Molecular Engineering for Chiroptical Smart Paints: Photo‐Tunable and Polymerizable Helical Nanostructures by Cyanostilbene‐Based Reactive Mesogens

Abstract

Many studies on the basis of cholesteric liquid crystal (CLC) have focused on dynamic optical changes utilizing photochemical reactions raising helical pitch variations. However, the conventional dyes doped in CLC system often show reversible isomerization such as trans‐cis, E‐Z, and open‐close forms that hampers the fixation of desired color cues. Here, a cyanostilbene‐based reactive mesogen (CSRM) for phototunable and subsequently polymerizable smart CLC paints is newly designed and synthesized. The structural color of the CLC paint can be precisely manipulated in remote controllable manner by inducing a photochemical reaction of CSRM. Due to the irreversible isomerization behaviors of CSRM, the constructed periodic structures with different helical pitches are intactly fixed via in situ photopolymerization under 460 nm light. In addition, the combination of helical nanostructures and cyanostilbene luminophore produces a circularly polarized light emission. The dissymmetry factor of circularly polarized luminescence is dependent on the photoregulated helical nanostructures in CLC films. The decoupled system of triple photofunctionality in a single component of chirophotonic crystalline material is applied to develop advanced optical information storages.

Smart Anti‐Counterfeiting Inks: Controlled Luminescence Polarization and Its Fixation by Photoisomerization and Subsequent Photopolymerization of Cyanostilbene‐Based Liquid Crystal Monomers

“…Weak input energies such as visible light irradiation and a little thermal fluctuation cannot induce the back isomerization due to its high energy barrier. [35,41,42] Attesting to this, the UV irradiated and subsequently vis irradiated CSRM at 55 °C exhibits no changes in UV-vis, PL, and 1 H NMR spectra (Figure S3, Supporting Information), compared with the spectra of the only UV irradiated sample (previously discussed in Figure 2d,e).…”

“…After UV irradiation for 15 min, the image becomes dark, indicating that the photoisomerization of CSRM in the ink induces the phase transition from the uniaxially oriented nematic LC phase to an Iso state. [35,37,38] The UV-induced phase transition of the smart ink is supported by2D wide angle X-ray diffraction (WAXD) patterns (Figure 3f). In the 2D WAXD pattern of only shear-coated ink, a diffraction peak at 2𝜃 = 17.94°(corresponding to d-spacing = 0.49 nm) is located in the meridian, indicative of uniaxially oriented nematic LC state.…”

“…As the water fraction becomes 90%, the definitely higher PL intensity than the other solutions is induced, which indicates AIE effect (Figure 2b). [35,[37][38][39][40] When CSRM dissolved in THF is irradiated under 365 nm UV light (concentration: 30 μm, UV intensity: 10 mW cm −2 , and irradiation time: 5 min), a gradual reduction in absorption intensity at 𝜆 = 350 nm is observed (Figure 2d). On the other hand, the emission band is gradually blue-shifted and enhanced during UV light irradiation (inset of Figure 2d).…”

“…These ultraviolet-visible (UV-vis) and PL spectral changes are originated from trans-to-cis isomerization (Figure 2c). [35,[37][38][39][40] The increase of the cis isomer with a shorter conjugation length causes the blue-shifted absorption and emission band. The enhanced PL intensity is resulted from the restricted rotational motion of the conformationally rigid cis isomer.…”

“…To chemically stabilize the UV light treated ink, blue light irradiation was performed under nitrogen atmosphere for 60 min. [35,47] The blue light-induced initiation mechanism is described in detail (Figure S6, Supporting Information). The chemical crosslinking of the ink by blue light initiation system was identified by Fourier transform infrared (FT IR) spectroscopy (Figure 4a).…”