Active Bicomponent Nanoparticle Assembly with Temporal,Microstructural, and Functional Control

Transient supramolecular self-assembly has evolved as a tool to create temporally programmable smart materials. Yet, so far single-component self-assembly has been mostly explored. In contrast, multicomponent self-assembly provides an opportunity to create unique nanostructures exhibiting complex functional outcomes, newer and different than individual components. Even two-component can result in multiple organizations, such as self-sorted domains or co-assembled heterostructures, can occur, thus making it highly complex to predict and reversibly modulate these microstructures. In this study, we attempted to create active bicomponent nanoparticle assemblies of orthogonally pH-responsive-group-functionalized gold and cadmium selenide nanoparticles with temporal microstructural control on their composition (self-sorted or co-assembly) in order to harvest their emergent transient photocatalytic activity by coupling to temporal changes in pH. Moving towards multicomponent systems can deliver next level control in terms of structural and functional outcomes of supramolecular systems.     

Supramolecular gels formed from multi-component low molecular weight species

Low molecular weight supramolecular gels consist of small molecules (gelators) that in an appropriate solvent self-assemble into nano- or micro-scale network structures resulting in the formation of a gel. Most supramolecular gels consist of two parts, namely the solvent and the gelator. However, the concept of multi-component supramolecular gels, in which more than one compound is added to the solvent, offers a facile way (e.g. by changing the ratio of the different components) to tailor the properties of the gel. The simplest multi-component gels consist of two components added to the solvent and are the most widely studied to date. There are three general classes of such multi-component gels that have been investigated. The first class requires all the added components to access the gel; that is, no component forms a gel on its own. A second class uses two (or more) gelators which can either co-assemble or self-sort into distinct assemblies and the final class consists of one (or more) gelator and one (or more) non-gelling additive which can impact the assembly process of the gelator and therefore the gel's properties.     

超分子凝胶在催化有机反应中的应用

超分子凝胶是有机小分子通过分子间非共价作用形成的使溶剂固定的三维网络结构胶体。 综述关注了近年来超分子凝胶在催化有机反应中一个新的应用方向,依据凝胶剂结构特征和催化反应的类型对迄今报道的小分子凝胶催化剂进行了深入的总结与归类,揭示凝胶催化的独特性,并对其存在的问题和发展趋势进行了讨论。     

The remarkable role of hydrogen bond,halogen, and solvent effect on self-healing supramolecular gel

Self-healing supramolecular gels of low-molecular-weight (LMW) molecules are smart soft materials; however, the development of self-healing LMW gelator is still a challenging task because of the lack of in-depth studies about self-healing mechanisms of LMW gels and the solvent effect on gel properties. Therefore, herein a different perspective was used to study a family of D-gluconic acetal-based gelators with variable structural fragments in 14 different solvents, and a more detailed understanding of self-assembly and self-healing mechanism of supramolecular gels was attained. Based on the critical gelation concentration, phase transition temperature, and rheological data, A8 bearing an amide group in side chain and two chlorine atoms linked to benzene ring was found to be an outstanding gelator, which could form gels with good self-healing ability in a variety of solvents. Interestingly, A8 gel formed in n-BuOH demonstrates high transparency, good mechanical strength, self-supporting behavior, and great self-healing ability from mechanical damage. Based on the Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and theoretical calculation analysis, the self-assembly and self-healing mechanisms of A8 gel were proposed, indicating that a combination of hydrogen bonding and halogen effect was responsible for the efficient self-healing behavior of supramolecular gel. Furthermore, the analysis of solvent parameters indicated that the dispersion force of solvent favored gelators to self-assemble, hydrogen bonding donor ability of solvent mainly affected the formation of one-dimensional assembly, and hydrogen bonding receptor ability and polarity of solvent mainly influenced the supramolecular interactions among assemblies, significantly intervening the self-healing ability of gels. Overall, this study provides a new perspective to the understanding of gelator structure–property correlation in solvents and sheds light for future development of self-healing supramolecular gels.     

Thermo-reversibility of the fluorescence enhancement of acridine orange induced by supramolecular self-assembly

Fluorescence enhancement of acridine orange (AO) in supramolecular hydrogels formed by self-assembly of the gelators 3-{[(2R)-2-(octadecylamino)-3-phenylpropanoyl]amino}butyrate (TC₁₈PheBu) and 1,3:2,4-di-O-benzylidene-d-sorbitol (DBS) was investigated by steady-state and varying temperature fluorescence, polarized fluorescence and time-resolved fluorescence techniques. The results showed that the fluorescence intensities of AO in the gels remarkably increased in comparison with AO aqueous solutions, and increased with an increase of the gelator concentrations. The varying temperature fluorescence analysis indicated that fluorescence intensities of AO in the gels decreased upon an increase of temperature, and vice versa. This can be attributed to aggregation and dissociation of the gelators in the systems, since the fluorescence enhancement of AO was induced by self-assembly of the gelators. Polarized fluorescence analysis indicated that the values of anisotropy (r) of AO are significantly higher than that in water. This further confirmed that the three-dimensional network formed by the gelator aggregates constrained the rotation of AO entrapped within the gels, resulting in high values of anisotropy. Time-resolved fluorescence analysis indicated that the rates of fluorescence decay in the gels are lower than that in water. These results reveal thermo-reversibility of the fluorescence enhancement of AO in supramolecular hydrogels.     

Stigmasterol-based novel low molecular weight/mass organic gelators

Conjugates consisting of stigmasterol and L-phenylalanine, interconnected through short-chained dicarboxylic acyls by ester and amide bonds, respectively, were synthesized as potential low molecular weight/mass organic gelators (LMWGs/LMMGs). Their physico-chemical properties were subjected to investigation, especially their ability to form gels reversibly based on changes of the environmental conditions. Other self-assembly properties detectable by UV-VIS traces were measured in systems consisting of two miscible solvents (water/acetonitrile) with varying solvent ratios and using constant concentrations of the studied compounds. Partition and diffusion coefficients and solubility in water were calculated for the target conjugates. The conjugate 3a was the only compound from this series capable of forming a gel in 1-octanol. All three conjugates 3a-3c displayed supramolecular characteristics in the UV-VIS spectra.     

Sequential Assembly of Mutually Interactive Supramolecular Hydrogels and Fabrication of Multi-Domain Materials

A two-component self-sorting hydrogel based on acylhydrazide and carboxylic acid derivatives of 1,3:2,4-dibenzylidene-d -sorbitol (DBS-CONHNH₂ and DBS-COOH) is reported. A heating–cooling cycle induces the self-assembly of DBS-CONHNH₂, followed by the self-assembly of DBS-COOH induced by decreasing pH. Although the networks are formed sequentially, there is spectroscopic evidence of interactions between them, which impact on the mechanical properties and significantly enhance the ability of these low-molecular-weight gelators (LMWGs) to form gels when mixed. The DBS-COOH network can be switched “off” and “on” within the two-component gel through a pH change. By using a photo-acid generator, the two-component gel can be prepared combining the thermal trigger with photo-irradiation. Photo-patterned self-assembly of DBS-COOH within a pre-formed DBS-CONHNH₂ gel under a mask yields spatially controlled multi-domain gels. Different gel domains can have different functions, for example, controlling the rate of release of heparin incorporated into the gel, or directing gold nanoparticle assembly. Such photo-patterned multi-component hydrogels have potential applications in regenerative medicine or bio-nano-electronics.     

Access to Metastable Gel States Using Seeded Self‐Assembly of Low‐Molecular‐Weight Gelators

Here we report on how metastable supramolecular gels can be formed through seeded self‐assembly of multicomponent gelators. Hydrazone‐based gelators decorated with non‐ionic and anionic groups are formed in situ from hydrazide and aldehyde building blocks, and lead through multiple self‐sorting processes to the formation of heterogeneous gels approaching thermodynamic equilibrium. Interestingly, the addition of seeds composing of oligomers of gelators bypasses the self‐sorting processes and accelerates the self‐assembly along a kinetically favored pathway, resulting in homogeneous gels of which the network morphologies and gel stiffness are markedly different from the thermodynamically more stable gel products. Importantly, over time, these metastable homogeneous gel networks are capable of converting into the thermodynamically more stable state. This seeding‐driven formation of out‐of‐equilibrium supramolecular structures is expected to serve as a simple approach towards functional materials with pathway‐dependent properties.     

Unique nanoscale morphologies underpinning organic gel-phase materials

This study investigates the self-assembly of simple aliphatic diamines with a dendritic peptide. By controlling the molar ratio of this two-component system, new nanoscale morphologies were generated. In the presence of relatively long aliphatic chains (C10, C12) a transition from nanoscale fibres to platelets was observed on changing the molar ratio, whereas, for shorter spacer chains (e.g., C9 and C8), interesting and unique morphological changes were observed by low voltage field emission gun scanning electron microscopy (SEM), with "nanosquares" or nanoscale "rosette" structures being formed. Remarkably, these discrete nanoscale structures were able to form sample-spanning networks capable of supporting a gel-phase material; whereas, most gels are usually based on fibrillar assemblies. In addition to SEM, the gels were characterised by using thermal measurements and circular dichroism spectroscopy. The length of the diamine spacer and the molar ratio of components controlled the self-assembly process by modifying the spatial organisation of the dendritic head groups at the molecular level, which is transcribed into the aspect ratio of the self-assembled state at the microscopic level. Ultimately, this led to diamine-induced control of the macroscopic material's behaviour. When present in excess, the diamine controlled the observed nanoscale morphology as a consequence of undergoing a dendritically controlled nanocrystallisation process to form a network, an unusual and significant result.     

Organogels with Fe(III) complexes of phosphorus-containing amphiphiles as two-component isothermal gelators

The properties of thermally reversible organogels in which the gelators consist of a phosphonic acid monoester, phosphonic acid, or phosphoric acid monoester and a ferric salt are probed by IR and NMR spectroscopies, optical microscopy, X-ray diffraction, rheology, and light and small-angle neutron scattering (SANS) techniques. This is one of a small number of two-component molecular gelator systems in which gelation can be induced isothermally. The data indicate that complexation between the phosphonate moieties and Fe(III) is accompanied by their in situ polymerization to form self-assembled fibrillar networks that encapsulate and immobilize macroscopically the organic liquid component. From SANS measurements, the cross-sectional radii of the cyclindrical fibers are ca. 15 A. The efficiencies of the gelators (based on the diversity of the liquids gelated, the minimum concentration of gelator required to make a gel at room temperature, and the temporal and thermal stabilities of the gels) have been determined. With a common ferric salt and liquid component, phosphonic acid monoesters are generally more efficient than phosphinic acids or phosphoric acid esters. Of the phosphonic acid monoesters, monophosphonates are better gelator components than bisphosphonates, and introduction of an omega-hydroxy group on the alkyl chain directly attached to phosphorus reduces significantly gelation ability. Several of the gels are stable for very long periods at room temperature. When heated, they revert to sols over wide temperature ranges. The structures of the gelator complexes and the mechanism of their formation and transformation to gels in selected liquids are examined as well.     

亚苄基山梨醇衍生物在有机溶剂中的自组装及凝胶化研究

从分子结构的差异、亲溶剂作用、分子几何构型、相转变热焓以及溶剂极性等方面研究了三种亚苄基山梨醇衍生物凝胶剂在有机溶剂中的自组装和凝胶化机理. 三种衍生物凝胶剂在结构上的差别仅在于亚苄基上甲基取代基数量不同. 结果表明: 由于亲溶剂作用的增加和分子几何构型的优化, 含甲基多的凝胶剂在有机溶剂中的自组装能力强, 表现在具有低的最低凝胶化浓度和高的相转变温度. 而溶剂极性的增强, 使三种衍生物凝胶剂形成的凝胶相转变温度降低. 偏光显微镜照片表明该凝胶剂在正辛醇凝胶中的聚集体晶型不同. 场发射扫描电镜照片表明三种衍生物凝胶剂自组装形成相互缠绕的纤维束网络结构. 紫外吸收光谱表明, 对比其溶液态, 三种衍生物聚集体苯环的K带发生红移, 表明π-π堆积作用是亚苄基山梨醇衍生物凝胶剂自组装的驱动力之一; 红移的幅度随苯环上甲基数量的增加而增加, 这与三种衍生物形成的分子凝胶的热稳定性相吻合.     

Progress of 3-aminopyridine-based amide,urea, imine and azo derivatives in supramolecular gelation

Supramolecular gels derived from low molecular weight gelators are considered to be fascinating soft and smart materials. Gelators of this class form gel networks involving noncovalent interactions and show various applications in many areas. The structural softness and the arrangement of the gelator molecules in the aggregated state have the collaborative effect to intensify the properties of the molecules for their potential applications in material chemistry. Of the various properties, stimulus responsibility is a desired property of supramolecular gel that finds profound application in sensing. In this review, a comprehensive summary of the work on 3-aminopyridine-based amide, urea, imine and azo gelators of different architectures indicating their different uses in supramolecular chemistry has been focused.     

Light-Responsive Arylazopyrazole Gelators: From Organic to Aqueous Media and from Supramolecular to Dynamic Covalent Chemistry

Versatile photoresponsive gels based on tripodal low molecular weight gelators (LMWGs) are reported. A cyclohexane-1,3,5-tricarboxamide (CTA) core provides face-to-face hydrogen bonding and a planar conformation, inducing the self-assembly of supramolecular polymers. The CTA core was substituted with three arylazopyrazole (AAP) arms. AAP is a molecular photoswitch that isomerizes reversibly under alternating UV and green light irradiation. The E isomer of AAP is planar, favoring the self-assembly, whereas the Z isomer has a twisted structure, leading to a disassembly of the supramolecular polymers. By using tailor-made molecular design of the tripodal gelator, light-responsive organogels and hydrogels were obtained. Additionally, in the case of the hydrogels, AAP was coupled to the core through hydrazones, so that the hydrogelator and, hence, the photoresponsive hydrogel could also be assembled and disassembled by using dynamic covalent chemistry.     

Dynamic covalent gels assembled from small molecules: from discrete gelators to dynamic covalent polymers

Dynamic covalent chemistry has emerged recently to be a powerful tool to construct functional materials. This article reviews the progress in the research and development of dynamic covalent chemistry in gels assembled from small molecules. First dynamic covalent reactions used in gels are reviewed to understand the dynamic covalent bonding. Afterwards the catalogues of dynamic covalent gels are reviewed according to the nature of gelators and the interactions between gelators. Dynamic covalent bonding can be involved to form low molecular weight gelators. Low molecular weight molecules with multiple functional groups react to form dynamic covalent cross-linked polymers and act as gelators. Two catalogues of gels show different properties arising from their different structures. This review aims to illustrate the structure-property relationships of these dynamic covalent gels.     

Recent Progress in Azobenzene-Based Supramolecular Materials and Applications

Azobenzene-containing small molecules and polymers are functional photoswitchable molecules to form supramolecular nanomaterials for various applications. Recently, supramolecular nanomaterials have received enormous attention in material science because of their simple bottom-up synthesis approach, understandable mechanisms and structural features, and batch-to-batch reproducibility. Azobenzene is a light-responsive functional moiety in the molecular design of small molecules and polymers and is used to switch the photophysical properties of supramolecular nanomaterials. Herein, we review the latest literature on supramolecular nano- and micro-materials formed from azobenzene-containing small molecules and polymers through the combinatorial effect of weak molecular interactions. Different classes including complex coacervates, host-guest systems, co-assembled, and self-assembled supramolecular materials, where azobenzene is an essential moiety in small molecules, and photophysical properties are discussed. Afterward, azobenzene-containing polymers-based supramolecular photoresponsive materials formed through the host-guest approach, polymerization-induced self-assembly, and post-polymerization assembly techniques are highlighted. In addition to this, the applications of photoswitchable supramolecular materials in pH sensing, and CO₂ capture are presented. In the end, the conclusion and future perspective of azobenzene-based supramolecular materials for molecular assembly design, and applications are given.     

双组分有机低分子凝胶的研究进展

双组分有机低分子凝胶的研究已成为纳米化学新兴研究领域的热点之一, 形成双组分有机低分子凝胶的有机分子具有多种结构, 例如: 巴比妥酸衍生物、氨基酸衍生物、糖类衍生物、金属有机化合物、胆固醇类衍生物和树枝状分子等. 较系统地综述了目前已知的双组分凝胶及其所涉及的有机低分子凝胶因子的主要结构类型, 并展望了双组分有机低分子凝胶的应用前景.     

Trackable Supramolecular Fusion: Cage to Cage Transformation of Tetraphenylethylene-Based Metalloassemblies

Herein, the trackable supramolecular transformation of a two-component molecular cage to a three-component cage through supramolecular fusion with another two-component molecular square is described. The use of tetraphenylethene (TPE), a chromophore with aggregation-induced emission (AIE) character, as a component for the molecular cages enables facile fluorescence monitoring of the transformation process: while both cages exhibit fluorescence emission via the restriction of intramolecular motion of the TPE motif, the interactions between TPE and 4,4′-bipyridine introduced in the supramolecular fusion process result in partial fluorescence quenching and shifts in the emission maximum. This study provides a simple and efficient approach towards complex supramolecular cages with emergent functions and demonstrates that AIE features could provide unique opportunities for the characterization of complex, dynamic supramolecular transformation processes.     

Low molecular mass cationic gelators derived from deoxycholic acid: remarkable gelation of aqueous solvents

During the past decade, the study of molecular self-assembly and network formation from small molecule gelators has become one of the most active areas of supramolecular chemistry. A serendipitous discovery of the gelation of a cationic bile salt (4) led us to investigate the aggregation properties of this new class of cationic hydrogelators. This article summarizes the recent efforts on the study of side chain structure-aggregation property relationship of cationic bile salts. Bile acid analogs with a quaternary ammonium group on the side chain were found to efficiently gel aqueous salt solutions. Some of the cationic bile salts gelled water alone and many of them gelled aqueous salt solutions even in the presence of organic co-solvents (≤20%) such as ethanol, methanol, DMSO, and DMF. These gels showed interconnected fibrous networks. Unlike natural anionic bile salt gels (reported for NaDC and NaLC), the cationic gels reported here are pH independent. Cationic gels derived from DCA showed more solid-like rheological response compared to natural NaDC gels studied earlier by Tato et al.