Absorption shifts in carotenoids—influence of index of refraction and submolecular electric fields

Most biological functions of carotenoids relate to strong visible absorption. In a quest to protein matrix effects on spectra, the shifts produced by solvent refractive index (n) and static dielectric permittivity (ɛ) were quantified for visible absorption maxima of β-carotene and a model compound with sharper bands, tetra-tert-butylnonaene ttbP9. The dispersive red shift that depends on optical dielectric constant n2 is insensitive to polarity, in disagreement with previous work (H. Nagae, et al., J. Chem. Phys. 101 (1994) 6750). Small residual red shift in polar liquids obtained after subtraction of the dispersive component, obeys a static susceptibility function (ɛ − n2)/(ɛ − n2 + 4). Because polyenes show little inhomogeneous band broadening in polar solvents, the shift was assigned mainly to quadratic Stark effect, rather than to a reaction field of quadrupolar origin, proposed earlier (N. Ghoneim, P. Suppan, Spectrochim. Acta A, 51 (1995) 1043). It is highly plausible that the chromophore is polarized in short-range “edge fields”, according to molecular dynamics/semiempirical simulations (F. Cichos, et al., J. Chem. Phys. 114 (2001) 6824), not in a uniform cavity field, as usually presumed.