Changes in Molecular Structure upon Triplet Excitation of All-trans-Spheroidene in n-Hexane Solution and 15-cis-Spheroidene Bound to the Photo-Reaction Center from Rhodobacter sphaeroides As Revealed by Resonance-Raman Spectroscopy and Normal-Coordinate Analysis†

The S0 and T1 Raman spectra of all-trans-spheroidene and its deuterio derivatives (10-d, 12-d, 14-d, 15-d, 15‘-d, 14‘-d, and 15,15‘-d2) were recorded in n-hexane solution. The T1 state was generated by the use of a sensitizer, anthracene. An empirical normal-coordinate analysis of the spectral data was performed by using Urey−Bradley−Shimanouchi force field; non-UBS cross terms were also introduced. By the use of each carbon−carbon stretching force constant as a scale of bond order, large changes in bond order upon triplet excitation were identified in the central part of the conjugated chain:  decrease in bond order was in the order, C13C14 > C11C12 > C9C10 > C1515‘, whereas increase in bond order was in the order, C12C13 > C14C15 ≈ C14‘C15‘. In other words, the triplet-excited region where the largest changes in bond order take place was located, not at the center of the entire carbon skeleton, but at the center of the conjugated chain. The S0 and T1 Raman spectra of 15-cis-spheroidene and its deuterio ...