Four-wave-mixing spectroscopy of peridinin in solution and in the peridinin-chlorophyll-a protein

A model for the third order optical response of carotenoids is used to analyse transient grating and pump-probe data of peridinin in solution and bound in the peridinin-chlorophyll protein (PCP). For peridinin in solution, the transient grating signal detected at 505 nm exhibits a bi-exponential recovery whose fast phase is assigned to relaxation from the S₂ state that has a lifetime of 75 ± 25 fs. The slower, solvent-dependent rise component is assigned to equilibration of the (S₁/ICT) state, taking place on a time scale of 0.6 and ∼2.5 ps in acetontrile and benzene, respectively. These dynamics match those obtained from pump-probe measured in the spectral region of the ICT state, implying that the ICT state contributes to the signal at 505 nm. In PCP, the transient grating signal shows distinctly different kinetics, and the signal shows no recovery. This difference is explained by energy transfer from peridinin to chlorophyll-a.