Visible absorption, fluorescence and thermal equilibration of electronic excitation energy in langmuir-blodgett multilayers of chlorophyll a AT 85 AND 300 K

Absorption and fluorescence spectra of chlorophyll a in Langmuir-Blodgett films were found to depend on the water content in these samples. Fluorescence maxima of multilayers in water-saturated nitrogen gas are at 740 nm at room temperature and shift to 760 nm on cooling to 85 K, while only a narrowing of the components in absorption spectra, at 678 and 710 nm. is observed at 85 K. The spectra of dried multilayers exhibit maxima at 678 nm in absorption and at 698 nm in fluorescence. Lowering the temperature leads to the decrease of 698 nm component and to the appearance of a long-wavelength component which position depends on temperature. The linearity of plots of the Kennard-Stepanov relation between absorption and fluorescence spectra and calculated effective temperatures reveal full thermal equilibration of electronic excitation energy in these multicomponent systems at 300 K. At 85 K, the Kennard-Stepanov relation is only approximately fulfilled, possibly due to sample imperfections. The thermal equilibration in a system of electronic excited levels is discussed with reference to temperature-induced fluorescence changes in photosynthetic systems.