Self‐Assembly of a Chiral Lipid Gelator Controlled by Solvent and Speed of Gelation

Glutamine derivative 1 with two‐photon absorbing units has been synthesized and was found to show gelation ability in some solvents. Its self‐assembly in the gel phase could be controlled by the solvent and speed of gelation. For example, in DMSO the organogelator self‐assembled into H‐aggregates with weak exciton coupling between the aromatic moieties. On the other hand, in DMSO/diphenyl ether (1:9, v/v) the molecules formed 1D aggregates, but with strong exciton coupling due to the small distance between the chromophores. Moreover, the formation of these two kinds of aggregates could be adjusted by the ratio of DMSO to diphenyl ether. In DMSO/toluene, DMSO/butanol, DMSO/butyl acetate, and DMSO/acetic acid systems similar results were observed. Therefore, conversion of the packing model occurs irrespective of the nature of the solvent. Notably, a unique sign inversion in the CD spectra could be realized by controlling the speed of gelation in the DMSO/diphenyl ether (1:9, v/v) system. It was found that a low speed of gelation induces the gelator to adopt a packing model with strong π–π interactions between the aromatic units. Moreover, the gels, when excited at 800 nm, emit strong green fluorescence and the quantum chemical calculations suggest that intramolecular charge transfer leads to two‐photon absorption of the gelator molecule.     

Multistimuli‐Responsive Supramolecular Organogels Formed by Low‐Molecular‐Weight Peptides Bearing Side‐Chain Azobenzene Moieties

This work demonstrates that the incorporation of azobenzene residues into the side chain of low‐molecular‐weight peptides can modulate their self‐assembly process in organic solvents leading to the formation of stimuli responsive physical organogels. The major driving forces for the gelation process are hydrogen bonding and π–π interactions, which can be triggered either by thermal or ultrasound external stimuli, affording materials having virtually the same properties. In addition, a predictive model for gelation of polar protic solvent was developed by using Kamlet–Taft solvent parameters and experimental data. The obtained viscoelastic materials exhibited interconnected multistimuli responsive behaviors including thermal‐, photo‐, chemo‐ and mechanical responses. All of them displayed thermoreversability with gel‐to‐sol transition temperatures established between 33–80 °C and gelation times from minutes to several hours. Structure–property relationship studies of a designed peptide library have demonstrated that the presence and position of the azobenzene residue can be operated as a versatile regulator to reduce the critical gelation concentration and enhance both the thermal stability and mechanical strength of the gels, as demonstrated by comparative dynamic rheology. The presence of N‐Boc protecting group in the peptides showed also a remarkable effect on the formation and properties of the gels. Despite numerous examples of peptide‐based gelators known in the literature, this is the first time in which low‐molecular‐weight peptides bearing side chain azobenzene units are used for the synthesis of “intelligent” supramolecular organogels. Compared with other approaches, this strategy is advantageous in terms of structural flexibility since it is compatible with a free, unprotected amino terminus and allows placement of the chromophore at any position of the peptide sequence.     

Self‐Assembly and Gelation of Poly(aryl ether) Dendrons Containing Hydrazide Units: Factors Controlling the Formation of Helical Structures

Self‐assembly of AB₂ and AB₃ type low molecular weight poly(aryl ether) dendrons that contain hydrazide units were used to investigate mechanistic aspects of helical structure formation during self‐assembly. The results suggest that there are three important aspects that control helical structure formation in such systems with acyl hydrazide/hydrazone linkage: i) J‐type aggregation, ii) the hydrogen‐bond donor/acceptor ability of the solvent, and iii) the dielectric constant of the solvent. The monomer units self‐assemble to form dimer structures through hydrogen‐bonding and further assembly of the hydrogen‐bonded dimers leads to macroscopic chirality in the present case. Dimer formation was confirmed by NMR spectroscopy and by mass spectrometry. The self‐assembly in the system was driven by hydrogen‐bonding and π–π stacking interactions. The morphology of the aggregates formed was examined by scanning electron microscopy, and the analysis suggests that aprotic solvent systems facilitate helical fibre formation, whereas introduction of protic solvents results in the formation of flat ribbons. This detailed mechanistic study suggests that the self‐assembly follows a nucleation–elongation model to form helical structures, rather than the isodesmic model.     

Ordered Hybrids from Template‐Free Organosilane Self‐Assembly

Despite considerable achievements over the last two decades, nonporous organic–inorganic hybrid materials are mostly amorphous, especially in the absence of solvothermal processes. The organosilane self‐assembly approach is one of the few opportunities for creating a regular assembly of organic and inorganic moieties. Additionally, well‐established organosilicon chemistry enables the introduction of numerous organic functionalities. The synthesis of periodically ordered hybrids relies on mono‐, bis‐, or multisilylated organosilane building blocks self‐assembling into hybrid mesostructures or superstructures, subsequently cross‐linked by siloxane Si‐O‐Si condensation. The general synthesis procedure is template‐free and one‐step. However, three concurrent processes underlie the generation of self‐organized hybrid networks: thermodynamics of amphiphilic aggregation, dynamic self‐assembly, and kinetically controlled sol–gel chemistry. Hence, the set of experimental conditions and the precursor structure are of paramount importance in achieving long‐range order. Since the first developments in the mid‐1990s, the subject has seen considerable progress leading to many innovative advanced nanomaterials providing promising applications in membranes, pollutant remediation, catalysis, conductive coatings, and optoelectronics. This work reviews, comprehensively, the primary evolution of this expanding field of research.     

Function‐Led Design of Aerogels: Self‐Assembly of Alloyed PdNi Hollow Nanospheres for Efficient Electrocatalysis

One plausible approach to endow aerogels with specific properties while preserving their other attributes is to fine‐tune the building blocks. However, the preparation of metallic aerogels with designated properties, for example catalytically beneficial morphologies and transition‐metal doping, still remains a challenge. Here, we report on the first aerogel electrocatalyst composed entirely of alloyed PdNi hollow nanospheres (HNSs) with controllable chemical composition and shell thickness. The combination of transition‐metal doping, hollow building blocks, and the three‐dimensional network structure make the PdNi HNS aerogels promising electrocatalysts for ethanol oxidation. The mass activity of the Pd₈₃Ni₁₇ HNS aerogel is 5.6‐fold higher than that of the commercial Pd/C catalyst. This work expands the exploitation of the electrocatalysis properties of aerogels through the morphology and composition control of its building blocks.     

Creation of Chiral Thixotropic Gels through a Crown–Ammonium Interaction and their Application to a Memory‐Erasing Recycle System

A unique class of oligothiophene‐based organogelator bearing two crown ethers at both ends was synthesized. This compound could gelatinize several organic solvents, forming one‐dimensional fibrous aggregates. From the observation of circular dichroism, it was confirmed that the helical handedness of the fibrous assembly is controllable by the chirality of 1,2bisammonium guests, thus suggesting that one guest molecule bridges two gelator molecules through the crown–ammonium interaction. Interestingly, we have found that such chirality is created by thermal gelation, whereas it disappears by thixotropic gelation. The new finding implies that the present organogel system is applicable as a reversible switching memory device, featuring memory creation by a heat mode and memory erasing by a mechanical mode.     

Dual Stimuli‐Responsive Self‐Assembled Supramolecular Nanoparticles

Supramolecular nanoparticles (SNPs) encompass multiple copies of different building blocks brought together by specific noncovalent interactions. The inherently multivalent nature of these systems allows control of their size as well as their assembly and disassembly, thus promising potential as biomedical delivery vehicles. Here, dual responsive SNPs have been based on the ternary host–guest complexation between cucurbit[8]uril (CB[8]), a methyl viologen (MV) polymer, and mono‐ and multivalent azobenzene (Azo) functionalized molecules. UV switching of the Azo groups led to fast disruption of the ternary complexes, but to a relatively slow disintegration of the SNPs. Alternating UV and Vis photoisomerization of the Azo groups led to fully reversible SNP disassembly and reassembly. SNPs were only formed with the Azo moieties in the trans and the MV units in the oxidized states, respectively, thus constituting a supramolecular AND logic gate.     

An “Ingredients” Approach to Functional Self‐Synthesizing Materials: A Metal‐Ion‐Selective,Multi‐Responsive,Self‐Assembled Hydrogel

New methodology for making novel materials is highly desirable. Here, an “ingredients” approach to functional self‐assembled hydrogels was developed. By designing a building block to contain the right ingredients, a multi‐responsive, self‐assembled hydrogel was obtained through a process of template‐induced self‐synthesis in a dynamic combinatorial library. The system can be switched between gel and solution by light, redox reactions, pH, temperature, mechanical energy and sequestration or addition of Mg^II salt.     

Preservation of the Morphology of a Self‐Encapsulated Thin Titania Film in a Functional Multilayer Stack: An X‐Ray Scattering Study

Looks matter: Generally, the morphology of titania thin films is crucial for their performance, hence much effort is spent to tailor the desired morphology. X‐ray scattering enables the monitoring of the crystalline titania layer morphology during build‐up of the functional multilayer stack (see Figure). Herein evidence is provided that the morphology is preserved throughout the fabrication process.

     

Self‐Assembly of Two‐Component Gels: Stoichiometric Control and Component Selection

Two‐component systems capable of self‐assembling into soft gel‐phase materials are of considerable interest due to their tunability and versatility. This paper investigates two‐component gels based on a combination of a L ‐lysine‐based dendron and a rigid diamine spacer (1,4‐diaminobenzene or 1,4‐diaminocyclohexane). The networked gelator was investigated using thermal measurements, circular dichroism, NMR spectroscopy and small angle neutron scattering (SANS) giving insight into the macroscopic properties, nanostructure and molecular‐scale organisation. Surprisingly, all of these techniques confirmed that irrespective of the molar ratio of the components employed, the “solid‐like” gel network always consisted of a 1:1 mixture of dendron/diamine. Additionally, the gel network was able to tolerate a significant excess of diamine in the “liquid‐like” phase before being disrupted. In the light of this observation, we investigated the ability of the gel network structure to evolve from mixtures of different aromatic diamines present in excess. We found that these two‐component gels assembled in a component‐selective manner, with the dendron preferentially recognising 1,4‐diaminobenzene (>70 %), when similar competitor diamines (1,2‐ and 1,3‐diaminobenzene) are present. Furthermore, NMR relaxation measurements demonstrated that the gel based on 1,4‐diaminobenzene was better able to form a selective ternary complex with pyrene than the gel based on 1,4‐diaminocyclohexane, indicative of controlled and selective π–π interactions within a three‐component assembly. As such, the results in this paper demonstrate how component selection processes in two‐component gel systems can control hierarchical self‐assembly.     

Stimuli‐Triggered Sol–Gel Transitions of Polypeptides Derived from α‐Amino Acid N‐Carboxyanhydride (NCA) Polymerizations

The past decade has witnessed significantly increased interest in the development of smart polypeptide‐based organo‐ and hydrogel systems with stimuli responsiveness, especially those that exhibit sol–gel phase‐transition properties, with an anticipation of their utility in the construction of adaptive materials, sensor designs, and controlled release systems, among other applications. Such developments have been facilitated by dramatic progress in controlled polymerizations of α‐amino acid N‐carboxyanhydrides (NCAs), together with advanced orthogonal functionalization techniques, which have enabled economical and practical syntheses of well‐defined polypeptides and peptide hybrid polymeric materials. One‐dimensional stacking of polypeptides or peptide aggregations in the forms of certain ordered conformations, such as α helices and β sheets, in combination with further physical or chemical cross‐linking, result in the construction of three‐dimensional matrices of polypeptide gel systems. The macroscopic sol–gel transitions, resulting from the construction or deconstruction of gel networks and the conformational changes between secondary structures, can be triggered by external stimuli, including environmental factors, electromagnetic fields, and (bio)chemical species. Herein, the most recent advances in polypeptide gel systems are described, covering synthetic strategies, gelation mechanisms, and stimuli‐triggered sol–gel transitions, with the aim of demonstrating the relationships between chemical compositions, supramolecular structures, and responsive properties of polypeptide‐based organo‐ and hydrogels.     

Self‐Assembled Fluorescent Organic Nanoparticles for Live‐Cell Imaging

Fluorescent, cell‐permeable, organic nanoparticles based on self‐assembled π‐conjugated oligomers with high absorption cross‐sections and high quantum yields have been developed. The nanoparticles are generated with a tuneable density of amino groups for charge‐mediated cellular uptake by a straightforward self‐assembly protocol, which allows for control over size and toxicity. The results show that a single amino group per ten oligomers is sufficient to achieve cellular uptake. The non‐toxic nanoparticles are suitable for both one‐ and two‐photon cellular imaging and flow cytometry, and undergo very efficient cellular uptake.     

Complexation and Catenation in Aqueous Media Using a Self‐Assembled PdII Metallacyclic Receptor

A M₂L₂ rectangular‐shaped metallacycle, obtained by metal‐directed self‐assembly of a 2‐(pyridin‐4‐ylmethyl)‐2,7‐diazapyrenium salt and [(en)Pd (NO₃)₂] (en=ethylenediamine), has been investigated as a molecular receptor for a wide range of aromatic substrates in water. Complexation and catenation of the receptor with selected mono‐ and polycyclic aromatic substrates produced 1:1 inclusion complexes and [2]catenanes in a highly efficient fashion, as determined by NMR and UV/Vis spectroscopic techniques, as well as single‐crystal X‐ray crystallography. Furthermore, the thermodynamic and kinetic features of the complexation processes have been analyzed for selected model guests.     

Unusual Photoreaction of Triquinacene within Self‐Assembled Hosts

Triquinacene is a concave tricyclic hydrocarbon with diverse photoreactivity. In the cavity of an electron‐accepting molecular host, triquinacene was specifically photooxidized at the peripheral allylic position into an alcohol, 1‐hydroxytriquinacene, via guest‐to‐host electron transfer. The unusual reactivity stems from the extremely electron‐deficient triazine panel ligand of the host cage, which allows the cage to function as a good electron acceptor. Thus, self‐assembled coordination cages can serve not only as molecular‐sized reaction vessels but also function electronically as redox media. Dissolved molecular oxygen is indispensable for the photoreaction and immediately traps a photogenerated radical.