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 S2 (1(1)Bu(+)) → S1 (2(1)Ag(-)) 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 S2 state of peridinin is promoted by large-amplitude torsional motions. The decay of an intermediate state termed Sx, which we assign to a twisted form of the S2 state, is substantially slowed by solvent friction in peridinin due to its intramolecular charge transfer (ICT) character.

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The orange carotenoid protein (OCP) mediates nonphotochemical quenching (NPQ) mechanisms in cyanobacteria. A bound ketocarotenoid serves as a sensor of midvisible light intensity and as a quencher of phycocyanobilin excitons in the phycobilisome. The photochemical mechanism that triggers conversion of the protein from a resting, orange state (OCP) to an active, red state (OCP) after optical preparation of the S state of the carotenoid remains an open question. We report here that the fluorescence spectrum and quantum yield of the bound carotenoids in OCP report important details of the motions that follow optical preparation of the S state. The fluorescence spectra from OCP preparations containing 3'-hydroxyechinenone (3hECN) or canthaxanthin (CAN) are markedly mirror asymmetric with respect to the absorption line shape and more than an order of magnitude more intense than for carotenoids in solution. Further, 3hECN exhibits a narrower fluorescence line shape and a larger quantum yield than CAN because its excited-state motions are hindered by a hydrogen bonding interaction between the 3'-hydroxyl group on its β2 ring and Leu37 in the N-terminal domain. These results show that large-amplitude motions of the carotenoid's β2-cyclohexene ring and of the conjugated polyene backbone initiate photochemistry in OCP.

Carotenoids are usually only weakly fluorescent despite being very strong absorbers in the mid-visible region because their first two excited singlet states, S 1 and S 2 , have very short lifetimes. To probe the structural mechanisms that promote the nonradiative decay of the S 2 state to the S 1 state, we have carried out a series of fluorescence lineshape and anisotropy measurements with a prototype carotenoid, β-carotene, in four aprotic solvents. The anisotropy values observed in the fluorescence emission bands originating from the S 2 and S 1 states reveal that the large internal rotations of the emission transition dipole moment, as much as 50°relative to that of the absorption transition dipole moment, are initiated during ultrafast evolution on the S 2 state potential energy surface and persist upon nonradiative decay to the S 1 state. Electronic structure calculations of the orientation of the transition dipole moment account for the anisotropy results in terms of torsional and pyramidal distortions near the center of the isoprenoid backbone. The excitation wavelength dependence of the fluorescence anisotropy indicates that these out-of-plane conformational motions are initiated by passage over a low-activation energy barrier from the Franck−Condon S 2 structure. This conclusion is consistent with detection over the 80−200 K range of a broad, red-shifted fluorescence band from a dynamic intermediate evolving on a steep gradient of the S 2 state potential energy surface after crossing the activation barrier. The temperature dependence of the oscillator strength and anisotropy indicate that nonadiabatic passage from S 2 through a conical intersection seam to S 1 is promoted by the out-of-plane motions of the isoprenoid backbone with strong hindrance by solvent friction.

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