Supramolecular Polymerization Promoted and Controlled through Self‐Sorting

A new method in which supramolecular polymerization is promoted and controlled through self‐sorting is reported. The bifunctional monomer containing p‐phenylene and naphthalene moieties was prepared. Supramolecular polymerization is promoted by selective recognition between the p‐phenylene group and cucurbit[7]uril (CB[7]), and 2:1 complexation of the naphthalene groups with cucurbit[8]uril (CB[8]). The process can be controlled by tuning the CB[7] content. This development will enrich the field of supramolecular polymers with important advances towards the realization of molecular‐weight and structural control.     

Hydrogen Bonding Directed Supramolecular Polymerisation of Oligo(Phenylene‐Ethynylene)s: Cooperative Mechanism,Core Symmetry Effect and Chiral Amplification

The design of supramolecular motifs with tuneable stability and adjustable supramolecular polymerisation mechanisms is of crucial importance to precisely control the properties of supramolecular assemblies. This report focuses on constructing π‐conjugated oligo(phenylene ethynylene) (OPE)‐based one‐dimensional helical supramolecular polymers that show a cooperative growth mechanism. Thus, a novel set of discotic molecules comprising a rigid OPE core, three amide groups, and peripheral solubilising wedge groups featuring C₃ and C₂ core symmetry was designed and synthesised. All of the discotic molecules are crystalline compounds and lack a columnar mesophase in the solid state. In dilute methylcyclohexane solution, one‐dimensional supramolecular polymers are formed stabilised by threefold intermolecular hydrogen bonding and π–π interactions, as evidenced by ¹H NMR measurements. Small‐angle X‐ray and light scattering measurements reveal significant size differences between the columnar aggregates of C₃‐ and C₂‐symmetrical discotics, that is, the core symmetry strongly influences the nature of the supramolecular polymerisation process. Temperature‐dependent CD measurements show a highly cooperative polymerisation process for the C₃‐symmetrical discotics. In contrast, the self‐assembly of C₂‐symmetrical discotics shows a smaller enthalpy release upon aggregation and decreased cooperativity. In all cases, the peripheral stereogenic centres induce a preferred handedness in the columnar helical aggregates. Moreover, one stereogenic centre suffices to fully bias the helicity in the C₂‐symmetrical discotics. Finally, chiral amplification studies with the C₃‐symmetrical discotics were performed by mixing chiral and achiral discotics (sergeants‐and‐soldiers experiment) and discotics of opposite chirality (majority‐rules experiment). The results demonstrate a very strong sergeants‐and‐soldiers effect and a rather weak majority‐rules effect.     

Two‐Photon‐Responsive Supramolecular Hydrogel for Controlling Materials Motion in Micrometer Space

Spatiotemporal control of fluidity inside a soft matrix by external stimuli allows real‐time manipulation of nano/micromaterials. In this study, we report a two‐photon‐responsive peptide‐based supramolecular hydrogel, the fluidity of which was dramatically controlled with high spatial resolution (10 μm×10 μm×10 μm). The off–on switching of the Brownian motion of nanobeads and chemotaxis of bacteria by two‐photon excitation was successfully demonstrated.     

Solvent‐Polarity‐Tuned Morphology and Inversion of Supramolecular Chirality in a Self‐Assembled Pyridylpyrazole‐Linked Glutamide Derivative: Nanofibers,Nanotwists, Nanotubes,and Microtubes

The self‐assembly of a low‐molecular‐weight organogelator into various hierarchical structures has been achieved for a pyridylpyrazole linked L ‐glutamide amphiphile in different solvents. Upon gel formation, supramolecular chirality was observed, which exhibited an obvious dependence on the polarity of the solvent. Positive supramolecular chirality was obtained in nonpolar solvents, whereas it was inverted into negative supramolecular chirality in polar solvents. Moreover, the gelator molecules self‐assembled into a diverse array of nanostructures over a wide scale range, from nanofibers to nanotubes and microtubes, depending on the solvent polarity. Such morphological changes could even occur for the xerogels in the solvent vapors. We found that the interactions between the pyridylpyrazole headgroups and the solvents could subtly change the stacking of the molecules and, hence, their self‐assembled nanostructures. This work exemplifies that organic solvents can significantly involve the gelation, as well as tune the structure and properties, of a gel.     

Nanopatterned Superlattices in Self‐Assembled C2‐Symmetric Oligodimethylsiloxane‐Based Benzene‐1,3,5‐Tricarboxamides

The synthesis of C₃‐ and C₂‐symmetric benzene‐1,3,5‐tricarboxamides (BTAs) containing well‐defined oligodimethylsiloxane (oDMS) and/or alkyl side chains has been carried out. The influence of the bulkiness of the oDMS chains in the aggregation behavior of dilute solutions of the oDMS‐BTAs in methylcyclohexane was studied by temperature‐dependent UV spectroscopy. The formation of hierarchically self‐assembled aggregates was observed at different BTA concentrations, the tendency of aggregation increases by shortening or removing oDMS chains. Chiral BTAs were investigated with circular dichroism (CD) spectroscopy, showing a stronger tendency to aggregate than the achiral ones. Majority rules experiments show a linear behavior consistent with the existence of a high mismatch penalty energy. The most efficient oDMS‐BTAs organogelators have the ability to form stable organogels at 5 mg mL^?1 (0.75 wt %) in hexane. Solid‐state characterization techniques indicate the formation of an intermolecular threefold hydrogen bonding between adjacent molecules forming thermotropic liquid crystals, exhibiting a hexagonal columnar organization from room temperature to above 150 °C. A decrease of the clearing temperatures was observed when increasing the number and length of the oligodimethylsiloxane chains. In addition to the three‐fold hydrogen bonding that leads to columnar liquid crystalline phase, segregation between the oDMS and aliphatic chains takes place in the BTA functionalized with two alkyl and one oDMS chain leading to a superlattice within the hexagonal structure with potential applications in lithography.     

Influence of the Solvent and the Enantiomeric Purity on the Transition between Different Supramolecular Polymers

The self‐assembly of two enantiomerically pure hexa(oligo (p‐phenylene vinylene))‐substituted benzenes having 24 stereocenters was studied in pure methylcyclohexane (MCH) and in a mixture of MCH/toluene (4:1). Irrespective of the solvent a cooperative supramolecular polymerization mechanism was determined for these star‐shaped molecules by using temperature‐dependent CD and UV/Vis spectroscopy. Quite remarkably, a transition from one helical supramolecular state (A) to a second more thermodynamically stable supramolecular helical assembly (B) was observed. The rate of the A→B transition was strongly dependent on the nature of the solvent; being faster in the solvent mixture than in pure MCH. By using size exclusion chromatography we could relate the increased rate to a decreased stability of the supramolecular A state in the solvent mixture. Next, we mixed the two enantiomerically pure hexa‐substituted benzene derivatives in a so‐called majority‐rules experiment, which lead to the anitcipated chiral amplification in the A state. More importantly it appeared that the A→B transition was significantly hampered in these mixed systems. Furthermore, the absence of chiral amplification in the B state revealed the formation of separated enantiomerically pure assemblies. Therefore, by using a wide variety of spectroscopic and chromatographic techniques we determined the influence of solvent and enantiomeric purity on the transition between different supramolecular states.     

Peptide Conjugates of a Nonsteroidal Anti‐Inflammatory Drug as Supramolecular Gelators: Synthesis,Characterization, and Biological Studies

Indomethacin ( IND ), which is a well‐known nonsteroidal anti‐inflammatory drug (NSAID), was conjugated with various naturally occurring amino acids. Most of these bioconjugates were capable of gelling pure water, a solution of NaCl (0.9 wt %), and phosphate‐buffered saline (pH 7.4), as well as a few organic solvents. The gels were characterized by table‐top and dynamic rheology, and electron microscopy. Variable‐temperature ¹H NMR spectroscopy studies on a selected gel were performed to gain insights into the self‐assembly process during gel formation. Both 1D and 2D hydrogen‐bonded networks were observed in the single‐crystal structures of two of the gelators. Plausible biological applications of the hydrogelators were evaluated with the ultimate aim of drug delivery in a self‐delivery fashion. All hydrogelators were stable in phosphate‐buffered saline at pH 7.4 at 37 °C, and biocompatible in mouse macrophage RAW 264.7 cell line (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay). Two of the most biocompatible hydrogelators displayed an anti‐inflammatory response comparable to that of the parent drug IND in prostaglandin E₂ assay. Release of the bioconjugates into the bulk solvent interfaced with the corresponding hydrogels indicated their plausible future application in drug delivery.     

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.     

Self‐Assembly of Five 8‐Hydroxyquinolinate‐Based Complexes: Tunable Core,Supramolecular Structure,and Photoluminescence Properties

Five new Zn^II complexes, namely [Zn₃(L)₆] ( 1 ), [Zn₂(Cl)₂(L)₂(py)₂] ( 2 ), [Zn₂(Br)₂(L)₂(py)₂] ( 3 ), [Zn(L)₂(py)] ( 4 ), and [Zn₂(OAc)₂(L)₂(py)₂] ( 5 ), were prepared by the solvothermal reaction of ZnX₂ (X^?=Cl^?, Br^?, F^?, and OAc^?) salts with a 8‐hydroxyquinolinate ligand (HL) that contained a trifluorophenyl group. All of the complexes were characterized by elemental analysis, IR spectroscopy, and powder and single‐crystal X‐ray crystallography. The building blocks exhibited unprecedented structural diversification and their self‐assembly afforded one mononuclear, three binuclear, and one trinuclear Zn^II structures in response to different anions and solvent systems. Complexes 1 – 5 featured four types of supramolecular network controlled by non‐covalent interactions, such as π???π‐stacking, C? H???π, hydrogen‐bonding, and halogen‐related interactions. Investigation of their photoluminescence properties exhibited disparate emission wavelengths, lifetimes, and quantum yields in the solid state.     

Charge‐Transfer Complex Formation in Gelation: The Role of Solvent Molecules with Different Electron‐Donating Capacities

A naphthalenediimide (NDI)‐based synthetic peptide molecule forms gels in a particular solvent mixture (chloroform/aromatic hydrocarbon, 4:1) through charge‐transfer (CT) complex formation; this is evident from the corresponding absorbance and fluorescence spectra at room temperature. Various aromatic hydrocarbon based solvents, including benzene, toluene, xylene (ortho, meta and para) and mesitylene, have been used for the formation of the CT complex. The role of different solvent molecules with varying electron‐donation capacities in the formation of CT complexes has been established through spectroscopic and computational studies.     

pH‐Regulated Selectivity in Supramolecular Polymerizations: Switching between Co‐ and Homopolymers

A strategy is presented to regulate the selectivity in aqueous supramolecular polymerizations by changes in pH. In neutral buffered conditions, oppositely charged phenylalanine‐based dendritic peptide amphiphiles self‐assemble into (A?B)_n alternating copolymers of low polydispersity when mixed in a 1:1 comonomer feed ratio. Via pH switch of the glutamic acid and lysine side chains, attractive Coulomb interactions in the coassembled materials are screened and selective polymerization occurs to form (A)_n homopolymers of the acidic comonomer at low pH and (B)_n homopolymers of the basic comonomer at high pH, while the complementary comonomer is released during the transition. Reversible switching is demonstrated between these three different polymeric states, which were characterized by CD and fluorescence spectroscopy, using a peptide based minimalistic fluorophore/quencher pair, and transmission electron microscopy.     

Two‐Dimensional Assembly Based on Flow Supramolecular Chemistry: Kinetic Control of Molecular Interactions Under Solvent Diffusion

Self‐assembly of porphyrin molecules can be controlled kinetically to form structures with lengths extending from the nano‐ to the micrometer scale, through a programmed solvent‐diffusion process in designed microflow spaces. Temporal solvent structures generated in the microflow were successfully transcribed into molecular architectures.