Intrinsic Photophysics of Light-harvesting Charge-tagged Chlorophyll a and b Pigments.

Chlorophylls a and b (Chla/b) are responsible for light-harvesting by photosynthetic proteins in plants. They display broad absorption in the visible region with multiple bands, due to the asymmetry of the macrocycle and strong vibronic coupling. Their photophysics relies on the microenvironment, with regard to transition energies as well as quenching of triplet states. Here, we firmly establish the splitting of the Q and Soret bands into x- and y- polarized bands for the isolated molecules in vacuo, and resolve vibronic features. Storage-ring experiments reveal that dissociation of photoexcited charge-tagged complexes occurs over several milliseconds, but with two different time constants. A fast decay is ascribed to dissociation after internal conversion and a slow decay to the population of a triplet state that acts as a bottleneck. Support for the latter is provided by pump-probe experiments, where a second laser pulse probes the long-lived triplet state.