Charge separation in 9,9?-dianthryl and a special pair treated by a semi-empirical reaction field method

Reaction fields are introduced to treat charge transfer excitations in chromophore systems. The methods are tested on the 9,9′-dianthryl dimeric molecule. The energy shift of the low energy component in dual fluorescence is a function of the polarity of the solvent and is correctly reproduced if an interpolation formula is used between the Mataga-Nishimoto and Ohno–Klopman approximations for the bi-centric γ integrals. The model is then applied to the special pair of the photosynthetic reaction center of Rhodopseudomonas viridis. This special pair is found to have an S1 state with dipolar character, corresponding to partial M ← L charge transfer. The bias is due to the bent shape of the M chromophore.