Anion-π interaction at the solid/water interfaces

Synergistic anion-π interactions in peptidomimetic polyethers

Anion-π interactions are crucial in various biological processes, such as enzyme catalysis and ion transport. Despite their significance, the exploitation of anion-π interactions in synthetic polymer systems remains underexplored. This study investigates anion-π interactions using chemically well-defined peptidomimetics guided by the composition of mussel foot proteins. Specifically, polyether-based polymers were designed utilizing two functional epoxide monomers-catechol acetonide glycidyl ether and 4,4-dimethyl-2-oxazoline glycidyl ether-to mimic the key amino acids 3,4-dihydroxyphenylalanine and aspartic acid, respectively. A surface forces apparatus was employed to study the anion-π interaction between the polymers, considering the effects of relative monomer composition and pH conditions. The maximum cohesion energy of 15.0 mJ/m2 was observed at an equimolar monomer composition at pH 7. Incorporating a phenyl group instead of the catechol group and introducing competing anions confirmed the dominant role of anion-π interactions. This study highlights the significance of anion-π interactions, posing a high potential in the design and synthesis of functional materials.

Mechanoelectrical Transduction through Anion Recognition with Naphthalenediimide Monolayers at the Air-Water Interface

In biological systems, various stimuli and energies are transduced into membrane potentials via ion transport or binding. The application of this concept to artificial devices may result in biomimetic signal transmitters and energy harvesters. In this study, we investigated the mechanical control of fluoride anion recognition with naphthalenediimide (NDI) monolayers at the air-water interface. Similar to the mechanosensitive ion channels in biological membranes, mechanical stimuli modulated the packing manner of the NDI monolayers, which reproducibly triggered anion binding and concomitant shifts in the membrane potential. Furthermore, mechanical stimuli resulted in anion binding or release, depending on the structure of the alkyl side chains attached to the NDI core, which was explained by the difference in the packing manner of the NDI monolayers. These findings provide insights into the development of novel mechanoelectrical transduction systems that mimic biological processes.

Anion-π Interaction in a Diketopyrrolopyrrole Derivative

In this work, an N-substituted diketopyrrolopyrrole (DPP) derivative Ph-DPP was synthesized, showing interaction toward Lewis alkaline anions such as F-. The typical electron-transfer-dominated anion-π interaction product Ph-DPP•- and unexpected isomer product i-Ph-DPP were both observed, and their formation mechanism was studied by density functional theory calculations, suggesting that a deprotonation initiation route is favored, which gives interesting insight for understanding the debatable role of F- in such non-covalent intermolecular interactions.

An anomalous anion transfer order in graphene oxide membranes induced by anion-π interactions

Selective transport of anions across membranes has become an important goal in chemistry and biology. Here, we found an anomalous anion transfer order within the graphene oxide membrane: Cl^- > Br^- > F^- > I^-. This is at odds with the conventional ranking of the transfer order, which usually decreases as the radii of the anions increase, i.e., F^- > Cl^- > Br^- > I^-. The abnormal transportation of F^- can be ascribed to the strong anion-π interactions between F^- and graphene oxide sheets. Such unexpectedly strong anion-π interaction resulted in the lower movement of F^- in the graphene oxide membrane and caused the anomalous anion transfer order. Our findings not only provide experimental evidence of anion-π interactions, but also improve our understanding of anion-π interactions in the selective transport of anions across a two-dimensional membrane.

Anion-π Interactions in Monolayers Formed by Amphiphilic Electron-Deficient Aromatic Compounds at Air/Water Interfaces

Biological systems precisely and selectively control ion binding through various chemical reactions, molecular recognition, and transport by virtue of effective molecular interactions with biological membranes and proteins. Because ion binding is inhibited in highly polar media, recognition systems for anions in aqueous media, which are relevant to biological and environmental systems, are still limited. In this study, we explored the anion binding of Langmuir monolayers formed by amphiphilic naphthalenediimide (NDI) derivatives with a series of substituents at air/water interfaces via anion-π interactions. Density functional theory (DFT) simulations revealed that the binding of anions originating from anion-π interactions is related to the electron density of the anions. At the air/water interfaces, amphiphilic NDI derivatives formed Langmuir monolayers, and the addition of anions caused expansion of the Langmuir monolayers. The anions with larger hydration energies related to electron density showed larger binding constants (K_a) for 1:1 stoichiometry with the NDI derivatives. The loosely packed monolayer formed by the amphiphilic NDI derivatives with bromine groups showed a better anion response. In contrast, the binding of NO₃^- was significantly enhanced in the highly packed monolayer. These results indicate that the packing of NDI derivatives with rigid aromatic rings influenced the binding of the anions. These results provide insight into ion binding using the air/water interface as a promising recognition site for mimicking biological membranes. In future, sensing devices can be developed using Langmuir-Blodgett films on electrodes. Furthermore, the capture of anions on electron-deficient aromatic compounds can lead to doping or composition technologies for n-type semiconductors.

Inner and Interfacial Environmental Nanoarchitectonics of Supramolecular Assemblies Formed by Amphiphiles: from Emergence to Application

The inner and interfacial environments of self-assemblies provide fascinating nano-space for selective and efficient chemical reactions and processes. In biological systems, various chemical reactions, molecular recognition, and transport occur precisely and selectively by virtue of effective molecular interactions on biological membranes and proteins. Considering these advantages and the concept of nanoarchitectonics, we demonstrated that the photochromism of a lophine dimer was accelerated by using confined nano-spaces formed by surfactant micelles. The photoresponsive micelles were used for the rapid controlled release of a model drug upon ultraviolet light irradiation. Furthermore, selective ion recognition inside the self-assembled molecular films at the interfaces was investigated. The anion-π interaction between the anion and an electron-deficient aromatic ring was evaluated on a solid substrate modified with a naphthalenediimide (NDI) analog. Force curve measurements afforded a quantitative analysis of anion-π interactions on the NDI film. The strength of anion-π interactions is regulated by the electric fields on the electrode. An optical probe was developed to visualize the distribution of Cs ions in the soil, plant bodies, and aqueous media using an optode system. Advances in the development of molecular functional systems are expected based not only on molecular structures but also on the spaces and environments produced by them.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.