Associative pyridinium electrolytes for air-tolerant redox flow batteries.

Pyridinium electrolytes are promising candidates for flow-battery-based energy storage1-4. However, the mechanisms underlying both their charge-discharge processes and overall cycling stability remain poorly understood. Here we probe the redox behaviour of pyridinium electrolytes under representative flow battery conditions, offering insights into air tolerance of batteries containing these electrolytes while providing a universal physico-chemical descriptor of their reversibility. Leveraging a synthetic library of extended bispyridinium compounds, we track their performance over a wide range of potentials and identify the singlet-triplet free energy gap as a descriptor that successfully predicts the onset of previously unidentified capacity fade mechanisms. Using coupled operando nuclear magnetic resonance and electron paramagnetic resonance spectroscopies5,6, we explain the redox behaviour of these electrolytes and determine the presence of two distinct regimes (narrow and wide energy gaps) of electrochemical performance. In both regimes, we tie capacity fade to the formation of free radical species, and further show that π-dimerization plays a decisive role in suppressing reactivity between these radicals and trace impurities such as dissolved oxygen. Our findings stand in direct contrast to prevailing views surrounding the role of π-dimers in redox flow batteries1,4,7-11 and enable us to efficiently mitigate capacity fade from oxygen even on prolonged (days) exposure to air. These insights pave the way to new electrolyte systems, in which reactivity of reduced species is controlled by their propensity for intra- and intermolecular pairing of free radicals, enabling operation in air.

Single-Molecule Stoichiometry of Supramolecular Complexes.

The use of single-molecule microscopy is introduced as a method to quantify the photophysical properties of supramolecular complexes rapidly at ultra low concentrations (<1 nM), previously inaccessible. Using a model supramolecular system based on the host-guest complexation of cucurbit[n]uril (CB[n]) macrocycles together with a fluorescent guest (Ant910Me), we probe fluorescent CB[n] host-guest complexes in the single molecule regime. We show quantification and differentiation of host-guest photophysics and stoichiometries, both in aqueous media and noninvasively in hydrogel, by thresholding detected photons. This methodology has wide reaching implications in aiding the design of next-generation materials with programmed and controlled properties.

Highly compressible glass-like supramolecular polymer networks.

Supramolecular polymer networks are non-covalently crosslinked soft materials that exhibit unique mechanical features such as self-healing, high toughness and stretchability. Previous studies have focused on optimizing such properties using fast-dissociative crosslinks (that is, for an aqueous system, dissociation rate constant kd > 10 s-1). Herein, we describe non-covalent crosslinkers with slow, tuneable dissociation kinetics (kd < 1 s-1) that enable high compressibility to supramolecular polymer networks. The resultant glass-like supramolecular networks have compressive strengths up to 100 MPa with no fracture, even when compressed at 93% strain over 12 cycles of compression and relaxation. Notably, these networks show a fast, room-temperature self-recovery (< 120 s), which may be useful for the design of high-performance soft materials. Retarding the dissociation kinetics of non-covalent crosslinks through structural control enables access of such glass-like supramolecular materials, holding substantial promise in applications including soft robotics, tissue engineering and wearable bioelectronics.

Augmenting the Performance of Hydrogenase for Aerobic Photocatalytic Hydrogen Evolution via Solvent Tuning.

This work showcases the performance of [NiFeSe] hydrogenase from Desulfomicrobium baculatum for solar-driven hydrogen generation in a variety of organic-based deep eutectic solvents. Despite its well-known sensitivity towards air and organic solvents, the hydrogenase shows remarkable performance under an aerobic atmosphere in these solvents when paired with a TiO2 photocatalyst. Tuning the water content further increases hydrogen evolution activity to a TOF of 60±3 s-1 and quantum yield to 2.3±0.4 % under aerobic conditions, compared to a TOF of 4 s-1 in a purely aqueous solvent. Contrary to common belief, this work therefore demonstrates that placing natural hydrogenases into non-natural environments can enhance their intrinsic activity beyond their natural performance, paving the way for full water splitting using hydrogenases.

On-Resin Recognition of Aromatic Oligopeptides and Proteins through Host-Enhanced Heterodimerization.

Peptide dimerization is ubiquitous in natural protein conjugates and artificial self-assemblies. A major challenge in artificial systems remains achieving quantitative peptide heterodimerization, critical for next-generation biomolecular purification and formulation of therapeutics. Here, we employ a synthetic host to simultaneously encapsulate an aromatic and a noncanonical l-perfluorophenylalanine-containing peptide through embedded polar-π interactions, constructing an unprecedented series of heteropeptide dimers. To demonstrate the utility, this heteropeptide dimerization strategy was applied toward on-resin recognition of N-terminal aromatic residues in peptides as well as insulin, both exhibiting high recycling efficiency (>95%). This research unveils a generic approach to exploit quantitative heteropeptide dimers for the design of supramolecular (bio)systems.

Host-Enhanced Phenyl-Perfluorophenyl Polar-π Interactions.

Phenyl-perfluorophenyl polar-π interactions have been revisited for the design and fabrication of functional supramolecular systems. The relatively weak associative interactions (ΔG ≈ -1.0 kcal/mol) have limited their use in aqueous self-assembly to date. Herein, we propose a strategy to strengthen phenyl-perfluorophenyl polar-π interactions by encapsulation within a synthetic host, thus increasing the binding affinity to ΔG= -15.5 kcal/mol upon formation of heteroternary complexes through social self-sorting. These heteroternary complexes were used as dynamic, yet strong, cross-linkers in the fabrication of supramolecular gels, which exhibited excellent viscoelasticity, stretchability, self-recovery, self-healing, and energy dissipation. This work unveils a general approach to exploit host-enhanced polar-π interactions in the design of robust aqueous supramolecular systems.

A comparison of choline:urea and choline:oxalic acid deep eutectic solvents at 338 K.

1:2 choline chloride:urea and 1:1 choline chloride:oxalic acid deep eutectic solvents are compared at 338 K using liquid-phase neutron diffraction with H/D isotopic substitution to obtain differential neutron scattering cross sections and fitting of models to the experimental data using Empirical Potential Structure Refinement. In comparison to the previously reported study of choline chloride:urea at 303 K, we observed significant weakening and lengthening of choline-OH⋯Cl- and choline-OH⋯hydrogen-bond acceptor correlations.

DESolution of CD and CB Macrocycles.

Supramolecular chemistry utilizing the macrocyclic hosts cyclodextrins (CDs) and cucurbit[n]urils (CB[n]s) is traditionally performed in aqueous media; however, their solubility is typically poor, especially for the family of CB[n]s. Through derivatization of these macrocycles their solubility can be augmented to enable enhanced solubility in water and in some organic solvents. The increase in solubility of these derivatized macrocycles allows for their use in a wider range of chemical environments and giving rise to myriad potential applications. The dissolution of parent CDs (α-, ß- and γ-) and CB[n]s (n=6-8) in deep eutectic solvents (DES) is reported, showing dramatic enhanced solubility of the larger species in both families, CB[7] and CB[8] as well as ß- and γ-CD, respectively. Furthermore, the host-guest properties are maintained in this new solvation medium.

Modulating the oxidation of cucurbit[n]urils.

The functionalisation of cucurbit[n]uril macrocycles carried out through an oxidative approach in water using ammonium persulfate was studied. Through complexation with a doubly-charged bisimidazolium guest we were able to detect, distinguish and quantify the presence of each CB[n]-(OH)x (where 1 ≤ x ≤ 2n) derivative for the first time. The impact of oxidation on each CB[n] (n = 6-8) was studied individually, as well as in the presence of other competing CB[n] species. We were able to understand the reactivity of the parent CB[n] alongside its hydroxylated derivatives, CB[n]-(OH)x, and show that the oxidation of CB[n] through a free-radical approach cannot result in stoichiometric hydroxylation despite previous literature reports by Bardelang, Ouari and co-workers, J. Am. Chem. Soc., 2015, 137, 10238. Furthermore, an in-depth study on hydroxylation of CB[7] was conducted. Through DFT calculations we were able to show that the second hydroxy substituent is preferentially located on the same glycoluril unit. Moreover, through optimisation of the reaction conditions we were able to access a protocol for controlled oxidation to yield a chemically monofunctional CB[7] derivative.

Association and liquid structure of pyridine-acetic acid mixtures determined from neutron scattering using a 'free proton' EPSR simulation model.

The liquid structure of pyridine-acetic acid mixtures have been investigated using neutron scattering at various mole fractions of acetic acid, χHOAc = 0.33, 0.50, and 0.67 and compared to the structures of neat pyridine and acetic acid. Data has been modelled using empirical potential structure refinement (EPSR) with a 'free proton' reference model, which has no prejudicial weighting towards either the existence of molecular or ionised species. Analysis of the neutron scattering results shows the existence of hydrogen-bonded acetic acid chains with pyridine inclusions, rather than the formation of an ionic liquid by proton transfer.

Brønsted acids in ionic liquids: how acidity depends on the liquid structure.

Gutmann Acceptor Number (AN) values have been determined for Brønsted acid-ionic liquid mixtures, over a wide compositional range. Four systems of general formula [C2mim][A]-HA (A(-) = bistriflamide, [NTf2](-); triflate, [OTf](-); mesylate, [OMs](-); or acetate, [OAc](-), [C2mim](+) = 1-ethyl-3-methylimidazolium cation) were studied. A library of Brønsted acidic systems of varying acidity was constructed and the AN parameter was found to be a convenient approach for quantifying their acidity. HOAc, HOMs and HOTf, when dissolved in ionic liquids, were found to associate with the respective anions to form hydrogen-bonded anionic clusters, [A(HA)x](-). In contrast, HNTf2 was solubilised as a discrete, undissociated molecule. AN values were sensitive to the presence of anionic clusters; acidity could be buffered to a particular AN by binding the solubilised acid in the anionic cluster form. Overall, a simple way to manipulate and quantify the Brønsted acidity of acid-ionic liquid mixtures was demonstrated, and measured AN values were related to liquid speciation.

Highly stretchable dynamic hydrogels for soft multilayer electronics.

Recent progress in the development of synthetic polymer networks has enabled the next generation of hydrogel-based machines and devices. The ability to mimic the mechanical and electrical properties of human tissue gives great potential toward the fields of bioelectronics and soft robotics. However, fabricating hydrogel devices that display high ionic conductivity while maintaining high stretchability and softness remains unmet. Here, we synthesize supramolecular poly(ionic) networks, which display high stretchability (>1500%), compressibility (>90%), and rapid self-recovery (<30 s), while achieving ionic conductivities of up to 0.1 S cm -1. Dynamic cross-links give rise to inter-layer adhesion and a stable interface is formed on account of ultrahigh binding affinities (>1013 M-2). Superior adherence between layers enabled the fabrication of an intrinsically stretchable hydrogel power source, paving the way for the next generation of multi-layer tissue mimetic devices.

Tissue-Mimetic Supramolecular Polymer Networks for Bioelectronics.

Addressing the mechanical mismatch between biological tissue and traditional electronic materials remains a major challenge in bioelectronics. While rigidity of such materials limits biocompatibility, supramolecular polymer networks can harmoniously interface with biological tissues as they are soft, wet, and stretchable. Here, an electrically conductive supramolecular polymer network that simultaneously exhibits both electronic and ionic conductivity while maintaining tissue-mimetic mechanical properties, providing an ideal electronic interface with the human body, is introduced. Rational design of an ultrahigh affinity host-guest ternary complex led to binding affinities (>1013  M-2 ) of over an order of magnitude greater than previous reports. Embedding these complexes as dynamic cross-links, coupled with in situ synthesis of a conducting polymer, resulted in electrically conductive supramolecular polymer networks with tissue-mimetic Young's moduli (<5 kPa), high stretchability (>500%), rapid self-recovery and high water content (>84%). Achieving such properties enabled fabrication of intrinsically-stretchable stand-alone bioelectrodes, capable of accurately monitoring electromyography signals, free from any rigid materials.

Facile, Energy-Efficient Microscale Fibrillation of Polyacrylamides under Ambient Conditions.

Insight into fiber formation can provide new rationale for the design and preparation of fibers with programmed mechanical properties. While synthetic bioinspired fibers have shown impressive tensile properties, the fiber formation process remains poorly understood. Moreover, these systems are highly complex and their formation is environmentally and economically costly. Controlled fiber formation under ambient conditions from polyacrylamide solutions with properties comparable to natural fibers such as wool and coir is demonstrated. Photopolymerization and subsequent microscale fibrillation of different acrylamides in water/ethanol mixtures yield a simple and energy-efficient route to fiber formation. This strategy reduces required processing energy by two-to-three orders of magnitude. Through extensive experimental elucidation, insight into precise fiber forming conditions of polymeric solutions is achieved. Ethanol is utilized as a chain transfer agent to control the molecular weight of the polyacrylamides, and the entanglement regimes of the solutions are determined through rheological characterization showing fiber formation above the entanglement concentration. Unique from previously reported hydrogel microfibers, it is shown that fibers with good mechanical properties can be obtained without the need for composites or crosslinkers. The reported approach offers a platform for fiber formation under ambient conditions with molecular-level understanding of their assembly.

Nanoparticle surfactants for kinetically arrested photoactive assemblies to track light-induced electron transfer.

Nature controls the assembly of complex architectures through self-limiting processes; however, few artificial strategies to mimic these processes have been reported to date. Here we demonstrate a system comprising two types of nanocrystal (NC), where the self-limiting assembly of one NC component controls the aggregation of the other. Our strategy uses semiconducting InP/ZnS core-shell NCs (3 nm) as effective assembly modulators and functional nanoparticle surfactants in cucurbit[n]uril-triggered aggregation of AuNCs (5-60 nm), allowing the rapid formation (within seconds) of colloidally stable hybrid aggregates. The resultant assemblies efficiently harvest light within the semiconductor substructures, inducing out-of-equilibrium electron transfer processes, which can now be simultaneously monitored through the incorporated surface-enhanced Raman spectroscopy-active plasmonic compartments. Spatial confinement of electron mediators (for example, methyl viologen (MV2+)) within the hybrids enables the direct observation of photogenerated radical species as well as molecular recognition in real time, providing experimental evidence for the formation of elusive σ-(MV+)2 dimeric species. This approach paves the way for widespread use of analogous hybrids for the long-term real-time tracking of interfacial charge transfer processes, such as the light-driven generation of radicals and catalysis with operando spectroscopies under irreversible conditions.

Design Principles for Aqueous Interactive Materials: Lessons from Small Molecules and Stimuli-Responsive Systems.

Interactive materials are at the forefront of current materials research with few examples in the literature. Researchers are inspired by nature to develop materials that can modulate and adapt their behavior in accordance with their surroundings. Stimuli-responsive systems have been developed over the past decades which, although often described as "smart," lack the ability to act autonomously. Nevertheless, these systems attract attention on account of the resultant materials' ability to change their properties in a predicable manner. These materials find application in a plethora of areas including drug delivery, artificial muscles, etc. Stimuli-responsive materials are serving as the precursors for next-generation interactive materials. Interest in these systems has resulted in a library of well-developed chemical motifs; however, there is a fundamental gap between stimuli-responsive and interactive materials. In this perspective, current state-of-the-art stimuli-responsive materials are outlined with a specific emphasis on aqueous macroscopic interactive materials. Compartmentalization, critical for achieving interactivity, relies on hydrophobic, hydrophilic, supramolecular, and ionic interactions, which are commonly present in aqueous systems and enable complex self-assembly processes. Relevant examples of aqueous interactive materials that do exist are given, and design principles to realize the next generation of materials with embedded autonomous function are suggested.

A simple supramolecular assay for drug detection in urine.

A supramolecular colorimetric assay utilising the macrocyclic host cucurbit[7]uril (CB[7]) with a commercial dye molecule, neutral red (NR), was evaluated as a novel method for drug detection in urine of a model therapeutic peptide drug Octreotide.