Norbornene-Based Dendronized Polymer with Azobenzene for Remote-Controllable Switches

For the fabrication of remote-controllable switches, we newly designed and successfully synthesized norbornene-based macromonomers. The macromonomer consists of hydrophilic linkers, photochromic azobenzene mesogens, and hydrophobic alkyl chains. Hydrophilic ethylene oxide linker and hydrophobic alkyl chain were introduced to form an ordered structure by π- π interaction. Azobenzene mesogens were introduced to fabricate photoresponsive actuators. The azobenzene-based dendronized polymer (AZ-P) were polymerized by ring-opening metathesis polymerization (ROMP) to prepare high molecular weight and low polydispersity index (PDI). The high molecular weight (Mn = 173 kDa) with low polydispersity (PDI = 1.05) was confirmed by size exclusion chromatography (SEC) trace of the polymer. The photochemical properties were investigated by ultraviolet and visible light spectroscopy (UV-Vis). The initial π-π* absorption represented <i>trans</i>-rich state of azobenzene. When exposed to 365 nm UV light and 450 nm Visible light, reversible photoisomerization was confirmed through an increased π-π* and n-π* absorption peaks, respectively. We analyzed thermal phase behaviors of the AZ-P and three different phases (SmF, SmC and SmA) were identified through differential scanning calorimetry (DSC) thermograms and polarized optical microscopy (POM) images of the finally synthesized azobenzene-based polynorbornene. Uniaxially oriented AZ-P films were prepared by mechanical shearing process. The ordered packing structures were investigated through small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) at different temperatures. 1D SAXS and WAXD patterns indicated that the order is lowered as the temperature decreased, and the side chains are packed closely in the layers. The detailed molecular packing structures were investigated through two-dimensional wide-angle X-ray diffraction (2D WAXD), resulting that the backbones were arranged perpendicular to the film normal (FN) and the side chains were oriented parallel to FN, tilted as the temperature decreased. Irradiation of UV and visible light to the AZ-P film resulted in <i>trans</i>-to-<i>cis</i> isomerization indicating photodynamic reversible behaviors. When UV light was exposed to the film, it becomes a <i>cis</i>-rich state by <i>trans</i>-to-<i>cis</i> isomerization in the light source direction and actuated by anisotropic volume contraction. In order to impart electrical conductivity, silver layer was coated on the AZ-P film and an electrical circuit was formed to fabricate wireless switches by light. When a film was exposed to UV light, it was bent by photo-induced actuation and the electric circuit switched with the light-emitting diode (LED) color changes. Upon irradiation of visible light, the film returned to its original shape and switched to the initial electric circuit and LED color. As a result, the AZ-P film has shown that it can be used as remote-controllable switches, and the photoresponsive azobenzene denpol will be practical in remote-controllable devices. This work was supported by the BK21 FOUR, Mid-Career Researcher Program (2021R1A2C2009423) and Basic Research Laboratory Program (2020R1A4A1018259).