Design of Chemoresponsive Liquid Crystals Using Metal-Coordinating Polymer Surfaces

Abstract

Liquid crystals (LCs), when interfaced with chemically functionalized surfaces, can amplify a range of chemical and physical transformations into optical outputs. While metal cation-binding sites on surfaces have been shown to provide a basis for the design of chemoresponsive LCs, the cations have been found to dissociate from the surfaces and dissolve slowly into LCs, resulting in time-dependent changes in the properties of LC–solid interfaces (which impacts the reliability of devices incorporating such surfaces). Here, we explore the use of surfaces comprising metal-coordinating polymers to minimize the dissolution of metal cations into LCs and characterize the impact of the interfacial environment created by the coordinating polymer on the ordering and time-dependent properties of LCs. In particular, by combining theoretical (electronic structure calculations) and experimental (polarization-modulation infrared reflection-adsorption spectroscopy) results, we determine that the pyridine groups of a thin film of poly(4-vinylpyridine-co-divinylbenzene) (P(4VP-co-DVB)) coordinate with Ni²⁺ when Ni(ClO₄)₂ is deposited onto the film. We provide evidence that the Ni²⁺-pyridine coordination weakens the binding of Ni²⁺ with 4′-n-pentyl-4-biphenylcarbonitrile (5CB), a room-temperature nematic LC, as compared to Ni(ClO₄)₂ supported on glass, although binding is still sufficiently strong to induce a homeotropic (perpendicular) orientation of the LC. Exposure of the 5CB films supported on Ni(ClO₄)₂-decorated P(4VP-co-DVB) substrates to parts-per-million vapor concentrations of dimethylmethylphosphonate (DMMP) was found to trigger orientational transitions (to planar (parallel) orientations) in the LC films. In contrast, 5CB supported on Ni(ClO₄)₂-decorated glass surfaces exhibited no response, even though displacement of 5CB by DMMP is predicted by computations to be thermodynamically favored in both cases. We propose that the distinct LC responses measured on glass and the coordinating polymer substrates are governed by the kinetics of displacement of 5CB by DMMP, a proposal that is supported by measurements performed with increasing temperature. Importantly, by using Ni²⁺ supported on P(4VP-co-DVB), we measured the ordering of 5CB to be stable and long-lived (>7 days), in contrast to unstable LC ordering (<14 h) when using Ni²⁺ supported on glass under dry conditions and at room temperature. We further demonstrate the stability of Ni(ClO₄)₂ supported on P(4VP-co-DVB) toward higher temperatures and humidity using E7 as the LC. Overall, these results demonstrate that metal-coordinating polymer films are a promising class of substrates for fabricating robust and long-lived chemoresponsive LCs.