Infrared Spectroscopy of Competitive Interactions between Liquid Crystals, Metal Salts, and Dimethyl Methylphosphonate at Surfaces

Abstract

We report the use of Fourier transform polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) to characterize the influence of dimethyl methylphosphonate (DMMP) on the molecular interactions occurring within thin films of nitrile-containing liquid crystals supported on surfaces presenting metal perchlorate salts. Infrared spectra obtained using thin films of 4'-octyl-4-biphenylcarbonitrile (8CB) supported on copper(II) perchlorate salts reveal the nitrile groups of 8CB to be coordinated to the copper(II) on these surfaces, and subsequent exposure of the system to DMMP to result in the elimination of these coordinated nitrile groups. Concurrently, evidence of coordination of the phosphoryl group of DMMP with copper(II) is provided by measurement of a shift of the phosphoryl stretch from 1246 to 1198 cm(-1). In contrast, surfaces presenting nickel(II) perchlorate salts only weakly coordinate with DMMP [the phosphoryl peak shifts from 1246 to 1213 cm(-1) in the presence of nickel(II)], and exposure of 8CB to DMMP results in only partial loss of coordination of the nitrile groups of 8CB with nickel(II). These PM-IRRAS measurements and others reported in this article provide insights into the molecular origins of macroscopic ordering transitions that are observed when micrometer-thick films of nitrile-containing liquid crystals supported on copper(II) or nickel(II) perchlorate are exposed to DMMP: Upon exposure to DMMP, nematic phases of 4'-pentyl-4-biphenylcarbonitrile (5CB) supported on copper(II) perchlorate salts undergo ordering transitions, whereas 5CB supported on nickel(II) perchlorate salts do not. Our IR results support the hypothesis that these ordering transitions reflect the relative strengths of coordination interactions occurring between the 5CB, DMMP, and the metal salts at these interfaces.

Liquid Crystal‐Based Chemical Sensors

“…Overall, these results along with similar results reported in the literature [31,38] provide strong support for the proposition that metal salts can be used to prepare a class of chemically functionalized surfaces at which LCs can be oriented through metal ion-LC coordination interactions involving the nitrile group of the LC. As discussed below, these coordination interactions can be disrupted by targeted chemical analytes, thus leading to a class of LC-based sensors for coordinating compounds.…”

“…Several key characteristics of the DMMP-induced ordering transitions are described in Figures 15.7E and F. Firstly, Figure 15.7E demonstrates that the DMMP-induced ordering transitions are reversible [25,31,33,36,39]. Secondly, Figure 15.7F reveals that the LC responds to concentrations of DMMP that are in the 10s of parts-per-billion range [25].…”

“…Copyright 2001 AAAS. revealed that surfaces decorated with metal salts that coordinate with the nitrile groups of LCs, can be used to achieve the initial homeotropic orientation of the LC (Figure 15.2B). Upon exposure of the system to an analyte that coordinates to the metal ions with an affinity that is greater than the nitrile group (e.g., a molecule with a phosphoryl group), the nitrile-metal ion coordination interaction is lost, resulting in an ordering transition of the LC toward a planar orientation [25,[29][30][31][32][33][34][35][36][37][38][39].…”

Beyond displays: The recent progress of liquid crystals for bio/chemical detections

“…Inspired by that, bio/chemically modified substrates have been explored to control LC ordering and designed for detecting biomolecules [9][10][11][12][13][14] or chemicals [15][16][17][18][19][20]. For example, as shown in Figure 3(a), the gold substrate was first treated with a mixture of biotin-(CH 2 ) 2 [(CH 2 ) 2 O] 2 NHCO(CH 2 ) 11 SH (BiSH) and CH 3 (CH 2 ) 7 SH (C 8 SH).…”

“…Apart from decorating the solid substrates with ligands for detecting proteins, it is also feasible to modify the surface with chemical receptors for chemical detection [15][16][17][18][19][20]. Figure 4 showed that depending on the surface topology and chemical composition, 5CB aligned on the plane with orientation depicted in Figure 4(c).…”