Design of Chemoresponsive\nLiquid Crystals Using Metal-Coordinating\nPolymer Surfaces

Liquid crystals (LCs), when interfaced with chemically\nfunctionalized\nsurfaces, can amplify a range of chemical and physical transformations\ninto optical outputs. While metal cation-binding sites on surfaces\nhave been shown to provide a basis for the design of chemoresponsive\nLCs, the cations have been found to dissociate from the surfaces and\ndissolve slowly into LCs, resulting in time-dependent changes in the\nproperties of LC–solid interfaces (which impacts the reliability\nof devices incorporating such surfaces). Here, we explore the use\nof surfaces comprising metal-coordinating polymers to minimize the\ndissolution of metal cations into LCs and characterize the impact\nof the interfacial environment created by the coordinating polymer\non the ordering and time-dependent properties of LCs. In particular,\nby combining theoretical (electronic structure calculations) and experimental\n(polarization-modulation infrared reflection-adsorption spectroscopy)\nresults, we determine that the pyridine groups of a thin film of poly(4-vinylpyridine<i>-co-</i>divinylbenzene) (P(4VP-<i>co</i>-DVB)) coordinate\nwith Ni<sup>2+</sup> when Ni(ClO<sub>4</sub>)<sub>2</sub> is deposited\nonto the film. We provide evidence that the Ni<sup>2+</sup>-pyridine\ncoordination weakens the binding of Ni<sup>2+</sup> with 4′-<i>n</i>-pentyl-4-biphenylcarbonitrile (5CB), a room-temperature\nnematic LC, as compared to Ni(ClO<sub>4</sub>)<sub>2</sub> supported\non glass, although binding is still sufficiently strong to induce\na homeotropic (perpendicular) orientation of the LC. Exposure of the\n5CB films supported on Ni(ClO<sub>4</sub>)<sub>2</sub>-decorated P(4VP<i>-co-</i>DVB) substrates to parts-per-million vapor concentrations\nof dimethylmethylphosphonate (DMMP) was found to trigger orientational\ntransitions (to planar (parallel) orientations) in the LC films. In\ncontrast, 5CB supported on Ni(ClO<sub>4</sub>)<sub>2</sub>-decorated\nglass surfaces exhibited no response, even though displacement of\n5CB by DMMP is predicted by computations to be thermodynamically favored\nin both cases. We propose that the distinct LC responses measured\non glass and the coordinating polymer substrates are governed by the\nkinetics of displacement of 5CB by DMMP, a proposal that is supported\nby measurements performed with increasing temperature. Importantly,\nby using Ni<sup>2+</sup> supported on P(4VP<i>-co-</i>DVB),\nwe measured the ordering of 5CB to be stable and long-lived (>7\ndays),\nin contrast to unstable LC ordering (<14 h) when using Ni<sup>2+</sup> supported on glass under dry conditions and at room temperature.\nWe further demonstrate the stability of Ni(ClO<sub>4</sub>)<sub>2</sub> supported on P(4VP<i>-co-</i>DVB) toward higher temperatures\nand humidity using E7 as the LC. Overall, these results demonstrate\nthat metal-coordinating polymer films are a promising class of substrates\nfor fabricating robust and long-lived chemoresponsive LCs.