Origin of the bathochromic shift in the early photointermediates of the rhodopsin visual cycle: A CASSCF/CASPT2 study

CASSCF/CASPT2 calculations have been applied to study the excited‐state properties of the chromophore in rhodopsin and in the early photointermediate, photorhodopsin. For rhodopsin, the chromophore geometry was taken from an optimized X‐ray structure while for photorhodopsin the structure proposed by Bifone et al. J Phys Chem B 1997, 101, 2954 was employed. The experimentally observed bathochromic shift of 72 nm is faithfully reproduced by the calculation. Analysis of a model chromophore in which the transition between the geometries is performed stepwise reveals that this shift is predominantly caused by double bond torsion.