Dynamics of excited states of the carotenoid peridinin in polar solvents: Dependence on excitation wavelength, viscosity, and temperature

The dynamics of the excited states of the carotenoid peridinin in polar solvents were studied using femtosecond transient absorption spectroscopy in the spectral range of 500-1900 nm. A broadening of the absorption spectrum in polar solvents is caused by a distribution of conformers having different ground-state properties. In addition, the dependence of the peridinin lifetime on the excitation wavelength reveals that two peridinin forms coexist in protic solvents, where a "red"-absorbing form is produced by hydrogen bonding via the carbonyl group. The observed dynamics show that the Si and intramolecular charge transfer (ICT) states of peridinin are strongly coupled, forming a collective S₁/ICT state whose lifetime is determined by the degree of ICT character. In nonpolar solvent, pure S₁ character with a lifetime of ∼ 160 ps is observed, whereas in polar solvents an increase in the ICT character leads to a lifetime as short as 10 ps in methanol and 13 ps in ethylene glycol. In protic solvents, the ICT character of the S₁/ICT state of the red peridinin form is further enhanced by hydrogen bonding, resulting in lifetimes shorter than 6 ps. A weak dependence of peridinin dynamics on viscosity shows that the ICT state is not formed via a twisted ICT mechanism. At 190 K in methanol, a significant increase in the S₁/ICT lifetime is observed, suggesting that thermal coupling is involved in the S₁/ICT state mixing. At 77 K in ethylene glycol glass, a multiexponential decay is revealed, indicating the presence of several conformers with different S₁/ICT state properties.