Effect of central metal substitution on linear dichroism of porphyrins: evidence of out-of-plane transition moments

Absorption anisotropy and emission anisotropy measurements in poly(vinyl) alcohol (PVA) films of different porphyrin derivatives are reported, Wavelength dependent absorption anisotropy in oriented PVA films, and wavelength dependent excitation spectrum of emission anisotropy of fluorescent porphyrin derivatives in isotropic PVA films indicate the presence of multiple transition moments with different well-defined orientation. Comparison of linear dichroism and orientation behavior in stretched PVA films of deuteroporphyrin III (C-2V symmetry) and its iron derivative reveals significant out-of-plane transition moment components. A considerable participation of out-of-plane polarized absorption components is also observed for metal derivatives of non-symmetrical protoporphyrin IX. It appears that central metal substitutions in porphyrin rings do not produce 'circular' degeneration of electronic transition moments. Instead, the presence of metal induces absorption components orthogonal to the porphyrin plane. (C) 1997 Elsevier Science B.V.