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Abstract— In this paper we describe the determination of the orientation of the absorption and emission transition dipoles of chlorophyll a and pheophytin a in their molecular frame. For this purpose we have embedded the pigments in anhydrous nitrocellulose films with a concentration of 2 × 10‐7 mol/g. We have shown previously that under these conditions the pigments are in a purely monomeric state, are distributed uniformly both before and after stretching and that no intermolecular energy transfer among the molecules takes place. Using a combination of steady‐state anisotropy experiments on unstretched films and angle‐resolved fluorescence depolarization measurements on stretched films, we obtain the orientation of the transition dipole moments of both pigments in their molecular frame and the orientational distribution function of the molecules relative to the stretching direction of the film. The steady‐state anisotropy measurements indicate that chlorophyll a has two distinct emission dipole moments and that excitation in the Soret‐region results in simultaneous excitation of two or more absorption transition dipole moments. On the other hand, excitation in the QY‐band involves only a single dipole moment. The directions of the transition dipole moments in the molecular frame are obtained from the angle‐resolved measurements. Pheophytin a also exhibits two emission dipole moments, but the angle between them is much smaller than that between the corresponding dipoles for chlorophyll a. As a consequence the dipole moments contributing to the Soret‐region could not be resolved and only an effective absorption transition dipole moment in the Soret‐region is extracted.

The orientational ordering of beta-carotene and crocetin embedded in lamellar model membranes has been investigated by angle-resolved resonance Raman scattering at a temperature well above the phase transition of the lipid chains. It is shown that the ordering of the carotenoids is dependent on the chemical composition of the lipid bilayers. The orientational distribution functions found clearly show that beta-carotene is oriented parallel to the bilayer plane (dioleoyl lecithin) or perpendicular to it (soybean lecithin). For dimyristoyl lecithin at 40 degrees C, egg-lecithin, and digalactosyl diacylglycerol two maxima were found in the orientational distribution: one parallel and one perpendicular to the bilayer surface. Crocetin embedded in soybean lecithin bilayers yields a similar bimodal distribution function. Because of rapid photodegradation no results could be obtained for spirilloxanthin.

Abstract— Model systems for the study of energy transfer processes are useful for the elucidation of the various factors governing the mechanism of energy transfer in photosynthetic systems. Here we describe the characterization of two systems, consisting of chlorophyll a incorporated in anhydrous nitrocellulose and polyvinylalcohol films. First, optical spectroscopy and time‐resolved fluorescence techniques are used to characterize the state of the chlorophyll molecules in the films. We find that in nitrocellulose films the state of chlorophyll a depends strongly on the ratio of nitrocellulose to dimeth‐ylsulfoxide in the solutions from which the films are cast. The state of chlorophyll a in polyvinylalcohol films does not depend on the amount of polymer originally dissolved in dimethylsulfoxide. In these films the pigment is monomeric at low concentrations of chlorophyll a, but aggregates are formed at much lower concentrations than in nitrocellulose. The latter fact is explained by the existence of pockets in polyvinylalcohol, leading to high local concentrations. To further test the suitability of the nitrocellulose polymer films as model systems for energy transfer processes, time‐resolved fluorescence anisotropy profiles are measured in dependence of the concentration of pigments in the matrix. Fits of the observed decay profiles to the predicted decay show good correspondence, as long as no traps are present. Furthermore, the fitted decay times yield the correct value of the Forster radius R0 as compared to the value obtained spectroscopically. We thus conclude that the chlorophyll a‐nitrocellulose system can be very appropriate for the study of energy transfer processes between photosynthetic pigment, since the pigments are uniformally distributed in the matrix.