Interactions between chlorophyll a and β-carotene in nematic liquid crystals

Abstract

Fluorescence lifetime in the ps range, polarized absorption, polarized fluorescence spectra and delayed luminescence time resolved spectra were measured for chlorophyll a solutions with and without β-carotene addition in nematic liquid crystal. Photoacoustic spectra of the same samples at various frequencies of light modulation were also taken. The frequency dependence of the photoacoustic spectra suggests that part 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 used concentration some aggregation of the chlorophyll a occurs. The chlorophyll a molecules interact strongly with the β-carotene forming some nonfluorescent or weakly fluorescent aggregates characterized by having various thermal deactivation yields and orientations in anisotropic matrix when compared to those of separated pigments. It seems that the aggregated forms of the chlorophylls are partially disrupted as a result of the their interaction with the β-carotene. Singlet excitation of β-carotene is not transferred to the fluorescent form of chlorophyll a. Delayed (in μs time range) luminescence of chlorophyll a is quenched by β-carotene. This luminescence is located in the same spectral region as prompt fluorescence. Interactions between chlorophyll a and β-carotene depend on the degree of pigment orientation and their aggregation.