The Influence of Binding Site Geometry on Anion-Binding Selectivity: A Case Study of Macrocyclic Receptors Built on the Azulene Skeleton

Exploiting the Mechanical Bond Effect for Enhanced Molecular Recognition and Sensing

The ubiquity of charged species in biological and industrial processes has resulted in ever-increasing interest in their selective recognition, detection, and environmental remediation. Building on the established coordination chemistry principles of the chelate and macrocyclic effects, and host preorganization, supramolecular chemists seek to construct specific 3D binding cavities reminiscent of biotic systems to enhance host-guest binding affinity and selectivity. Mechanically interlocked molecules (MIMs) present a wholly unique platform for synthetic host design, wherein topologies afforded by the mechanical bond enable the decoration of 3D cavities for non-covalent interactions with a range of target guest geometries. Notably, MIM host systems exhibit mechanical bond effect augmented affinities and selectivities for a variety of charged guest species, compared to non-interlocked acyclic and macrocycle host analogs. Furthermore, the modular nature of MIM synthesis facilitates incorporation of optical and electrochemical reporter groups, enabling fabrication of highly sensitive and specific molecular sensors. This review discusses the development of recognition and sensing MIMs, from the first reports in the late 20th century through to the present day, delineating how their topologically preorganized and dynamic host cavities enhance charged guest recognition and sensing, demonstrating the mechanical bond effect as a potent tool in future chemosensing materials.

Revealing the Hidden Costs of Organization in Host-Guest Chemistry Using Chloride-Binding Foldamers and Their Solvent Dependence

Preorganization is a key concept in supramolecular chemistry. Preorganized receptors enhance binding by minimizing the organization costs associated with adopting the conformation needed to orient the binding sites toward the guest. Conversely, poorly organized receptors show affinities below what is possible based on the potential of their specific binding interactions. Despite the fact that the organization energy is paid each time like a tax, its value has never been measured directly, though many compounds have been developed to measure its effects. We present a method to quantify the hidden costs of receptor organization by independently measuring the contribution it makes to chloride complexation by a flexible foldameric receptor. This method uses folding energy to approximate organization energy and relies on measurement of the coil-helix equilibrium as a function of solvent. We also rely on the finding, established with rigid receptors, that affinity is inversely related to the solvent dielectric and expect the same for the foldamer's helically organized state. Increasing solvent polarity across nine dichloromethane-acetonitrile mixtures we see an unusual V-shape in affinity (decrease then increase). Quantitatively, this shape arises from weakened hydrogen-bonding interactions with solvent polarity followed by solvent-driven folding into an organized helix. We confirm that dielectric screening impacts the stability of host-guest complexes of flexible foldamers just like rigid receptors. These results experimentally verify the canonical model of binding (affinity depends on the sum of organization and noncovalent interactions). The picture of how solvent impacts complex stability and conformational organization thereby helps lay the groundwork for de novo receptor design.

Macrocyclic versus open-chain carbazole receptors for carboxylate binding

Anion recognition properties of six synthetic acyclic and macrocyclic carbazole-based receptors have been studied by 1H-NMR as well as with COSMO-RS calculations towards acetate, benzoate, lactate, sorbate and formate. The...

Azulene-based fluorescent chemosensor for adenosine diphosphate

AzuFluor® 435-DPA-Zn, an azulene fluorophore bearing two zinc(II)-dipicolylamine receptor motifs, exhibits fluorescence enhancement in the presence of adenosine diphosphate. Selectivity for ADP over ATP, AMP and PPi results from appropriate positioning of the receptor motifs, since an isomeric sensor cannot discriminate between ADP and ATP.

Synthesis and Halochromic Properties of 1,2,6-Tri- and 1,2,3,6-Tetra-aryl Azulenes

A series of novel 2,6-functionalized azulene molecules Azu1-3 with varied fluorene substituents at the 1- and 3-positions of azulene as well as at the 5'-position of 2-thiophene group were synthesized. Their electronic absorption and emission spectra at neutral and protonated states were examined. It was found that after functionalization with fluorenyl groups, Azu1-3 exhibited absorption maxima at 445, 451 to 468 nm, respectively. In contrast, their corresponding protonated species showed much redshifted absorption maxima at 560, 582 to 643 nm, respectively, mainly due to the extension of conjugation length and the large dipole moment along the C_2v axis of 2,6-substituted azulene molecules. Azu1-3 are non-fluorescent in their neutral forms, but became emissive in their protonated states. Analysis of absorption and emission spectra shows that substitution of the 1- or 3-position of azulene led to decrease in response to trifluoroacetic acid.

C4-aldehyde of guaiazulene: synthesis and derivatisation

Guaiazulene is an alkyl-substituted azulene available from natural sources and is a much lower cost starting material for the synthesis of azulene derivatives than azulene itself. Here we report an approach for the selective functionalisation of guaiazulene which takes advantage of the acidity of the protons on the guaiazulene C4 methyl group. The aldehyde produced by this approach constitutes a building block for the construction of azulenes substituted on the seven-membered ring. Derivatives of this aldehyde synthesised by alkenylation, reduction and condensation are reported, and the halochromic properties of a subset of these derivatives have been studied.

Azulene-A Bright Core for Sensing and Imaging

Azulene is a hydrocarbon isomer of naphthalene known for its unusual colour and fluorescence properties. Through the harnessing of these properties, the literature has been enriched with a series of chemical sensors and dosimeters with distinct colorimetric and fluorescence responses. This review focuses specifically on the latter of these phenomena. The review is subdivided into two sections. Section one discusses turn-on fluorescent sensors employing azulene, for which the literature is dominated by examples of the unusual phenomenon of azulene protonation-dependent fluorescence. Section two focuses on fluorescent azulenes that have been used in the context of biological sensing and imaging. To aid the reader, the azulene skeleton is highlighted in blue in each compound.

Azulene Functionalization by Iron-Mediated Addition to a Cyclohexadiene Scaffold

The functionalization of azulenes via reaction with cationic η5-iron carbonyl diene complexes under mild reaction conditions is demonstrated. A range of azulenes, including derivatives of naturally occurring guaiazulene, were investigated in reactions with three electrophilic iron complexes of varying electronic properties, affording the desired coupling products in 43-98% yield. The products were examined with UV-vis/fluorescence spectroscopy and showed interesting halochromic properties. Decomplexation and further derivatization of the products provide access to several different classes of 1-substituted azulenes, including a conjugated ketone and a fused tetracycle.

Colorimetric detection of Hg2+ with an azulene-containing chemodosimeter via dithioacetal hydrolysis

Azulene is a bicyclic aromatic chromophore that absorbs in the visible region. Its absorption maximum undergoes a hypsochromic shift if a conjugated electron-withdrawing group is introduced at the C1 position. This fact can be exploited in the design of a colorimetric chemodosimeter that functions by the transformation of a dithioacetal to the corresponding aldehyde upon exposure to Hg2+ ions. This chemodosimeter exhibits good chemoselectivity over other metal cations, and responds with an unambiguous colour change clearly visible to the naked eye. Its synthesis is concise and its ease of use makes it appropriate in resource-constrained environments, for example in determing mercury content of drinking water sources in the developing world.

A Colorimetric Chemosensor Based on a Nozoe Azulene That Detects Fluoride in Aqueous/Alcoholic Media

Colorimetry is an advantageous method for detecting fluoride in drinking water in a resource-limited context, e. g., in parts of the developing world where excess fluoride intake leads to harmful health effects. Here we report a selective colorimetric chemosensor for fluoride that employs an azulene as the reporter motif and a pinacolborane as the receptor motif. The chemosensor, NAz-6-Bpin, is prepared using the Nozoe azulene synthesis, which allows for its rapid and low-cost synthesis. The chemosensor gives a visually observable response to fluoride both in pure organic solvent and also in water/alcohol binary solvent mixtures.

Azulene-Derived Fluorescent Probe for Bioimaging: Detection of Reactive Oxygen and Nitrogen Species by Two-Photon Microscopy

Two-photon fluorescence microscopy has become an indispensable technique for cellular imaging. Whereas most two-photon fluorescent probes rely on well-known fluorophores, here we report a new fluorophore for bioimaging, namely azulene. A chemodosimeter, comprising a boronate ester receptor motif conjugated to an appropriately substituted azulene, is shown to be an effective two-photon fluorescent probe for reactive oxygen species, showing good cell penetration, high selectivity for peroxynitrite, no cytotoxicity, and excellent photostability.

Stabilization and Extraction of Fluoride Anion Using a Tetralactam Receptor

A neutral tetralactam macrocycle was prepared in a few minutes in one pot and at high concentration using commercially available starting materials. NMR titration studies in DMSO revealed an anion affinity order of F^- > AcO^- > Cl^- > Br^-. The receptor affinity for F^- is very high due in part to formation of a self-complementary dimer comprised of two "saddle shaped" complexes. An X-ray crystal structure showed that the two F^- ions within the dimer are separated by 3.39 Å. The electrostatic penalty for this close proximity is compensated by attractive interactions provided by the surrounding tetralactam molecules. Reactivity experiments showed that stabilization of F^- as a supramolecular complex abrogated its capacity to induce elimination and substitution chemistry. This finding raises the idea of using tetralactam macrocycles to stabilize fluoride-containing liquid electrolytes within redox devices such as room-temperature fluoride-ion batteries. A lipophilic version of the tetralactam macrocycle was prepared and used to extract F^- from water into a chloroform layer with high efficiency. The favorable extraction is due to the architecture of the extracted dimeric complex, with all the polarity located within the core of the self-associated dimer and all the nonpolar functionality on the exterior surface.

Azulenes with aryl substituents bearing pentafluorosulfanyl groups: synthesis, spectroscopic and halochromic properties

Azulenes with SF₅-containing substituents gave significantly different spectroscopic responses to protonation depending on the regioisomer in question.