A theoretical study of the excited states of chlorophyll a and pheophytin a

The first six excited states of pheophytin a and chlorophyll a with the phytyl chain replaced by a hydrogen atom have been calculated using a combination of density functional theory and the multireference configuration interaction method (DFT/MRCI). The calculations accurately reproduce experimental absorption energies and intensities. Four and three excited states are calculated in the visible region of pheophytin a and chlorophyll a, respectively. We thus predict the existence of more than the two Q(x) and Q(y) electronic states in this energy range. These additional states are of significant double-excitation character and cannot be explained within the four frontier orbital model. Two strongly allowed states are calculated in the Soret region at 2.9 eV.