Use of Ultrafast Dispersed Pump−Dump−Probe and Pump−Repump−Probe Spectroscopies to Explore the Light-Induced Dynamics of Peridinin in Solution

Abstract

Optical pump-induced dynamics of the highly asymmetric carotenoid peridinin in methanol was studied by dispersed pump−probe, pump−dump−probe, and pump−repump−probe transient absorption spectroscopy in the visible region. Dispersed pump−probe measurements show that the decay of the initially excited S₂ state populates two excited states, the S₁ and the intramolecular charge-transfer (ICT) state, at a ratio determined by the excitation wavelength. The ensuing spectral evolution occurs on the time scale of a few picoseconds and suggests the equilibration of these states. Dumping the stimulated emission of the ICT state with an additional 800-nm pulse after 400- and 530-nm excitation preferentially removes the ICT state contribution from the broad excited-state absorption, allowing for its spectral characterization. At the same time, an unrelaxed ground-state species, which has a subpicosecond lifetime, is populated. The application of the 800-nm pulse at early times, when the S₂ state is still populated, led to direct generation of the peridinin cation, observed for the first time in a transient absorption experiment. The excited and ground electronic states manifold of peridinin has been reconstructed using target analysis; this approach combined with the measured multipulse spectroscopic data allows us to estimate the spectra and time scales of the corresponding transient states.