Soft Materials Constructed Using Calix[4]pyrrole- and "Texas-Sized" Box-Based Anion Receptors

Rational Design and Bulk Synthesis of Water-Containing Supramolecular Polymers

The utilization of structural water in chemical self-assembly has not only effectively eliminated the negative influences of solvents from solutions or gels but has also provided new insight into the fabrication of new materials in bulk. However, up to now, supramolecular polymerization triggered by structural water has been dominated more by serendipity than rational design. After carefully analyzing the chemical structures of artificial monomers and gaining a deep understanding of the water-triggered assembly process, we report herein the bulk formation of polymeric materials from water and low-molecular weight monomers by rational design instead of serendipity.

BODIPY-based macrocycles

During the past few years, the construction of BODIPY-based macrocycles has attracted extensive interest due to the widespread applications of these materials in sensing, bioimaging, molecular machines, and photodynamic therapy (PDT). Since significant progress has been made in this field, it is time to summarize the recent developments involving BODIPY-based macrocycles. In this review, we will briefly introduce the synthesis routes of BODIPY-based macrocycles, including a covalent synthetic protocol and a noncovalent self-assembly protocol. In addition, we will discuss the photophysical and photochemical properties and the applications of these BODIPY-based macrocycles in the areas of sensing, bioimaging, photodynamic therapy, etc.

Heteroatom-bridged molecular belts as containers

Hoop-shaped or belt-like molecules have been fascinating not only due to their challenging synthesis, but also unique physical and chemical properties. The incorporation of heteroatoms (N, O, S, etc.) into these belts could alter both molecular structures and electronic properties which will lead to versatile applications, from advanced host-guest systems to functional materials. Despite numerous computational studies, the synthesis and characterization of heteroatom-bridged double-stranded molecular belts remains scarce. Here we report the synthesis, crystal structure, and host-guest chemistry of two novel heteroatom-bridged belt-like macrocycles composed of phenoxathiin. The bowl-shaped belt demonstrates a strong binding affinity (K_a = 3.6 × 10⁹ M^‒2) towards fullerene C₆₀ and forms a 2:1 capsule-like complex with the aid of C‒H···S hydrogen bonds. The column-like belt can bind the cyclic guest [2,2]paracyclophane to form a ring-in-ring complex. The modular synthesis, structural specificity, and diverse host-guest chemistry of cyclophenoxathiins markedly expands the known chemistry of molecular belts.

Highly Selective Fluoride Recognition by a Small Tris-Urea Covalent Cage

A highly selective recognition of fluoride was achieved through the design of a small hemicryptophane cage (3) presenting a southern tris-urea hosting moiety. The resulting host-guest complex has been characterized by electrospray ionization-high-resolution mass spectrometry, ¹H and ¹⁹F NMR, and X-ray diffraction techniques. In particular, X-ray diffraction analysis of [3·F^-] reveals that the encapsulation of one fluoride, within 3, occurs through NH···F^- H-bonding with the six NH residues of the tris-urea ligand. An association constant of 1200 M^-1 was extracted from ¹H NMR titration experiments, indicating that efficient fluoride binding also occurs in solution. Finally, in sharp contrast with previously reported urea-based hemicryptophane hosts, the small preorganized cavity found in 3 allows for an exclusive selectivity for fluoride over other competing halides.

Active Anion Delivery by Self-Propelled Microswimmers

Self-propelled micro- and nanomachines are at the forefront of materials research, branching into applications in biomedical science and environmental remediation. Cationic frameworks enabling the collection and delivery of anionic species (A^-) are highly required, due to the large variety of life-threatening pollutants, such as radioactive technetium and carcinogenic chromium, and medicines, such as dexamethasone derivatives with negative charges. However, such autonomous moving carriers for active transport of the anions have been barely discussed. A polymeric viologen (PV⁺⁺)-consisting of electroactive bicationic subunits-is utilized in a tubular autonomous microswimmer to selectively deliver A^- of different sizes and charge densities. The cargo loading is based on a facile anion exchange mechanism. The packed crystal structure of PV⁺⁺ allows removal of an exceptionally high quantity of anions per one microswimmer (2.55 × 10^-13 mol anions per microswimmer), a critical factor often neglected regarding the real-world application of microswimmers. Notably, there was virtually no leakage of anions during the delivery process or upon keeping the loaded microswimmers under ambient conditions for at least 4 months. Multiple release mechanisms, compatible with different environments, including electrochemical, photochemical, and a metathesis reaction, with high efficiencies up to 98% are introduced. Such functional autonomous micromachines provide great promise for the next generation of functional materials for biomedical and environmental applications.

Increasing the complexity of oxoporphyrinogen colorimetric sensing chromophores: N-alkylation and β-substitution

Meso-5,10,15,20-tetrakis-3,5-di-tert-butyl-4-oxocyclohexadienylideneporphyrinogen, OxP, is a versatile, highly colored chromophore derived from meso-5,10,15,20-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin. It exhibits a wide range of chromogenic responses to solvents (solvatochromism), anions and acidic media (halochromism) making it potentially useful as an analytical reagent. The chromogenic responses of OxP can be modulated by varying its chemical structure, and this is reviewed here based on the introduction of substituents at central nitrogen atoms or pyrrolic [Formula: see text]-positions. OxP and its N-alkylated derivates Bn2OxP and Bn4OxP have been used to estimate acidity in non-polar solvents. Bn2OxP can also be used to determine enantiomeric excesses of chiral substances. N-alkylation has also been used to introduce higher functional groups such as porphyrins to prepare self-assembling systems. [Formula: see text]-Substitution has been used to introduce selectivity of anion interactions including towards basic anions (fluoride, cyanide) and polyoxoanions (nitrate, perchlorate, etc.). These aspects make OxP a highly adaptable tetrapyrrole molecule for sensing and other applications.

New dimensions in calix[4]pyrrole: the land of opportunity in supramolecular chemistry

The quest for receptors endowed with the selective complexation and detection of negatively charged species continues to receive substantial consideration within the scientific community worldwide. This study is encouraged by the utilization of anions in nature in a plethora of biological systems such as chloride channels and proteins and as polyanions for genetic information. The molecular recognition of anionic species is greatly interesting in terms of their favourable interactions. In this comprehensive review, in addition to giving accounts of some selected syntheses, we illustrated diverse applications ranging from molecular containers to ion transporters and drug carriers of a supramolecular receptor named calix[4]pyrrole. We believe that the present review may act as a catalyst in enhancing the novel applications of calix[4]pyrrole and its congeners in the other dimensions of science and technology.

The quest for receptors endowed with the selective complexation and detection of negatively charged species continues to receive substantial consideration within the scientific community worldwide.