Selective Anion-Induced Crystal Switching and Binding in Surface Monolayers Modulated by Electric Fields from Scanning Probes

Anion-selective (Br^– and I^–) and voltage-driven crystal switching between two differently packed phases (α ⇆ β) was observed in 2D crystalline monolayers of aryl-triazole receptors ordered at solution–graphite interfaces. Addition of Br^– and I^– was found to stimulate the α → β phase transformation and to produce ion binding to the β phase assembly, while Cl^– and BF₄^– addition retained the α phase. Unlike all other surface assemblies of either charged molecules or ion-templated 2D crystallization of metal-ligand or receptor-based adsorbates, the polarity of the electric field between the localized scanning tip and the graphite substrate was found to correlate with phase switching: β → α is driven at −1.5 V, while α → β occurs at +1.1 V. Ion-pairing between the countercations and the guest anions was also observed. These observations are supported by control studies including variation of anion species, relative anion concentration, surface temperature, tip voltage, and scanning time

Polarized Naphthalimide CH Donors Enhance Cl^– Binding within an Aryl-Triazole Receptor

The dipolar character of 1,8-naphthalimide together with polarization of the C⁴–H and C⁵–H donors has been utilized in receptor <b>1</b> to effectively bind chloride alongside triazole and phenylene units. The Cl^– binding strength of <b>1</b> shows that the naphthalimide provides greater anion stabilization than an unactivated phenylene, and DFT calculations show that its collinear donor array can be a “urea-like” analog for CH···anion interactions

Electrostatic and Allosteric Cooperativity in Ion-Pair Binding: A Quantitative and Coupled Experiment–Theory Study with Aryl–Triazole–Ether Macrocycles

Cooperative binding of ion pairs to receptors is crucial for the manipulation of salts, but a comprehensive understanding of cooperativity has been elusive. To this end, we combine experiment and theory to quantify ion-pair binding and to separate allostery from electrostatics to understand their relative contributions. We designed aryl–triazole–ether macrocycles (<b>MC</b>) to be semiflexible, which allows ion pairs (NaX; X = anion) to make contact, and to be monocyclic to simplify analyses. A multiequilibrium model allows us to quantify, for the first time, the experimental cooperativity, α, for the equilibrium <b>MC</b>·Na⁺ + <b>MC</b>·X^– ⇌ <b>MC</b>·NaX + <b>MC</b>, which is associated with contact ion-pair binding of NaI (α = 1300, Δ<i>G</i>_α = −18 kJ mol^–1) and NaClO₄ (α = 400, Δ<i>G</i>_α = −15 kJ mol^–1) in 4:1 dichloromethane–acetonitrile. We used accurate energies from density functional theory to deconvolute how the electrostatic effects and the allosteric changes in receptor geometry individually contribute to cooperativity. Computations, using a continuum solvation model (dichloromethane), show that allostery contributes ∼30% to overall positive cooperativity. The calculated trend of electrostatic cooperativity using pairs of spherical ions (NaCl > NaBr > NaI) correlates to experimental observations (NaI > NaClO₄). We show that intrinsic ionic size, which dictates charge separation distance in contact ion pairs, controls electrostatic cooperativity. This finding supports the design principle that semiflexible receptors can facilitate optimal electrostatic cooperativity. While Coulomb’s law predicts the size-dependent trend, it overestimates electrostatic cooperativity; we suggest that binding of the individual anion and cation to their respective binding sites dilutes their effective charge. This comprehensive understanding is critical for rational designs of ion-pair receptors for the manipulation of salts