Theory of optical spectra involving charge transfer states: Dynamic localization predicts a temperature dependent optical band shift

The influence of charge transfer states on the optical line shape of chromophore complexes is investigated in a minimal model that includes a coupling between an excited state and an optically dark charge transfer state. In the calculations of the absorption spectrum, an intensity borrowing by the charge transfer state, strong vibrational sidebands, and a temperature dependent shift of optical transitions are obtained. The theory is applied to the bacterial photosynthetic reaction center to explain a 30 nm blueshift of the low-energy absorption band with increasing temperature.