Cucurbit[8]uril Mediated Donor–Acceptor Ternary Complexes: A Model System for Studying Charge-Transfer Interactions

A supramolecular self-assembly approach is described which allows for the convenient preparation of a wide range of charge-transfer (CT) donor–acceptor complexes in aqueous solutions. When one equiv of the macrocyclic host cucurbit[8]­uril (CB[8]) is added to an aqueous donor and acceptor solution, a heteroternary complex forms inside the host’s cavity with a well-defined face-to-face π–π-stacking geometry of the donor and acceptor. This heteroternary, CB[8]-mediated complex offers the opportunity to study the CT phenomena at low concentrations and free from complications arising from any donor–donor and acceptor–acceptor interactions as a result of the large binding affinities and the very high selectivity over the formation of these homoternary complexes. Thus, this supramolocular self-assembly strategy is a practical donor–acceptor mix-and-match approach with synthetic advantages over much more cumbersome tethering schemes. While the characteristic UV/vis features of a few CB[8] ternary systems had been described as a CT band, we present for the first time systematic evidence for the existence of CT interactions between several donor–acceptor pairs that are mediated by the host CB[8]. Correlation of the experimentally obtained CT λ_max to computed HOMO–LUMO energies demonstrated that the CT process in the host’s cavity can be described by the Mulliken model. Furthermore, the literature claim of a “CT driving force” for the formation of CB[8] ternary complexes was scrutinized and evaluated by calorimetric (ITC) and ESI-MS measurements. The findings indicated that neither in the aqueous medium nor in the “gas-phase” is CT of energetic relevance to the Gibbs free binding energy. In contrast, electrostatic considerations combined with solvation effects are much better suited to rationalize the observed trends in binding affinities. Additionally, the CT λ_max was found to be much more red-shifted (≥75 nm) inside the CB[8] cavity than in any polar organic solvents or water, indicating a significant stabilization of the CT excited state within the host cavity, further demonstrating the unique electrostatic, polar properties of the host cavity

A selective supramolecular photochemical sensor for dopamine

<div><p>Dopamine (DA) is an important biomarker for diseases and biological disorders. Existing techniques for DA detection suffer from drawbacks including low sensitivity and selectivity as well as interfering signals from non-target molecules. A simple and selective photochemical sensor for the determination of DA in a supramolecular manner is presented. This approach utilises the complexation properties of a highly fluorescent water-soluble complex of perylene bis(diimide) dye with the macrocyclic host cucurbit[8]uril. The method can be used for the determination of DA in aqueous media, with detection limits below 2 × 10^− 5 M, even in the presence of known interferents including ascorbic acid and the catecholamines epinephrine and norepinepherine.</p></div

Facile Method for Preparing Surface-Mounted Cucurbit[8]uril-Based Rotaxanes

Surface-immobilized rotaxanes are of practical interest for myriad applications including molecular rotors and analytical sensing. Herein, we present a facile method for the preparation of cucurbit[8]­uril (CB[8])-based rotaxanes on gold (Au) surfaces threaded onto a viologen (MV²⁺) axle. The surface-bound CB[8] rotaxanes were characterized by contact angle measurements and optical microscopy. Direct imaging of the rotaxanes was accomplished by attaching either azobenzene-functionalized silica (Si-azo) colloids or fluorescein-labeled dopamine that were bound to the Au surface through a supramolecular heteroternary (1:1:1) complex with CB[8]. The surface density of CB[8] rotaxanes was examined based on their detection of dopamine. The calculated surface density is 4.8 × 10¹³ molecules·cm^–2, which is only slightly lower than the theoretical value of 5.0 × 10¹³ molecules·cm^–2. Surface-functionalized rotaxanes can be reversibly switched using external stimuli to bind electron-rich second guests for CB[8], including both small molecules such as dopamine and appropriately-functionalized colloidal particles. Such controlled reversibility gives rise to potential applications including selective sensing or reusable templates for preparing well-defined colloidal arrays. The formation of the surface-bound rotaxane structure is critical for successfully anchoring CB[8] host molecules onto Au substrates, yielding an interlocked architecture and preventing the dissociation of binary host–guest complex MV²⁺⊂CB­[8]. The MV²⁺⊂CB­[8] rotaxane structure thus effectively maintains the material density on the Au surface and dramatically enhances the stability of the functional surface

Quantitative SERS Using the Sequestration of Small Molecules Inside Precise Plasmonic Nanoconstructs

We show how the macrocyclic host, cucurbit[8]­uril (CB[8]), creates precise subnanometer junctions between gold nanoparticles while its cavity simultaneously traps small molecules; this enables their reproducible surface-enhanced Raman spectroscopy (SERS) detection. Explicit shifts in the SERS frequencies of CB[8] on complexation with guest molecules provides a direct strategy for absolute quantification of a range of molecules down to 10^–11 M levels. This provides a new analytical paradigm for quantitative SERS of small molecules

Formation of Cucurbit[8]uril-Based Supramolecular Hydrogel Beads Using Droplet-Based Microfluidics

Herein we describe the use of microdroplets as templates for the fabrication of uniform-sized supramolecular hydrogel beads, assembled by supramolecular cross-linking of functional biopolymers with the macrocyclic host molecule, cucurbit[8]­uril (CB[8]). The microdroplets were formed containing diluted hydrogel precursors in solution, including the functional polymers and CB[8], in a microfluidic device. Subsequent evaporation of water from collected microdroplets concentrated the contents, driving the formation of the CB[8]-mediated host–guest ternary complex interactions and leading to the assembly of condensed three-dimensional polymeric scaffolds. Rehydration of the dried particles gave monodisperse hydrogel beads. Their equilibrium size was shown to be dependent on both the quantity of material loaded and the dimensions of the microfluidic flow focus. Fluorescein-labeled dextran was used to evaluate the efficacy of the hydrogel beads as a vector for controlled cargo release. Both passive, sustained release (hours) and triggered, fast release (minutes) of the FITC-dextran was observed, with the rate of sustained release dependent on the formulation. The kinetics of release was fitted to the Ritger-Peppas controlled release equation and shown to follow an anomalous (non-Fickian) transport mechanism

Release of High-Energy Water as an Essential Driving Force for the High-Affinity Binding of Cucurbit[<i>n</i>]urils

Molecular dynamics simulations and isothermal titration calorimetry (ITC) experiments with neutral guests illustrate that the release of high-energy water from the cavity of cucurbit­[<i>n</i>]­uril (CB<i>n</i>) macrocycles is a major determinant for guest binding in aqueous solutions. The energy of the individual encapsulated water molecules decreases with increasing cavity size, because larger cavities allow for the formation of more stable H-bonded networks. Conversely, the total energy of internal water increases with the cavity size because the absolute number of water molecules increases. For CB7, which has emerged as an ultrahigh affinity binder, these counteracting effects result in a maximum energy gain through a complete removal of water molecules from the cavity. A new design criterion for aqueous synthetic receptors has therefore emerged, which is the optimization of the size of cavities and binding pockets with respect to the energy and number of residing water molecules

Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers

Herein we report the photocontrol of cucurbit[8]­uril (CB[8])-mediated supramolecular polymerization of azobenzene-containing monomers. The CB[8] polymers were characterized both in solution and in the solid state. These host–guest complexes can be reversibly switched between highly thermostable photostationary states. Moreover, a remarkable stabilization of <i>Z</i>-azobenzene was achieved by CB[8] complexation, allowing for structural characterization in the solid state

Ultrahigh-Water-Content Supramolecular Hydrogels Exhibiting Multistimuli Responsiveness

Hydrogels are three-dimensional networked materials that are similar to soft biological tissues and have highly variable mechanical properties, making them increasingly important in a variety of biomedical and industrial applications. Herein we report the preparation of extremely high water content hydrogels (up to 99.7% water by weight) driven by strong host–guest complexation with cucurbit[8]­uril (CB[8]). Cellulosic derivatives and commodity polymers such as poly­(vinyl alcohol) were modified with strongly binding guests for CB[8] ternary complex formation (<i>K</i>_eq = 10¹² M^–2). When these polymers were mixed in the presence of CB[8], whereby the overall solid content was 90% cellulosic, a lightly colored, transparent hydrogel was formed instantaneously. The supramolecular nature of these hydrogels affords them with highly tunable mechanical properties, and the dynamics of the CB[8] ternary complex cross-links allows for rapid self-healing of the materials after damage caused by deformation. Moreover, these hydrogels display responsivity to a multitude of external stimuli, including temperature, chemical potential, and competing guests. These materials are easily processed, and the simplicity of their preparation, their availability from inexpensive renewable resources, and the tunability of their properties are distinguishing features for many important water-based applications

Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers

Herein we report the photocontrol of cucurbit[8]­uril (CB[8])-mediated supramolecular polymerization of azobenzene-containing monomers. The CB[8] polymers were characterized both in solution and in the solid state. These host–guest complexes can be reversibly switched between highly thermostable photostationary states. Moreover, a remarkable stabilization of <i>Z</i>-azobenzene was achieved by CB[8] complexation, allowing for structural characterization in the solid state

Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers

Herein we report the photocontrol of cucurbit[8]­uril (CB[8])-mediated supramolecular polymerization of azobenzene-containing monomers. The CB[8] polymers were characterized both in solution and in the solid state. These host–guest complexes can be reversibly switched between highly thermostable photostationary states. Moreover, a remarkable stabilization of <i>Z</i>-azobenzene was achieved by CB[8] complexation, allowing for structural characterization in the solid state