SOLVENT EFFECTS AND VIBRATIONAL DEPENDENCE IN ELECTROCHROMIC SPECTRA OF CAROTENOIDS

Electrochromic (Stark effect) spectra of three carotenoids, beta-carotene, lutein and violaxanthin, were obtained in glassy matrices at low temperature. When analyzed in the framework of the theory of electrochromism they were found to contain a remarkable contribution from the second derivative of the absorption spectrum, equivalent to a substantial change in dipole moment (3-5 D) on electronic excitation, in addition to the usual polarizability term. These dipole moments only weakly depend on solvent polarity; this puts in doubt the induced dipole model. In the case of violaxanthin, a variability of the electro-optical parameters along the electrochromic spectrum was found, which is related to the type of vibration involved in the electronic transition. An analogous effect was also noted for tetradecaheptaene chromophore in amphotericin B. These observations strongly indicate an essential role of vibronic coupling in determining the electro-optical parameters of carotenoids.