Photoacoustic Spectra and Flurescence Lifetimes of Chlorophyll a and Chlorophyll b in Nematic Liquid Crystal

With an aim to evaluate the yield of excitation energy transfer between ordered chlorophyll a and chlorophyll b molecules, the nematic liquid crystal (LC) mixture MBBA (p-methoxybenzylidene ṕ-butylaniline) and EBBA (p-ethoxybenzylidene ṕ-butylaniline) is used. The LC is oriented by deposition of the sample between stretched polyvinylalcohol films or between silicon oxide orienting layers; chlorophyll molecules are oriented similarly due to their strong interaction with LC molecules. Chlorophylls (chls) in LC are monomeric at concentrations lower than 10 −2 M. The pigment fluorescence yields (ν) were established from measurements of fluorescence lifetimes (τ) of chl a and chl b in LC and in ethyl ether. ν of chl a is about twice that of chl b; therefore the yield of excitation energy transfer between pigments was obtained from photoacoustic spectra (PAS). In PAS measurements, natural and polarized light were used for sample illumination, in order to study the orientation of different species. Photoacoustic spectra and analysis of τ of pigment mixtures suggest that part of the energy absorbed in the Soret band of chl a is transferred to chl b. In fluorescence spectra, a new peak at 695 nm is observed, probably related to mixed chl a-chl b-LC "aggregates" which are differently oriented than the monomeric pigment. The energy absorbed by these aggregates is dissipated more efficiently than that absorbed by chl a alone. The formation of some mixed aggregates can explain why the τ and PAS values of pigment mixtures are different than the values calculated by assuming that chl a and chl b are contributing to fluorescence and deactivation effects independently of each other.