Shinsuke Takagi

Energy transfer reaction of cationic porphyrin complexes on the clay surface: effect of sample preparation method

Photochemical energy transfer of non-aggregated cationic porphyrins on an anionic-type clay (Smecton SA) surface was investigated. The efficiency of energy transfer and excited-state quenching in the absence of energy transfer were evaluated at various loading levels of porphyrin on the clay surface and were found to be significantly affected by the loading level. As the latter increased, both energy transfer efficiency and excited-state quenching increased. Judging from the dependency of energy-transfer efficiency on the porphyrin loading level, a partially clustered structure, but without aggregation, of porphyrins on the clay surface is proposed.

Preparation and photochemical behavior of polyfluorinated cationic azobenzene-titanoniobate intercalation compounds

A novel photofunctional material composed of polyfluorinated cationic azobenzene and layered potassium titanoniobate was synthesized and its photochemical behavior was investigated. Although polyfluorinated cationic azobenzene could not be intercalated into the interlayer region of layered potassium titanoniobate directly, the intercalation compound was obtained by guest–guest-exchange with the hexylammonium-TiNbO5 intercalation compound. X-Ray diffraction, TGA, IR, TEM, UV-visible spectroscopy and elemental analysis results indicated that the polyfluorinated cationic azobenzene was intercalated into the interlayer spaces of the potassium titanoniobate. The AFM image for the intercalated compound with a basal spacing of 3.90 nm was consistent with the X-ray diffraction data. The spectral properties as well as X-ray diffraction results have revealed that the adsorbed polyfluorinated cationic azobenzene molecules form J-like aggregates with bi-layers in the interlayer space of the potassium titanoniobate. The intercalation compound exhibited an excellent reversible cis–transphotoisomerization by successive illumination with UV light at 365 nm and visible light at 458 nm. The basal spacing changed reversibly upon photoisomerization of the intercalated polyfluorinated cationic azobenzene.

Microscopic structures of adsorbed cationic porphyrins on clay surfaces: molecular alignment in artificial light-harvesting systems

The intercalation behavior of cationic porphyrin derivatives within the interlayer spaces of nano-layered clay minerals has been investigated. The porphyrins were successfully intercalated by the newly adopted method of repeated freeze-thaw cycles. The absorption spectra of the porphyrins were compared in the solution phase, adsorbed onto the exfoliated clay nano-sheets, intercalated within the interlayer spaces of clay sheets dispersed in water and intercalated in dry films. Substantial red shifts of the λmax values in the absorption spectra of the porphyrins were observed on the exfoliated clay sheets, and further red shifts were induced within the interlayer space. The dry films of the intercalated samples exhibited the largest red shifts. X-ray diffraction studies revealed that the clearance space between the layers in these intercalated hybrid compounds is only large enough for the porphyrins to be rigidly packed parallel to the clay layer. For the exfoliated clay nano-sheets, theoretical calculations were carried out on the correlation between the dihedral angle of the meso-substituted pyridiniumyl plane vs. the porphyrin ring and the λmax of the porphyrin Soret band. An extrapolation of the experimental λmax value to the correlation curve, afforded the dihedral angle to be 61.6°. The microscopic structure of the adsorbed state of the cationic porphyrins on the exfoliated clay nano-sheets was, thus, proposed to involve an orientation parallel to the clay surface, with a distance of 0.15 nm from the surface, which implies the expulsion of the solvent water molecules.