0000000000349082
AUTHOR
Jari A. I. Oksanen
Investigations of Chl a aggregates cross-linked by dioxane in 3-methylpentane
In this work, dioxane-bound aggregates of chlorophyll a are prepared in 3-methylpentane. The properties of the aggregates are studied by using steady-state and time-resolved spectroscopies. The Q -region absorption spectrum of the y chlorophyll a-dioxane aggregate shows four clearly resolvable narrow bands with comparable intensities. The band maxima are located at 683, 689, 698 and 702 nm. The emission spectrum consists of two emission bands centred at 699 and 702 nm suggesting the presence of two types of aggregates. High degree of fluorescence polarization is detected yielding the angles between the absorption transition moments with respect to the 702 nm emission transition moment. The …
Characterisation of Chlorophyll a and Chlorophyll b Monomers in Various Solvent Environments with Ultrafast Spectroscopy
In photosynthesis the energy from the sun is captured by light harvesting chlorophyll pigments and converted to stable chemical energy, by the photochemical reaction center. Photosynthetic energy transfer in the antenna systems of green plants has previously been studied by ultrafast time resolved spectroscopy. The characteristics of the chlorophyll pigments itself is important to study in order to understand the dynamics on a femtosecond timescale. One way to study the energy transfer is to use transient absorption spectroscopy and follow the increase or decrease in the transient absorption signal with time (1). Another way to study the energy transfer is to monitor the change in dichroism…
Dynamics of ground and excited state chlorophylla molecules in pyridine solution probed by femtosecond transient absorption spectroscopy
Abstract Femtosecond pump–probe spectroscopy was used to investigate the ground and excited state dynamics of chlorophyll a (Chl a ) in pyridine following excitation by a 100 fs optical pulse. The transient absorption spectrum and kinetics reveal spectral evolution on two ultrafast time scales: ∼100 fs and ∼3 ps. We attribute these dynamics to ground-state transient hole-burning and solvation dynamics. Transient absorption anisotropy at early times (∼500 fs) was measured for Chl a in pyridine and shows a pronounced wavelength dependence, where anisotropy varies between 0 and 0.5. Strong contribution from excited state absorption is the origin of the variation.
Exciton interactions in self-organised bacteriochlorophyll a - aggregates
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 Qy 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 molecul…
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 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 substru…