SOLVENT EFFECT ON SPHEROIDENE IN NONPOLAR AND POLAR SOLUTIONS AND THE ENVIRONMENT OF SPHEROIDENE IN THE LIGHT‐HARVESTING COMPLEXES OFRhodobacter sphaeroides2.4.1 AS REVEALED BY THE ENERGY OF THE¹Ag⁻→¹Bu⁺ABSORPTION AND THE FREQUENCIES OF THE VIBRONICALLY COUPLED C=C STRETCHING RAMAN LINES IN THE¹Ag⁻AND¹Bu⁻STATES

Abstract –The 1 A g − → 1 B u + electronic absorption band and the vibronically coupled, C=C stretching Raman lines in the 1 A g − and 2 1 A g − states were recorded for spheroidene free in nonpolar and polar solvents as well as for spheroidene bound to the LH1 and LH2 complexes of Rhodobacter sphaeroides 2.4.1. The 1 B u + energy exhibited a linear dependence on R(n) = (n 2 ‐ 1)/(n 2 + 2) in both nonpolar and polar solvents; the line for polar solvents had a gentler slope and crossed the line for nonpolar solvents at R(n) = 0.3. The above characteristic of polar solvents was ascribed to the electric field generated by fluctuation of the solvent permanent dipoles; it stabilizes the 1 B u + energy and reduces the polarizability of the solvent. The vibronically coupled, C=C stretching frequencies in the 1 A g − and 2 1 A g − states [ν(A g ) and [ν(2A g )] also showed similar dependence on R(n), which is explained in terms of vibronic coupling among the 1 A g − , 2 1 A g − and 3 1 A g − states. The environment of spheroidene in the LH2 and LH1 complexes was assessed on the basis of the 1 B u + energy and the ν(A g ) and [ν(2A g ) frequencies: Spheroidene in the LH2 complex is located in an environment with high polarizability, while spheroidene in the LH1 complex is located in an environment with lower polarizability.