylene A Direct Reaction Field Study. J.Phys.Chem.A (2001) 105 3583-3590. 

In this case both e.t. processes are intramolecular, the distance between the porphyrin and the quinone being fixed by a short space. The highest rate constants of the photo-induced charge separation reach about 4 x 1011 s J and these would not be observable in a diffusion controlled intermolecular reaction. In these measurements, two solvents were used — nonpolar toluene and polar butyronitrile — and the AG values are therefore subject to the uncertainties discussed in Sect. 2.1. In particular, the Coulomb term for butyronitrile assumes the point charge model with full solvent screening, which may well underestimate the free energy values, especially in such an intramolecular system where it is clear that the solvent cannot be inserted between the chromophores. 

The Mataga-Kakitani (M-K) theory is based on the rather general observation that e.t. processes which show the M.I.R. are mostly charge recombinations and charge shifts, whereas the photo-induced charge separations which start from neutral reactants follow Rehm-Weller behaviour. It is then suggested that the difference is due to the electric field which acts on the solvent in the field of ions or ion pairs, partial dielectric saturation of polar solvents would be reached, and this would restrict solvent motion. No such dielectric saturation effect would exist in the solvent shell of neutral reactants, so that solvent motion remains unhindered. 

Fig. 6 Arrhenius plots for the rate of photo-induced charge separation in 1(8) in two solvents...

M. Zander and W. Rettig, Fluorescence studies on solvent-induced intramolecular charge separation in symmetric biaryls, Chem. Phys. Lett. 110, (Ml (1984). 

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