Singlet and triplet excited state properties of natural chlorophylls and bacteriochlorophylls

Ten naturally occurring chlorophylls (a, b, c ₂, d) and bacteriochlorophylls (a, b, c, d, e, g) were purified and studied using the optical spectroscopic techniques of both steady state and time-resolved absorption and fluorescence. The studies were carried out at room temperature in nucleophilic solvents in which the central Mg is hexacoordinated. The comprehensive studies of singlet excited state lifetimes show a clear dependency on the structural features of the macrocycle and terminal substituents. The wide-ranging studies of triplet state lifetime demonstrate the existence of an energy gap law for these molecules. The knowledge of the dynamics and the energies of the triplet state that were obtained in other studies allowed us to construct an energy gap law expression that can be used to estimate the triplet state energies of any (B)chlorophyll molecule from its triplet lifetime obtained in a liquid environment.