Femtosecond Heterodyne Transient Grating Detection of Conformational Dynamics in the S$_0$ (1$^1$A$_g$$^-$) State of Carotenoids After Nonradiative Decay of the S$_ 2$ (1$^1$B$_u$$^+$) State*

Transient grating spectroscopy was used to study the dynamics of nonradiative decay of the S$_1$ (2$^1$A$_g$$^-$) state in ß-carotene and peridinin after optical preparation of the S$_2$) state. The kinetics of the recovery of the absorption and dispersion components of the third-order signal exhibit significantly different time constants. For $\beta$-carotene in benzonitrile, the absorption and dispersion recovery time constants are 11.6 and 10.2 ps. For peridinin in methanol, the time constants are 9.9 and 7.4 ps. These results indicate that the initial product of the decay of the S$_1$ state is a conformationally displaced structure. The decay rate for the S$_1$ state and the conformational relaxation rate are both slowed in peridinin as the polarity of the solvent decreases; in ethyl acetate, the conformational relaxation time constant is 45 ps, which rules out a dominant contribution from vibrational cooling. These results indicate that the S$_1$ state develops intramolecular charge transfer character owing to distortions along torsional and out-of-plane coordinates, with a pyramidal structure favored as the most stable conformation. Recovery of the photoselected ground state conformation involves a reverse charge-transfer event followed by relaxation to a planar structure.