Exciton Interactions and Femtosecond Relaxation in Chlorophyll a−Water and Chlorophyll a−Dioxane Aggregates

Abstract

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...

Exciton Hamiltonian for the Bacteriochlorophyll System in the LH2 Antenna Complex of Purple Bacteria

“…17,32,38 The semiemperical INDO/S/CIS approach tends to overestimate transition dipole moments in porphyrin-like systems as well. 66 This is probably the reason for the larger computed couplings 40,42,43 compared to other calculations. The CEO computed Q y transition dipole moment of Bchl monomer is 1.44 e‚Å, which compares well with the experimental value of 1.33 e‚Å.…”

“…More accurate computational techniques that can in principle take into account the Coulomb interaction between the actual charge distributions have been employed. 16,32,39,40 The point-monopole approach that computes interaction between transition monopoles distributed on the atomic centers was used at the CIS level using the simplified PPP Hamiltonian. 16 A different study evaluated the interaction between transition densities of each chromophore, which are calculated at the ab initio level with configuration interaction singles (CIS).…”

Deactivation of Excited States in Nanostructures Containing Cu-Porphyrin Subunit

“…[85][86][87][88] The size and the morphology of these aggregates and their spectroscopic properties (polarized fluorescence and high optical activity) depend strongly on the linker molecule, concentration, ratio of the solvent mixture and temperature. [87][88][89] In binary mixture of solvents we realized conditions where the formation of mixed aggregates of different molecules but having the general structural elements, was thermodynamically more preferable. [85,87] The dioxane as a linker molecule keeps, according to calculations, the interplane distance between the adjacent pigment molecules at about 7 Å.…”

“…B: Computed minimum energy structures of (Chl a-dioxane) 24 aggregates (stick, arc, and ring). Optimized structures were minimized by using a molecular mechanics method with the CHARMm force field and Newton Raphson optimization (adopted from [89] ). C: absorption (1), fluorescence (2), circular dichroism (3) and fluorescence polarization spectra (4, l det =693 nm) of chlorophyll a aggregate.…”

“…[88] Additionally, the existence of the delocalized excitation is indicated also by the short lifetime observed for fluorescence emission of Chl a-dioxane aggregates (t~40 -90 ps), and the absence of the temperature effect on the fluorescence lifetimes indicates the excitation delocalization length to be not large enough. [88] According to experimental results (analysis of the splitting in CD spectra of [87,88] ) and quantum chemical calculations [89] excitonic (dipole-dipole) coupling in the dimeric cell of the PChl a-dioxane and Chl a-dioxane aggregates is estimated to be V 12~4 0 -70 cm -1…”