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

Abstract

We have synthesised a series of phthalocyanine-based discotic liquid crystals, (m-C_nOPhO)₈PcCu (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-C_nOPhO)₈PcCu 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 ≅ 5.2 Å due to flip-flop of the bulky aromatic substituents, and that the Col_ho mesophase in 2o (n = 20) gave a halo denoted as Halo_alkyl at d ≅ 4.6–4.8 Å 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)₈PcCu (2h) derivative having moderately long alkyl chains gave Halo_alkyl at about 4.8 Å in the lower temperature mesophase of Col_ho, but Halo_arom. at about 5.2 Å in the higher temperature mesophase of Col_ro(P2₁/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(P2₁/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.