INTERACTIONS BETWEEN CHLOROPHYLL-A AND BETA-CAROTENE IN NEMATIC LIQUID-CRYSTALS

Fluorescence lifetime in the ps range, polarized absorption, polarized fluorescence spectra and delayed luminescence time resolved spectra w ere measured for chlorophyll a solutions with and without beta-caroten e addition in nematic liquid crystal. Photoacoustic spectra of the sam e samples at various frequencies of light modulation were also taken. The frequency dependence of the photoacoustic spectra suggests that pa rt of the excitation is converted into heat in a slow process (with a decay time of the order of ms). The lifetime results suggest that at u sed concentration some aggregation of the chlorophyll a occurs. The ch lorophyll a molecules interact strongly with the beta-carotene forming some nonfluorescent or weakly fluorescent aggregates characterized by having various thermal deactivation yields and orientations in anisot ropic matrix when compared to those of separated pigments. It seems th at the aggregated forms of the chlorophylls are partially disrupted as a result of the their interaction with the beta-carotene. Singlet exc itation of beta-carotene is not transferred to the fluorescent form of chlorophyll a. Delayed (in mu s time range) luminescence of chlorophy ll a is quenched by beta-carotene. This luminescence is located in the same spectral region as prompt fluorescence. Interactions between chl orophyll a and beta-carotene depend on the degree of pigment orientati on and their aggregation.