Nonlinear optical (NLO) materials whose optical properties change in accordance with incident light intensity are
attracting much attention in various fields. Liquid crystals (LCs) exhibit the largest nonlinearity among functional
materials due to their photoinduced molecular reorientation. In particular, doping dichroic oligothiophene dye into LCs
increases the light sensitivity of materials based on the interaction between dyes and an optical-electric field.
Furthermore, the absorbance of this LC system drastically increases through the dye molecules' reorientation, promising.
for application to the optical limiter; however, practical applications require better light sensitivity. In this study, we
investigated the effect of the host LC structure such as fluorinated LCs on optical limiting behavior derived from
nonlinear molecular reorientation. Irradiation of dye-doped LCs with a laser beam brought about molecular reorientation,
and the transmittance decreased with an incident light intensity. Furthermore, the threshold light intensity for optical
limiting behavior depended on the host LCs structure. Trifluorinated LCs effectively increased the light sensitivity of the
dye-doped LCs compared to LCs without fluorine substituents. This result contributes to the material design for the low threshold optical devices utilizing the NLO of dye-doped LCs.
Functional soft materials with controlled molecular alignment are attracting much attention in various fields due to their excellent flexibility and functional properties. Among conventional alignment methods, mechanical methods such as rubbing the polymer surface are well-known as a facile route to align various molecules. Besides, photoalignment methods, using photoresponsive molecules and polarized light, enable precise alignment control towards advanced functions. As a novel alignment method combining the advantages of both mechanical and photoalignment methods, we have developed scanning wave photopolymerization (SWaP) where phototriggered molecular diffusion is applied to align molecules. Since it uses the molecular diffusion as a driving force for alignment control, SWaP has the potential to align a variety of molecules. For further exploration of the mechanism, it is necessary to understand the polymer properties; thus, the synthesis of polymers applicable to solution-based analyses is highly desired. In this study, we conducted SWaP to synthesize soluble liquid-crystalline polymer films with one-dimensional alignment. Furthermore, we compared the molecular alignment behavior between SWaP and the conventional rubbing alignment technique using a soluble polymer, and revealed that only SWaP can induce a unidirectional molecular alignment in film.
Liquid crystal (LC) is the promising material for the fabrication of high-performance soft, flexible devices. The fascinating and useful properties arise from their cooperative effect that inherently allows the macroscopic integration and control of molecular alignment through various external stimuli. To date, light-matter interaction is the most attractive stimuli and researchers developed photoalignment through photochemical or photophysical reactions triggered by linearly polarized light. Here we show the new choice based on molecular diffusion by photopolymerization. We found that photopolymerization of a LC monomer and a crosslinker through a photomask enables to direct molecular alignment in the resultant LC polymer network film. The key generating the molecular alignment is molecular diffusion due to the difference of chemical potentials between irradiated and unirradiated regions. This concept is applicable to various shapes of photomask and two-dimensional molecular alignments can be fabricated depending on the spatial design of photomask. By virtue of the inherent versatility of molecular diffusion in materials, the process would shed light on the fabrication of various high-performance flexible materials with molecular alignment having controlled patterns.
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