THE ORIENTATION OF THE TRANSITION DIPOLE MOMENTS OF CHLOROPHYLL a and PHEOPHYTIN a IN THEIR MOLECULAR FRAME

Abstract

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.