Hydrated chlorophyll a oligomers in solutions, monolayers, and thin films

The interfacial tension measurements of dry and wet chlorophyll a at an octane-water interface show differences in adsorption behavior of the pigment depending on its aggregation state. Optical properties were also investigated using absorption and Fourier transform-infrared spectroscopy, along with fluorescence (excitation and emission) measurements in non-polar solvents and in thin films. The absorption and fluorescence spectral characteristics, and fluorescence lifetime measurements of dry and wet samples clearly indicate the difference in aggregation properties of this photosynthetic pigment. A strong decrease in the energy of Q_y electronic transitions (Δν = 1700cm^-1), due to the formation of hydrated oligomers, was accompanied by a decrease in fluorescence lifetime from 5.75 ± 0.02 ns (diluted dry chlorophyll a in n-hexane) to 0.12 ± 0.03 ns (wet chlorophyll a in thin films). Discussion of these results is based on the key role of the central magnesium atom in the porphyrin ring in intermolecular interactions and self-organization of chlorophyll a molecules.