DISK-LIKE LIQUID-CRYSTALS OF TRANSITION-METAL COMPLEXES - PART 20 - PURSUIT OF CHEMISTRY TO DIRECTLY VISUALIZE VAN-DER-WAALS INTERACTIONS

This work reports on mesomorphic thermochromism caused by metal centre s leading to visualization of van der Waals interactions. We succeeded in obtaining columnar liquid crystals exhibiting very interesting the rmochromism by introduction of long chains into disk-like bis(glyoxima to) d(8) metal(II) complexes. These long-chain-substituted Ni-II, Pd-I I and Pt-II complexes, (CnO)(8)-M, containing one-dimensional metal ch ain change color according to the metal-metal stacking distance. With increasing temperature, the metal-metal distance increases leading to an enlargement of the band gap between the filled nd(z2) Valence band and the empty (n + 1)p(z) conduction band of the central metal. Genera lly speaking, when mesomorphic materials are heated, the peripheral al kyl chains melt, whereas the central aromatic part remains rigid. Adja cent peripheral long chains tend to lie within the van der Waals radiu s at lower temperatures, which results in a shortening of the distance between the central d(8) metals in the present (CnO)(8)-M systems. Th is 'fastener effect' of the van der Waals interaction of peripheral lo ng chains is weakened with increasing temperature, leading to visible color changes due to the nd(z2)(n + l)p(z) interaction in the central metal chain. The 'apparent pressures' of the long chains in the (CnO)( 8)-M complexes have been estimated from the shift coefficient of the d -p band vs. pressure for the corresponding non-substituted metal compl exes.