Exciton interactions and femtosecond relaxation in chlorophyll a water and chlorophyll a dioxane aggregates

Chlorophyll a (Chl a) in hydrocarbon solution with a small amount of dioxane or water shows red-shifted absorption bands at 686 nm and at 700 nm (dioxane) and at 745 nm (water), indicative of self-organized aggregate structures in solution. To study the relationship between the structure and spectral properties of the aggregates, several one-dimensional model structures of Chi a-dioxane and Chi a-water aggregates were computed by the molecular mechanics method. Three overall structures ranging from stick to a ring shape were energetically favored for the dioxane system. All these structures contain structural heterogeneity that consists of repeating dimers that further form tetramer substructures. For the Chi a-water system a one-dimensional homogenous helical structure was obtained. By using the model structures, the transition energies and fluorescence excitation polarizations were computed. Exciton theory with dipole-dipole interaction approximation and semiempirical quantum mechanical CI calculations were used. Excitonic splittings of the aggregate transition energies were calculated by diagonalizing 10 x 10 interaction matrixes. For the Chi a-dioxane dimers, trimers, and tetramers the exciton theory with dipole-dipole interaction approximation produced blue-shifted exciton transitions of the computed structures, while the semiempirical calculation gave red-shifted transitions for all these species, the tetramer shifts being closest to the experimental shifts. The quantum chemical calculations of the two tetramers appearing in the computed one-dimensional model structure predict the quartet structure of the absorption spectrum. The calculations also produce fluorescence excitation polarizations that are very similar to the values observed at low temperatures for the Chi a-dioxane aggregate. In the case of the Chi a-water aggregate both dipole approximation and semiempirical exciton shifts were only one-half of the observed spectral shift. It is suggested that the remainder is due to the environmental effects not included in the calculations, like two-dimensional chromophore-chromophore interactions and solvent effects in the Chi a-water aggregate. Calculations on the Chi a-water and Chf a-dioxane model aggregates demonstrate that at close chromophore-chromophore distances the dipole-dipole approximation and the semiempirical calculation give very different results. The results from the model calculations are compared with available spectral data of each aggregate together with new femtosecond results for the Chi a-water aggregate. In one-color absorption recovery experiments the Chi a-water aggregate shows an obvious wavelength dependence of electric relaxation. The decay has a strong femtosecond component (about 300 fs) in the blue side of the Q(y) band that is not present in the red side. No rise time could be observed when the Q(y) band was pumped in the blue side and probed in the red side with 350 fs time resolution. Our results suggest that thermalization of the excitation energy of the Chi a-water aggregate takes place in less than 350 fs.