The role of anions in adsorbate-induced anchoring transitions of liquid crystals on surfaces with discrete cation binding sites

Abstract

We report a combined theoretical and experimental effort to elucidate systematically for the first time the influence of anions of transition metal salt-decorated surfaces on the orientations of supported films of nematic liquid crystals (LCs) and adsorbate-induced orientational transitions of these LC films. Guided by computational chemistry predictions, we find that nitrate anions weaken the binding of 4'-n-pentyl-4-biphenylcarbonitrile (5CB) to transition metal cations, as compared to perchlorate salts, although binding is still sufficiently strong to induce homeotropic (perpendicular) orientations of 5CB. In addition, we find the orientations of the LC to be correlated across all metal cations investigated by a molecular anchoring energy density that is calculated as the product of the single-site binding energy and metal cation binding site density on the surface. The weaker single-site binding energy caused by nitrate also facilitates competitive binding of adsorbates to the metal cations, leading to more facile orientational transitions induced by adsorbates. Finally, our analysis suggests that nitrate anions recruit water via hydrogen bonding to the metal binding sites, modulating further the relative net binding energies of 5CB and adsorbates to surfaces decorated with metal nitrates. After accounting for the presence of water, we find a universal exponential relationship between the calculated displacement free energies and measured dynamic response of LCs to adsorbates for all metal salts studied, independent of the metal salt anion.

Redox‐Triggered Orientational Responses of Liquid Crystals to Chlorine Gas

“…Additional discussion of the origin of the sigmoidal shape of the response curve evident in Figure 3ais given in the Supporting Information. [40] Previously, [17,29] we demonstrated that the dynamic response of LC films to chemical targets can be accelerated by lowering the density of the metal cation binding sites on surfaces to the minimum density required to achieve the initial homeotropic orientation of the LC.F urthermore,w e…”

“…Zuschriften showed that the cation surface density required to achieve ahomeotropic orientation of aLCdecreases with an increase in the strength of binding between the mesogen and metal cation binding site. [29] Guided by these prior design rules,w e investigated the use of LCs containing 4-(4-pentylphenyl)pyridine (PD,S cheme 1) [28] to generate ar esponse to Cl 2 . Table 1s hows that the binding energy of pyridine,t he surrogate for PD used in our computations,t oM n(ClO 4 ) 2 to be À1.75 eV,avalue that is 0.41 eV more negative than that of PhCN.Significantly,however,the binding energy of pyridine to MnO 2 is only À0.22 eV.Accordingly,these calculations led us to predict that LCs containing PD should transition from ahomeotropic orientation on Mn(ClO 4 ) 2 to aplanar orientation on MnO 2 upon exposure to Cl 2 with dynamics and sensitivity that exceed nitrile-containing LCs.B ecause PD does not form aL Ca sap ure component, we mixed it with 85 wt %nematic TL205 (Scheme 1) and confirmed mixing by using differential scanning calorimetry (Supporting Information, Figure S9).…”

“…In the absence of PD,TL205 does not assume ah omeotropic orientation on metal salt surfaces. [28,29] However,t he PD + TL205 mixture assumed homeotropic orientations on surfaces with densities of Mn 2+ as low as 3.7 AE 0.2 pmol mm À2 .T his contrasts to nitrile containing LCs such as 5CB that exhibit homeotropic orientations only when the surface density of Mn 2+ exceeds 7.0 AE 0.3 pmol mm À2 (see details in the Supporting Information). By using 3.7 AE 0.2 pmol mm À2 Mn(ClO 4 ) 2 and the PD + TL205 mixture,w e found that the response time of the LC to 1ppm Cl 2 was lowered to 4mins compared to 10 mins for 5CB on 10.6 AE 0.3 pmol mm À2 for 1ppm of Cl 2 (Figure 4; Supporting Information, Figure S7).…”

“…We illustrate the approach using Cl 2 gas.Thef irst step in our approach used computational chemistry methods to select metal cations that exhibit oxidation-state-dependent binding interactions with mesogens. [26][27][28][29] Specifically,w es ystematically studied the binding of benzonitrile (PhCN), asurrogate for the common mesogen 4-cyano-4'-pentylbiphenyl (5CB;S cheme 1) with ar ange of metal cations (as perchlorate salts;T able 1). Ther esults Scheme 1.…”

“…shown in Table 1r eveal that nitrile-mediated binding of PhCN to Fe 2+ ,C r 3+ ,a nd Mn 2+ occurs with an energy release of À1.54, À0.95, and À1.34 eV,r espectively.G uided by past results,t his strength of nitrile-mediated interaction is predicted to be sufficient to cause ahomeotropic (perpendicular) orientation of 5CB on surfaces presenting these metal cations. [28,29] We also calculated the binding of PhCN to Fe 3+ to be strong (À0.93 eV) and thus we concluded that the Fe 2+ / Fe 3+ cations were unlikely to define useful redox-triggered binding sites that permit LCs to respond to Cl 2 .I nc ontrast, for Mn 4+ ,w hich exists as MnO 2 in the presence of humidity, an extended surface model was used to calculate the binding of PhCN to the Mn 4+ cation to be weak (À0.40 eV). Similarly, for Cr 6+ ,which was modeled as aCrO 4 2À oxide cluster,wealso calculated weak binding of PhCN (À0.12 eV,r espectively).…”