Discotic liquid crystals of transition metal complexes. Part 26: Supramolecular structures of long-chain-substituted octaphenyltetrapyrazinoporphyrazine derivatives

Abstract

Ten novel columnar liquid crystals, [octakis(4-alkoxyphenyl)tetrapyrazinoporphyrazinato]metal(II) (abbreviated as (C_nO)₈–M; n = 10, 12; M = Cu, Ni) and [octakis(3,4-dialkoxyphenyl)tetrapyrazinoporphyrazinato]metal(II) (abbreviated as (C_nO)₁₆–M; n = 8, 10, 12; M = Cu, Ni), have been synthesized and characterized. It was found that the mesophase structures of (C_nO)₈–M are very sensitive to the central metal and closely related to the aggregate structures in the solution. The (C_nO)₁₆–M derivatives exhibit a D_hd mesophase at lower temperatures and a D_rd (C2/m) phase at higher temperatures. Thus, the mesophase with higher symmetry appears at lower temperatures for these (C_nO)₁₆–M derivatives. This is quite opposite to the general tendency for the higher symmetry mesophase to appear at higher temperatures. In order to further clarify the structures of both the mesophases and the aggregate in solutions, the electronic and magnetic circular dichromism (MCD) spectra were measured. The Q band of (C_nO)₁₆–M in n-hexane showed a wide Davidov splitting. It was proven that such a wide splitting of the Q band can be attributed to the formation of dimers. The dimerization was confirmed by vapor pressure osmometric (VPO) measurements in n-hexane solution. Furthermore, the spectrum of the thin film in the mesophase in the absence of solvent at room temperature was similar to that of then-hexane solution. From these electronic absorption spectra, MCD spectra, VPO measurements and temperature-dependent X-ray diffraction studies, it was clarified for (C_nO)₁₆–M that the dimer structure in n-hexane solution is closely related to those in the thermotropic mesophases.

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