Solvent Acts as the Referee in a Match-Up Between Charged and Preorganized Receptors

Expanding the toolbox for supramolecular chemistry: probing host-guest interactions and binding with in situ FTIR spectroscopy

Association constant (K a) measurements provide fundamental information on host-guest interactions in supramolecular chemistry and other areas of science. Here we report the use of in situ FTIR spectroscopy to measure the K a values across three classes of host-guest complexes that involve hydrogen bonding and halogen bonding. This approach can be performed with minimal sample preparation, does not require deuterated solvents, can measure association based on changes in host or guest vibrations, and benefits from a much shorter timescale than NMR spectroscopy. Due to its fast timescale, FTIR spectroscopy also provides details on host/guest conformational changes, such as the presence of unsymmetrical host conformations that are not in the ideal binding conformation until treatment with a suitable guest. These changes would not be observable by standard time-averaged NMR titration measurements. Using this approach, we demonstrated the capabilities and challenges of this technique to investigate host-guest interactions of three anion receptors that use hydrogen or halogen bonding with both mono- and polyatomic anions. In addition to directly observing how host-guest interactions impact bonding within the individual molecules, we also demonstrate that global fitting of the FTIR spectra is an effective and robust approach to measure K a values of these host-guest complexes. We anticipate that this method will provide a new and useful approach to investigating the dynamics and specific interactions across broad areas of science.

Computer-aided design of triazolo-cages as anion receptors

Molecular cages with three-dimensional cavities have garnered significant interest due to their enhanced encapsulation abilities. In this study, we computationally investigate the binding behavior of a triazolo-cage receptor composed of alternating triazole and phenyl building blocks. With six different anions, including atomic (F-, Cl-, Br-, and I-), linear (SCN-), and trigonal planar (NO3-) geometries, we analyze the binding selectivity of the parent cage with DFT calculations. The influence of solvation on binding strength is investigated by calculating binding free energies in both gas phase and six solvent environments of progressively increasing dielectric constants. Symmetry-Adapted Perturbation Theory (SAPT) analysis reveals that electrostatic interactions dominate the binding process. Additionally, we perform computer-aided design to generate a series of new cage receptors with diverse functionalities, and our findings highlight the tunable chloride affinity achieved by adjusting various cage properties. Overall, this study offers insights into the design of novel cage receptors with versatile functionalities and provides a strategic approach to the rational design of anion receptors.

Helical aromatic foldamers with tubular cavities capable of accommodating multiple halide ions

A series of pyridinium-indolocarbazole oligomers with different lengths, capable of adopting helical conformations with tubular cavities, has been synthesized. These oligomers can accommodate two, three, or four iodide ions in a linear arrangement within their cavities, which can be exchanged with other halides.

Solvent effects in anion recognition

Anion recognition is pertinent to a range of environmental, medicinal and industrial applications. Recent progress in the field has relied on advances in synthetic host design to afford a broad range of potent recognition motifs and novel supramolecular structures capable of effective binding both in solution and at derived molecular films. However, performance in aqueous media remains a critical challenge. Understanding the effects of bulk and local solvent on anion recognition by host scaffolds is imperative if effective and selective detection in real-world media is to be viable. This Review seeks to provide a framework within which these effects can be considered both experimentally and theoretically. We highlight proposed models for solvation effects on anion binding and discuss approaches to retain strong anion binding in highly competitive (polar) solvents. The synthetic design principles for exploiting the aforementioned solvent effects are explored.