Bacteriochlorophyll <i>a</i> Franck−Condon Factors for the S₀ → S₁(Q<i>_y</i>) Transition

Abstract

Pseudovibronic satellite hole-burning spectroscopy of bacteriochlorophyll a (BChl a ) in two glasses at 5 K was used to determine the Franck−Condon (FC) factors for 56 one-quantum (0 → 1) vibrational transitions that lie between 160 and 1600 cm -1 . As in the case of Chl a (Pieper, J.; et al. J. Phys. Chem. B 1999, 103, 2319), the FC factors are small, ranging between 0.05 and 0.0007 (uncertainty ≈ ±20%). The FC factors, together with the experimentally determined inhomogeneous site excitation distribution function for the zero-phonon line and linear electron−phonon coupling parameters, account well for the S 0 → Q y absorption spectra. Thus, the FC factors are accurate enough to be used in quantum mechanical calculations of excitation energy transfer rates in photosynthetic antenna complexes (with their intrinsic structural heterogeneity) exhibiting excitonic coupling that ranges between weak and strong. The BChl a FC factors determined by hole burning are compared with those obtained by fluorescence line narrowing spectroscopy (Wendling, M.; et al. J. Phys. Chem. B 2000, 104, 5825). The latter, which are about a factor of 5 smaller than those determined by hole burning, are too small to account for the vibronic contribution to the S 0 → Q y absorption spectrum. The discrepancy between the two sets of FC factors is discussed.