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

Abstract

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.

Anion‐(π)n‐π Catalytic Micelles

“…An excellent recent study reports anion-π interactions in monolayers of NDI amphiphiles at the air/water interface. [25] The delocalized yet directional nature of anion-π interactions suggests that anion-π catalysis would be most suitable for reactions that involve electron displacement over long distances in coupled transition states. With the stabilization of multiple carbocation stabilization over πbasic arrays during terpene and steroid cyclization, lessons from nature support this expectation by analogy with canonical cation-π biocatalysis.…”

Anion‐(π)n‐π Catalytic Micelles

“…Pioneering examples for catalysis in NDI micelles include Suzuki–Miyaura couplings, [21,22] an early aldolization catalyzed by an NDI nanotube with tethered prolines, [23] and bisphenol A photodegradation in homologous PDI micelles, [19] all possibly benefitting from anion‐π interactions, although this possibility was neither mentioned nor explored. An excellent recent study reports anion‐π interactions in monolayers of NDI amphiphiles at the air/water interface [25] …”

Anion–π catalysis on carbon allotropes

“…Particularly MWCNTs have the potential to couple anion-π and cation-π catalysis with electricfield-assisted catalysis [14]. While anion-π (and cation-π [15,16]) catalysis, compared to other unorthodox interactions, has been less impactful than expected [3,4,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31], this combination has the potential to revolutionize organic catalysis [32][33][34][35][36][37][38][39][40][41][42][43]. These high expectations have never been realized for technical reasons.…”