Obvious vapochromic color changes of a pillar[6]arene containing one benzoquinone unit with a mechanochromic change before vapor exposure

Separation of cyclohexanol from cyclohexanol/cyclohexene mixtures by crystals of pillar[6]arene containing three benzoquinone units

Here, we develop a new absorbent for efficient separation of cyclohexanol (CHA-ol) and cyclohexene (CHA-ene) by using crystals of pillar[6]arene with three benzoquinone units (P3QA). P3QA crystals are found to show remarkable selectivity for CHA-ol in 50 : 50 (v/v) CHA-ol : CHA-ene mixtures with a purity of 95.2%, along with vapochromic behavior.

Macrocycle-Based Charge Transfer Cocrystals with Dynamically Reversible Chiral Self-Sorting Display Chain Length-Selective Vapochromism to Alkyl Ketones

Chirality-driven self-sorting plays an essential role in controlling the biofunction of biosystems, such as the chiral double-helix structure of DNA from self-recognition by hydrogen bonding. However, achieving precise control over the chiral self-sorted structures and their functional properties for the bioinspired supramolecular systems still remains a challenge, not to mention realizing dynamically reversible regulation. Herein, we report an unprecedented saucer[4]arene-based charge transfer (CT) cocrystal system with dynamically reversible chiral self-sorting synergistically induced by chiral triangular macrocycle and organic vapors. It displays efficient chain length-selective vapochromism toward alkyl ketones due to precise modulation of optical properties by vapor-induced diverse structural transformations. Experimental and theoretical studies reveal that the unique vapochromic behavior is mainly attributed to the formation of homo- or heterochiral self-sorted assemblies with different alkyl ketone guests, which differ dramatically in solid-state superstructures and CT interactions, thus influencing their optical properties. This work highlights the essential role of chiral self-sorting in controlling the functional properties of synthetic supramolecular systems, and the rarely seen controllable chiral self-sorting at the solid-vapor interface deepens the understanding of efficient vapochromic sensors.

Keto-Adamantane-Based Macrocycle Crystalline Supramolecular Assemblies Showing Selective Vapochromism to Tetrahydrofuran

Here, we report the synthesis of adamantane-based macrocycle 2 by combining adamantane building blocks with π-donor 1,3-dimethoxy-benzene units. An unpredictable keto-adamantane-based macrocycle 3 was obtained by the oxidation of 2 using DDQ as an oxidant. Moreover, a new type of macrocyclic molecule-based CT cocrystal was prepared through exo-wall CT interactions between 3 and DDQ. The cocrystal material showed selective vapochromism behavior towards THF, specifically, among nine volatile organic solvents commonly used in the laboratory. Powder X-ray diffraction; UV-Vis diffuse reflectance spectroscopy; 1H NMR; and single crystal X-ray diffraction analyses revealed that color changes are attributed to the vapor-triggered decomplexation of cocrystals.

Smart organic materials based on macrocycle hosts

Innovative design of smart organic materials is of great importance for the advancement of modern technology. Macrocycle hosts, possessing cyclic skeletons, intrinsic cavities, and specific guest binding properties, have demonstrated pronounced potential for the elaborate fabrication of a variety of functional organic materials with smart stimuli-responsive characteristics. In this tutorial review, we outline the current development of smart organic materials based on macrocycle hosts as key building blocks, focusing on the design principles and functional mechanisms of the tailored systems. Three main types of macrocycle-based smart organic materials are exemplified as follows according to the distinct forms of construction patterns: (1) supramolecular polymeric materials and nanoassemblies; (2) adaptive molecular crystals; (3) smart porous organic materials. The responsive performances of macrocycle-containing smart materials in versatile aspects, including mechanically adaptive polymers, soft optoelectronic devices, data encryption, drug delivery systems, artificial transmembrane channels, crystalline-state gas adsorption/separation, and fluorescence sensing, are illustrated by discussing the representative studies as paradigms, where the roles of macrocycles in these systems are highlighted. We also provide in the conclusion part the perspectives and remaining challenges in this burgeoning field.

Grinding-induced supramolecular charge-transfer assemblies with switchable vapochromism toward haloalkane isomers

Synthetic macrocycles have proved to be of great application value in functional charge-transfer systems in the solid state in recent years. Here we show a switchable on-off type vapochromic system toward 1-/2-bromoalkane isomers by constructing solid-state charge-transfer complexes between electron-rich perethylated pillar[5]arene and electron-deficient aromatic acceptors including 4-nitrobenzonitrile and 1,4-dinitrobenzene. These charge-transfer complexes with different colors show opposite color changes upon exposure to the vapors of 1-bromoalkanes (fading) and 2-bromoalkanes (deepening). Single-crystal structures incorporating X-ray powder diffraction and spectral analyses demonstrate that this on-off type vapochromic behavior is mainly attributed to the destruction (off) and reconstruction (on) of the charge-transfer interactions between perethylated pillar[5]arene and the acceptors, for which the competitive host-guest binding of 1-bromoalkanes and the solid-state structural transformation triggered by 2-bromoalkanes are respectively responsible. This work provides a simple colorimetric method for distinguishing positional isomers with similar physical and chemical properties.

Macrocycle-Based Crystalline Supramolecular Assemblies Built with Intermolecular Charge-Transfer Interactions

Synthetic macrocycles have served as principal tools for supramolecular chemistry, have greatly extended the scope of organic charge transfer (CT) complexes, and have proved to be of great practical value in the solid state during the past few years. In this Minireview, we summarize the research progress on the macrocycle-based crystalline supramolecular assemblies primarily driven by intermolecular CT interactions (a.k.a. macrocycle-based crystalline CT assemblies, MCCAs for short), which are classified by their donor-acceptor (D-A) constituent elements, including simplex macrocyclic hosts, heterogeneous macrocyclic hosts, and host-guest D-A pairs. Particular attention will be focused on their diverse functions and applications, as well as the underlying CT mechanisms from the perspective of crystal engineering. Finally, the remaining challenges and prospects are outlined.

Design and synthesis of piezochromic materials exploring intermolecular charge transfer: chalconoids bound to the p-sulfonatocalix[6]arene macrocycle

Solid-state systems composed of chalconoid encapsulated within p-sulfonatocalix[6]arene (SCX6) scaffolds that exhibit mechanochromism and thermochromism have been developed. An introduction of a supramolecular host promises a variety of applications in diverse areas, which makes them fascinating. Largely hydrogen bonding as well as π···π interactions are responsible for the host-guest complexation. The complex shows partial encapsulation of the guest with one of the phenyl rings of chalcone (guest) is held inside the SCX6 cavity, whilst other phenyl rings that exclude the cavity are hydrogen-bonded to sulfonate portals of the host. The hydrogen bonding conducing such complexation triggers proton transfer engendering a mechanochromic switch. The complexes are further characterized by a variety of experiments such as cyclic voltammetry (CV), steady-state fluorescence, vibrational spectroscopy, and ¹H or 2D NMR (NOESY) spectroscopy experiments. Detailed structure furnished through the NMR shows deshielding of the Ha-e (guest) protons whereas, the hydroxyl protons from the host experience shielding as evidenced from the ¹H NMR spectra. These inferences have further been corroborated through the density functional theory. Electrochemical investigations suggested an irreversible one-electron transfer in the host-guest binding. The characteristic 'frequency shift' for the intense carbonyl vibration in the infrared spectra, which can be correlated to the kinetic energy density parameter, G(r), in the quantum theory of atoms in molecules (QTAIM).

State- and water repellency-controllable molecular glass of pillar[5]arenes with fluoroalkyl groups by guest vapors

Molecular glasses are low-molecular-weight organic compounds that are stable in the amorphous state at room temperature. Herein, we report a state- and water repellency-controllable molecular glass by n-alkane guest vapors. We observed that a macrocyclic host compound pillar[5]arene with the C₂F₅ fluoroalkyl groups changes from the crystalline to the amorphous state (molecular glass) by heating above its melting point and then cooling to room temperature. The pillar[5]arene molecular glass shows reversible transitions between amorphous and crystalline states by uptake and release of the n-alkane guest vapors, respectively. Furthermore, the n-alkane guest vapor-induced reversible changes in the water contact angle were also observed: water contact angles increased and then reverted back to the original state by the uptake and release of the n-alkane guest vapors, respectively, along with the changes in the chemical structure and roughness on the surface of the molecular glass. The water repellency of the molecular glass could be controlled by tuning the uptake ratio of the n-alkane guest vapor.

Pillar[5]arenes with C₂F₅ substituents showed reversible amorphous–crystal transitions by uptake and release of n-alkane vapors. The amorphous–crystal transitions triggered macroscopic property change such as water repellency.

Detection of aliphatic aldehydes by a pillar[5]arene-based fluorescent supramolecular polymer with vaporchromic behavior

The detection of volatile aliphatic aldehydes is of significance because of their chemical toxicity, physical volatility and widespread applications in chemical industrial processes. In this work, the direct detection of aliphatic aldehydes is tackled using a fluorescent supramolecular polymer with vaporchromic behavior which is contructed by pillar[5]arene-based host-guest intereactions. Thin films with strong orange-yellow fluorescence are prepared by coating the linear supramolecular polymer on glass sheets. When the thin films are exposed to aliphatic aldehydes with different carbon chain lengths, they can selectivly sensing  n -butyraldehyde ( C 4 ) and caprylicaldehyde ( C 8 ), accompanied by fluorescence quenching, indicating that the supramolecular polymer is a highly selective vapochromic response material for aliphatic aldehydes with long alkyl chains.

A triphenylamine derivative and its Cd(ii) complex with high-contrast mechanochromic luminescence and vapochromism

A triphenylamine derivative and its Cd(ii) complex exhibited predominant mechanochromism and vapochromism with high-contrast color and emission changes.

Hierarchical self-assembly of discrete bis-[2]pseudorotaxane metallacycle with bis-pillar[5]arene via host-guest interactions and their redox-responsive behaviors

A discrete rhomboidal metallacycle R functionalized with bis-[2]pseudorotaxane of [Cu(phenanthroline)2]+ derivatives was successfully synthesized via coordination-driven self-assembly. Furthermore, the host-guest complexation of such a bis-[2]pseudorotaxane metallacycle with a bis-pillar[5]arene (bisP5) allowed for the formation of a new family of cross-linked supramolecular polymers R⊃(bisP5)2, which displayed interesting redox-responsive properties. By taking advantage of the substantial structural differences between the coordination geometries of [Cu(phenanthroline)2]+ and [Cu(phenanthroline)2]2+, the weight-average diffusion coefficients D of the supramolecular polymer were adjusted through changing the redox state of the Cu(i)/Cu(ii) complexes.

Vapochromism of Organic Crystals Based on Macrocyclic Compounds and Inclusion Complexes

Vapochromic materials, which change color and luminescence when exposed to specific vapors and gases, have attracted considerable attention in recent years owing to their potential applications in a wide range of fields such as chemical sensors and environmental monitors. Although the mechanism of vapochromism is still unclear, several studies have elucidated it from the viewpoint of crystal engineering. In this mini-review, we investigate recent advances in the vapochromism of organic crystals. Among them, macrocyclic molecules and inclusion complexes, which have apparent host–guest interactions with analyte molecules (specific vapors and gases), are described. When the host compound is properly designed, its cavity size and symmetry change in response to guest molecules, influencing the optical properties by changing the molecular inclusion and recognition abilities. This information highlights the importance of structure–property relationships resulting from the molecular recognition at the solid–vapor interface.