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Nanoparticle-liquid crystal (NP-LC) composites based on hydrogen bonding were explored using a model system. The ligand shells of 3 nm diameter zirconium dioxide nanoparticles (ZrO2 NPs) were varied to control their interaction with 4-n-hexylbenzoic acid (6BA). The miscibility and effect of the NPs on the nematic order as a function of particle concentration was characterized by polarized optical microscopy (POM), fluorescence microscopy and (2)H NMR spectroscopy. Nonfunctionalized ZrO2 NPs have the lowest miscibility and strongest effect on the LC matrix due to irreversible binding of 6BA to the NPs via a strong zirconium carboxylate bond. The ZrO2 NPs were functionalized with 6-phosphonohexanoic acid (6PHA) or 4-(6-phosphonohexyloxy)benzoic acid (6BPHA) which selectively bind to the ZrO2 NP surface via the phosphonic acid groups. The miscibility was increased by controlling the concentration of the pendant CO2H groups by adding hexylphosphonic acid (HPA) to act as a spacer group. Fluorescence microscopy of lanthanide doped ZrO2 NPs showed no aggregates in the nematic phase below the NP concentration where aggregates are observed in the isotropic phase. The functionalized NPs preferably concentrate into LC defects and any remaining isotropic liquid but are still present throughout the nematic liquid at a lower concentration.

Synthesis of (3-(pyridin-4-yl)propyl)phosphonic acid.Synthesis of (3-(pyridin-4-yl)propyl)phosphonic acid was performed in two steps as shown in scheme S1.Synthesis of diethyl (3-(pyridine-4-yl)propylphosphonate. Lithium diisopropyl amide (LDA) (18.5 ml, 1.2 eq.) was added dropwise to a solution 4-methyl pyridine (Sigma Aldrich) (1.5 ml, 15.25 mmol, 1 eq.) in 30 ml dry THF at -30 o C under an inert atmosphere. After the reaction mixture was stirred at low temperature for 3 h, diethyl 2-bromoethyl phosphonate (2.85 ml, 15.25 mmol, 1 eq.) was added as a solution in 20 ml dry THF. The reaction mixture was allowed to reach room temperature overnight then was portioned between water and chloroform. The organic layer was separated, dried over MgSO4 and concentrated. The crude product was purified by column chromatography (SiO2, dichloromethane: methanol, 24 : 1 (v : v)). The fractions with Rf = 0.15 were separated to give 1.91 g of diethyl (3-(pyridine-4-yl)propylphosphonate (h = 0.48).

Nanoparticle/liquid crystal (NP/LC) composites can be stabilized by hydrogen bonding, a relatively strong, yet reversible interaction allowing for the annealing of defects.

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