Structural Control of Plasmon Resonance in Molecularly Linked Metal Oxide Nanocrystal Gel Assemblies.

Nanocrystal gels exhibit collective optical phenomena based on interactions among their constituent building blocks. However, their inherently disordered structures have made it challenging to understand, predict, or design properties such as optical absorption spectra that are sensitive to the coupling between the plasmon resonances of the individual nanocrystals. Here, we bring indium tin oxide nanocrystal gels under chemical control and show that their infrared absorption can be predicted and systematically tuned by selecting the nanocrystal sizes and compositions and molecular structures of the link-mediating surface ligands. Thermoreversible assemblies with metal-terpyridine links form reproducible gel architectures, enabling us to derive a plasmon ruler that governs the spectral shifts upon gelation, predicated on the nanocrystal and ligand compositions. This empirical guide is validated using large-scale, many-bodied simulations to compute the optical spectra of gels with varied structural parameters. Based on the derived plasmon ruler, we design and demonstrate a nanocrystal mixture whose spectrum exhibits distinctive line narrowing upon assembly.

Peptide Macrocyclization Guided by Reversible Covalent Templating.

The creation of complementary products via templating is a hallmark feature of nucleic acid replication. Outside of nucleic acid-like molecules, the templated synthesis of a hetero-complementary copy is still rare. Herein we describe one cycle of templated synthesis that creates homomeric macrocyclic peptides guided by linear instructing strands. This strategy utilizes hydrazone formation to pre-organize peptide oligomeric monomers along the template on a solid support resin, and microwave-assisted peptide synthesis to couple monomers and cyclize the strands. With a flexible templating strand, we can alter the size of the complementary macrocycle products by increasing the length and number of the binding peptide oligomers, showing the potential to precisely tune the size of macrocyclic products. For the smaller macrocyclic peptides, the products can be released via hydrolysis and characterized by ESI-MS.

Plasmonic Response of Complex Nanoparticle Assemblies.

Optical properties of nanoparticle assemblies reflect distinctive characteristics of their building blocks and spatial organization, giving rise to emergent phenomena. Integrated experimental and computational studies have established design principles connecting the structure to properties for assembled clusters and superlattices. However, conventional electromagnetic simulations are too computationally expensive to treat more complex assemblies. Here we establish a fast, materials agnostic method to simulate the optical response of large nanoparticle assemblies incorporating both structural and compositional complexity. This many-bodied, mutual polarization method resolves limitations of established approaches, achieving rapid, accurate convergence for configurations including thousands of nanoparticles, with some overlapping. We demonstrate these capabilities by reproducing experimental trends and uncovering far- and near-field mechanisms governing the optical response of plasmonic semiconductor nanocrystal assemblies including structurally complex gel networks and compositionally complex mixed binary superlattices. This broadly applicable framework will facilitate the design of complex, hierarchically structured, and dynamic assemblies for desired optical characteristics.

11B NMR Spectroscopy: Structural Analysis of the Acidity and Reactivity of Phenyl Boronic Acid-Diol Condensations.

Phenyl boronic acids are valuable for medical diagnostics and biochemistry studies due to their ability to readily bind with carbohydrates in water. Incorporated in carbohydrates are 1,2-diols, which react with boronic acids through a reversible covalent condensation pathway. A wide variety of boronic acids have been employed for diol binding with differing substitution of the phenyl ring, with the goals of simplifying their synthesis and altering their thermodynamics of complexation. One method for monitoring their pKa's and binding is 11B NMR spectroscopy. Herein, we report a comprehensive study employing 11B NMR spectroscopy to determine the pKa of the most commonly used phenyl boronic acids and their binding with catechol or d,l-hydrobenzoin as prototypical diols. The chemical shift of the boronic acid transforming into the boronate ester was monitored at pHs ranging from 2 to 10. With each boronic acid, the results confirm (1) the necessity to use pHs above their pKa's to induce complexation, (2) that the pKa's change in the presence of diols, and (3) that 11B NMR spectroscopy is a particularly convenient tool for monitoring these interconnected acidity and binding phenomena.

Colorimetric quantification of linking in thermoreversible nanocrystal gel assemblies.

Nanocrystal gels can be responsive, tunable materials, but designing their structure and properties is challenging. By using reversibly bonded molecular linkers, gelation can be realized under conditions predicted by thermodynamics. However, simulations have offered the only microscopic insights, with no experimental means to monitor linking leading to gelation. We introduce a metal coordination linkage with a distinct optical signature allowing us to quantify linking in situ and establish structural and thermodynamic bases for assembly. Because of coupling between linked indium tin oxide nanocrystals, their infrared absorption shifts abruptly at a chemically tunable gelation temperature. We quantify bonding spectroscopically and use molecular simulation to understand temperature-dependent bonding motifs, revealing that gel formation is governed by reaching a critical number of effective links that extend the nanocrystal network. Microscopic insights from our colorimetric linking chemistry enable switchable gels based on thermodynamic principles, opening the door to rational design of programmable nanocrystal networks.

A self-degradable hydrogel sensor for a nerve agent tabun surrogate through a self-propagating cascade.

Nerve agents that irreversibly deactivate the enzyme acetylcholinesterase are extremely toxic weapons of mass destruction. Thus, developing methods to detect these lethal agents is important. To create an optical sensor for a surrogate of the nerve agent tabun, as well as a physical barrier that dissolves in response to this analyte, we devise a network hydrogel that decomposes via a self-propagating cascade. A Meldrums acid-derived linker is incorporated into a hydrogel that undergoes a declick reaction in response to thiols, thereby breaking network connections, which releases more thiols, propagating the response throughout the gel. A combination of chemical reactions triggered by the addition of the tabun mimic initiates the cascade. The dissolving barrier is used to release dyes, as well as nanocrystals that undergo a spontaneous aggregation. Thus, this sensing system for tabun generates a physical response and the delivery of chemical agents in response to an initial trigger.

A Colorimetric Method for Quantifying Cis and Trans Alkenes Using an Indicator Displacement Assay.

A colorimetric indicator displacement assay (IDA) amenable to high-throughput experimentation was developed to determine the percentage of cis and trans alkenes. Using 96-well plates two steps are performed: a reaction plate for dihydroxylation of the alkenes followed by an IDA screening plate consisting of an indicator and a boronic acid. The dihydroxylation generates either erythro or threo vicinal diols from cis or trans alkenes, depending upon their syn- or anti-addition mechanisms. Threo diols preferentially associate with the boronic acid due to the creation of more stable boronate esters, thus displacing the indicator to a greater extent. The generality of the protocol was demonstrated using seven sets of cis and trans alkenes. Blind mixtures of cis and trans alkenes were made, resulting in an average error of ±2 % in the percentage of cis or trans alkenes, and implementing E2 and Wittig reactions gave errors of ±3 %. Furthermore, we developed variants of the IDA for which the color may be tuned to optimize the response for the human eye.

Effects of linker flexibility on phase behavior and structure of linked colloidal gels.

Colloidal nanocrystal gels can be assembled using a difunctional "linker" molecule to mediate bonding between nanocrystals. The conditions for gelation and the structure of the gel are controlled macroscopically by the linker concentration and microscopically by the linker's molecular characteristics. Here, we demonstrate using a toy model for a colloid-linker mixture that linker flexibility plays a key role in determining both phase behavior and the structure of the mixture. We fix the linker length and systematically vary its bending stiffness to span the flexible, semiflexible, and rigid regimes. At fixed linker concentration, flexible-linker and rigid-linker mixtures phase separate at low colloid volume fractions, in agreement with predictions of first-order thermodynamic perturbation theory, but the semiflexible-linker mixtures do not. We correlate and attribute this qualitatively different behavior to undesirable "loop" linking motifs that are predicted to be more prevalent for linkers with end-to-end distances commensurate with the locations of chemical bonding sites on the colloids. Linker flexibility also influences the spacing between linked colloids, suggesting strategies to design gels with desired phase behavior, structure, and, by extension, structure-dependent properties.

Effect of Extended Benzannelation Orientation on Bergman and Related Cyclizations of Isomeric Quinoxalenediynes.

A combined computational and experimental study was conducted to examine the effect of extended benzannelation orientation on C1-C5 and C1-C6 cyclization of acyclic quinoxalenediynes. Calculations (mPW1PW91/cc-pVTZ//mPW1PW91/6-31G(d,p)) on terminal and phenylethynyl-substituted 5,6-diethynylquinoxaline and 6,7-diethynylquinoxaline showed C1-C6 Bergman cyclization as the favored thermodynamic reaction pathway, with larger C1-C6 preference for the angular quinoxalenediynes due to gain of a new aromatic sextet. Kinetic studies, as a function of 1,4-cyclohexadiene concentration, revealed retro-Bergman ring opening predominates over hydrogen atom abstraction (k-1 > k2) for 6,7-diethynylquinoxaline while 5,6-diethynylquinoxaline undergoes irreversible Bergman cyclization indicative of a large retro-Bergman ring opening barrier (k2 > k-1). The effect of extended linear versus angular benzannelation on reaction pathway shows in the contrasting photocyclizations of phenylethynyl derivatives. While angular 5,6-diethynylquinoxalines gave exclusive C1-C6 photocyclization, linear 6,7-diethynylquinoxaline afforded C1-C5 fulvene products. Computed singlet-triplet gaps and biradical stabilization energies indicated weak interaction between the nitrogen lone pair and proximal radical center in angular 5,6-diethynylquinoxalines. The overall data indicates extended angular benzannelation effectively renders Bergman cyclization irreversible due to favorable aromatic stabilization energy, while extended linear benzannelation results in increased retro-Bergman ring opening, allowing C1-C5 cyclization to become a competitive reaction channel.