Remote-Controllable Molecular Knob with Photochromic Chiral Dopant for Optically Tunable Devices

To fabricate the optically tunable device, photo-responsive chiral dopant (AZ₄ICD) was newly designed and successfully synthesized. AZ₄ICD is composed of photo-responsive azobenzene moiety and chiral isosorbide central core. Therefore, AZ₄ICD can be used as a remote-controllable molecular knob for the control of molecular orientation in the helical mesophases. First, the phase structures are investigated by using thermal, microscopic, and scattering analyses. Based on the experimental results, AZ₄ICD exhibits one liquid crystalline phase (SmA*) at high temperatures and two crystalline phases (SmCr₁ & SmCr₂) at lower temperatures. To find out detail molecular structures and symmetries in the ordered phases, 2D WAXD patterns are obtained at different temperatures. From the analysis of 2D WAXD patterns, the two AZ₄ICD molecules are self-assembled to the anticlinically tilted crystalline structure in SmCr₁ and SmCr₂ phases. In SmA* phase, anticlinically tilted bilayer structure transforms to the simple layered structure. Due to the photo-responsiveness of azobenzene mesogen, a photo-reversible molecular conformational change can be induced by UV and visible light. Upon irradiating UV light, the uniaxially oriented SmA* phase is transformed into an isotropic phase by <i>trans</i>-to-<i>cis</i> isomerization of AZ₄ICD. The isotropic phase is returned to the SmA* phase in the absence of UV irradiation, and this process can be accelerated by irradiation of visible light. AZ₄ICD dopant exhibits good compatibility with the nematic LC host (E7) due to the mesogenic properties of azobenzene and its side chain. To fabricate a chiral nematic (N*) thin film, the mixture composed of chiral dopant R811, AZ₄ICD, and E7 was injected into the sandwich cell. The chirality is inversely proportional to the <i>trans</i>-to-<i>cis</i> photoisomerization of AZ₄ICD, indicating the helical pitch is controlled by the UV and visible light irradiation. Another chiral mesophase that normally formed between N* and Iso phase is called by blue LC phase (BP). As the content of AZ₄ICD in the BP mixture increases, the temperature range of BP is expanded. To further investigate the photoisomerization behavior of AZ₄ICD in a 3D helical superstructure, a BP film was also prepared. Under irradiation with UV, the double twisted cylindrical alignment of BP building block is broken by the metastable <i>cis</i>-AZ₄ICD molecules. Such N* and BP films can induce the reversible phase transformation and change of reflected color without any degradation. The AZ₄ICD LC films with remote-controllable molecular knob can provide a lot of opportunities in the various applications. This work was supported by the BK21 FOUR, Mid-Career Researcher Program (2021R1A2C2009423) and Basic Research Laboratory Program (2020R1A4A1018259).