Dynamic coloration of polymerized cholesteric liquid crystal networks by infiltrating organic compounds

Yu Cao; Li Chong; Ke-Hui Wu; Lu-Qian You; Sen-Sen Li; Lu-Jian Chen

doi:10.3788/COL202220.091602

Abstract

We demonstrate the dynamic coloration of polymerized cholesteric liquid crystal (PCLC) networks templated by the “wash-out/refill” method in the presence of organic compounds. The reflection colors were modulated by two key approaches, that is, the injection of mutually soluble organic fluids into a microfluidic channel and the diffusion of volatile organic compounds (VOCs). The reversible tuning of reflected colors with central wavelengths between

\sim 450 nm

and

\sim 600 nm

was achieved by alternative injection of nematic liquid crystal E7 (n_av = 1.64) and benzyl alcohol (n = 1.54) using syringe pumps. The fascinating iridescence with reflection centers from

\sim 620 nm

\sim 410 nm

was presented from the volatilization and diffusion of alcohol as a model VOC. Additionally, the flow velocity of fluid and the diffusion time were adjusted to explore the underlying mechanism for the dynamic coloration of cholesteric networks. This work is expected to extend the study of PCLCs as a dynamically tunable optofluidic reflector, visually readable sensor, or compact anti-counterfeit label in response to organic compounds.

During the past few decades, cholesteric liquid crystals (CLCs) with intrinsic helical configuration of molecular directors have great perspectives towards a wide range of advanced photonic applications such as brightness-enhancement devices of liquid crystal (LC) displays, diffractive optical elements, smart windows, mirrorless lasers, and sensors^[1–5]. Thanks to the Bragg reflection of CLCs, which confers nontrivial optical functionalities, significant structural color can be generated to reflect circularly polarized (CP) light with identical handedness in the visible spectrum. In general, the colors of CLCs, exhibited by the selective reflection wavelength, depend on the helical pitch length ( $p$ ) or/and the average refractive index (average RI, $n_{av}$ )^[6–8] in response to the external stimuli, such as temperature, electric field, and light irradiation. Also, some organic compounds can be incorporated into CLCs to alter the optical properties directly. The molecular structure of CLCs can be further modified with recognition fragments and utilized to absorb specific analytes. The uptake of specific analytes will affect $p$ and $n_{av}$ and reveal different reflected colors consequently^[9–11]. This feature is beneficial for the detection of volatile samples such as alcohol, amine, and acetone^[12–14], providing a versatile sensing platform with several advantages like low cost, being power-free, and naked-eye detection.