Polarization of electronic transitions in the disodium complex of tetraphenylbacteriochlorin and its monoanion studied by means of photochemically induced dichroism

Abstract

Information about the polarization of electronic transitions in molecules and molecular ions can be obtained from studying fluorescence polarization or from studies of dichroism induced photochemically or in any other way. However, the former method is not suitable for studying the monoanions of tetrapyrrole pigments because they exhibit only weak fluorescence and it appears in a region which is not easily accessible (800-1100 nm). Because of this, we have used the method of photochemically induced dichroism to study the disodium complex of tetraphenylbacte riochlorin (Na2-TPBC) with crossed arrangement of the hydrogenated pyrrole rings. This method makes it possible to determine not only the mutual arrangement of the molecular oscillators of the pigment anion but also the arrangement of the ionic oscillators with respect to the oscillators of the neutral molecule. The 7r -electron system of the Nae-TPB C molecule has the symmetry of the point group D2h [1]. In systems of this symmetry there are only three mutually perpendicular directions (second-order symmetry axes) along which the allowed electronic transitions can be polarized (including out-of-plane transitions). This considerably facilitates the interpretation of experimental data. Photochemical Dichroism and Polarization of Electronic Transitions The existence of dichroism in isotropic media caused by photochemical reactions occurring in these media was discovered by Weigert [2]. Later on, a similar phenomenon observed in solid frozen solutions of organic dyes [3] was suggested as a technique to obtain information on the nature of electronic transitions in molecules. This method has been successfully used to study negative ions derived from aromatic hydrocarbons [4]. The method is based on the following effects. When irradiating solutions of molecular negative ions