Experimental determination of the orientation of excited state transition dipoles in tetrapyrroles with different molecular symmetries

Abstract

So far, the orientations of transition dipole moments in tetrapyrroles have been investigated mainly by fluorescence anisotropy spectroscopy that allows the determination of the orientations of S 0–S n absorption transition dipoles with respect to a fixed emission transition dipole. However, little is known about the orientation of excited state absorption (ESA) transition dipole moments. Transient absorption spectroscopy (TAS) with polarized probing permits access to these transitions. We present photoinduced fluorescence and TAS anisotropy data of three tetrapyrroles of different molecular symmetries. The orientation of the S 0–S n and S 1–S n transition dipoles were extracted from these data. A distinctive influence of the molecular symmetry was found for tetra( t-butyl)-phthalocyanine and its zinc complex due to a doubly degenerate S 1 state in the latter. For a reduced porphyrin, pheophorbide-a, the higher asymmetry of the molecule influences the orientation of the electronic transition dipole moment only slightly.