M. Van Dijk

Self-assembling properties of non-ionic tetraphenylporphyrins and discotic phthalocyanines carrying oligo(ethylene oxide) alkyl or alkoxy units

The thermotropic phase behaviour and self-assembling features of some non-ionic tetraphenylporphyrins and phthalocyanines containing oligo(ethylene oxide) alkoxy or alkyl units have been investigated. From DSC measurements and polarization microscopy it was concluded that none of the tetraphenylporphyrins was mesomorphic while the phthalocyanines displayed discotic hexagonal phases even at room temperature. The aggregation of the compounds in aqueous media was studied by means of UV–VIS and fluorescence spectroscopy and it has been found that in water the tetraphenylporphyrins form J- or head-to-tail type of aggregates while phthalocyanines form H- or face-to-face type of aggregates. The luminescence properties of the tetraphenylporphyrin and phthalocyanine aggregates are explained on the basis of the molecular exciton approximation. Steric constraints and orientational disorder in the tetraphenylporphyrin aggregates determine the luminescence yield relative to the monomeric species. The cofacial arrangement of the macrocycles in phthalocyanine aggregates results in a forbidden S 1 –S 0 transition and thus in a complete disappearance of the luminescence.

Electrical and catalytic properties of some oxides with the fluorite or pyrochlore structure: Part 1: Synthesis, characterization and conductivity

In view of their possible use as electrocatalysts, several oxides with the fluorite or pyrochlore structure were investigated. Part I of this paper deals with the synthesis, characterization and electrical conductivity of these materials. All lanthanide titanates investigated (partly doped with Bi) show the pyrochlore structure and are poor conductors, except for the compounds containing Tb. For Nd-zirconates, partly substituted with Ce, the degree of order depends on the amount of Ce and temperature treatment. Ionic and in some cases mixed conductivity was observed. Part II reports about the catalytic activity for CO oxidation.

Thermotropic phase transitions in 5,15-bis(4-alkoxyphenyl) octaalkylporphyrins.

Nineteen novel alkyl substituted porphyrins have been synthesized and their thermal phase behaviour has been investigated in detail. Twelve compounds showed a reversible phase transition below the isotropization temperature. From time resolved microwave conductivity (TRMC) measurements and powder X-ray diffractometry it was concluded that the molecular packing does not change significantly at the lower phase transition temperature and that the porphyrin cores occupy isolated positions. Single X-ray diffraction measurements showed that the porphyrins are arranged in a layered structure and that the space between the layers is occupied by the alkyl substituents of the pyrrole units. The phase transitions at the lower temperature were therefore identified as changes in the crystal ordering of the porphyrins.