Heat-Set Gel-like Networks of Lipophilic Co(II) Triazole Complexes in Organic Media and Their Thermochromic Structural Transitions

Symmetry Breaking in Supramolecular Gel Condensation

Supramolecular condensation during cooling cycles often transitions through multiple metastable phases before achieving a stable crystalline state. Metastability arises from various competing parameters like symmetrical arrangement, and supramolecular bonding and manifests at different temperatures. Symmetrical physical arrangements can minimize vibrational energy and stabilize the systems at higher temperatures. Further cooling promotes directional supramolecular bonding, such as charge-assisted hydrogen bonding, resulting in molecular periodicity within metastable structures. Frustration occurs when weaker van der Waals bonds form during further cooling, propagating perpendicularly to stronger one-dimensional charge-assisted hydrogen bonds and disrupting lateral periodicity in certain solvents. This makes parallel 1D fibers slidable, adding flexibility to the gel fiber. Eventually, some supramolecular systems attain thermodynamically stable crystalline states by perfectly arranging all the molecules. Throughout the process, metastability results from different symmetrical arrangements, and each rearrangement alters the supramolecular structure's symmetry, generating new physicochemical properties. Different supramolecular gels uniquely break symmetry, which can be monitored through various techniques. This perspective analyzes supramolecular thermoreversible, reverse thermal, liquid crystalline, thixotropic, and antisolvent-induced gels to illustrate spontaneous symmetry reduction processes. Reaching a suprasymmetry condensate can classify big data and be applied in unconventional analogue computing or data storage.

Cu(I)-Induced G-Quartets: Robust Supramolecular Polymers Exhibiting Heating-Induced Aqueous Phase Transitions Into Gel or Precipitate

Certain proteins and synthetic covalent polymers experience aqueous phase transitions, driving functional self-assembly. Herein, we unveil the ability of supramolecular polymers (SPs) formed by G4.Cu+ to undergo heating-induced unexpected aqueous phase transitions. For the first time, guided by Cu+, guanosine (G) formed a highly stable G-quartet (G4.Cu+)/G-quadruplex as a non-canonical DNA secondary structure with temperature tolerance, distinct from the well-known G4.K+. The G4.Cu+ self-assembled in water through π-π stacking, metallophilic and hydrophobic interactions, forming thermally robust SPs. This enhanced stability is attributed to the stronger coordination of Cu+ to four carbonyl oxygens of G-quartet and the presence of Cu+- - -Cu+ attractive metallophilic interactions in Cu+-induced G-quadruplex, exhibiting a significantly higher interaction energy than K+ as determined computationally. Remarkably, the aqueous SP solution exhibited heating-induced phase transitions-forming a hydrogel through dehydration-driven crosslinking of SPs below cloud temperature (Tcp) and a hydrophobic collapse-induced solid precipitate above Tcp, showcasing a lower critical solution temperature (LCST) behavior. Notably, this LCST behavior of G4.Cu+ SP originates from biomolecular functionality rather than commonly exploited thermo-responsive oligoethylene glycols with supramolecular assemblies. Furthermore, exploiting the redox reversibility of Cu+/Cu2+, we demonstrated control over the assembly and disassembly of G-quartets/G-quadruplex and gelation reversibly.

Heat-Set Supramolecular Hydrogelation by Regulating the Hydrophilic-Lipophilic Balance for a Tunable Circularly Polarized Luminescent Switch

Heat-set supramolecular gels exhibited totally opposite phase behaviors of dissolution upon cooling and gelation on heating. They are commonly discovered by chance and their rational design remains a great challenge. Herein, a rational design strategy is proposed to realize heat-set supramolecular hydrogelation through regulation of the hydrophilic-lipophilic balance of the system. A newly synthesized amphiphile hydrogelator with pyrene embedded in its lipophilic terminal can self-assemble into a hydrogel through a heating and cooling cycle. However, the host-guest complex of the gelator and hydrophilic γ-cyclodextrin (γ-CyD) results in a sol at room temperature. Thus, heat-set hydrogelation is realized from the sol state in a controllable manner. Heat-set gelation mechanism is revealed by exploring critical heat-set supramolecular gelation and the related findings provide a general strategy for developing new functional molecular gels with tunable hydrophilic-lipophilic balance.

Construction and screening of spin-crossover-sponge materials based on iron(II)-triazole coordination polymers

Iron(II)-triazole coordination polymers have attracted considerable interest for their synthetic versatility, which allows tuning their spin-crossover (SCO) properties. Embedding SCO solid particles in sponge matrices is a simple, powerful, and generic approach to construct processable SCO materials. Here, we have studied a series of magnetic frameworks based on partial ligand substitution by using different chemical mixtures of two organic ligands, yielding four isostructural coordination polymers. The integration of the hygroscopic SCO material has endowed the composite sponge with the ability to capture moisture under ambient conditions. In particular, not only does a spin-crossover transition during absorption occur, but also a color variation has been achieved by varying humidity. The consequences of cooperativity and the exposed surface of the composite sponge on the spin transition were evaluated and the most promising materials among them were screened. This work provides guiding significance for the fabrication and practical application of spin-crossover-sponge materials.

Emerging Aspects of Triazole in Organic Synthesis: Exploring its Potential as a Gelator

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) - commonly known as the "click reaction" - serves as the most effective and highly reliable tool for facile construction of simple to complex designs at the molecular level. It relates to the formation of carbon heteroatomic systems by joining or clicking small molecular pieces together with the help of various organic reactions such as cycloaddition, conjugate addition, ring-opening, etc. Such dynamic strategy results in the generation of triazole and its derivatives from azides and alkynes with three nitrogen atoms in the five-membered aromatic azole ring that often forms gel-assembled structures having gelating properties. These scaffolds have led to prominent applications in designing advanced soft materials, 3D printing, ion sensing, drug delivery, photonics, separation, and purification. In this review, we mainly emphasize the different mechanistic aspects of triazole formation, which includes the synthesis of sugar-based and non-sugar-based triazoles, and their gel applications reported in the literature for the past ten years, as well as the upcoming scope in different branches of applied sciences.

Cobalt-Adenosine Monophosphate Supramolecular Hydrogel with pH-Responsive Multi-Nanozymatic Activity

Self-assembled metal-ion cross-linked multifunctional hydrogels are gaining a lot of attention in the fields of biomedical and biocatalysis. Herein, we report a heat-triggered metallogel that was spontaneously formed by the self-assembly of adenosine 5'-monophosphate (AMP) and cobalt chloride, accompanied by a color transition depicting an octahedral to tetrahedral transition at high temperature. The hydrogel shows excellent stability in a wide pH window from 1 to 12. The metallogel is being exploited as a multienzyme mimic, exhibiting pH-responsive catalase and peroxidase activity. Whereas catalase mimicking activity was demonstrated by the hydrogel under neutral and basic conditions, it shows peroxidase mimicking activity in an acidic medium. The multifunctionality of the synthesized metallogel was further demonstrated by phenoxazinone synthase-like activities. Owing to its catalase-mimicking activity, the metallogel could effectively reduce the oxidative stress produced in cells due to excess hydrogen peroxide by degrading H2O2 to O2 and H2O under physiological conditions. The biocompatible metallogel could prevent cell apoptosis by scavenging reactive oxygen species. A green and simple synthetic strategy utilizing commonly available biomolecules makes this metallogel highly attractive for catalytic and biomedical applications.

Structural Conversion of Supramolecular Assembly in Solution by Thermally Induced Intramolecular Electron Transfer of [Co2 Fe2 ] Complex

Combining metal complexes with amphiphilic molecules leads to a wide variety of functional self-assembled nanostructures. Metal complexes exhibiting spin transitions can be good candidates as the trigger to cause structural conversion of such assembly because they respond to various external stimuli. In this work, we studied a structural conversion of a supramolecular assembly containing a [Co2 Fe2 ] complex through a thermally induced electron transfer-coupled spin transition (ETCST). With an amphiphilic anion, the [Co2 Fe2 ] complex formed reverse vesicles in solution and showed thermal ETCST. In contrast, thermal ETCST in the presence of a bridging hydrogen-bond donor caused structural conversion from the reverse vesicle structure to entangled one-dimensional chains through hydrogen bond formation.

Stimuli-Responsive DNA-Based Hydrogels on Surfaces for Switchable Bioelectrocatalysis and Controlled Release of Loads

The assembly of enzyme [glucose oxidase (GOx)]-loaded stimuli-responsive DNA-based hydrogels on electrode surfaces, and the triggered control over the stiffness of the hydrogels, provides a means to switch the bioelectrocatalytic functions of the hydrogels. One system includes the assembly of GOx-loaded, pH-responsive, hydrogel matrices cross-linked by two cooperative nucleic acid motives comprising permanent duplex nucleic acids and "caged" i-motif pH-responsive duplexes. Bioelectrocatalyzed oxidation of glucose leads to the formation of gluconic acid that acidifies the hydrogel resulting in the separation of the i-motif constituents and lowering the hydrogel stiffness. Loading of the hydrogel matrices with insulin results in the potential-triggered, glucose concentration-controlled, switchable release of insulin from the hydrogel-modified electrodes. The switchable bioelectrocatalyzed release of insulin is demonstrated in the presence of ferrocenemethanol as a diffusional electron mediator or by applying an electrically wired integrated matrix that includes ferrocenyl-modified GOx embedded in the hydrogel. The second GOx-loaded, stimuli-responsive, DNA-based hydrogel matrix associated with the electrode includes a polyacrylamide hydrogel cooperatively cross-linked by duplex nucleic acids and "caged" G-quadruplex-responsive duplexes. The hydrogel matrix undergoes K+-ions/crown ether-triggered stiffness changes by the cyclic K+-ion-stimulated formation of G-quadruplexes (lower stiffness) and the crown ether-induced separation of the G-quadruplexes (higher stiffness). The hydrogel matrices demonstrate switchable bioelectrocatalytic functions guided by the stiffness properties of the hydrogels.

Designing Heat-Set Gels for Crystallizing APIs at Different Temperatures: A Crystal Engineering Approach

An organic salt crystallizes through different kinds of charge-assisted hydrogen-bonded networks depending on carboxylic functionality number and the degree of amine. These H-bonded packing patterns are often robust and predictable, so one can design a supramolecular salt with a certain purpose. In some cases, two different crystalline packing patterns can be found in Primary Ammonium Dicarboxylate (PAD) salts at different temperatures. Two kinds of supramolecular bonding, namely, charge-assisted hydrogen bonding and weak van der Waals interactions stabilize the two states. A small increase in the carbon chain length in a primary amine enhances the additional van der Waals interactions with the packing so that the 2D hydrogen-bonded network (HBN) transforms into a 1D HBN at room temperature. Such van der Waals interactions can be controlled by external heat, so a temperature-dependent 1D to 2D phase change is feasible. When certain moieties, such as azo and bipyridine, are introduced into the carboxylic acid backbone, the acids become insoluble in most organic solvents, raising their melting point, and resulting in heat-set gels. In the presence of an API, temperature and solvent-dependent polymorphic crystals can be grown in the heat-set gel medium and by simply cooling down the mixture, the API crystals can be separated easily.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

Rapid Self-Healing and Thixotropic Organogelation of Amphiphilic Oleanolic Acid-Spermine Conjugates

Natural and abundant plant triterpenoids are attractive starting materials for the synthesis of conformationally rigid and chiral building blocks for functional soft materials. Here, we report the rational design of three oleanolic acid-triazole-spermine conjugates, containing either one or two spermine units in the target molecules, using the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction. The resulting amphiphile-like molecules 2 and 3, bearing just one spermine unit in the respective molecules, self-assemble into highly entangled fibrous networks leading to gelation at a concentration as low as 0.5% in alcoholic solvents. Using step-strain rheological measurements, we show rapid self-recovery (up to 96% of the initial storage modulus) and sol ⇔ gel transition under several cycles. Interestingly, rheological flow curves reveal the thixotropic behavior of the gels. To the best of our knowledge, this kind of behavior was not shown in the literature before, neither for a triterpenoid nor for its derivatives. Conjugate 4, having a bolaamphiphile-like structure, was found to be a nongelator. Our results indicate that the position and number of spermine units alter the gelation properties, gel strength, and their self-assembly behavior. Preliminary cytotoxicity studies of the target compounds 2-4 in four human cancer cell lines suggest that the position and number of spermine units affect the biological activity. Our results also encourage exploring other triterpenoids and their derivatives as sustainable, renewable, and biologically active building blocks for multifunctional soft organic nanomaterials.

MOF based engineered materials in water remediation: Recent trends

The significant upsurge in the demand for freshwater has prompted various developments towards water sustainability. In this context, several materials have gained remarkable interest for the removal of emerging contaminants from various freshwater sources. Among the currently investigated materials for water treatment, metal organic frameworks (MOFs), a developing class of porous materials, have provided excellent platforms for the separation of several pollutants from water. The structural modularity and the striking chemical/physical properties of MOFs have provided more room for target-specific environmental applications. However, MOFs limit their practical applications in water treatment due to poor processability issues of the intrinsically fragile and powdered crystalline forms. Nevertheless, growing efforts are recognized to impart macroscopic shapability to render easy handling shapes for real-time industrial applications. Furthermore, efforts have been devoted to improve the stabilities of MOFs that are subjected to fragile collapse in aqueous environments expanding their use in water treatment. Advances made in MOF based material design have headed towards the use of MOF based aerogels/hydrogels, MOF derived carbons (MDCs), hydrophobic MOFs and magnetic framework composites (MFCs) to remediate water from contaminants and for the separation of oils from water. This review is intended to highlight some of the recent trends followed in MOF based material engineering towards effective water regeneration.

Stimuli-Responsive Biomolecule-Based Hydrogels and Their Applications

This Review presents polysaccharides, oligosaccharides, nucleic acids, peptides, and proteins as functional stimuli-responsive polymer scaffolds that yield hydrogels with controlled stiffness. Different physical or chemical triggers can be used to structurally reconfigure the crosslinking units and control the stiffness of the hydrogels. The integration of stimuli-responsive supramolecular complexes and stimuli-responsive biomolecular units as crosslinkers leads to hybrid hydrogels undergoing reversible triggered transitions across different stiffness states. Different applications of stimuli-responsive biomolecule-based hydrogels are discussed. The assembly of stimuli-responsive biomolecule-based hydrogel films on surfaces and their applications are discussed. The coating of drug-loaded nanoparticles with stimuli-responsive hydrogels for controlled drug release is also presented.

Luminescent Vesicles Self-assembled Directly from an Amphiphilic Europium Complex in an Ionic Liquid

Novel luminescent vesicles with enhanced emission were successfully achieved for the first time by an amphiphilic europium complex through its spontaneously self-assembly in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF₆). The complex was prepared by europium ions coordinated with terpyridine ligands, which were modified with the hydrophilic ethoxy chains. The enhanced absolute quantum yield and prolonged fluorescence lifetime of complex in vesicles were observed because of the effective shielding of the quench effects caused by both solvent and complex concentration. Compared to the aggregates formed in other solvents, the vesicles obtained in [Bmim]PF₆ showed the best luminescence intensity with the quantum efficiency (37.74%) and luminescent emission lifetime (1.915 ms) both increased about 10 times more. Furthermore, this europium complex was designed to show unsaturated coordination, which made the vesicle luminescence easily quenched when contacting with water. The fluorescence sensing of water with this vesicle as probe was therefore possible, where several unique properties like high sensitivity, low detection limit (0.05 vol %), visible color change, and fast response had been observed. Such designed systems are expected to provide strategies to develop novel supramolecular aggregates in ionic liquids and offer guidance for luminescence detection with facile and wide applications.

A heat-set lanthanide metallogel capable of emitting stable luminescence under thermal, mechanical and water stimuli

A luminescence-stable lanthanide-based metallogel prepared by a heat-set procedure.

Solvent-Assisted Tyrosine-Based Dipeptide Forms Low-Molecular Weight Gel: Preparation and Its Potential Use in Dye Removal and Oil Spillage Separation from Water

Low-molecular weight gelators (supramolecular, or simply molecular gels) are highly important molecular frameworks because of their potential application in drug delivery, catalysis, pollutant removal, sensing materials, and so forth. Herein, a small dipeptide composed of N-(tert-butoxycarbonyl)pentafluoro-l-phenylalanine and O-benzyl-l-tyrosine methyl ester was synthesized, and its gelation ability was investigated in different solvent systems. It was found that the dipeptide was unable to form gel with a single solvent, but a mixture of solvent systems was found to be suitable for the gelation of this dipeptide. Interestingly, water was found to be essential for gelation with the polar protic solvent, and long-chain hydrocarbon units such as, petroleum ether, kerosene, and diesel, were important for gelation with aromatic solvents. The structural insights of these gels were characterized by field-emission scanning electronic microscopy, atomic force microscopy, Fourier transform infrared analysis, and X-ray diffraction studies, and their mechanical strengths were characterized by rheological experiments. Both of the gels obtained from these two solvent systems were thermoreversible in nature, and these translucent gels had potential application for the treatment of waste water. The gel obtained from dipeptides with methanol-water was used to remove toxic dyes (crystal violet, Eriochrome Black T, and rhodamine B) from water. Furthermore, the gel obtained from dipeptide with assistance from toluene-petroleum ether was used as a phase-selective gelator for oil-spill recovery.

Supramolecular gels from sugar-linked triazole amphiphiles for drug entrapment and release for topical application

A simple molecular framework obtained by cross-linking a hydrophobic chain with S,S- and R,R-tetritol by the copper-catalysed azide-alkyne cycloaddition reaction is found to serve as an excellent bioisostere for self-assembly. The hexadecyl-linked triazolyl tetritol composite spontaneously self-assembles in n-hepane and methanol to form hierarchical organogels. Microscopic analyses and X-ray diffraction studies demonstrate eventual formation of nanotubes through lamellar assembly of the amphiphiles. A rheological investigation shows solvent-dictated mechanical properties that obey power law behavior similar to other low molecular weight gelators (LMOGs). The gel network was then utilized for the entrapment of drugs e.g. ibuprofen and 5-fluorouracil, with tunable mechanical behaviour under applied stress. The differential release profiles of the drugs over a period of a few hours as a result of the relative spatio-temporal location in the supramolecular network can be utilized for topical formulations.

Multistimuli-Responsive Self-Healable and Moldable Nickel(II)-Based Gels for Reversible Gas Adsorption and Palladium Sequestration via Gel-to-Gel Transformation

We report the in situ formation of Ni-based supramolecular organogel and organic-aqueous gels using amine appended triazole ligand, having varying morphology and rheological properties. These gels are self-healable and moldable or injectable respectively depending on the absence or presence of water in the gelation medium. Our studies reveal that the formation and rupture of hydrogen bonds assisted by the solvent movement is responsible for the self-healing nature of the gels. The porous structure of the gel has been observed from the migration of dye molecules on the self-healed gel. In addition, the gels show dual function of reversible adsorption of toxic gases and sequestration of heavy metal ions, especially palladium via  gel-to-gel transformation. It is imperative to stress that such transformation is extremely rare for small molecule based metallogels. The dynamic nature of Ni-N_triazole interactions has been utilized in achieving the reversible gas/vapor responsive behavior of the metallogels, which could be suitable in developing colorimetric probes for the detection of toxic gases and heavy metal ions. Such multifunctional gels are exceptional in contemporary literature and are expected to find utility in fabricating smart multistimuli-responsive gel-based materials in the future.

Shape-memory and self-healing functions of DNA-based carboxymethyl cellulose hydrogels driven by chemical or light triggers

Photoresponsive nucleic acid-based carboxymethyl cellulose (CMC) hydrogels are synthesized, and their application as shape-memory and self-healing functional matrices are discussed. One system involves the preparation of a carboxymethyl cellulose hydrogel crosslinked by self-complementary nucleic acid duplexes and by photoresponsive trans-azobenzene/β-cyclodextrin (β-CD) supramolecular complexes. Photoisomerization of the trans-azobenzene to the cis-azobenzene results in a hydrogel exhibiting lower stiffness due to the separation of the azobenzene/β-CD bridging units. The hydrogel is switched between high and low stiffness states by the cyclic and reversible light-induced isomerization of the azobenzene units between the trans and cis states. The light-controlled stiffness properties of the hydrogel are used to develop a shape-memory hydrogel, where the duplex bridging units act as permanent memory in the quasi-liquid shapeless state of the hydrogel. A second system in the study is a carboxymethyl cellulose hydrogel crosslinked by the K+-stabilized G-quadruplex bridging units and by trans-azobenzene/β-CD complexes. The resulting hydrogel includes dual-trigger functionalities, where the trans-azobenzene/β-CD complexes can be reversibly formed and dissociated through the trans and cis photoisomerization of the azobenzene units, and the K+-stabilized G-quadruplexes can be reversibly dissociated and reformed in the presence of 18-crown-6-ether/K+-ions. The signal-responsive crosslinked hydrogel reveals controlled stiffness properties, where the hydrogel crosslinked by the trans-azobenzene/β-CD and K+-ion-stabilized G-quadruplex reveals high stiffness and the hydrogel crosslinked only by the K+-ion-stabilized G-quadruplexes or only by the trans-azobenzene/β-CD complexes reveals low stiffness properties. The controlled stiffness properties of the hydrogel are used to develop shape-memory hydrogels, where the trans-azobenzene/β-CD complexes or the K+-ion-stabilized G-quadruplexes act as permanent memories in the shapeless and quasi-liquid states of the hydrogels. In addition, the hydrogel that includes two types of stimuli-responsive crosslinking units is used as a self-healing matrix, where each of the triggers guides the self-healing processes.

A temperature-responsive supramolecular hydrogel: preparation, gel-gel transition and molecular aggregation

In this study, a novel supramolecular hydrogel, abbreviated as AGC16/NTS, was designed and constructed by the molecular self-assembly of a cationic gemini surfactant, 1,3-bis(N,N-dimethyl-N-cetylammonium)-2-propylacrylatedibromide (AGC16), and an anionic aromatic gelator, trisodium 1,3,6-naphthalenetrisulfonate (NTS). The AGC16/NTS hydrogel was able to form in a mass ratio range of AGC16 and NTS from 20 : 1 to 10 : 1. It was interestingly found that AGC16/NTS exhibited two phase transitions (gel-to-gel and gel-to-sol) observed by visual and rheological measurements during the heating process, which is rarely reported in the previous literature reports of hydrogels prepared using low molecular weight gelators. Cryogenic scanning electron microscopy (cryo-SEM), fluorescence emission spectroscopy and X-ray diffraction (XRD) were used to investigate the temperature-responsive properties and molecular self-assembly mechanism of the hydrogel AGC16/NTS. During the gel-to-gel transition process, the temperature-responsive changes in the visual appearance of AGC16/NTS (turbid to transparent) were clearly observed. Compared with the transparent gel, the turbid gel possesses higher mechanical strength and a much more compact network mophology due to stronger intermolecular hydrophobic association beetween gelators. The molecular self-assembly modes for the two different hydrogel states (turbid and transparent gel) were proposed, helping to further understand the hydrogel transition mechanisms at a molecular level.

Gels with sense: supramolecular materials that respond to heat, light and sound

Advances in the field of supramolecular chemistry have made it possible, in many situations, to reliably engineer soft materials to address a specific technological problem. Particularly exciting are "smart" gels that undergo reversible physical changes on exposure to remote, non-invasive environmental stimuli. This review explores the development of gels which are transformed by heat, light and ultrasound, as well as other mechanical inputs, applied voltages and magnetic fields. Focusing on small-molecule gelators, but with reference to organic polymers and metal-organic systems, we examine how the structures of gelator assemblies influence the physical and chemical mechanisms leading to thermo-, photo- and mechano-switchable behaviour. In addition, we evaluate how the unique and versatile properties of smart materials may be exploited in a wide range of applications, including catalysis, crystal growth, ion sensing, drug delivery, data storage and biomaterial replacement.

Preparation and Formation Process of Zn(II)-Coordinated Nanovesicles

Mixing a glycylglycine lipid and zinc acetate has been reported to form novel supramolecular Zn(II)-coordinated nanovesicles in ethanol. In this study, we investigate in detail the formation of nanovesicles by using three lipids at different temperatures and discuss their formation process. The original lipids show extremely low solubilities and appear as plate structures in ethanol. Within a small window of lipid solubility, the formation of lipid-Zn(II) complexes occurs mainly on the solid surfaces of plate structures. Controlling of the lipid solubility by temperature affects the kinetics of complex formation and the subsequent transformation of the complexes into nanovesicles and nanotubes. An improved method of two-step control of temperature is developed for preparing all the three kinds of nanovesicles. We provide new insights into the formation process of nanovesicles based on several control experiments. A tetrahedral lipid-cobalt(II) complex similarly produces nanovesicles, whereas an octahedral complex gives sheet structures. Mixing of zinc acetate with a β-alanyl-β-alanine lipid can only give sheet structures, which lack a polyglycine II hydrogen-bond network and induce no morphological changes. We conclude that the formation of the lipid-Zn(II) complexes on solid plate structures, tetrahedral geometry, and polyglycine II hydrogen-bond network in the complexes shall work cooperatively for the formation of Zn(II)-coordinated nanovesicles.

High Order in a Self-Assembled Iridium(III) Complex Gelator Towards Nanostructured IrO2 Thin Films

The preparation and characterization of a new metallogelator based on the Ir^III discrete cyclometalated complex [(ppy)₂ Ir(bpy)](CH₃ CH₂ OCH₂ CO₂ ) are reported, where H(ppy) is 2-phenylpiridine and bpy is 2,2'-bipyridine, which is used as an ancillary ligand. The compound is able to self-assemble in water in a range of concentrations between 3 % and 6 % w/w, creating a luminescent ordered supramolecular gel. The gel and xerogel architectures were investigated through polarized optical microscopy (POM), SEM and TEM microscopies coupled with powder X-ray diffraction. The gel supramolecular organization is characterized by columnar tetragonal strands, already present at high dilution conditions, of cations surrounded by counteranions. These strands, in turn, are self-assembled in an oblique columnar cell upon gelification. The xerogel thin films obtained upon complete dehydration maintained the gel supramolecular order and can be used as a precursor for the preparation of nanostructured IrO₂ thin films.

Stimuli-Responsive DNA-Based Hydrogels: From Basic Principles to Applications

The base sequence of nucleic acids encodes structural and functional information into the DNA biopolymer. External stimuli such as metal ions, pH, light, or added nucleic acid fuel strands provide triggers to reversibly switch nucleic acid structures such as metal-ion-bridged duplexes, i-motifs, triplex nucleic acids, G-quadruplexes, or programmed double-stranded hybrids of oligonucleotides (DNA). The signal-triggered oligonucleotide structures have been broadly applied to develop switchable DNA nanostructures and DNA machines, and these stimuli-responsive assemblies provide functional scaffolds for the rapidly developing area of DNA nanotechnology. Stimuli-responsive hydrogels undergoing signal-triggered hydrogel-to-solution transitions or signal-controlled stiffness changes attract substantial interest as functional matrices for controlled drug delivery, materials exhibiting switchable mechanical properties, acting as valves or actuators, and "smart" materials for sensing and information processing. The integration of stimuli-responsive oligonucleotides with hydrogel-forming polymers provides versatile means to exploit the functional information encoded in the nucleic acid sequences to yield stimuli-responsive hydrogels exhibiting switchable physical, structural, and chemical properties. Stimuli-responsive DNA-based nucleic acid structures are integrated in acrylamide polymer chains and reversible, switchable hydrogel-to-solution transitions of the systems are demonstrated by applying external triggers, such as metal ions, pH-responsive strands, G-quadruplex, and appropriate counter triggers that bridge and dissociate the polymer chains. By combining stimuli-responsive nucleic acid bridges with thermosensitive poly(N-isopropylacrylamide) (pNIPAM) chains, systems undergoing reversible solution ↔ hydrogel ↔ solid transitions are demonstrated. Specifically, by bridging acrylamide polymer chains by two nucleic acid functionalities, where one type of bridging unit provides a stimuli-responsive element and the second unit acts as internal "bridging memory", shape-memory hydrogels undergoing reversible and switchable transitions between shaped hydrogels and shapeless quasi-liquid states are demonstrated. By using stimuli-responsive hydrogel cross-linking units that can assemble the bridging units by two different input signals, the orthogonally-triggered functions of the shape-memory were shown. Furthermore, a versatile approach to assemble stimuli-responsive DNA-based acrylamide hydrogel films on surfaces is presented. The method involves the activation of the hybridization chain-reaction (HCR) by a surface-confined promoter strand, in the presence of acrylamide chains modified with two DNA hairpin structures and appropriate stimuli-responsive tethers. The resulting hydrogel-modified surfaces revealed switchable stiffness properties and signal-triggered catalytic functions. By applying the method to assemble the hydrogel microparticles, substrate-loaded, stimuli-responsive microcapsules are prepared. The signal-triggered DNA-based hydrogel microcapsules are applied as drug carriers for controlled release. The different potential applications and future perspectives of stimuli responsive hydrogels are discussed. Specifically, the use of these smart materials and assemblies as carriers for controlled drug release and as shape-memory matrices for information storage and inscription and the use of surface-confined stimuli-responsive hydrogels, exhibiting switchable stiffness properties, for catalysis and controlled growth of cells are discussed.

Coordination chemistry of N-picolyl-1,8-naphthalimides: colourful low molecular weight metallo-gelators and unique chelation behaviours

A family of N-picolyl-1,8-napthalimide ligands display fascinating coordination chemistry and produce a series of robust and chemically responsive supramolecular metallo-gels.

Hierarchical Self-Assembly of Luminescent Tartrate-Bridged Chiral Binuclear Tb(III) Complexes in Ethanol

A new family of supramolecular metalloamphiphiles carrying two metal centers is developed. They are formed by bridging two coordinatively unsaturated lipophilic Tb³⁺ complexes (TbL⁺) with chiral dicarboxylate anions. The formation of bridging coordination bonds is confirmed using UV spectroscopy, induced circular dichroism (ICD), increased luminescence intensity of TbL⁺, and electrospray ionization mass spectrometry (ESIMS) analysis. These supramolecular metalloamphiphiles hierarchically self-assemble in ethanol to give luminescent nanospheres, as observed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The two hydroxyl groups introduced in the bridging ligands of [TbL]₂(d-/l-tartrate) significantly promote self-assembly by increasing coherent forces via intermolecular hydrogen bonding. The observed self-assembly in ethanol also merits mention because such polar alcoholic media have been unfavorable for conventional molecular self-assemblies. The present approach offers a new molecular design strategy for composable metalloamphiphiles.

Self-Assembly and Drug Release Capacities of Organogels via Some Amide Compounds with Aromatic Substituent Headgroups

In this work, some amide compounds with different aromatic substituent headgroups were synthesized and their gelation self-assembly behaviors in 22 solvents were characterized as new gelators. The obtained results indicated that the size of aromatic substituent headgroups in molecular skeletons in gelators showed crucial effect in the gel formation and self-assembly behavior of all compounds in the solvents used. Larger aromatic headgroups in molecular structures in the synthesized gelator molecules are helpful to form various gel nanostructures. Morphological investigations showed that the gelator molecules can self-assembly and stack into various organized aggregates with solvent change, such as wrinkle, belt, rod, and lamella-like structures. Spectral characterizations suggested that there existed various weak interactions including π-π stacking, hydrogen bonding, and hydrophobic forces due to aromatic substituent headgroups and alkyl substituent chains in molecular structures. In addition, the drug release capacities experiments demonstrated that the drug release rate in present obtained gels can be tuned by adjusting the concentrations of dye. The present work would open up enormous insight to design and investigate new kind of soft materials with designed molecular structures and tunable drug release performance.

First report of charge-transfer induced heat-set hydrogel. Structural insights and remarkable properties

The remarkable ability of a charge-transfer (CT) complex prepared from a pyrene-based donor (Py-D) and a naphthalenediimide-based acceptor (NDI-A) led to the formation of a deep-violet in color, transparent hydrogel at room temperature (RT-gel). Simultaneously, the RT-gel was diluted beyond its critical gelator concentration (CGC) to obtain a transparent sol. Very interestingly, the resultant sol, on heating above 70 °C, transformed into a heat-set gel instantaneously with a hitherto unknown CGC value. Detailed studies revealed the smaller globular aggregates of the RT-gels fuse to form giant globules upon heating, which, in turn, resulted in heat-set gelation through further aggregation. The thermoresponsive property of Py-D alone and 1 : 1 Py-D : NDI-A CT complex was investigated in detail which revealed the hydrophobic collapse of the oxyethylene chains of the CT complex upon heating was mainly responsible for heat-set gelation. Thixotropy, injectability, as well as stimuli responsiveness of the RT-gels were also addressed. In contrast, heat-set gel did not show thixotropic behavior. The X-ray diffraction (XRD) patterns of the xerogel depicted lamellar packing of the CT stacks in the gel phase. Single crystal XRD studies further evidenced the 1 : 1 mixed CT stack formation in the lamellae and also ruled out orthogonal hydrogen bonding possibilities among the hydrazide unit in the CT gel although such interaction was observed in a single crystal of NDI-A alone. In addition, a Ag(+)-ion triggered metallogelation of NDI-A and nematic liquid-crystalline property of Py-D were also observed.

Cation Tuning toward the Inference of the Gelation Behavior of Supramolecular Gels

We serendipitously discovered that the tripeptide Asp-Phe-Phe trifluoroacetic acid salt (hereafter abbreviated as β-AspFF) formed a reversible thermotropic gel in chloroform solution (at temperatures higher than the boiling point of chloroform), and a stable gel in toluene solution (at equal to or lower than the room temperature). Experimental results indicate that doping metal ions into β-AspFF toluene gels can trigger morphological variations in the gel skeleton, thereby increasing gel volume and inducing the collapse of organogels. Investigation on the cation-tuned gelation behavior of β-AspFF can be used to elucidate heating-induced gel collapse (of normal gel) or reverse thermotropic gelation as well as select carbamide and acetamide as activators of β-AspFF gels in chloroform solution at room temperature.

A Co²⁺-selective and chirality-sensitive supermolecular metallohydrogel with a nanofiber network skeleton

This paper introduces a new metallohydrogel precursor that offers a peculiar gelation response to Co(2+) at pH 7-8. It is notable that the stability of this metallohydrogel is significantly dependent on its enantiomeric purity. In addition to the expected multi-stimuli responsive properties, including thixotropy, as well as re-assembly properties on adding HCl, this metallohydrogel possesses excellent self-healing behavior, which is uncommon in low-molecular-weight gelators. Electron microscopy (EM) studies revealed that the cryodried remains of this gel presented as well an organized three dimensional (3D) network of fibrils. This interesting metallohydrogel shows potential for developing new soft materials with drug delivery and catalysis properties.