Phthalocyanine-based discotic liquid crystals switching from a molten alkyl chain type to a flying-seed-like type

摘要

We have synthesised a series of phthalocyanine-based discotic liquid crystals, (m-CnOPhO)(8)PcCu (n = 1-20: 2a-o), and investigated their mesomorphism by using a polarizing optical microscope (POM), a differential scanning calorimeter (DSC) and a temperature-dependent small angle X-ray diffractometer. We found that each of the derivatives 2a-o shows mesomorphism. However, the mesomorphism of the (m-CnOPhO)(8)PcCu derivatives strongly depends on the alkoxy chain length (n). The mesomorphism of the short chain-substituted derivatives 2a-e for n = 1-5 is a flying-seed-like type induced by flip-flop of the peripheral bulky substituents, whereas the mesomorphism of the long chain-substituted derivatives 2j-o for n = 10-20 is a conventional molten alkyl chain type induced by melting of the long alkyl chains. The moderately long chain derivatives (2f-i) for n = 6-9 in between show both types of mesophases. The detailed temperature-dependent X-ray diffraction measurements were carried out for three representative derivatives, 2b (n = 2 for n = 1-5), 2h (n = 8 for n = 6-9), and 2o (n = 20 for n = 10-20). As a result, we revealed that the Col(ro)(P2m) mesophase in 2b (n = 2) gave a halo denoted as Halo(arom). at d congruent to 5.2 angstrom due to flip-flop of the bulky aromatic substituents, and that the Colho mesophase in 2o (n = 20) gave a halo denoted as Halo(alkyl) at d congruent to 4.6-4.8 angstrom due to melting of the long alkyl chains. Therefore, we can distinguish the type of mesophase from Halo(arom). and Halo(alkyl). Very interestingly, the (m-C8OPhO)(8)PcCu (2h) derivative having moderately long alkyl chains gave Halo(alkyl) at about 4.8 angstrom in the lower temperature mesophase of Col(ho), but Halo(arom). at about 5.2 angstrom in the higher temperature mesophase of Col(ro)(P2(1)/a). This means that melting of the alkyl chains induces the Col(ho) phase in the lower temperature region, but that flip-flop of the bulky aromatic substituents induces the Col(ro)(P2(1)/a) phase in the higher temperature region. This unusual reverse phase transition sequence from a higher symmetry of the Col(h) mesophase to a lower symmetry of the Col(r) mesophase on a heating stage is attributable to such a unique stepwise melting of these two different types of substituents. To the best of our knowledge, this mesogen (2h) is the first example switching mesomorphism from the molten alkyl chain type to the flying-seed-like type in a discotic liquid crystal.