Effect of the protein environment on the excited state phenomena in a bacteriophytochrome

The excited state processes of a bacteriophytochrome are studied using high-level multireference methods. The various non-radiative channels of deactivation are identified for the chromophore. The effects of the protein environment and substituents are elucidated for these excited state processes. It is observed that while the excited states are completely delocalized in the Franck-Condon (FC) region, they acquire significant charge transfer character near the conical intersections. Earlier studies have emphasized the delocalized nature of the excited states in the FC region, which leads to absorption spectra with minimal Stokes shift [Rumyantsev et al., Sci. Rep., 2015, 5, 18348]. The effect of the protein environment on the vertical excitation energies was minimal, while that on the conical intersection (CI) energetics was significant. This may lead one to believe that it is charge transfer driven. However, energy decomposition analysis shows that it is the effect of the dispersion of nearby residues and the steric effect on the rings and substituents that lead to the large effect of proteins on the energetics of the CIs.