Electroabsorption spectra of carotenoid isomers: Conformational modulation of polarizability vs. induced dipole moments

Abstract

Electroabsorption spectra of all- trans, 13- cis and 15- cis isomers of carotenoids violaxanthin and β-carotene frozen in organic solvents were analysed in terms of changes in permanent dipole moment, Δ μ , and in the linear polarizability, Δ α , on electronic excitation. The spectral range investigated covered the two carotenoid absorption bands in the VIS and UV, known to originate from differently oriented transition dipole moments. In contrast with the collinearity of the apparent Δ μ with Δ α in the lowest-energy allowed (VIS) transition 1 A g - → 1 B u + , the axis of the largest polarizability change in the UV transition 1 A g - → 1 A g + (“ cis band”) was found to make a large angle with the transition moment, while the direction of Δ μ appears to be much closer to it. These data support the view that Δ μ ’s inferred from electrochromic spectra of carotenoids are apparent and are not induced by the local matrix field in the solvent cavity, but merely result from conformational modulation of molecular polarizability.