Tailoring of Selective Responsiveness in Liquid Crystals via Organic Ionics

Abstract

Liquid crystals (LCs) have an amplification property by anisotropic nature and long-range molecular ordering, magnifying molecular-scale stimuli to a visible scale. Therefore, LCs have been widely used for the design of stimuli-responsive materials in a variety of fields, including smart phone displays (electric fields), medical thermography (variations in temperature), and sensors (chemical and biological stimuli). However, in chemo-responsive LC system, the relatively poor selectivity has been pointed out as a critical problem. Here we propose versatile design rules to control both selectivity and sensitivity of LC system by surface treatment with chemical selector, consist of headgroup, tailgroup and counter ion. The LC film integrated with chemical selector has optical transition upon the exposure of a specific gas chemical at very low concentration (< 1 ppm). At initial state, LC molecule in the film has vertical orientation due to electrostatic and physical interaction. In particular, our observations indicate that the electric dipole moment of the cyano-group within LCs results in their affinity towards the headgroup of chemical selector, causing vertical orientation. However, the specific gas chemical interrupts the existing interactions, causing LC reorientation. For example, carboxylic acid was found to be the most optimal and exclusive target chemical due to their strong electrostatic attraction to the headgroup. The length of tailgroup can also be used to regulate the LC anchoring strengths and chemo-responsive sensitivity via interdigitation of the tailgroup into the LC layer, allowing for fine control of the system&apos;s response to target chemicals. Moreover, the size of counter ion proportionally governs the both LC anchoring strength and sensitivity due to quadrupole moment change. Furthermore, we perform the molecular simulation to confirm the underlying mechanism and is in excellent agreement with experiment results. Overall, our works provide insight into the rational design of selectivity and sensitivity of the LC system for target chemical. This work was supported by NRF (RS-2023-00212739) and Samsung Electronics Co., Ltd (4.0026312).