Changes in Carbon−Carbon and Carbon−Nitrogen Stretching Force Constants in the Macrocycles of Bacteriochlorophyll a and Bacteriopheophytin a upon Triplet and Singlet Excitation: Resonance-Raman Spectroscopy and Normal-Coordinate Analysis of the Unlabeled and Totally 15N-, 13C-, and 2H-Labeled Species

Abstract

The Raman spectra of bacteriochlorophyll a and bacteriopheophytin a in the S0, T1, and S1 states were recorded for the unlabeled species and the totally-labeled species with the 15N, 13C, and 2H isotopes, and an empirical normal-coordinate analysis of the Raman data was performed to establish the assignments of Raman lines in the 1650−1200 cm-1 region and to determine the stretching force constants of the carbon−carbon and the carbon−nitrogen bonds within the macrocycle. By the use of the stretching force constants as a scale, changes in bond order upon triplet and singlet excitation were characterized in both BChl a and BPhe a as follows. (a) For BChl a, upon triplet excitation, the carbon−carbon bonds in the methine bridge (Ca−Cm) and those in the pyrrole rings I and III (Ca−Cb and Cb−Cb) decrease in bond order, whereas the carbon−nitrogen bonds in all the pyrrole rings (Ca−N) and the carbon−carbon bonds in rings II and IV (Ca‘−Cb‘) increase in bond order. Upon singlet excitation, the Cb−Cb bonds substa...