A UNIQUE "FLATTENING EFFECT" OF CLAY ON THE PHOTOCHEMICAL PROPERTIES OF METALLOPORPHYRINS(2010 TMC Special Paper)

Abstract

Clay-porphyrin complex formation and spectral behaviour of metalloporphyrins containing di-, tri- and tetravalent central metals with synthetic clay were examined. The spectral shift of porphyrin Soret band to higher wavelengths was observed upon the complex formation, where the clay sheets were either exfoliated or stacked in aqueous colloidal suspension. Moreover, the degree of the spectral shift decreased as the valence of porphyrin central metal increased. Since tri- and tetravalent metals have one and two axial ligands, respectively, their steric effect on porphyrin ring plane affected the extent of the "flattening" (i.e. co-planarization of porphyrin ring and peripheral aromatic rings) on the clay surface, thus influencing the spectral shift in porphyrin absorption spectra. These observations support the proposed hypothesis that "flattening" of porphyrin molecules on the clay surface induces the spectral shift of porphyrin Soret band. With this "flattening effect" of the clay surface, the λ_<max> value of Mg(II)TMPyP porphyrin reached up to 500nm in the stacked clay complex. These results indicate that the utilizing a combination of porphyrin with clay turned out to be unique to control the photochemical properties of porphyrins.