Water-Regulated Evolution of Inversion, Reinversion, and Amplification of Circularly Polarized Luminescence of Supramolecular Organogels Based on Glutamide-Cyanostilbene Amphiphile

Water incorporated with supramolecular building blocks in organic solvents can play a key role in the circularly polarized luminescence (CPL) inversion and amplification of supramolecular assemblies. Herein, we demonstrate that fine-tuning the water content regulated the assembly structure evolution and made the circular dichroism and CPL sign of the system undergo intriguing inversion, reinversion, and amplification processes based on a unique and interesting glutamide-cyanostilbene system, as supported by morphology, spectroscopic observations, and time-dependent density functional theory calculation.

Scrolling in Supramolecular Gels: A Designer's Guide

Gelation by small molecules is a topic of enormous importance in catalysis, nanomaterials, drug delivery, and pharmaceutical crystallization. The mechanism by which gelators self-organize into a fibrous gel network is poorly understood. Herein, we describe the crystal structures and gelation properties of a library of bis(urea) compounds and show, via molecular dynamics simulations, how gelator aggregation progresses from a continuous pattern of supramolecular motifs to a homogeneous fiber network. Our model suggests that lamellae with asymmetric surfaces scroll into uniform unbranched fibrils, while sheets with symmetric surfaces undergo stacking to form crystals. The self-assembly of asymmetric lamellae is associated with specific molecular features, such as the presence of narrow and flexible end groups with high packing densities, and likely represents a general mechanism for the formation of small-molecule gels.

Non-destructive real-time monitoring and investigation of the self-assembly process using fluorescent probes

Self-assembly has been considered as a strategy to construct superstructures with specific functions, which has been widely used in many different fields, such as bionics, catalysis, and pharmacology. A detailed and in-depth analysis of the self-assembly mechanism is beneficial for directionally and accurately regulating the self-assembly process of substances. Fluorescent probes exhibit unique advantages of sensitivity, non-destructiveness, and real-time self-assembly tracking, compared with traditional methods. In this work, the design principle of fluorescent probes with different functions and their applications for the detection of thermodynamic and kinetic parameters during the self-assembly process were systematically reviewed. Their efficiency, limitations and advantages are also discussed. Furthermore, the promising perspectives of fluorescent probes for investigating the self-assembly process are also discussed and suggested.

1D Supramolecular Assemblies That Crystallize and Form Gels in Response to the Shape-Complementarity of Alcohols

N-9-Fluorenylmethyloxycarbonyl (Fmoc)- and C-tertiary butyl (t-Bu)-protected glutamate (L-2), bearing a phenanthroline moiety at the side residue, forms 1D supramolecular assemblies via H-bonding as well as undergoing π-stacking interactions to afford crystals or gels that depend on the shape-complementarity of coexisting alcohols, as demonstrated by structural analyses on these assemblies by means of single-crystal X-ray diffractometry and supplemented with small- and wide-angle X-ray scattering data. Moreover, the rheological measurements on the gels help to define a model for when gels and crystals are expected and found. These observations and conclusions highlight an important, but not very appreciated, aspect of solute-solvent interactions within supramolecular assemblies that can allow the constituent-aggregating molecules in some systems to exhibit high selectivity toward the structures of their solvents. The consequences of this selectivity, as demonstrated here by single-crystal and powder X-ray diffraction data, can lead to self-assembled structures which alter completely the bulk phase properties and morphology of the materials. In that regard, rheological measurements have helped to develop a model to explain when gels and phase-separated mixtures of crystals and solvents are expected.

Evolution of Transient Luminescent Assemblies Regulated by Trace Water in Organic Solvents

Trace water in organic solvents can play a crucial role in the construction of supramolecular assemblies, which has not gained enough attention until very recent years. Herein, we demonstrate that residual water in organic solvents plays a decisive role in the regulation of the evolution of assembled structures and their functionality. By adding Mg(ClO₄)₂ into a multi-component organic solution containing terpyridine-based ligand 3Tpy and monodentate imidazole-based ligand M2, the system underwent an unexpected kinetic evolution. Metallo-supramolecular polymers (MSP) formed first by the coordination of 3Tpy and Mg²⁺, but they subsequently decomposed due to the interference of M2, resulting in a transient MSP system. Further investigation revealed that this occurred because residual water in the solvent and M2 cooperatively coordinated with Mg²⁺. This allowed M2 to capture Mg²⁺ from MSP, which led to depolymerization. However, owing to the slow reaction between trace water/M2/Mg²⁺, the formation of MSP still occurred first. Therefore, water regulated both the thermodynamics and kinetics of the system and was the key factor for constructing the transient MSP. Fine-tuning the water content and other assembly motifs regulated the assembly evolution pathway, tuned the MSP lifetime, and made the luminescent color of the system undergo intriguing transition processes over time.

Understanding gel-to-crystal transitions in supramolecular gels

Most supramolecular gels are stable or assumed to be stable over time, and aging effects are often not studied. However, some gels do show clear changes on aging, and a small number of systems exhibit gel-to-crystal transitions. In these cases, crystals form over time, typically at the expense of the network underpinning the gel; this leads to the gel falling apart. These systems are rare, and little is known about how these gel-to-crystal transitions occur. Here, we use a range of techniques to understand in detail a gel-to-crystal transition for a specific functionalised dipeptide based gelator. We show that the gel-to-crystal transition depends on the final pH of the medium which we control by varying the amount of glucon-δ-lactone (GdL) added. In the gel phase, at low concentrations of GdL, and at early time points with high concentrations of GdL, we are able to show the nanometre scale dimensions of the self-assembled fibre using SAXS; however there is no evidence of molecular ordering of the gel fibres in the WAXS. At low concentrations of GdL, these self-assembled fibres stiffen with time but do not crystallise over the timescale of the SAXS experiment. At high concentrations of GdL, the fibres are already stiffened, and then, as the pH drops further, give way to the presence of crystals which appear to grow preferentially along the direction of the fibre axis. We definitively show therefore that the gel and crystal phase are not the same. Our work shows that many assumptions in the literature are incorrect. Finally, we also show that the sample holder geometry is an important parameter for these experiments, with the rate of crystallisation depending on the holder in which the experiment is carried out.

Water-Mediated Folding Behaviors and Chiroptical Inversion of Ferrocene-Conjugated Dipeptides

The hydration effect on the folding behavior of oligopeptides is of vital importance both in the structure basis of biomolecules and in the rational design of peptide-based materials, which however has rarely been addressed. Here we present the hydration impact on the spontaneous folding of dipeptides conjugated by the ferrocene spacer. In organic phase, the ferrocene-glycine-phenylalanine dipeptide formed a parallel β-sheet structure and Herrick's conformation, which underwent conformational transformation encountering aqueous media, by significantly switching dipeptide arm angles around the ferrocene axis up to 72°. The conformational transformation behavior aroused inversion of the chiroptical activity. Solid X-ray structures, proton nuclear magnetic resonance, chiroptical spectroscopy, and the density functional theory calculation were employed to unveil the hydration effect in the secondary structure transition, in which the rearrangement of hydrogen bonds played the vital role. This work deepens the understanding of water functioning in the structure modulation of biomolecules and also provides an alternative protocol in designing novel chiroptical switches and adaptive peptide-based biomaterials.

Supramolecular gel formation regulated by water content in organic solvents: self-assembly mechanism and biomedical applications

As one of the most important and fruitful methods, supramolecular self-assembly has a significant advantage in designing and fabricating functional soft materials with various nanostructures. In this research, a low-molecular-weight gelator, N,N'-di(pyridin-4-yl)-pyridine-3,5-dicarboxamide (PDA-N4), was synthesized and used to construct self-assembled gels via a solvent-mediated strategy. It was found that PDA-N4 could form supramolecular gels in mixed solvents of water and DMSO (or DMF) at high water fraction (greater than or equal to 50%). By decreasing the water fraction from 50% to 30%, the gel, suspension and solution phases appeared successively, indicating that self-assembled aggregates could be efficiently modulated via water content in organic solvents. Moreover, the as-prepared PDA-N4 supramolecular gels not only displayed solid-like behavior, and pH- and thermo-reversible characteristics, but also showed a solution-gel-crystal transition with the extension of aging time. Further analyses suggested that both the crystal and gel had similar assembled structures. The intermolecular hydrogen bonding between amide groups and the π-π stacking interactions between pyridine groups played key roles in gel formation. Additionally, the release behavior of vitamin B12 (VB12) from PDA-N4 gel (H2O/DMSO, v/v = 90/10) was evaluated, and the drug controlled release process was consistent with a first-order release mechanism. The human umbilical venous endothelial cell culture results showed that the PDA-N4 xerogel has good cytocompatibility, which implied that the gels have potential biological application in tissue engineering and controlled drug release.

Sugar-Based Organogelators for Various Applications

In this Feature Article, we discuss the design strategy, syntheses, and the self-assembly of various sugar-based gelators to form organogels. We illustrate the use of organogels formed by these sugar-based gelators for various applications such as (a) development of scratch-free, shatter-free, soft-optical devices using oil gels formed by mannitol-based gelators, (b) marine oil-spill recovery using sugar-based phase selective organogelators, (c) preparation of semiconducting cotton cloths using a diyne functionalized sugar gelator, (d) development of sugar arrays on glass slides using a polymerizable diyne functionalized sugar gelator for efficient lectin binding, (e) development of sintering resistant hybrid CaO-silica material for the absorption of CO₂, (f) preparation of porous polystyrene-crown ether matrix for the selective alkali metal ions sequestration, and (g) preparation of porous polystyrene, structured silica, and fluorescent gels using a library of sugar-based gelators, and also the mechanism of gelation of some of these gelators have been discussed. We have also given our perspective toward exploring sugar-based gelators for advanced applications.

Halobenzyl alcohols as structurally simple organogelators

We report 11 simple halobenzyl alcohols, each comprising of only 16 atoms, as organogelators for aliphatic hydrocarbon solvents. Their self-assembly is similar in both gel and crystal states and involve OH⋯O, CH⋯O, CH⋯π, O⋯X, CH⋯X and X⋯X interactions.

Carbohydrate Derived Organogelators and the Corresponding Functional Gels Developed in Recent Time

Owing to their multifarious applicability, studies of molecular and supramolecular gelators and their corresponding gels have gained momentum, particularly in the last two decades. Hydrophobic⁻hydrophilic balance, different solvent parameters, gelator⁻gelator and gelator⁻solvent interactions, including different noncovalent intermolecular interactive forces like H-bonding, ionic interactions, π⁻π interactions, van der Waals interactions, etc., cause the supramolecular gel assembly of micro and nano scales with different types of morphologies, depending on the gelator, solvent, and condition of gelation. These gel structures can be utilized for making template inorganic superstructures for potential application in separation, generation of nanocomposite materials, and other applications like self-healing, controlled drug encapsulation, release and delivery, as structuring agents, oil-spill recovery, for preparation of semi-conducting fabrics, and in many other fields. Sugars, being easily available, inexpensive, and nontoxic natural resources with multi functionality and well-defined chirality are attractive starting materials for the preparation of sugar-based gelators. This review will focus on compilation of sugar derived organogelators and the corresponding gels, along with the potential applications that have been developed and published recently between January 2015 and March 2018.

Water-Binding-Mediated Gelation/Crystallization and Thermosensitive Superchirality

Determination of molecular structural parameters of hydrophobic cholesterol-naphthalimide conjugates for water binding capabilities as well as their moisture-sensitive supramolecular self-assembly were revealed. Water binding was a key factor in leading trace water-induced crystallization against gelation in apolar solvent. Ordered water molecules entrapped in self-assembly arrays revealed by crystal structures behave as hydrogen-bonding linkers to facilitate three-dimensional growth into crystals rather than one-dimensional gel nanofibers. Water binding was also reflected on the supramolecular chirality inversion of vesicle self-assembly in aqueous media via heating-induced dehydration. Structural parameters that favor water binding were evaluated in detail, which could help rationally design organic building units for advancing soft materials, crystal engineering, and chiral recognition.

Unravelling the secret of seed-based gels in water: the nanoscale 3D network formation

Chia (Salvia hispanica) and basil (Ocimum basilicum) seeds have the intrinsic ability to form a hydrogel concomitant with moisture-retention, slow releasing capability and proposed health benefits such as curbing diabetes and obesity by delaying digestion process. However, the underlying mode of gelation at nanoscopic level is not clearly explained or explored. The present study elucidates and corroborates the hypothesis that the gelling behavior of such seeds is due to their nanoscale 3D-network formation. The preliminary study revealed the influence of several conditions like polarity, pH and hydrophilicity/hydrophobicity on fiber extrusion from the seeds which leads to gelation. Optical microscopic analysis clearly demonstrated bundles of fibers emanating from the seed coat while in contact with water, and live growth of fibers to form 3D network. Scanning electron microscope (SEM) and transmission electron microscope (TEM) studies confirmed 3D network formation with fiber diameters ranging from 20 to 50 nm.

Lamellar-cubic transition of a dihydrazide derivative and its effect on the gel stability

N,N'-Bis(4-n-alkyloxybenzoyl)hydrazine (4D16) was demonstrated to show three different aggregates, i.e. a crystalline cubic phase and two kinds of lamellar structure with layer spacings of 34.20 Å (termed the L1 structure) and 40.85 Å (L2 structure) depending on the type of solvents. Lamellar (L1)-crystalline cubic transition during heating was confirmed for 4D16 showing the L1 structure. 4D16 organogels in cyclohexane and benzene exhibited either a mixture of the L1 structure and the crystalline cubic phase or only one of the two structures. 4D16 gels prepared at a higher concentration or a lower incubation temperature consisted of more lamellar L1 structures compared to those obtained at a lower concentration or a higher incubation temperature. Annealing of the as-prepared 4D16 gels at certain temperatures for different time periods caused gradual lamellar L1-cubic transition, and thus increased the content of the cubic phase in the gels, which showed lower Tgel compared to those of the as-prepared ones. The existence of the cubic phase in 4D16 gels in cyclohexane and benzene destabilized the gels.

Tunable multicolor emissions in a monocomponent gel system by varying the solvent, temperature and fluoride anion

The facile tuning of the fluorescent properties of organogels is highly desirable for optical switches, light-emitting diodes, chemosensors and bioprobes. The design of organic molecules with multiple emission colors but only one molecular platform remains challenging. Herein, a new cholesterol-based organogelator N1 containing D-A pairs (salicylaldehyde and naphthalimide units) was designed. We successfully obtained multiple solvent-tuned emission colors in both the solution and gel states using a unimolecular platform. Moreover, the effects of the solvent on the gel morphology, rheology and anion-responsive properties were studied. Finally, we showed that the gel in benzene displayed reversible thermochromic properties with changes in emission color from yellow-green to red. Several experiments suggested that a short-distance and ordered array of the D-A pairs facilitated the efficient intermolecular electron transfer of the fluorophores.

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.

Silver ions blocking crystallization of guanosine-based hydrogel for potential antimicrobial applications

In this work, the detailed crystallization process of 2′-deoxy-2′-fluoroguanosine (^FG_d) hydrogel has been studied using single crystal X-ray diffraction, variable-temperature nuclear magnetic resonance (VT-NMR), and scanning electron microscopy (SEM). Both solid and solution results indicated that the K⁺-mediated G-quartet structures were unstable and easily resulted in the breakdown of the hydrogel to form linear ribbon structures by forming mimic reverse Watson–Crick base pairs between the two faces with an intermolecular hydrogen-bond (N10H–O11). Accordingly, Ag⁺ was introduced to block the crystallization of ^FG_d to form long lifetime stable supramolecular hydrogel (>6 months) and possible silver-ions-mediated base pair motifs were suggested via NMR, UV, and mass spectroscopy (MS) in combination with powder X-ray diffraction (PXRD) and circular dichroism spectroscopy (CD). Furthermore, ^FG_dAg hydrogel exhibited low toxicity for normal oral keratinocyte cells (NOK-SI) and good antibacterial activities for Fusobacterium nucleatum in vitro.

Ag⁺ was introduced to block the crystallization of ^FG_d forming long lifetime stability supramolecular hydrogel (>6 months), which displayed low toxicity for NOK-SI cell and good antibacterial activities.

Environment-Adaptive Coassembly/Self-Sorting and Stimulus-Responsiveness Transfer Based on Cholesterol Building Blocks

Manipulating the property transfer in nanosystems is a challenging task since it requires switchable molecular packing such as separate aggregation (self-sorting) or synergistic aggregation (coassembly). Herein, a unique manipulation of self-sorting/coassembly aggregation and the observation of switchable stimulus-responsiveness transfer in a two component self-assembly system are reported. Two building blocks bearing the same cholesterol group give versatile topological structures in polar and nonpolar solvents. One building block (cholesterol conjugated cynanostilbene, CCS) consists of cholesterol conjugated with a cynanostilbene unit, and the other one (C₁₀CN) is comprised of cholesterol connected with a naphthalimide group having a flexible long alkyl chain. Their assemblies including gel, crystalline plates, and vesicles are obtained. In gel and crystalline plate phases, the self-sorting behavior dominates, while synergistic coassembly occurs in vesicle phase. Since CCS having the cyanostilbene group can respond to the light irradiation, it undergoes light-induced chiral amplification. C₁₀CN is thermally responsive, whereby its supramolecular chirality is inversed upon heating. In coassembled vesicles, it is interestingly observed that their responsiveness can be transferred by each other, i.e., the C₁₀CN segment is sensitive to the light irradiation, while CCS is thermoresponsive. This unprecedented behavior of the property transfer may shine a light to the precise fabrication of smart materials.

Sulfamide-Lattice Restructuring To Form Dimensionally Controlled Molecular Arrays and Gel-Forming Systems

A design approach that incorporates structural requirements for the formation of a 1D assembly, fibril stability, and fibril-fibril interactions for gelation was attempted by using amino acid-based sulfamides with the general structure Aa-NH-SO₂ -NH-Aa (Aa=amino acid). A preference for 1D assembly alone was not a sufficient condition for gelation, which became evident from studies involving sulfamide esters 1-5. Reducing the crystallization tendency without hindering unidirectional growth was executed through diacids of the sulfamide precursors with various amines that form an envelope around the sulfamide core through salt bridges. This strategy was fruitful, and gels of a wide variety of solvents could be formed by varying the acid and amine components. The use of dodecylamine or benzylamine, which could stabilize the molecular layers through alkyl-chain segregation or π-π interactions improved the gelation tendency, whereas the nature of the amino acid side chain, especially the rotational freedom and hydrophobicity, had a direct role in dictating the solvent preference. Crystallographic studies of these two-component systems gave molecular-level insight into the assembly and showed the importance of anisotropy in the distribution of secondary interactions in gelation.

Solvent-dependent gelation behaviour and liquid crystal properties of a bent-core dihydrazide derivative

Both liquid crystalline gel and crystalline gel are obtained by changing the solvent from non-polar to polar. Liquid crystalline gels exhibit better elastic property over crystalline gel from EtOH. And their corresponding xerogels show distinct mesophase behaviour.

Rapid self-healing and anion selectivity in metallosupramolecular gels assisted by fluorine–fluorine interactions

Metal complexes from perfluoroalkylamide terpyridine self-assemble into anion selective gels, which manifest self-healing and thermal rearrangement in aqueous dimethyl sulfoxide.

Trace Solvent as a Predominant Factor To Tune Dipeptide Self-Assembly

Solvent molecules such as water are of key importance for tuning self-assembly in biological systems. However, it remains a great challenge to detect the role of different types of noncovalent interactions between trace solvents and biomolecules such as peptides. In this work, we discover a dominant role of trace amounts of solvents for mediation of dipeptide self-assembly, in which solvent-bridged hydrogen bonding is demonstrated as a crucial force in directing fiber formation. Hydrogen-bond-forming solvents (including ethanol, N,N-dimethylformamide, and acetone) can affect the hydrogen bonding of C═O and N-H in diphenylalanine (FF) molecules with themselves, but this does not induce π-π stacking between FF molecules. The directional hydrogen bonding promotes a long-range-ordered arrangement of FF molecules, preferentially along one dimension to form nanofibers or nanobelts. Furthermore, we demonstrate that water with strong hydrogen-bond-forming capability can notably speed up structure formation with long-range order, revealing the importance of water as a trace solvent for regulation of persistent and robust fiber formation.

Hierarchical self-assembly of switchable nucleolipid supramolecular gels based on environmentally-sensitive fluorescent nucleoside analogs

Exquisite recognition and folding properties have rendered nucleic acids as useful supramolecular synthons for the construction of programmable architectures. Despite their proven applications in nanotechnology, scalability and fabrication of nucleic acid nanostructures still remain a challenge. Here, we describe a novel design strategy to construct new supramolecular nucleolipid synthons by using environmentally-sensitive fluorescent nucleoside analogs, based on 5-(benzofuran-2-yl)uracil and 5-(benzo[b]thiophen-2-yl)uracil cores, as the head group and fatty acids, attached to the ribose sugar, as the lipophilic group. These modified nucleoside-lipid hybrids formed organogels driven by hierarchical structures such as fibers, twisted ribbons, helical ribbons and nanotubes, which depended on the nature of fatty acid chain and nucleobase modification. NMR, single crystal X-ray and powder X-ray diffraction studies revealed the coordinated interplay of various non-covalent interactions invoked by modified nucleobase, sugar and fatty acid chains in setting up the pathway for the gelation process. Importantly, these nucleolipid gels retained or displayed aggregation-induced enhanced emission and their gelation behavior and photophysical properties could be reversibly switched by external stimuli such as temperature, ultrasound and chemicals. Furthermore, the switchable nature of nucleolipid gels to chemical stimuli enabled the selective two channel recognition of fluoride and Hg(2+) ions through visual phase transition and fluorescence change. Fluorescent organogels exhibiting such a combination of useful features is rare, and hence, we expect that this innovative design of fluorescent nucleolipid supramolecular synthons could lead to the emergence of a new family of smart optical materials and probes.

A thermo-sensitive supramolecular hydrogel derived from an onium salt with solution–gel–crystal transition properties

A self-assembled supramolecular hydrogel, which not only exhibits thermo-sensitive characteristics but has the property of solution–gel–crystal transition, is obtained by mixing the aqueous solutions of benzoic acid and melamine.

Tuning of gel morphology with supramolecular chirality amplification using a solvent strategy based on an Fmoc-amino acid building block

The self-assembly of an aromatic amino acid affords diverse aggregates from flat nanofibers to twist nanofibers with tunable supramolecular chirality.