All-organic liquid crystalline radicals with a spin unit in the outer position of a bent-core system

Abstract

All-organic paramagnetic liquid crystals offer the advantage of a long-range order of liquid crystalline phases and the magnetic properties of the individual molecules. In such systems, the magnetic properties can be modified by phase transition or the application of external fields. This manuscript reports on paramagnetic all-organic bent-core liquid crystals having the radical-bearing unit (TEMPO) in the terminal position of an elongating side arm. The mesomorphic properties of the materials are ensured by the optimized molecular structure. The paramagnetic nature of the mesogenic materials is investigated by electron paramagnetic resonance, the magnetic properties of the bulk materials are studied by SQUID magnetometry. It is shown that the materials preserve their magnetic properties within the whole temperature range of liquid crystalline behaviour. Moreover, a strong correlation between spin orientation and molecular alignment within different mesophases has been observed, however, SQUID measurements do not provide evidence about spin glass formation. Unlike materials presented thus far, the position of the spin unit has a plausible effect on the formation of mesophases leading to unique polymorphism of the studied paramagnetic compounds. For two six-ring hockey-stick-like compounds, polymorphism with four different mesophases, including two B_1Rev-type phases, has been found in a broad temperature range (about 20 °C each). Thus, for the first time, such behaviour is described for all-organic paramagnetic bent-core liquid crystals.