Torsional Dynamics and Intramolecular Charge Transfer in the S2 (11 Bu+) Excited State of Peridinin: A Mechanism for Enhanced Mid Visible Light Harvesting

Of the carotenoids known in photosynthetic organisms, peridinin exhibits one of the highest quantum efficiencies for excitation energy transfer to chlorophyll (Chl) a acceptors. The mechanism for this enhanced performance involves an order-of magnitude slowing of the S-2 (1(1)B(u)(+)) -> S-1 (2(1)A(g)(-)) nonradiative decay pathway compared to carotenoids lacking carbonyl substitution. Using femtosecond transient grating spectroscopy with optical heterodyne detection, we have obtained the first evidence that the nonradiative decay of the S-2 state of peridinin is promoted by large-amplitude torsional motions. The decay of an intermediate state termed S-x, which we assign to a twisted form of the S-2 state, is substantially slowed by solvent friction in peridinin due to its intramolecular charge transfer (ICT) character.