Gas-phase energy of the S2←S0 transition and electrostatic properties of the S2 state of carotenoid peridinin via a solvatochromic shift and orientation broadening of the absorption spectrum

Abstract

The solvent effect on the position and the shape of the absorption spectrum of peridinin for 12 protic and aprotic solvents as well as the temperature effect for methanol were studied using a solvatochromic theory based on the Onsager sphere cavity model. (Experimental data have been provided by T. Polivka and V. Sundström.) Solvatochromic calculations combined with estimations of orientation broadening of the absorption spectrum by convolution allowed the conclusion that the orientation (dipole–dipole), induction and dispersion solute–solvent interactions reasonably describes the position of the 0–0 frequency. The orientation interactions led to the blue solvatochromic shift, separating them from the induced and dispersion interactions, which produce a red shift. The FWHM of Gaussian of inhomogeneous broadening originated from the fluctuations of orientation interactions was demonstrated to be high (945 cm⁻¹) even for such a nonpolar solvent as hexane. The value of |Δμ|/cos φ of −18.7 D has been found (Δμ = μ₂ − μ_g, φ is the angle between Δμ and μ_g). By assigning peridinin to the idealized C_2v point group, the large change of dipole moment |Δμ| of 18.7 D under S₂←S₀ transition is obtained for peridinin in gas phase. Moreover, the S₂-excited state dipole moment μ₂ has the opposite orientation relative to that at the ground S₀ state μ_g. The determined gas-phase 0–0 energy of the S₂←S₀ transition, 22 910 cm⁻¹ (2.84 eV) is employed to calculate the polarizability change between the S₀ and S₂ states of 376 ų. The finding for the effective Onsager radius is of 9.4 Å. Obtained results for electrostatic properties of the S₂ state are compared with those known from Stark spectroscopy and quantum-mechanical calculations.