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 phy tyl chain replaced by a hydrogen atom have been calculated using a combinat ion of density functional theory and the multireference configuration inter action method (DFT/MRCI). The calculations accurately reproduce experimenta l absorption energies and intensities. Four and three excited states are ca lculated in the visible region of pheophytin a and chlorophyll a, respectiv ely. We thus predict the existence of more than the two Q(x) and Q(y) elect ronic states in this energy range. These additional states are of significa nt double-excitation character and cannot be explained within the four fron tier orbital model. Two strongly allowed states are calculated in the Soret region at 2.9 eV.