Thermosensitive Phase Behavior of Benzo-21-crown-7 and Its Derivatives

The aqueous supramolecular chemistry of crown ethers

This mini-review summarizes the seminal exploration of aqueous supramolecular chemistry of crown ether macrocycles. In history, most research of crown ethers were focusing on their supramolecular chemistry in organic phase or in gas phase. In sharp contrast, the recent research evidently reveal that crown ethers are very suitable for studying abroad range of the properties and applications of water interactions, from: high water-solubility, control of Hofmeister series, "structural water", and supramolecular adhesives. Key studies revealing more details about the properties of water and aqueous solutions are highlighted.

Nanostructuring with Surfactants: The Self-Assembly of a New Poly(thiophene-phenylene) Conjugated Polymer Bearing Azacrown Ether Pendant Groups

We report the synthesis of a new conjugated polymer bearing crown ether moieties, poly[(N(1-aza-[18]crown-6)carbamido)thiophene-2,5-diyl-alt-1,4-phenylene] (BG2). In water, BG2 forms a dispersion with a slightly cloudy appearance. We have studied the effect of adding surfactants, with different polar head groups, on these polymer-polymer aggregates. Special attention is given to the system with the anionic surfactant, sodium dodecyl sulfate (SDS). The combination of photophysical techniques with electrical conductivity, NMR (¹H, ¹³C, and ²⁷Na), DFT calculations, molecular dynamics simulations, and small-angle neutron scattering (SANS) provides a detailed picture on the behavior of the SDS/BG2 system in aqueous solution and in thin films. NMR, electric conductivity, and DFT results suggest that hydrophilic interactions occur between the polar headgroup of the surfactant (OSO₃^- Na⁺) and the aza-[18]-crown-6 moiety. DFT calculations confirmed the capability of BG2 to form stable complexes with the Na⁺ cations, where the cation can be either inside the azacrown cavity or sandwiched between the cavity and the polymer chain, which seem to determine the position of the surfactant hydrocarbon chain and, therefore, be responsible for the disruption of the BG2 aggregates and subsequent increase in the photoluminescence quantum yields. SANS measurements, made with hydrogenated and deuterated SDS in D₂O, clearly show how micron-sized aggregates of BG2 are broken down by SDS and then how BG2 becomes preferentially incorporated within joint colloidal particles of BG2 and SDS with increasing [SDS]/[BG2] molar ratio.

Pseudo Polyampholytes with Sensitively Ion-Responsive Conformational Transition Based on Positively Charged Host-Guest Complexes

Biological polyampholytes are ubiquitous in living organisms with primary functions including that serving as transporters for moving chemical molecular species across the cell membranes. Synthetic amphoteric macromolecules that can change their phase states depending on the environment to simulate some properties of natural polyampholytes are of great interests. Here, we explore implementation of synthetic pseudo polymeric ampholytes with ion-recognition-triggered conformational change. The phase transition behaviors of the ion-recognition-creative polyampholytes that containing deprotonated carboxylic acid groups as negative charges and 18-crown-6 units for forming positively charged host-guest complexes are systematically investigated. The ion-recognition-triggered phase transition behaviors of pseudo polyampholytes are significantly dependent on cation species and concentrations. Only those specific ions like K⁺ , Ba²⁺ , Sr²⁺ and Pb²⁺ ions that can form 1:1 host-guest complexes with 18-crown-6 units in polymers enable to control over the conformational change like that of the traditional pH-dependent polyampholytes. By regulating the content of the carboxylic acid groups to match the content of the ion-recognized positive charges provided by the host-guest complexes, the pseudo polyampholytes are more sensitive to the recognizable cations. Such ion-recognition-triggered amphoteric characteristics make the pseudo polyampholytes acting like biological proteins, nucleic acids and enzymes as molecular transporters, genetic code storage and biocatalysts in artificial systems. This article is protected by copyright. All rights reserved.

Direct synthetic routes to functionalised crown ethers

Crown ethers are macrocyclic hosts that can complex a wide range of inorganic and organic cations as well as neutral guest species. Their widespread utilization in several areas of fundamental and applied chemistry strongly relies on strategies for their functionalisation, in order to obtain compounds that could carry out multiple functions and could be incorporated in sophisticated systems. Although functionalised crown ethers are normally synthesised by templated macrocyclisation using appropriately substituted starting materials, the direct addition of functional groups onto a pre-formed macrocyclic framework is a valuable yet underexplored alternative. Here we review the methodologies for the direct functionalisation of aliphatic and aromatic crown ethers sporadically reported in the literature over a period of four decades. The general approach for the introduction of moieties on aliphatic crown ethers involves a radical mediated cross dehydrogenative coupling initiated either by photochemical or thermal/chemical activation, while aromatic crown ethers are commonly derivatised via electrophilic aromatic substitution. Direct functionalization routes can reduce synthetic effort, allow the later modification of crown ether-based architectures, and disclose new ways to exploit these versatile macrocycles in contemporary supramolecular science and technology.

Supramolecular Chemistry: Host-Guest Molecular Complexes

In recent times, researchers have emphasized practical approaches for capturing coordinated and selective guest entrap. The physisorbed nanoporous supramolecular complexes have been widely used to restrain various guest species on compact supporting surfaces. The host-guest (HG) interactions in two-dimensional (2D) permeable porous linkages are growing expeditiously due to their future applications in biocatalysis, separation technology, or nanoscale patterning. The different crystal-like nanoporous network has been acquired to enclose and trap guest molecules of various dimensions and contours. The host centers have been lumped together via noncovalent interactions (such as hydrogen bonds, van der Waals (vdW) interactions, or coordinate bonds). In this review article, we enlighten and elucidate recent progress in HG chemistry, explored via scanning tunneling microscopy (STM). We summarize the synthesis, design, and characterization of typical HG structural design examined on various substrates, under ambient surroundings at the liquid-solid (LS) interface, or during ultrahigh vacuum (UHV). We emphasize isoreticular complexes, vibrant HG coordination, or hosts functional cavities responsive to the applied stimulus. Finally, we critically discuss the significant challenges in advancing this developing electrochemical field.

Selenacrown Macrocycle in Aqueous Medium: Synthesis, Redox-Responsive Self-Assembly, and Enhanced Disulfide Formation Reaction

Organic selenides are famous for their coordination and catalytic functions in the organic phase, albeit challenging for aqueous medium. Herein, the combination of a hydrophilic body of crown ether and substitution of one oxygen atom with a selenium one provides a new type of design route for organic selenide entities with charming functions in aqueous solution. The selenacrown ether C9Se presented here intrinsically shows an amphiphile-like property. Its nanosphere structure in water readily expands the catalysis of organic selenide to aqueous substrates in thiol/disulfide conversion.

Selenium-containing heterodimeric crown ether acting as an unconventional multi-responsive amphiphile in water

A new heterodimeric crown amphiphile was fabricated, wherein the oxacrown and selencrown ethers provided the desired molecular framework for hydrophilicity and hydrophobicity, respectively. From an integrated perspective, the developed amphiphile possesses features of crown ethers, amines, and selenium-containing species, and its assembly in water can be responsive to diverse chemical effectors-H2O2 and CO2 in a switchable ON/OFF mode to achieve controlled release. It is the first case wherein the applications of cyclic polyethers with different solubilities drives the self-assembly in an aqueous medium.

Supramolecular microtubes based on 1,3,5-benzenetricarboxamides prepared by self-assembly upon heating

A series of three 1,3,5-benzenetricarboxamides with peripheral tertiary N,N-dialkyl-ethylamino substituents with different length of the alkyl groups is reported. In particular, the N¹,N³,N⁵-tris[2-(diethylamino)-ethyl]-1,3,5-benzenetricarboxamide exhibits phase separation followed by self-assembly upon heating from aqueous solution into well-defined supramolecular fiber-like structures in the form of microtubes.

Self-assembly behaviors of perylene- and naphthalene-crown macrocycle conjugates in aqueous medium

The synthesis of conjugates of perylene diimide (PDI) and naphthalene diimide (NDI) modified with two benzo-21-crown-7 ethers (B21C7) are herein described. Their self-assembly behavior in various solvents was investigated particularly in aqueous medium, due to the recently discovered hydrophilic properties of B21C7 crown macrocycle. An unexpected fluorescence quenching phenomenon was observed in the PDI-B21C7 macrocycle conjugate in chloroform. The detailed UV-vis absorption and fluorescence spectra of these PDI/NDI derivatives in different solvents as well as their morphologies were investigated.

LCST phase behavior of benzo-21-crown-7 with different alkyl chains

The introduction of hydrophobic units into crown ethers can dramatically decrease the critical transition temperature of LCST and realize macroscopic phase separation at low to moderate temperature and concentration. Minor modifications in the chemical structure of crown ethers (benzo-21-crown-7, B21C7s) can effectively control the thermo-responsive properties.

A degradable low molecular weight monomer system with lower critical solution temperature behaviour in water

A degradable thermo-responsive system was prepared and investigated. The degradation behaviour induced by the cleavage process of the thermo-sensitive crown ethers effectively altered the thermo-responsiveness.

Differences in solubilities, crystal structures, NMR spectra and fluorescence emissions induced by potassium cation/benzo-21-crown-7 molecular recognition

Changes of the fundamental properties of host–guest pairs induced by potassium cation complexation were investigated.

LCST behavior controlled by size-matching selectivity from low molecular weight monomer systems

LCST behavior was controlled by crown ether–cation recognition motifs via size-matching selectivity.

Soluble-Insoluble-Soluble Transitions of Thermoresponsive Cryptand-Containing Graft Copolymers

Cryptand-containing alternative copolymers were first made from copolymerization of styrenic derivatives and maleic anhydride and then chemically modified in this work by grafting methoxy poly(ethylene glycol) (MPEG) onto the maleic functional groups. These graft copolymers show interesting multistep soluble-insoluble-soluble (S-I-S) transitions in acidic aqueous media at a cloud point (T cp) and a subsequent mixing temperature (T mix). Turbidity measurements and dynamic light scattering studies indicate that such complex transitions may be attributed to the entropic contribution associated with the dehydration and aggregation of the MPEG groups and then the enthalpic contribution associated with the hydrogen bonding between ethylene glycol and carboxylic acid groups. More importantly, the phase transition temperatures and insoluble temperature ranges are very sensitive to changes in subtle hydrophobic-hydrophilic balance of the copolymers, such as the variation of pH, the cryptand size, and the length of the MPEG graft. The understanding of the S-I-S transition in relation to the structure of the copolymers and the external conditions may be useful in the design of smart materials and sensors.

Supramolecular control over pillararene-based LCST phase behaviour

Based on the supramolecular interactions between pillar[5]arenes and ionic liquids, supramolecular control was successfully introduced into thermo-responsive systems to adjust LCST phase behaviour in water.

Polymeric crown ethers: LCST behavior in water and stimuli-responsiveness

A crown ether-functionalized poly(vinyl alcohol) (PVA) system shows lower critical solution temperature (LCST) phase separation behavior in water.