Disk-like liquid crystals of transition metal complexes. Part 20.‡—Pursuit of chemistry to directly visualize van der Waals interactions

Abstract

This work reports on mesomorphic thermochromism caused by metal centres leading to visualization of van der Waals interactions. We succeeded in obtaining columnar liquid crystals exhibiting very interesting thermochromism by introduction of long chains into disk-like bis(glyoximato) d⁸ metal(ii) complexes. These long-chain-substituted Ni^II, Pd^II and Pt^II complexes, (C_nO)₈-M, containing one-dimensional metal chain 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_zconduction band of the central metal. Generally speaking, when mesomorphic materials are heated, the peripheral alkyl chains melt, whereas the central aromatic part remains rigid. Adjacent peripheral long chains tend to lie within the van der Waals radius at lower temperatures, which results in a shortening of the distance between the central d⁸ metals in the present (C_nO)₈-M systems. This ‘fastener effect’ of the van der Waals interaction of peripheral long chains is weakened with increasing temperature, leading to visible color changes due to the nd_z2–(n+1)p_z interaction in the central metal chain. The 4;mmol) and 85% potassium hydroxide (12.9 g, 185 mmol) were added and the mixture was stirred vigorously forca. 1 h and filtered to remove precipitates. 3,3′,4,4′-Tetradecyloxybenzil 5 (3.00 g, 2.83 mmol) was added to the filtrate and under a nitrogen atmosphere the mixture was refluxed with stirring for 12.5 h. To the hot reaction mixture was added potassium tetrachloroplatinate(ii) (0.64 g, 1.54 mmol) dissolved in a small amount of ethane-1,2-diol. Immediately the solution was neutralized with glacial acetic acid as confirmed by pH test paper. After neutralization, reflux was continued for more than 6 h during which dark green precipitates gradually appeared. After reflux, the hot reaction mixture was filtered and the dark green precipitate collected on filter paper. The product was dissolved in chloroform and the solvent evaporated to give dark green liquid crystals. Purification was carried out by column chromatography [silica gel, chloroform–benzene (1:1 v/v), R_f=0.70] and reprecipitation carried out by adding acetone to a hot solution of the product in chloroform to give dark green liquid crystals (0.37 g, yield: 10%).