Thermal and photoinduced liquid crystalline phase transitions with a rod–disc alternative change in the molecular shape

Abstract

A triphenylene derivative which has six 4-tetradecyloxyazobenzene units connected with propylene–ester linkages (1) was synthesised to study the liquid crystalline phase transitions in both thermotropic and photo-induced manners. It was found that 1 exhibits smectic A (SmA) and rectangular columnar (Col_r) mesophases in an enantiotropic way. This means that 1 can behave as both rod- and disc-like molecules depending on the thermal condition. To the best of our knowledge, this is the first case in thermotropic liquid crystals where one compound exhibits both calamitic and discotic mesomorphism with the change of molecular shape in anisotropy. Under the photo-irradiation at 365 nm of wavelength which induces a photoisomerisation of azobenzene units from trans to cis forms, the phase transitions take place in the same manner as in the thermotropic ones with a certain decrease of phase transition temperatures. It is also a new finding that at 202.5 °C at which 1 exhibits a Col_r mesophase without photo-irradiation, the successive phase changes are induced by increasing the light intensity to the isotropic liquid (Iso) phase by way of a SmA phase. This means that the isothermal phase transitions among Iso, SmA and Col_r phases can be controlled by light intensity. This is also the first example that the isothermal liquid crystalline phase transitions are controlled by photo-irradiation among three phases (Col_r, SmA and Iso), which are also accompanied with the shape change of molecules between rod and disc.