Theoretical study of the excited states of chlorin, bacteriochlorin, pheophytin a and chlorophyll a by the SAC/SAC-CI method

Excited states of free base chlorin (FBC), free base Bacteriochlorin (FBBC), pheophytin a (Pheo a), and chlorophyll a (Chlo a), which are derivatives of free base porphine (FBP), were calculated by the SAC (symmetry adapted cluster)/SAC-CI (configuration interaction) method. The results reproduced well the experimentally determined excitation energies. The reduction of the outer double bonds in the porphine ring in the order of FBP, FBC, and FBBC causes a breakdown of the symmetry and a narrowing of the HOMO-LUMO gap, which result in a red shift of the Q(x) band and an increase of its intensity. In the change from Pheo a to Chlo a, the Mg coordination reduces the quasidegeneracy in the Q(x) state and then increases the spectral intensity. The disappearance of the Q(y) humps from the absorption spectrum of Pheo a, compared with that of Chlo a, is due to the red shift of the Q(y) state.