Exciton interactions in self-organised bacteriochlorophylla - aggregates

Abstract

Exciton interactions of self-organised bacteriochlorophyll a - aggregates in non-polar solution linked via water and dioxane have been studied. Absorption and CD spectra of the aggregates show large red shifts typical of J-aggregates. Femtosecond excitation of the Q_y band of the aggregates is followed by wavelength dependent non-exponential picosecond relaxation and anisotropy decay takes place in subpicosecond time scale. To explain these observations exciton theory and semi-empirical MO/CI calculations, that constitute the basis of the CIEM-approach developed by Linnanto et al. (J. Phys. Chem. B, 1999, 103B, 8739) was used. Structural models of aggregates were created by using the molecular mechanics method. Absorption and CD spectra of the model structures were calculated from excitonic wavefunctions. A stable J-type helical structure of BChl a–water aggregate with a diameter of about 20 nm, in agreement with experiment, was obtained. Calculations for this structure produced the experimental absorption and CD spectra of the BChl a–water aggregate correctly. For BChl a–dioxane aggregates several stable H-type linear structures were calculated and blue shifted absorption and CD spectra with respect to the monomer Q_y transition were predicted. Almost a perfect match of the shape of the calculated CD spectrum with the experimental spectrum suggests that solvent interaction not included in the calculations is mostly responsible for the red shift observed experimentally for the dioxane aggregates. The results are discussed with reference to molecular interactions of BChl's in solution and in light harvesting antenna of photosynthetic bacteria.