Swelling and Gel/Sol Formation of Perchlorate-Type Layered Double Hydroxides in Concentrated Aqueous Solutions of Amino Acid-Related Zwitterionic Compounds

ClO₄^–MgAl-LDH3, a MgAl (Mg/Al = 3) layered double hydroxide (LDH) containing perchlorate, swells and forms colloidal suspensions (sols) via the gel state in concentrated aqueous solutions of zwitterionic compounds related to amino acids. In total, 36 zwitterionic compounds with different molecular structures and additional functional groups were examined at various concentrations, and the sol-formation ability was judged by the transmittance (at λ = 589 nm) of the resulting suspensions. At low concentration, the obtained suspensions were turbid, with transmittances of ∼0%. However, above the threshold concentration (0.3–1.0 M), osmotic swelling occurred and the transmittances of the suspensions increased sharply with increases in concentration to reach maximum values of 70–95%. The threshold concentration and maximum transmittance value depended on the structure and the location of the functional groups. The enhancement of the permittivity of water by the zwitterions and the formation of H-bond networks were assumed to be the reasons for the swelling phenomenon. Similar gel/sol formation was observed for ClO₄^–LDHs with Mg/Al = 2, Ni/Al = 2, 3, and Co/Al = 2 and some NO₃^–LDHs. Large ClO₄^–LDH films could be prepared by filtration of the colloidal suspensions followed by washing and drying processes

All-Metal Layer-by-Layer Films: Bimetallic Alternate Layers with Accessible Mesopores for Enhanced Electrocatalysis

We have prepared multilayer mesoporous bimetallic (Pt/Pd) alternating films by layer-by-layer (LbL) electrochemical deposition. Because of the high surface area and heterometallic interfacial atomic contacts, enhanced electrocatalytic activity for methanol oxidation reaction is realized. This novel LbL approach allows optimization of the electrocatalytic performance through precise tuning of the thickness of each layer

Ultratrace Detection of Toxic Chemicals: Triggered Disassembly of Supramolecular Nanotube Wrappers

Chemical sensors offer opportunities for improving personal security, safety, and health. To enable broad adoption of chemical sensors requires performance and cost advantages that are best realized from innovations in the design of the sensing (transduction) materials. Ideal materials are sensitive and selective to specific chemicals or chemical classes and provide a signal that is readily interfaced with portable electronic devices. Herein we report that wrapping single walled carbon nanotubes with metallo-supramolecular polymers creates sensory devices with a dosimetric (time- and concentration-integrated) increase in electrical conductivity that is triggered by electrophilic chemical substances such as diethylchlorophosphate, a nerve agent simulant. The mechanism of this process involves the disassembly of the supramolecular polymer, and we demonstrate its utility in a wireless inductively powered sensing system based on near-field communication technology. Specifically, the dosimeters can be powered and read wirelessly with conventional smartphones to create sensors with ultratrace detection limits

Amperometric Detection of Sub-ppm Formaldehyde Using Single-Walled Carbon Nanotubes and Hydroxylamines: A Referenced Chemiresistive System

We report amperometric detection of formaldehyde (HCHO) using hydroxylamine hydrochloride and single-walled carbon nanotubes (SWCNTs). Hydroxylamine hydrochloride reacts with HCHO to emit HCl vapor, which injects a hole carrier into semiconducting SWCNTs. The increase of conductivity in SWCNTs is easily monitored using an ohmmeter. The debundling of SWCNTs with a metallo-supramolecular polymer (MSP) increased the active surface area in the SWCNTs network, leading to excellent sensitivity to HCHO with a limit of detection (LoD) of 0.016 ppm. The response of sensor is reversible, and the sensor is reusable. The selectivity to HCHO is 10⁵–10⁶ times higher than interferences with other volatiles such as water, methanol, and toluene. Moreover, false-positive responses caused by a significant variation of humidity and/or temperature are successfully discriminated from true-positive responses by using two sensors, one with and the other without hydroxylamine hydrochloride, in a referenced system

Chiral Guest Binding as a Probe of Macrocycle Dynamics and Tautomerism in a Conjugated Tetra­pyrrole

We report chiral guest binding as a probe of prototropic tautomerism and macrocyclic inversion in a highly conjugated tetra­pyrrole studied using ¹H NMR spectroscopy in conjunction with mandelic acid as the chiral guest. Both tautomerism and macrocycle inversion can be influenced in a non-trivial way depending on temperature and the respective concentrations of tetra­pyrrole host, chiral guest or water. Chirality of the interacting guest is the key feature since it permits separation and detailed observation of macrocyclic inversion and tautomerism. Based on this, a methodology was developed to identify and characterize the dynamic processes. Our observations suggest that yields of products (e.g., of asymmetric reactions) can be affected by reactivity of functional groups (in molecules undergoing tautomerism or inversion) by varying solution properties including reagent concentrations and impurities such as water. This work establishes a connection between the important chemical concepts of chirality, tautomerism, and macrocyclic dynamics

NMR Spectroscopic Determination of Enantiomeric Excess Using Small Prochiral Molecules

The use of chiral auxiliaries, which derivatize enantiomers to diastereomers, is an established technique for NMR spectroscopic analysis of chirality and enantiomeric excess (<i>ee</i>). Here we report that some small prochiral molecules exhibit <i>ee</i>-dependent splitting of ¹H NMR signals at room temperature based on acid/base interactions with chiral analytes, especially when either a chiral or prochiral acid contains a phenoxy group at the α-position of the carboxylic acid. As a representative case, the benzylamine (<b>BA</b>)/2-phenoxylpropionic acid (<b>PPA</b>) complex was comprehensively investigated by using various methods. Notably, X-ray crystallographic analysis shows that there are multipoint interactions in the <b>BA</b>/<b>PPA</b> complex, implying that “fixing” of molecular conformation is critical for efficient intermolecular transfer of magnetic anisotropy. Our results suggest that a wide range of prochiral molecules are available for NMR determination of <i>ee</i> when intermolecular interactions between prochiral molecules and chiral analytes are adequately designed

Chiral Guest Binding as a Probe of Macrocycle Dynamics and Tautomerism in a Conjugated Tetra­pyrrole

We report chiral guest binding as a probe of prototropic tautomerism and macrocyclic inversion in a highly conjugated tetra­pyrrole studied using ¹H NMR spectroscopy in conjunction with mandelic acid as the chiral guest. Both tautomerism and macrocycle inversion can be influenced in a non-trivial way depending on temperature and the respective concentrations of tetra­pyrrole host, chiral guest or water. Chirality of the interacting guest is the key feature since it permits separation and detailed observation of macrocyclic inversion and tautomerism. Based on this, a methodology was developed to identify and characterize the dynamic processes. Our observations suggest that yields of products (e.g., of asymmetric reactions) can be affected by reactivity of functional groups (in molecules undergoing tautomerism or inversion) by varying solution properties including reagent concentrations and impurities such as water. This work establishes a connection between the important chemical concepts of chirality, tautomerism, and macrocyclic dynamics

Polymeric Micelle Assembly for Preparation of Large-Sized Mesoporous Metal Oxides with Various Compositions

Here we report the synthesis of mesoporous metal oxide materials with various compositions by assembly of spherical polymeric micelles consisting of triblock copolymer poly­(styrene-<i>b</i>-2-vinyl pyridine-<i>b</i>-ethylene oxide) (PS-<i>b</i>-PVP-<i>b</i>-PEO) with three chemically distinct units. The PVP block interacts strongly with the inorganic precursors for the target compositions. The hydrophobic PS block is kinetically frozen in the precursor solutions, enabling the spherical micelles to remain in a stable form. The frozen PS cores serve as templates for preparing robust mesoporous materials. The PEO corona helps the micelles to stay well dispersed in the precursor solutions, which plays a key role in the orderly arrangement of the micelles during solvent evaporation. This approach is based on assembly of the stable micelles using a simple, highly reproducible method and is widely applicable toward numerous compositions that are difficult for the formation of mesoporous structures

Chiral Guest Binding as a Probe of Macrocycle Dynamics and Tautomerism in a Conjugated Tetra­pyrrole

We report chiral guest binding as a probe of prototropic tautomerism and macrocyclic inversion in a highly conjugated tetra­pyrrole studied using ¹H NMR spectroscopy in conjunction with mandelic acid as the chiral guest. Both tautomerism and macrocycle inversion can be influenced in a non-trivial way depending on temperature and the respective concentrations of tetra­pyrrole host, chiral guest or water. Chirality of the interacting guest is the key feature since it permits separation and detailed observation of macrocyclic inversion and tautomerism. Based on this, a methodology was developed to identify and characterize the dynamic processes. Our observations suggest that yields of products (e.g., of asymmetric reactions) can be affected by reactivity of functional groups (in molecules undergoing tautomerism or inversion) by varying solution properties including reagent concentrations and impurities such as water. This work establishes a connection between the important chemical concepts of chirality, tautomerism, and macrocyclic dynamics

Thermo-/Mechano-Chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-Sorting

Although strong chiral self-sorting often emerges in extended covalent or supramolecular polymers, the phenomenon is generally weak in discrete assemblies (e.g., dimers and oligomers) of small molecules due to the lack of a cooperative growth mechanism. Consequently, chiral self-sorting has been overlooked in the design of switchable and metastable discrete supramolecular structures. Here, we report a butyl-benzo[h]quinoline-based iridium(III) complex (Bu-Ir) with helical chirality at its metal center, which forms preferentially a homochiral dimer and exhibits thermo-/mechano-chromism based on a monomer–dimer transformation. While a five-coordinate monomer is formed in a racemic or an enantiopure Bu-Ir solution at 25 °C, a six-coordinate homochiral dimer complex is formed almost exclusively at low temperatures, with a higher degree of dimerization in enantiopure Bu-Ir solution. Estimation of apparent dimerization binding constants (K) and thermodynamic parameters (ΔH and ΔS) based on variable temperature ultraviolet–visible (UV–vis) and 1H NMR spectra reveals a strong preference for homochiral dimerization (largest known value for the coordination complex, Khomo/Khetero > 50). Notably, crystals of the homochiral dimer are metastable, undergoing a distinct color change upon grinding (from yellow to red) due to mechanical cleavage of coordination bonds (i.e., a dimer to monomer transformation). A comparison with control compounds having different substituents (proton, methyl, isopropyl, and phenyl groups) reveals that Bu-Ir dimerization involves both strong homochiral self-sorting preference and connected thermo-/mechano-chromic behavior, which is based on matched propeller-shaped chirality and subtle steric repulsion between alkyl substituents that render the homochiral dimer switchable and metastable. These findings provide substantial insights into the emergence of dynamic functionality based on the rational design of discrete chiral assemblies