Controlling Spatiotemporal Mechanics of Supramolecular Hydrogel Networks with Highly Branched Cucurbit[8]uril Polyrotaxanes

Attempts to rationally tune the macroscopic mechanical performance of supramolecular hydrogel networks through noncovalent molecular interactions have led to a wide variety of supramolecular materials with desirable functions. While the viscoelastic properties are dominated by temporal hierarchy (crosslinking kinetics), direct mechanistic studies on spatiotemporal control of supramolecular hydrogel networks, based on host–guest chemistry, have not yet been established. Here, supramolecular hydrogel networks assembled from highly branched cucurbit[8]uril‐threaded polyrotaxanes (HBP‐CB[8]) and naphthyl‐functionalized hydroxyethyl cellulose (HECNp) are reported, exploiting the CB[8] host–guest complexation. Mechanically locking CB[8] host molecules onto a highly branched hydrophilic polymer backbone, through selective binary complexation with viologen derivatives, dramatically increases the solubility of CB[8]. Additionally, the branched architecture enables tuning of material dynamics of the supramolecular hydrogel networks via both topological (spatial hierarchy) and kinetic (temporal hierarchy) control. Relationship between macroscopic properties (time‐ and temperature‐dependent rheological properties, thermal stability, and reversibility), spatiotemporal hierarchy, and chain dynamics of the highly branched polyrotaxane hydrogel networks is investigated in detail. Such kind of tuning of material mechanics through spatiotemporal hierarchy improves our understanding of the challenging relationship between design of supramolecular polymeric materials and their complex viscoelasticity, and also highlights a facile strategy to engineer dynamic supramolecular materials.     

A Smart Supramolecular Hydrogel Exhibiting pH‐Modulated Viscoelastic Properties

The macroscopic viscoelastic properties of a physical hydrogel are reversibly modulated by tuning the microscopic hydrogen‐bonding interactions with pH. The hydrogel forms at a rather low concentration of the multi‐pyridyl‐based gelator, N, N′, N″‐tris(3‐pyridyl)trimesic amide. The yield stress of the hydrogel is greatly enhanced from 10 to 769 Pa by changing the pH from 7.0 to 5.0. At pH 7.0, the amide molecules are assembled into an ordered structure as a result of the hydrogen bonds between the amide N–H bond and the nitrogen on the pyridyl group (N–H…Py). Fourier transform (FT) IR spectroscopy indicates that hydrogen bonds of N–H…Py are partially broken because the pyridyl groups are partly protonated at pH 5.0. This condition leads to a highly branched and homogeneous fibrillar network, which is confirmed by X‐ray diffraction (XRD) measurements and field‐emission scanning electron microscopy (FESEM) images. Highly branched fibrillar networks create more compartments and greatly increase the interfacial tension that is required to hold the solvent in the gel, thereby increasing the yield stress to 769 Pa. By further increasing the acidity of the hydrogel to pH < 3.0, the gel becomes a sol. Both the change in the viscoelastic properties and the sol–gel transition are reversible and controllable in the material.     

Photoswitchable Nanomaterials Based on Hierarchically Organized Siloxane Oligomers

Materials with highly ordered molecular arrangements have the capacity to display unique properties derived from their nanoscale structure. Here, the synthesis and characterization of azobenzene (AZO)‐functionalized siloxane oligomers of discrete length that form photoswitchable supramolecular materials are described. Specifically, synergy between phase segregation and azobenzene crystallization leads to the self‐assembly of an exfoliated 2D crystal that becomes isotropic upon photoisomerization with UV light. Consequently, the material undergoes a rapid athermal solid‐to‐liquid transition which can be reversed using blue light due to the unexpectedly fast 2D crystallization that is facilitated by phase segregation. In contrast, enabling telechelic supramolecular polymerization through hydrogen bonding inhibits azobenzene crystallization, and nanostructured pastes with well‐ordered morphologies are obtained based on phase segregation alone, thus demonstrating block copolymer‐like behavior. Therefore, by tailoring the balance of self‐assembly forces in the azobenzene‐functionalized siloxane oligomers, fast and reversible phase‐changing materials can be engineered with various mechanical properties for applications in photolithography or switchable adhesion to lubricant properties.     

Reversible Bioadhesives Using Tannic Acid Primed Thermally‐Responsive Polymers

A two‐layer approach is reported for the formation of a thermally triggered reversible adhesive, involving a thermally‐responsive polymer matrix coated on tannic acid‐pretreated substrates/tissues. Interfacial adhesion originates from strong molecular interactions of tannic acid with both the polymer matrix and the substrate/tissue. The reversibility is due to a temperature‐triggered phase transition of the polymer matrix, leading to cohesive failure. Depending on different gelation mechanisms, the polymer forms a highly cohesive gel or soft solid upon either warming or cooling, leading to a strong adhesion to the tissues at physiological temperatures. Detachment of the adhesive is triggered by a temperature‐induced compromise of cohesive strength of the polymer matrix, by the opposite gel‐to‐sol transition. This facile, low‐cost, and modular design offers a reversible adhesive platform which is useful for biomedical and industrial applications.     

Multiple H-Bonding Chain Extender-Based Ultrastiff Thermoplastic Polyurethanes with Autonomous Self-Healability,Solvent-Free Adhesiveness,and AIE Fluorescence

Developing an autonomous room temperature self-healing supramolecular polyurethane (PU) with toughness and stiffness remains a great challenge. Herein, a novel concept that utilizes a T-shaped chain extender with double amide hydrogen bonds in a side chain to extend PU prepolymers to construct highly stiff and tough supramolecular PU with integrated functions is reported. Mobile side-chain H-bonds afford a large flexibility to modulate the stiffness of the PUs ranging from highly stiff and tough elastomer (105.87 MPa Young's modulus, 27 kJ m⁻² tearing energy), to solvent-free hot-melt adhesive, and coating. The dynamic side-chain multiple H-bonds afford an autonomous self-healability at room temperature (25  ° C). Due to the rapid reconstruction of hydrogen bonds, this PU adhesive demonstrates a high adhesion strength, fast curing, reusability, long-term adhesion, and excellent low-temperature resistance. Intriguingly, the PU emits intrinsic blue fluorescence presumably owing to the aggregation-induced emission of tertiary amine domains induced by side-chain H-bonds. The PU is explored as a counterfeit ink coated on the predesigned pattern, which is visible-light invisible and UV-light visible. This work represents a universal and facile approach to fabricate supertough supramolecular PU with tailorable functions by chain extension of PU prepolymers with multiple H-bonding chain extenders.     

Room Temperature Fluorescent Conjugated Polymer Gums

High molecular weight poly(diphenylacetylene) [PDPA] derivatives are introduced as fluorescent, soft conjugated polymers that exist in the gum state at room temperature. The gum‐like behavior of the polymers is easily modified according to the side alkyl chain length and substitution position. Long alkyl chain‐coupled PDPA derivatives provide soft and sticky gums at room temperature. Manual kneading of gum polymers produce soft films with very smooth surfaces. The gum polymers show an endothermic transition due to the melting of long alkyl chains. The X‐ray diffraction of gum polymers reveals a new signal due to the molten aliphatic chains. The gum polymers show significant viscoelastic relaxation at the melting temperature of the alkyl side chains. The dynamic thermo‐mechanical analysis (DTMA) of gum polymers at room temperature suggest that the meta‐substituted polymer is softer and stickier than para‐polymer. Rheological analysis suggests that the meta‐polymer has less entanglement than para‐polymer. The fluorescence emission of gum polymer is quite intense in the film and solution. The gum polymer film is readily stretched to produce a uniaxually oriented film. Stretching and subsequent relaxation of elastomer‐supported gum polymer film generate buckles perpendicular to the axis of strain. The gum polymer film accommodates the large strain without cracking and delamination.     

Tuning Molecular Adhesion via Material Anisotropy

Cell adhesion with extracellular matrix depends on the collective behaviors of a large number of receptor‐ligand bonds at the compliant cell‐matrix interface. While most biological tissues and structures, including cells and extracellular matrices, exhibit strongly anisotropic material properties, existing studies on molecular adhesion via receptor‐ligand bonds have been largely limited to isotropic materials. Here the effects of transverse isotropy, a common form of material anisotropy in biological systems, in modulating the adhesion behavior of a cluster of receptor‐ligand bonds are investigated. The results provide a theoretical basis to understand cell adhesion on anisotropic extracellular matrices and to explore the possibility of controlling cell adhesion via anisotropy design in material properties. The combined analysis and simulations show that the orientation of material anisotropy strongly affects the apparent softness felt by the adhesive bonds, thereby altering their ensemble lifetime by several orders of magnitude. An implication of this study is that distinct cellular behaviors can be achieved through remodeling of material anisotropy in either extracellular matrix or cytoskeleton. Comparison between different loading conditions, together with the effects of material anisotropy, yields a rich array of out‐of‐equilibrium behaviors in the molecular interaction between reactant‐bearing soft surfaces, with important implications on the mechanosensitivity of cells.     

Non‐Equilibrium,Light‐Adaptive,Steady‐State Reconfiguration of Mechanical Patterns in Bioinspired Nanocomposites

Bioinspired nanocomposites have made great progress for the fabrication of mechanical high‐performance structural materials, but their properties have thus far been engineered with a focus on static behavior. This contrasts profoundly with the dynamic reconfiguration often observed in living tissues. Here, a first concept is introduced for steady‐state, light‐adaptive reconfiguration of mechanical patterns in bioinspired nanocomposites under dissipative out‐of‐equilibrium conditions. This is realized for green, waterborne cellulose nanofibril/polymer nanopapers by achieving a heterogeneous activation of a photothermal effect. To this end, predefined mechanical patterns are designed by top‐down lamination of bottom‐up engineered bioinspired nanocomposites containing thermoreversible hydrogen bonds, as well as spatially selectively incorporated single‐walled carbon nanotubes for photothermal response. Global irradiation leads to localized photothermal softening by cascading light to heat, and to the dynamization and breakage of the thermoreversible supramolecular bonds, leading to macroscopic reconfiguration and even inversion of mechanical stiff/soft patterns. The altered configuration is only stable in a dissipative steady state and relaxes to the ground state once light is removed. The strategy presents a new approach harnessing the capabilities from top‐down and bottom‐up structuring, and by interfacing it with non‐equilibrium adaptivity concepts, it opens avenues for hierarchical bioinspired materials with anisotropic response in global fields.     

Solvent‐Mediated Plasmon Tuning in a Gold‐Nanoparticle–Poly(Ionic Liquid) Composite

The design, synthesis, and characterization of a hierarchically ordered composite whose structure and optical properties can be reversibly switched by adjustment of solvent conditions are described. Solvent‐induced swelling and de‐swelling is shown to provide control over the internal packing arrangement and hence, optical properties of in situ synthesized metal nanoparticles. Specifically, a gold‐nanoparticle‐containing ionic‐liquid‐derived polymer is synthesized in a single step by UV irradiation of a metal‐ion‐precursor‐doped, self‐assembled ionic liquid gel, 1‐decyl‐3‐vinylimidazolium chloride. Small‐angle X‐ray scattering (SAXS) studies indicate that in the de‐swollen state, the freestanding polymer adopts a perforated lamellar structure. Optical spectroscopy of the dried composite reveals plasmon resonances positioned in the near‐IR. Strong particle–particle interactions arise from matrix‐promoted formation of aggregated 1D clusters or chains of gold nanoparticles. Upon swelling in alcohol, the composite undergoes a structural conversion to a disordered structure, which is accompanied by a color change from purple to pale pink and a shift in the surface plasmon resonance to 527 nm, consistent with isolated, non‐interacting particles. These results demonstrate the far‐field tuning of the plasmonic spectrum of gold nanoparticles by solvent‐mediated changes in its encapsulating matrix, offering a straightforward, low‐cost strategy for the fabrication of nanophotonic materials.     

A Supramolecule‐Triggered Mechanochromic Switch of Cyclodextrin‐Jacketed Rhodamine and Spiropyran Derivatives

An innovative approach for covalent‐bond‐activated mechanoresponse by complexing rhodamine or spiropyran with cyclodextrin (CD) is reported. This approach endows diverse fluorophores with perfect mechanochromism by introducing a supramolecular system. Unique characteristics such as noncovalent chemical modification and convenient preparation make this approach promising for practical applications. The strong hydrogen bonds provided by CD play a crucial role in triggering the mechanochromic switch. First, the hydrogen bonds seize both sides of the fluorophore's weak chemical bonds and tightly lock the fluorophore in the cavity of CD. Second, the hydrogen bonds prompt the aggregation of complex inclusions in large ordered arrays and strengthen the molecular interactions. In this way, the weak chemical bonds can focus more external force and stretch more easily upon shearing (quantified). This is the first report of supramolecule‐triggered mechanochromic switches. This study opens an avenue to correlate a mechanochemical reaction with a supramolecular system.     

Room‐Temperature Self‐Organizing Characteristics of Soluble Acene Field‐Effect Transistors

We report on the room‐temperature self‐organizing characteristics of thin films of the organic small‐molecule semiconductor triethylsilylethynyl‐anthradithiophene (TES‐ADT) and its effect on the electrical properties of TES‐ADT‐based field‐effect transistors (FETs). The morphology of TES‐ADT films changed dramatically with time, and the field‐effect mobility of FETs based on these films increased about 100‐fold after seven days as a result of the change in molecular orientation from a tilted structure in the as‐prepared film to a well‐oriented structure in the final film. We found that the molecular movement is large enough to induce a conformational change to an energetically stable state in spin‐coated TES‐ADT films, because TES‐ADT has a low glass‐transition temperature (around room temperature). Our findings demonstrate that organic small‐molecule semiconductors that exhibit a low crystallinity immediately after spin‐coating can be changed into highly crystalline structures by spontaneous self‐organization of the molecules at room temperature, which results in improved electrical properties of FETs based on these semiconductors.     

Room Temperature Multifunctional Organophosphorus Gels and Liquid Crystals

A new series of extended dithieno[3,2‐b:2',3'‐d]phospholes with dendritic and non‐dendritic architectures involving phenylenevinylene as well as Fréchet‐type dendrons is presented. Modification of the phosphorus center with Pd allows for the generation of a dimeric dendrimer with Pd‐center. The synthetic strategy employed balances the rigid main scaffold with the flexibility of the dendrons in order to keep control of supramolecular self‐organization features. All the structures show a high photoluminescence in both solution and solid state, which is further intensified via energy transfer from the dendrons to the core. In terms of self‐organization in solution, three of the derivatives which bear an extended phosphole unit as common moiety are able to gel a variety of organic solvents at room temperature independently from the nature of their substituents. Notwithstanding, the dimeric dendrimer with Fréchet‐type dendrons is only able to display gel properties at low temperature. All gels exhibit pronounced photoluminescence properties that can be tuned by variation of the solvent and the temperature. In absence of solvent, the phosphole derivatives exhibit, moreover, liquid‐crystalline mesomorphism features. While three of the compounds present stable and highly luminescent columnar hexagonal phases at room temperature, the fourth species was found to be crystalline in the thermal range up to its isotropic state. Finally as proof of concept, the multifunctionality of these materials is demonstrated in an electrochromic device.     

Producing Supramolecular Functional Materials Based on Fiber Network Reconstruction

Here, the creation of new supramolecular functional materials based on the reconstruction of three‐dimensional interconnecting self‐organized nanofiber networks by a surfactant is reported. The system under investigation is N‐lauroyl‐L ‐glutamic acid di‐n‐butylamide in propylene glycol. The architecture of networks is implemented in terms of surfactants, e.g. sorbitan monolaurate. The elastic performance of the soft functional material is either weakened or strengthened (up to 300% for the current system) by reconstructing the topology of a fiber network. A topology transition of gel fiber network from spherulite‐like to comb‐like to spherulite‐like is performed with the introduction of this surfactant. The Span 20 molecules are selectively adsorbed on the side surfaces of the crystalline fibers and promote the nucleation of side branches, giving rise to the transformation of the network architecture from spherulite‐like topology to comb‐like topology. At high surfactant concentrations, the occurrence of micelles may provide an increasing number of nucleation centers for spherulitic growth, leading to the reformation of spherulite‐like topology. An analysis on fiber network topology supports and verifies a perfect agreement between the topological behavior and the rheological behavior of the functional materials. The approach identified in this study opens up a completely new avenue in designing and producing self‐supporting supramolecular functional materials with designated macroscopic properties.     

Impact of adhesive rheology on stress-distortion of bonded plastic substrates for flexible electronics applications

For fabrication of flexible electronics using standard microelectronics toolsets, a temporary bond-debond method has been developed that requires minimization of the distortion of bonded flexible substrate and bow of bonded system (flexible substrate-adhesive-carrier) during processing. To elucidate the critical parameters of the adhesive used in the bonding that control the stress (bow) and distortion, adhesives with different viscoelastic properties are examined systematically. By blending a high modulus adhesive into a low modulus adhesive, the storage modulus, loss modulus and loss factor of the adhesive can be tuned by orders of magnitude. Detailed examination of the impact of these three rheological parameters on the stress and distortion of bonded system reveals that the relative viscoelastic flow properties of the bonding adhesive to that of the bonded flexible substrate are directly correlated to bow and distortion. When the loss factor of the adhesive is less than that for the plastic substrate, precise registration of layers during photolithography is observed. These results provide insight into the rheological parameters critical to the adhesive formulations for the temporary bond-debond method in the fabrication of flexible electronics.     

Versatile Thermochromic Supramolecular Materials Based on Competing Charge Transfer Interactions

Stimuli‐responsive supramolecular materials are of paramount importance for a broad range of applications. It is essential to impart versatility, sustainability, and scalability into these materials. Herein the authors report the design and synthesis of a new class of thermochromic supramolecular materials, which can easily be processed from water via a reversible sol–gel transition. The supramolecular materials are composed of a bis‐bipyridinium acceptor, a π‐electron‐rich naphthalene derivative donor, and halogen counterions. Long helical nanofibers can be assembled in water, gelating at room temperature. Inked designs, thin films, and aerogels are solution‐processed to exhibit thermochromic behavior based on competing π → π* and n → π* charge transfer interactions. By using different π‐electron rich donors, and counterions, the authors demonstrate that both the color observed at room temperature and at high temperatures can be tailored. The results open up the door to develop novel amphiphile‐based thermochromes with water processability and a large tunable color palette.     

Supramolecular Nanostructuring of Silver Surfaces via Self‐Assembly of [60]Fullerene and Porphyrin Modules

Recent achievements in our laboratory toward the “bottom‐up” fabrication of addressable multicomponent molecular entities obtained by self‐assembly of C₆₀ and porphyrins on Ag(100) and Ag(111) surfaces are described. Scanning tunneling microscopy (STM) studies on ad‐layers constituting monomeric and triply linked porphyrin modules showed that the molecules self‐organize into ordered supramolecular assemblies, the ordering of which is controlled by the porphyrin chemical structure, the metal substrate, and the surface coverage. Specifically, the successful preparation of unprecedented two‐dimensional porphyrin‐based assemblies featuring regular pores on Ag(111) surfaces has been achieved. Subsequent co‐deposition of C₆₀ molecules on top of the porphyrin monolayers results in selective self‐organization into ordered molecular hybrid bilayers, the organization of which is driven by both fullerene coverage and porphyrin structure. In all‐ordered fullerene–porphyrin assemblies, the C₆₀ guests organize, unusually, into long chains and/or two‐dimensional arrays. Furthermore, sublimation of C₆₀ on top of the porous porphyrin network reveals the selective long‐range inclusion of the fullerene guests within the hosting cavities. The observed mode of the C₆₀ self‐assembly originates from a delicate equilibrium between substrate–molecule and molecule–molecule interactions involving charge‐transfer processes and conformational reorganizations as a consequence of the structural adaptation of the fullerene–porphyrin bilayer.     

Tunable Gas Sensing Gels by Cooperative Assembly

The cooperative assembly of biopolymers and small molecules can yield functional materials with precisely tunable properties. Here, the fabrication, characterization, and use of multicomponent hybrid gels as selective gas sensors are reported. The gels are composed of liquid crystal droplets self‐assembled in the presence of ionic liquids, which further coassemble with biopolymers to form stable matrices. Each individual component can be varied and acts cooperatively to tune gels' structure and function. The unique molecular environment in hybrid gels is explored for supramolecular recognition of volatile compounds. Gels with distinct compositions are used as optical and electrical gas sensors, yielding a combinatorial response conceptually mimicking olfactory biological systems, and tested to distinguish volatile organic compounds and to quantify ethanol in automotive fuel. The gel response is rapid, reversible, and reproducible. These robust, versatile, modular, pliant electro‐optical soft materials possess new possibilities in sensing triggered by chemical and physical stimuli.     

Polyamidine supramolecular structures—a new class of photosensitive polymeric semiconductors

Photophysical properties of a new class of polymers, polyamidines, are investigated in relation to their molecular structure. It is shown that the photosensitivity in the systems investigated is a consequence of the formation of supramolecular structures in them. These structures emerge due to the formation of an ordered system of hydrogen bonds. It is found that the materials mentioned are characterized by reasonably high quantum yields of photogeneration of charge carriers and photosensitivity at a level of 10⁴ cm²/J.     

Self‐Assembled Conjugated Thiophene‐Based Rotaxane Architectures: Structural,Computational, and Spectroscopic Insights into Molecular Aggregation

A comparative study of the self‐assembly at a variety of surfaces of a dithiophene rotaxane 1 ?β‐CD and its corresponding dumbbell, 1, by means of atomic force microscopy (AFM) imaging and scanning tunneling microscopy (STM) imaging on the micrometer and nanometer scale, respectively. The dumbbell is found to have a greater propensity to form ordered supramolecular assemblies, as a result of π–π interactions between dithiophenes belonging to adjacent molecules, which are hindered in the rotaxane. The fine molecular structure determined by STM was compared to that obtained by molecular modelling. The optical properties of both rotaxane and dumbbell in the solid state were investigated by steady‐state and time‐resolved photoluminescence (PL) experiments on spin‐cast films and diluted solutions. The comparison between the optical features of the threaded and unthreaded systems reveals an effective role of encapsulation in reducing aggregation and exciton migration for the rotaxanes with respect to the dumbbells, thus leading to higher PL quantum efficiency and preserved single‐molecule photophysics for longer times after excitation in the threaded oligomers.     

Thermoreversible Siloxane Networks: Soft Biomaterials with Widely Tunable Viscoelasticity

Polysiloxane elastomers represent a widely utilized soft material with excellent rubber‐like elasticity, biocompatibility, and biodurability; however, there is a lack of an effective and straightforward approach to manipulate the material's viscoelastic response. A facile hydrosilylation reaction is employed to integrate ureidopyrimidinone hydrogen‐bonding side‐groups into linear and crosslinked siloxane polymers to achieve biocompatible soft materials with a highly tunable viscoelastic relaxation timescale. Stacking of H‐bonded moieties is avoided in the designed macromolecular architectures with tight, side‐groups substituents. The obtained siloxane network features the presence of both covalent crosslinks and truly thermoreversible crosslinks, and can be formulated across a broad material design space including elastic solids, recoverable viscoelastic solids, and viscous liquids. The elastomers exhibit unique temperature‐dependent shape‐memory capability and show good cytocompatibility. Importantly, a deformed material's shape‐recovery occurs regardless of external triggering, and through manipulation of network formulations, the shape‐recovery timescale can be easily tuned from seconds to days, opening new possibilities for biomedical, healthcare, and soft material applications.