The orientation of molecular magnets in LCs can be controlled by magnetic and electric fields, through the magnetic and dielectric anisotropies of the LC-forming molecules. Due to the high viscosity of metal-containing LCs, the necessary fields to position the molecular magnets are quite strong (> 1 T). Metal-free LCs are easier to control, however, their magnetic properties can be hard to measure, as their inherent field strength is extremely low.
In a recent Communications Chemistry publication, Yoshiaki Uchida and colleagues from Osaka University study a new, all-organic LC material built up from comparably small, stable, chiral, and radical nitroxide molecules they call 8NO8_2 and an azobenzene derivative. These components show ferromagnetic spin-spin dipole interactions in magnetic fields – referred to as the "positive magneto-LC effect". The change in the overall magnetic properties at the LC melting point is large enough to be readily detectable by SQUID magnetometry in a magnetic field of only 0.5 T.
The inclusion of an azobenzene photoswitch allows the magnetic properties of the LC to be controlled by light. Azobenzenes have been widely used to enable photoswitching of LCs. Ultraviolet and visible light irradiation respectively induce trans-to-cis and cis-to-trans photoisomerization of the azobenzene. Here, when the azobenzene is in its trans conformation, the LC material in its ordered state is paramagnetic with a normal magnetic susceptibility. When irradiated with UV light, the azobenzene photoisomerizes to its cis conformation, which disturbs the LC structure. This induces a phase transition to the disordered liquid state, which shows high magnetic susceptibility. Additionally, the material naturally changes in its magnetic properties when melting and solidifying.
As a result of this elegant design, unlike previous magnetically active LCs, this system is not only influenced by temperature and the presence of even weak magnetic fields. The easily detectable magnetic properties of this metal-free LC material can be turned up or down simply by flipping the light switch, as it loses and regains its LC structure through photoisomerization.