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The syntheses of eight new star-shaped D(3)-symmetric arrays in which three 15-(pyrid-4-yl)porphyrin subunits are attached to the 1, 3, and 5 positions of a benzene core through linkers consisting of collinear repetitive phenylethynyl units have been carried out using Pd(0)-catalyzed coupling reactions. By the same procedure, an analogous 10-(4-pyridin-yl)porphyrin hexamer in which all positions of the benzene core are substituted has been obtained. Likewise, the preparation of suitably sized cyclic porphyrin hexamers, in which all six or at least three alternate porphyrin rings are complexed with Zn(II) ions, is described in detail. In solution, such cyclic porphyrin hexamers form supramolecular assemblies with the star-shaped polyporphyrins in which the latter are held in the interior of the macrocycle through coordination of the apical pyridine rings with the Zn(II) ions. The suggested structures are supported by (1)H NMR spectroscopic and MALDI-TOF mass spectrometric measurements. They agree with the high values of the binding constants of the corresponding supramolecules, which range between K = 1.1 x 10(10) and 1.4 x10(9) M(-1).

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Ag2S and Ag2Se undergo polymorphic, first-order transitions at about 133°C and 179°C respectively. Above the transition temperatures both compounds are cubic body centred and characterized by a strongly disturbed sublattice of Ag+ ions in a quasi molten state. This disorder is of course associated with a relatively large ionic conductivity. However, electronic conduction is always predominant and this crystallographic transition is associated with a striking change from semiconductor to metallic character. The electrical and optical properties of the α and β phases have been discussed by many authors, however a fully acceptable explanation is still lacking. In order to clarify the situation, we have reinvestigated the magnetic, electical and optical properties of both, the α and β phases of silver chalcogenides. A discussion of these results based essentially on the theory of non crystalline materials (Mott and Davis, 1971) is presented. Two possible models will be discussed. According to the first model, both the valence and conduction band get tails, due to the fluctuation- and random positions of the silver ions, and overlap. The valence band gets more tail than the conduction band, so that the Fermienergy lies to the right of the minimum in the conduction band. In the second model, we postulate an impurity band due to non-stoichiometric excess silver, the energy levels of such donors being spread in a band by the random field of the disorded silver ions. The second model seems more reliable. A comparison of the measured electrical conductivities with the minimum metallic conductivity introduced by Mott (1970) demonstrates that Anderson localization (Anderson, 1958) does not occur in these systems. A detailed publication will appear in Philosophical Magazine