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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 Chl a−dioxane and Chl 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 Chl 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 × 10 interaction matrixes. For the Chl 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 Chl a−dioxane aggregate. In the case of the Chl 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 Chl a−water aggregate. Calculations on the Chl a−water and Chl 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 Chl a−water aggregate. In one-color absorption recovery experiments the Chl 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 Chl a−water aggr...

We present a femtosecond transient absorption study of chlorophyll b monomers in various solvent environments. Transient absorption spectra and kinetics of chlorophyll b were measured in neat pyridine and acetone. The kinetics were measured also for three samples of chlorophyll b dissolved in 3-methylpentane and titrated with pyridine. Characteristic for all chlorophyll b samples is a very strong excited-state absorption, which significantly affects time-resolved anisotropy values in the wavelength region of our study (600−750 nm). The excited-state absorption spectrum was simulated, and on the basis of time-resolved anisotropy measurements the broad and smooth excited-state absorption band was found to be composed of several transitions to higher excited states. For all of the chlorophyll b samples studied, we observed two fast lifetime components (∼100 fs and 1−3 ps). The former component is assigned to transient hole burning of the inhomogeneously broadened ground-state absorption spectrum, while the latter component, which is sensitive to properties of the solvent, is suggested to arise mainly from solvent relaxation.

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A circular (CD) and linear dichroism (LD) study of the water adducts of the green plant chlorophylls a (Chl a) and b (Chl b) in hydrocarbon solvents 3-methylpentane and paraffin oil is presented. A strong red shift of the Q,-absorption band from 663 to 746 nm (1678 cm-I) is observed as the water adduct of Chl a is formed. The Chl a-water adduct shows a strong, nonconservative CD signal, which is characterized by a positive peak at 748 nm and two negative peaks at 720 and 771 nm. The maximum CD (AL -AR) is only one order of magnitude smaller than the isotropic absorption maximum. We propose that this exceptionally strong signal is the so-called psi-type CD. The LD spectrum was measured in a flow of paraffin oil. The isotropic absorption maximum peaks at 742 nm in paraffin oil, whereas the maximum of the LD signal is at 743 nm. The LD signal is positive over the whole water-adduct absorption band indicating that the transition dipole of the 742 M I transition is preferentially oriented along the long axis of the aggregate. The structure of the Chl b-water adduct is less well defined. The preparations of the Chl b-water adduct are unstable. The Chl b-water adduct absorption band maximum is at 683 nm. The CD signal of the Chl a-water adduct is about 200-fold the CD of the Chl b-water adduct. We could not orient the Chl b-water adducts by flow, which suggests that the adducts are small or disordered.