Abstract
The design, synthesis, properties, and performance of a new class of promesogenic calamitic side-tethering organic ligands used to direct quantum dot nanoparticle self-assembly are described. This work was motivated by inadequate modularity, step count, and yield associated with syntheses of existing ligands. Attaching the new ligands to quantum dots and dispersing them in a liquid crystal host affords hollow micrometer-sized capsules via phase transition templating. The capsules resist thermal decomposition up to 350 °C—significantly higher than any previously reported microcapsules assembled from side-tethering calamitic ligand-functionalized nanoparticles. These novel ligands can be used for encapsulation applications where stability under high temperature is required. Evaluation of the capsules by small-angle X-ray scattering shows that interparticle spacing varies from 10 to 13 nm depending on the ligand used and is correlated to aminoalkyl chain length.
Highlights
A wide variety of small organic molecules have been employed as surface-modifying ligands for metallic or semiconducting nanoparticles.[1−5] Metal nanoparticles functionalized with mesogenic or promesogenic organic ligands bearing a motif consisting of multiple closely linked aromatic rings likened to a molecular “rod” and an orthogonal nucleophilic tethering arm self-assemble[6] into different two- and three-dimensional mesoscale morphologies like rods, spheres, and capsules (Figure 1).[3,7,12]
We designed a ten-membered library of ligands (4a−4j, Table 1) to evaluate their ability to attach to CdSe/ZnS core− shell quantum dots and direct their self-assembly
We have developed a rapid, modular synthesis of new side-attaching calamitic promesogenic ligands. Their attachment to quantum dots and subsequent self-assembly in a liquid crystal host demonstrates the broad applicability of nematic templating
Summary
A wide variety of small organic molecules have been employed as surface-modifying ligands for metallic or semiconducting nanoparticles.[1−5] Metal nanoparticles functionalized with mesogenic (liquid crystalline) or promesogenic organic ligands bearing a motif consisting of multiple closely linked aromatic rings likened to a molecular “rod” (calamitic) and an orthogonal nucleophilic tethering arm self-assemble[6] into different two- and three-dimensional mesoscale morphologies like rods, spheres, and capsules (Figure 1).[3,7,12] The derived morphologies are envisioned for use in biochemical sensors,[13,14] optoelectronic and photovoltaic devices,[15−18] and light-emitting diodes.[19]. We have experienced many challenges with the synthesis and purification of C, which is prepared in eights steps with a longest linear sequence of five steps.[8] Like A and B, ligand C contains esters in the calamitic arm, which are prone to decompose upon attack by a nitrogen atom in particular. These esters are not critical for nanoparticle self-assembly (they may be detrimental); they are likely relics of the condensation reactions used for their synthesis. We describe the development of a library of calamitic side-tethering ligands by modifying the calamitic arm, amine tether length, and ethereal linker lengths, and we demonstrate their ability to direct quantum dot self-assembly into microcapsules stable up to 350 °C via nematic templating
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