CHLOROPHYLL A AGGREGATES IN HYDROCARBON SOLUTION, A PICOSECOND SPECTROSCOPY AND MOLECULAR MODELING STUDY

Chlorophyll a aggregates in 3-methylpentane solution have been studied by using picosecond absorption and fluorescence spectroscopy and mole cular modeling. Chlorophyll a aggregates give rise to reversible tempe rature changes in the absorption and fluorescence spectra. Time-depend ent anisotropies were used to estimate rotational correlation times of the aggregates. These were indicative of the sizes of the aggregates. The rotational diffusion of the monomer and the two identified aggreg ates was hydrodynamic over the viscosity range studied (0.29-1.8 cP). Molecular mechanics calculations were used to predict the minimum ener gy structures of several chlorophyll a dimers suggested earlier in the literature. These structures were used to estimate exciton splitting ofthe Q(y0-0) transition and compared to the experimentally observed s pectral shifts.