Computational Chemistry‐Guided Design of Selective Chemoresponsive Liquid Crystals Using Pyridine and Pyrimidine Functional Groups

Abstract

We report computational chemistry-guided designs of chemoresponsive liquid crystals (LCs) with pyridine or pyrimidine groups that bind to metal cation-functionalized surfaces to provide improved selective responses to targeted vapor species (dimethylmethylphosphonate (DMMP)) over non-targeted species (water). We test the LC designs against experiments by synthesizing 4-(4-pentyl-phenyl)pyridine and 5-(4-pentyl-phenyl)-pyrimidine and quantifying LC responses to DMMP and water. Consistent with the computations, pyridine-containing LCs bind to metal cation-functionalized surfaces too strongly to permit a response to either DMMP or water whereas pyrimidine-containing LCs undergo a surface-driven orientational transition in response to DMMP without interference from water. The computation predictions are not strongly dependent on assumptions regarding the degree of coordination of the metal ions but are limited in their ability to predict LC responses when using cations with mostly empty d orbitals. Overall, this work identifies a promising new class of chemoresponsive LCs based on pyrimidine that exhibits enhanced tolerance to water, a result that is important because water is a ubiquitous and particularly challenging chemical interferent in chemical sensing strategies based on LCs. The work also provides further evidence of the transformative utility of computational chemistry methods to design of LC materials that exhibit selective orientational responses in specific chemical environments.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

Redox‐Triggered Orientational Responses of Liquid Crystals to Chlorine Gas

“…The first step in our approach used computational chemistry methods to select metal cations that exhibit oxidation‐state‐dependent binding interactions with mesogens . Specifically, we systematically studied the binding of benzonitrile (PhCN), a surrogate for the common mesogen 4‐cyano‐4′‐pentylbiphenyl (5CB; Scheme ) with a range of metal cations (as perchlorate salts; Table ).…”

Redox‐Triggered Orientational Responses of Liquid Crystals to Chlorine Gas

“…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).…”