Direct Detection of Hydrogen Bonds in Supramolecular Systems Using 1H-15N Heteronuclear Multiple Quantum Coherence Spectroscopy

Efficient detection of 1H, 15N correlations in hydrogen bonded low molecular catalyst-substrate intermediates without selective 15N-labelling

To date, SOFAST approaches have generally been limited to biomolecules. We present the applicability of SOFAST-HMQC techniques to small molecules in the slow-tumbling regime offering a time-efficient characterization of catalyst substrate hydrogen bonds with nitrogen at natural abundance. This extends NMR access to a broader range of catalyst substrate combinations.

Influence of Protonation and Iron(II) Complexation on Magnetic Interactions in Spin-Labeled Mechanically Interlocked Molecules

This study reports the effects of protonation and Fe(II) complexation on the exchange coupling and magnetic properties of a [2]-rotaxane and its corresponding free-axle biradicals. The rotaxane features nitronyl-nitroxide units as bulky stopper groups, a triazole-pyridine-triazole ligand as the axle component, and a bipyridine moiety within the macrocyclic ring. Our findings, obtained by Electron Paramagnetic Resonance spectroscopy in solution and magnetometry in solid state, demonstrate that rotaxanation and protonation induce profound changes in the magnetic interactions of the biradical both in its free form and as complexed with Fe(II). These results highlight the transformative impact of the mechanical bond on the electronic structure of interlocked radical systems.

Symmetry Breaking in Supramolecular Gel Condensation

Supramolecular condensation during cooling cycles often transitions through multiple metastable phases before achieving a stable crystalline state. Metastability arises from various competing parameters like symmetrical arrangement, and supramolecular bonding and manifests at different temperatures. Symmetrical physical arrangements can minimize vibrational energy and stabilize the systems at higher temperatures. Further cooling promotes directional supramolecular bonding, such as charge-assisted hydrogen bonding, resulting in molecular periodicity within metastable structures. Frustration occurs when weaker van der Waals bonds form during further cooling, propagating perpendicularly to stronger one-dimensional charge-assisted hydrogen bonds and disrupting lateral periodicity in certain solvents. This makes parallel 1D fibers slidable, adding flexibility to the gel fiber. Eventually, some supramolecular systems attain thermodynamically stable crystalline states by perfectly arranging all the molecules. Throughout the process, metastability results from different symmetrical arrangements, and each rearrangement alters the supramolecular structure's symmetry, generating new physicochemical properties. Different supramolecular gels uniquely break symmetry, which can be monitored through various techniques. This perspective analyzes supramolecular thermoreversible, reverse thermal, liquid crystalline, thixotropic, and antisolvent-induced gels to illustrate spontaneous symmetry reduction processes. Reaching a suprasymmetry condensate can classify big data and be applied in unconventional analogue computing or data storage.

Visualizing Dynamic Weak Interaction Networks of Fluorine Atoms within Proteins via NMR

The utilization of fluorine probes in protein research has advanced our understanding of the nature of macromolecules. The diverse activities of proteins are governed by intricate weak interaction networks, yet the experimental detection presents a challenge. Herein, we have developed an NMR analytical method based on quantum coherent operations to characterize the weak chemical bonds of fluorine atoms within proteins that are bound to metal fluorides or labeled with fluorinated amino acids. Our approach offers a fast-screening method for identifying a weak interaction network without requiring crystallization.

Active template synthesis

The active template synthesis of mechanically interlocked molecular architectures exploits the dual ability of various structural elements (metals or, in the case of metal-free active template synthesis, particular arrangements of functional groups) to serve as both a template for the organisation of building blocks and as a catalyst to facilitate the formation of covalent bonds between them. This enables the entwined or threaded intermediate structure to be covalently captured under kinetic control. Unlike classical passive template synthesis, the intercomponent interactions transiently used to promote the assembly typically do not 'live on' in the interlocked product, meaning that active template synthesis can be traceless and used for constructing mechanically interlocked molecules that do not feature strong binding interactions between the components. Since its introduction in 2006, active template synthesis has been used to prepare a variety of rotaxanes, catenanes and knots. Amongst the metal-ion-mediated versions of the strategy, the copper(I)-catalysed alkyne-azide cycloaddition (CuAAC) remains the most extensively used transformation, although a broad range of other catalytic reactions and transition metals also provide effective manifolds. In metal-free active template synthesis, the recent discovery of the acceleration of the reaction of primary amines with electrophiles through the cavity of crown ethers has proved effective for forming an array of rotaxanes without recognition elements, including compact rotaxane superbases, dissipatively assembled rotaxanes and molecular pumps. This Review details the active template concept, outlines its advantages and limitations for the synthesis of interlocked molecules, and charts the diverse set of reactions that have been used with this strategy to date. The application of active template synthesis in various domains is discussed, including molecular machinery, mechanical chirality, catalysis, molecular recognition and various aspects of materials science.

Local weak hydrogen bonds induced dipole-dipole interactions in polymer for enhancing photocatalytic oxidation

Functionalizing organic polymers is an effective strategy for enhancing their photocatalytic performance. However, this approach is currently limited by specific motifs, complex preparation methods, and an unclear electron transfer mechanism. Here, we present a meticulously designed structure of perylene diimide connected with poly (barbituric acid trimer) through self-assembled hydrogen bonding. In particular, the local chemical environment of the two components is adjusted by hydrogen bond-induced dipole-dipole interactions, leading to the emergence of a significant inherent electric field. Additionally, the formation of hydrogen bonds provides electronic pathways that facilitate charge transfer from perylene to adjacent units. Moreover, the distinctive electronic structure enhances polarity transfer and improves activation and adsorption capabilities for reactive molecules. Ultimately, B-PDI exhibits outstanding oxidation rates for benzylamine to N-benzylidene-benzylamine (10.03 mmol g-1h-1) and selectivity (>99.99 %). Our work offers a widely popular approach for enhancing the photocatalytic activity of organic semiconductor materials by constructing hydrogen bonds in heterogeneous molecules.

Visible-Light Driven Control Over Triply and Quadruply Hydrogen-Bonded Supramolecular Assemblies

Supramolecular polymers offer tremendous potential to produce new "smart" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs. I, Incorporating a triply hydrogen-bonded heterodimer, was found to exhibit concentration dependent switching between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. II, Based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both monomers II and III, demonstrating the ability to control supramolecular assembly and physical properties triggered by light.