Liquid crystalline (LC) materials show high responsivity to various stimuli due to their collective molecular motion. The high responsivity can lead to developments of LC devices; in fact, LC displays need the control of light propagation by means of the molecular reorientation in applied electric fields. It had been natural that magnetic properties of metal-free compounds are undetectably weak in high-temperature LC phases until some metal-free LC compounds with a stable nitroxide radical moiety in the mesogen core like 8NO8 were reported to exhibit the detectable change in magnetic properties at their melting points, which is called "Magneto-LC effect" (Fig. 1a). We have reported that magneto-LC effects originate from spin glass-like inhomogeneous magnetic interactions and the molecular motion in fluid phases: LC and isotropic (Iso) phases. To apply this phenomenon to any devices, the main problems are that transitions from or to Cr phases are usually irreversible and too slow. Since LC-to-Iso phase transitions are reversible and fast, these can immediately occur in response to external stimuli like not only temperature but also light. Thus, LC materials consisting of the magnetic LC compounds could exhibit light-induced reversible switching of magnetic properties as a new photomagnetic effect (Fig. 1b). However, since the magnetic properties of the previously reported LC-NRs hardly change at the LC-to-Iso phase transitions, we have designed a new compound 8NO8₂ showing the large change of magnetic properties at the chiral smectic C LC (SmC*)-to-Iso phase transitions to realize the light-induced reversible switching.

Fig. 1. Molecules and properties of the present LC mixture. a Molecular structures of (2S,5S) enantiomers of compounds 8NO8, 8NO8₂, and BMAB. b Photomagnetic effect in a metal-free LC system consisting of (2S,5S)-8NO8₂ and an azobenzene analog, BMAB. SmC* denotes chiral smectic C, and Iso denotes isotropic liquid. Ultraviolet (UV) and visible (Vis) lights induce trans-to-cis and cis-to-trans photoisomerization of BMAB, respectively. When SmC* material dissolving BMAB is irradiated with UV light, cis-BMAB disturbs the SmC* structure, and therefore, SmC*-to-Iso phase transition can occur. When the SmC* material consists mainly of nitroxide radicals, the magnetic susceptibility (χ) can increase at the photo-induced SmC*-to-Iso phase transition.

With the effective use of this large change of magnetic properties at SmC*-to-Iso phase transition, temperature change could induce the reversible switching of magnetic properties. We focused on light irradiation as one of the other external stimuli to induce the SmC*-to-Iso and Iso-to-SmC* phase transitions because light stimuli can be controlled much more quickly than temperature. According to previous reports, for the photo-induced switching of the magnetic properties, we doped 4-butyl-4’-methoxyazobenzene (BMAB) into (2S,5S)-8NO8₂. The mixture shows an enantiotropic SmC* phase from room temperature to 51.2 °C in the DSC chart; the doped BMAB does not extinguish the intrinsic LC nature of (2S,5S)-8NO8₂. The switching of magnetic properties is reversible and stable against repeated irradiation of UV and visible lights as well as temperature change (Fig. 2).

Fig. 2. Photomagnetic effects in LC mixture of (2S,5S)-8NO8₂ and BMAB. a Molecular structure and photoisomerization of BMAB. b Temperature dependence of χrel for photo-responsive magnetic LC mixture of (2S,5S)-8NO8₂ and BMAB by EPR spectroscopy in a magnetic field of around 0.33 T in the heating processes in dark (closed circles) and under UV light irradiation (open circles). Error bars are not shown because they are sufficiently small. Vertical dashed line denotes the clearing point under UV light irradiation expected by DSC analysis. Switching of c χ_rel, d g-value, e ΔB_pp^L, and f ΔB_pp^G for photo-responsive magnetic LC mixture was measured by EPR spectroscopy in a magnetic field of 0.33 T. The data were obtained from EPR spectra measured under UV and visible light irradiation at 51 °C

To read more, please see our paper: “Photomagnetic effects in metal-free liquid crystals” published in Communications Chemistry:

https://www.nature.com/articles/s42004-019-0165-9