Spectroscopic Properties of Mg−Chlorin, Mg−Bacteriochlorin, and Bacteriochlorophylls a, b, c, d, e, f, g, and h Studied by Semiempirical and Ab Initio MO/CI Methods

Abstract

The semiempirical PM3 method has been used to calculate fully optimized structures of bacteriochlorophylls a, b, c, d, e, f, g, and h, magnesium−chlorin, and magnesium−bacteriochlorin. Several configuration interaction (CI) methods, the PM3 (5,5) CIS and CISD, ZINDO/S CIS (n,n) with 2 < n <30, and ab initio CIS (5,5)/6-31G* methods, were tested for their predictive power in estimation of spectroscopic properties of bacteriochlorophylls. The ZINDO/S CIS (15,15) method turned out the best results for overall simulation of absorption spectra. Both the transition energies and relative intensities of the Qy, Qx, and Soret bands were correctly predicted. The effect of solvent coordination on the transition energies and oscillator strengths was also studied. The calculations predict solvent-induced spectroscopic shifts of the Qx and Soret bands, whereas the Qy transitions remain practically unshifted, in accord with experimental findings. The shifts are due to solvent-induced energy level shifts and charge redistribution of the magnesium atom in the excited states of bacteriochlorophylls. The results are discussed with reference to spectroscopic properties of bacteriochlorophylls in solution, in aggregates, and in photosynthetic light harvesting antenna.