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.