Excitation Energy Transfer by Coherent and Incoherent Mechanisms in the Peridinin–Chlorophyll a Protein

Excitation energy transfer from peridinin to chlorophyll (Chl) a is unusually efficient in the peridinin–chlorophyll a protein (PCP) from dinoflagellates. This enhanced performance is derived from the long intrinsic lifetime of 4.4 ps for the S 2 (1 1 B u + ) state of peridinin in PCP, which arises from the electron-withdrawing properties of its carbonyl substituent. Results from heterodyne transient grating spectroscopy indicate that S 2 serves as the donor for two channels of energy transfer: a 30 fs process involving quantum coherence and delocalized peridinin–Chl states and an incoherent, 2.5 ps process initiated by dynamic exciton localization, which accompanies the formation of a conformationally distorted intermediate in 45 fs. The lifetime of the S 2 state is lengthened in PCP by its intramolecular charge-transfer character, which increases the system–bath coupling and slows the torsional motions that promote nonradiative decay to the S 1 (2 1 A g – ) state.