Cucurbit[8]uril/Cucurbit[7]uril Controlled Off/On Fluorescence of the Acridizinium and 9-Aminoacridizinium Cations in Aqueous Solution

Controlled oligomeric guest stacking by cucurbiturils in water

Previously, we reported a guest molecule containing a viologen (V), a phenylene (P) and an imidazole (I) fragment (VPI) forming a host : guest 2 : 2 complex with cucurbit[8]uril (CB[8]) and an unprecedented 2 : 3 complex with cucurbit[10]uril (CB[10]). To better address the structural features required to form these complexes, two VPI analogues were designed and synthesized: the first with a tolyl (T) group grafted on the V part (T-VPI) and the second with a naphthalene (N) fused on the imidazole (I) part (VPI-N). While VPI-N afforded a discrete well-defined 2 : 2 complex with CB[8] and a 2 : 3 complex with CB[10], T-VPI organized also as a 2 : 2 complex with CB[8] but no well-defined complex was obtained with CB[10]. These complexes were studied by NMR spectroscopy, notably DOSY, which allowed us to estimate binding constants for 2 : 2 complex formation with CB[8], pointing to more stable 2 : 2 complexes with more hydrophobic guests. UV-vis and fluorescence spectroscopy confirmed complex formation, suggesting host-stabilized charge-transfer interactions. Therefore, the simple addition of CB[8] or CB[10] enabled us to control the level of guest stacking (dimer or trimer) using relevant pairs of synthetic hosts through spontaneous host : guest quaternary or quinary self-assembly.

Metal-Assisted Complexation of Fluorogenic Dyes by Cucurbit[7]uril and Cucurbit[8]uril: A DFT Evaluation of the Key Factors Governing the Host-Guest Recognition

With the emergence of host-guest systems, a novel branch of complexation chemistry has found wide application in industries such as food, pharmacy, medicine, environmental protection and cosmetics. Along with the extensively studied cyclodextrins and calixarenes, the innovative cucurbiturils (CB) have enjoyed increased popularity among the scientific community as they possess even better qualities as cavitands as compared to the former molecules. Moreover, their complexation abilities could further be enhanced with the assistance of metal cations, which can interestingly exert a dual effect on the complexation process: either by competitively binding to the host entity or cooperatively associating with the CB@guest structures. In our previous work, two metal species (Mg²⁺ and Ga³⁺) have been found to bind to CB molecules in the strongest fashion upon the formation of host-guest complexes. The current study focuses on their role in the complex formation with three dye molecules: thiazole orange, neutral red, and thioflavin T. Various key factors influencing the process have been recognized, such as pH and the dielectric constant of the medium, the cavity size of the host, Mⁿ⁺ charge, and the presence/absence of hydration shell around the metal cation. A well-calibrated DFT methodology, solidly based and validated and presented in the literature experimental data, is applied. The obtained results shed new light on several aspects of the cucurbituril complexation chemistry.

Assembly and Applications of Macrocyclic-Confinement-Derived Supramolecular Organic Luminescent Emissions from Cucurbiturils

Cucurbit[n]urils (Q[n]s or CB[n]s), as a classical of artificial organic macrocyclic hosts, were found to have excellent advantages in the fabricating of tunable and smart organic luminescent materials in aqueous media and the solid state with high emitting efficiency under the rigid pumpkin-shaped structure-derived macrocyclic-confinement effect in recent years. This review aims to give a systematically up-to-date overview of the Q[n]-based supramolecular organic luminescent emissions from the confined spaces triggered host-guest complexes, including the assembly fashions and the mechanisms of the macrocycle-based luminescent complexes, as well as their applications. Finally, challenges and outlook are provided. Since this class of Q[n]-based supramolecular organic luminescent emissions, which have essentially derived from the cavity-dependent confinement effect and the resulting assembly fashions, emerged only a few years ago, we hope this review will provide valuable information for the further development of macrocycle-based light-emitting materials and other related research fields.

Supramolecular spectral/visual detection of urinary polyamines through synergetic/competitive complexation with γ-CD and CB[7]

A supramolecular strategy for detecting the concentration of polyamines has been established through competitive/synergetic complexation among polyamines, CB[7], γ-CD, and pyrene derivatives, which allows for convenient, rapid, and high throughput spectral/visual detection of the concentration of urinary polyamines based on the switching on/off of the pyrene excimer fluorescence.

Tuning the emission of a water-soluble 3-hydroxyflavone derivative by host-guest complexation

3-Hydroxyflavone derivatives have great potential as fluorescent probes for bio-labeling in aqueous medium. They were extensively studied in various organic solvents for the "excited state intramolecular proton transfer" process, but seldom addressed in aqueous solution due to the poor water solubility. Herein, an amphiphilic molecule bearing 3-hydroxyflavone and oligo(ethylene oxide) (denoted as 3HF-EO) was designed and synthesized. Different from the fluorescence in organic solvents, 3HF-EO in aqueous solution showed a remarkable single fluorescence emission, which is ascribed to the fluorescence of its anionic species. We found that the fluorescence intensity could be efficiently tuned via host-guest complexation. α-CD has little effect on the emission, while β-CD and γ-CD lead to enhanced and reduced emissions of 3HF-EO, respectively. The 1H NMR and 2D NOESY NMR spectra indicate that α-CD barely had any interaction with 3HF-EO, while β-CD and γ-CD formed complexes with one and two 3HF-EO molecules, respectively. These results provide a sound explanation for the modulated fluorescence intensity.

Fluorescence enhancement and pKa shift of a rho kinase inhibitor by a synthetic receptor

Fasudil (FSD), a selective rho kinase (ROCK) inhibitor, was found to form 1 : 1 host-guest inclusion complexes with a synthetic macrocyclic receptor, cucurbit[7]uril (CB[7]), in aqueous solutions, as evidenced by ¹H NMR, photoluminescence and UV-visible spectroscopic titrations, isothermal titration calorimetry (ITC) titration, and electrospray ionization (ESI) mass spectrometry, as well as density functional theory (DFT) molecular modeling. Upon encapsulation, whereas the UV-vis absorbance of FSD experienced a moderate decrease and bathochromic shift, the fluorescence intensity of FSD at 354 nm was dramatically enhanced for up to 69-fold at neutral pH, which could potentially be applied in fluorescent tracking of the drug delivery and release. More interestingly, the binding affinity (K_a = (4.28 ± 0.21) × 10⁶ M^-1), of FSD-CB[7] complexes under acidic conditions (pH = 2.0), is approximately three orders of magnitude higher than that (2.2∼6.6 × 10³ M^-1) under neutral pH conditions (pH = 7.0). Accordingly, UV-visible spectroscopic titration of the free and complexed FSD under various pH conditions has demonstrated that the encapsulation of FSD by CB[7] shifted the pK_a of the isoquinoline-N upward from 3.05 to 5.96 (ΔpK_a of 2.91). The significantly higher binding affinity of the complexes under acidic conditions may be applied in developing the "enteric" formulation of FSD. Furthermore, our in vitro study of the bioactivity of FSD in the absence and presence of CB[7] on a neural cell line, SH-SY5Y, showed that the complexation preserved the drug's pro-neurite efficacy. Thus this discovery may lead to a fluorescence-trackable, orally administered enteric formulation of rho kinase inhibitors that are stable under gastric conditions, without compromising bioactivity of the drugs.

White Light Emission from Cucurbituril-Based Host-Guest Interaction in the Solid State: New Function of the Macrocyclic Host

Energy transfer and interchange are central for fabricating white light-emitting organic materials. However, increasing the efficiency of light energy transfer remains a considerable challenge because of the occurrence of "cross talk". In this work, by exploiting the unique photophysical properties of cucurbituril-triggered host-guest interactions, the two complementary luminescent colors blue and yellow for white light emission were independently obtained from a single fluorophore dye rather than energy transfer. Further study suggested that the rigid cavity of cucurbiturils efficiently prevented the aggregation of the dye and improved its thermal stability in the solid state by providing a regular nanosized fence for each encapsulated dye molecule. As a result, a novel macrocycle-assisted supramolecular approach for obtaining solid, white light-emitting organic materials with low cost, high efficiency, and easy scale-up was successfully demonstrated.

Perturbation of cationic equilibrium by cucurbit-7-uril

The effect of cucurbit-7-uril (CB-7) on a cationic mixture with same charge has been investigated by studying monocationic mixtures of 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b) and 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-c]pyridine (DMAPIP-c). The pK_a of both the guests increases in CB-7. DMAPIP-b forms all three monocations in the ground and the excited states in aqueous as well as in CB-7 solution. However, CB-7 shifts the equilibrium more towards the less polar MC2 and MC3. On the other hand, DMAPIP-c exists only as MC1 and MC3 in aqueous solution, however, in CB-7 it exists not only as MC1 and MC3 but also as MC2 in CB-7 in the ground state. In the excited state, as in aqueous solution MC1 forms MC2 by biprotonic phototautomerism in CB-7. The association constants of monocations suggest a pyridyl nitrogen position dependence. All the MC-CB-7 complexes are optimized by density functional theory (DFT).

Interactions between photoacidic 3-hydroxynaphtho[1,2-b]quinolizinium and cucurbit[7]uril: Influence on acidity in the ground and excited state

3-Hydroxynaphtho[1,2-b]quinolizinium was synthesized by cyclodehydration route and its optical properties in different media were investigated. The absorption and emission spectra of this compound depend on the pH of the solution. Thus, at higher pH values the deprotonation yields a merocyanine-type dye that exhibits significantly red-shifted absorption bands and causes a dual emisson, i.e., a combination of emission bands of the hydroxyquinolizinium and its deprotonated form. Whereas this compound is a weak acid in the ground state (pK_a = 7.9), it has a strongly increased acidity in the excited state (pK_a^* = 0.4). As a result, the blue-shifted fluorescence of the hydroxyquinolizinium becomes dominant only under strongly acidic conditions. In addition, it is shown that 3-hydroxynaphtho[1,2-b]quinolizinium binds to cucurbit[7]uril (CB[7]) with moderate affinity (K_b = 1.8 × 10⁴ M^-1, pH 5) and that the pK_a and pK_a^* values of this ligand increase by about two to three orders of magnitude, respectively, when bound to CB[7].

Complexation induced aggregation and deaggregation of acridine orange with sulfobutylether-β-cyclodextrin

The present study illustrates intriguing switching of multi-mode binding interactions of acridine orange dye with a sulfobutylether-β-cyclodextrin host.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

A supramolecular keypad lock

The reversible photoswitching between an anthracene derivative and its [4+4] dimer, using the template effect of the CB8 macrocycle, was demonstrated. This example of supramolecular chemistry in water was harnessed to demonstrate the operation of a keypad lock device that is driven by means of light and chemicals as inputs.

pH-Assisted control over the binding and relocation of an acridine guest between a macrocyclic nanocarrier and natural DNA

Controlled binding and relocation of a dye–drug between a macrocyclic nanocarrier and natural DNA is demonstrated using pH as a stimulus.

Triple emission from p-dimethylaminobenzonitrile-cucurbit[8]uril triggers the elusive excimer emission

The intriguing dual-emission behavior of p- dimethylaminobenzonitrile (DMABN) and the identity of the associated excited states is, arguably, the most extensively investigated and also controversially discussed molecule- specific phenomenon of modern photochemistry. We have now found a new, third fluorescence band when DMABN is encapsulated within the water-soluble molecular container cucurbit[8]uril (CB8). It is centered between the previously observed emissions and assigned to the elusive excimer emission from DMABN through 1:2 CB8:DMABN complex formation. Heating of the CB8⋅(DMABN)2 complex from 0 to 100 °C results in the dissociation of the ternary complex and restoration of the dual-emission properties of the monomer. Alternatively, monomer emission can be obtained by selecting cucurbit[7]uril (CB7), a host homologue that is too small to accommodate two DMABN molecules, or by introducing ethyl instead of methyl groups at the amino terminus of the aminobenzonitrile guest.

A simple assay for quality binders to cucurbiturils

A new approach towards the rapid identification of quality binders to cucurbiturils--those that combine high affinity with high selectivity for a particular homologue--was developed. The assay exploits macrocycle-specific optical fingerprints (colorimetric or fluorimetric) of carefully selected indicators dyes. The screening of a guest library revealed known (e.g., adamantane derivatives) and new (e.g., terpenes) quality binders. The predictive power of the assay was underpinned by the modeling of the involved thermodynamic equilibria.

Recognition-mediated light-up of thiazole orange with cucurbit[8]uril: exchange and release by chemical stimuli

This article reports a convenient supramolecular strategy to construct fluorescent photoswitchable molecular assemblies between a macrocyclic host, cucurbit[8]uril (CB8), and a fluorogenic dye, thiazole orange (TO). The interaction mechanism and the stable stoichiometric host-guest arrangements have been claimed on the basis of the optical absorption, steady-state and time-resolved fluorescence lifetime and anisotropy measurements, and also the geometry optimization studies. The CB8 recognized TO in its 2:2 stoichiometry exhibited spectacular fluorescence enhancement of the order of 1700 fold, which is the largest directly determined value so far reported for a dye in an organic macrocyclic system. This prospective 2CB8:2TO assembly responded to selected chemical stimuli such as metal ions, adamantylamine, and tryptophan, providing different dissociation mechanisms and demonstrating a controlled exchange and release action desired with such noncovalently linked assemblies. Positively, considering the aqueous solubility and biocompatibility of the host-guest constituents, this methodology can evolve into a general approach to deliver and operate intracellularly functional molecular components under chemical/thermal/optical trigger control, especially for therapeutic applications.

Greatly enhanced binding of a cationic porphyrin towards bovine serum albumin by cucurbit[8]uril

Binding affinity towards serum albumin and intracellular proteins is of importance for a photodynamic therapy (PDT) sensitizer to selectively localize in tumours and efficiently induce cell death. In this paper, it was found that cucurbit[8]uril (CB8) can greatly improve the binding affinity of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP), a promising PDT photosensitizer, towards bovine serum albumin (BSA). Absorption, fluorescence emission, (1)H NMR, dynamic light scattering, atomic force microscope, as well as protein photocleavage measurements suggest that the binding enhancement originates from the formation of a ternary complex of CB8·TMPyP·tryptophan residues. This finding opens up a new approach for the development of more efficient PDT agents.

Host-guest complexes and pseudorotaxanes of cucurbit[7]uril with acetylcholinesterase inhibitors

Pseudorotaxanes may be assembled in aqueous solution using dicationic acetylcholinesterase inhibitors, such as succinylcholine, BW284c51, and alpha,omega-bis(trialkylammonium)alkane dications (or their phosphonium analogues), as bolaform axles and cucurbit[7]uril (CB[7]) as the wheel. With the exceptions of the shorter [(CH(3))(3)N(CH(2))(n)N(CH(3))(3)](2+) (n = 6, 8) dications, the addition of a second CB[7] results in the translocation of the first CB[7], such that the hydrophobic -NR(3)(+) and -PR(3)(+) end groups (R = Me or Et) are located in the cavities of the wheels, while the central portion of the axles extend through the CB[7] portals into the bulk solvent. In the case of the [Quin(CH(2))(10)Quin](2+) (Quin = quinuclidinium) dication, the CB[7] host(s) resides only on the quinuclidinium end group(s). The 1:1 host-guest stability constants range from 8 x 10(6) to 3 x 10(10) M(-1) and are dependent on both the nature of the end group as well as the length and hydrophobicity of the central linker. The magnitude of the stability constants for the 2:1 complexes closely follow the trend observed previously for CB[7] binding with the NR(4)(+) and PR(4)(+) cations.