Fluorescence anisotropy detection of barrier crossing and ultrafast conformational dynamics 
in the S 2 state of β-carotene.

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, beta-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 degrees 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.