Femtosecond Heterodyne Transient Grating Studies of Nonradiative Decay of the S$_2$ (1$^1$B$_u$$^+$) State of Peridinin: Detection and Spectroscopic Assignment of an S$_x$ Intermediate State

Femtosecond heterodyne transient grating spectroscopy was employed to investigate the nonradiative relaxation dynamics of peridinin from the S$_2$ state to the S$_1$ (2$^1$A$_g$$^-$) state in methanol. A global target analysis indicates that S$_2$ decays in 12 fs to populate an intermediate state, S$_x$. The absorption and dispersion components of the transient grating signal exhibit a response that is very similar to that of $\beta$-carotene in benzonitrile solution. Numerical simulation of the experimental data indicates that the excited state absorption transition from S$_x$ has a larger oscillator strength than that of S$_1$, which rules out an assignment of S$_x$ to a vibrationally excited S$_1$ state. The lifetime of S$_x$ is found to be strongly dependent on the polar solvation timescale. This result indicates that nonradiative decay from S$_x$ to S$_1$ involves large-amplitude torsional motions and a concomitant formation of intramolecular charge transfer character. The present work provides the first evidence that peridinin has an ultrashort S$_2$ lifetime owing to the onset of torsional motions and shows that the S$_x$ acts as an active state for excitation energy transfer to chlorophyll in light-harvesting proteins.