Toroidal Nanoobjects from Rosette Assemblies of Melamine-Linked Oligo(p-phenyleneethynylene)s and Cyanurates

Pathway Selection in Temporal Evolution of Supramolecular Polymers of Ionic π-Systems: Amphiphilic Organic Solvent Dictates the Fate of Water

Understanding solvent-solute interactions is essential to designing and synthesising soft materials with tailor-made functions. Although the interaction of the solute with the solvent mixture is more complex than the single solvent medium, solvent mixtures are exciting to unfold several unforeseen phenomena in supramolecular chemistry. Here, we report two unforeseen pathways observed during the hierarchical assembly of cationic perylene diimides (cPDIs) in water and amphiphilic organic solvent (AOS) mixtures. When the aqueous supramolecular polymers (SPs) of cPDIs are injected into AOS, initially kinetically trapped short SPs are formed, which gradually transform into thermodynamically stable high aspect ratio SP networks. Using various experimental and theoretical investigations, we found that this temporal evolution follows two distinct pathways depending on the nature of the water-AOS interactions. If the AOS is isopropanol (IPA), water is released from cPDIs into bulk IPA due to strong hydrogen bonding interactions, which further decreases the monomer concentration of cPDIs (Pathway-1). In the case of dioxane AOS, cPDI monomer concentration further increases as water is retained among cPDIs (Pathway-2) due to relatively weak interactions between dioxane and water. Interestingly, these two pathways are accelerated by external stimuli such as heat and mechanical agitation.

Flow-Chemistry-Enabled Synthesis of 5-Diethylboryl-2,3'-bipyridine and Its Self-Assembly Dynamics

5-Diethylboryl-2,3'-bipyridine (1), which is inaccessible by conventional batch methods, was synthesized by using a flow microreactor. Compound 1 was obtained as an equilibrium mixture of a cyclic trimer and a cyclic tetramer in solution, the latter of which was crystallized in benzene by vapor diffusion of hexane at 7 °C. The dynamic nature of this system was confirmed by solvent- and concentration-dependent experiments. Notably, the dynamics was verified by using flow NMR spectroscopy, which revealed that the time required to reach equilibrium was influenced by the solvent ratio (<18 s, 24-28 s, and 34-42 s in 2 : 1, 1 : 1, and 1 : 2 mixtures of [D₆ ]acetone and C₆ D₆ , respectively). Compound 1 and 3-[4'-(diethylboryl)phenyl]pyridine (2) exhibited different self-assembly behavior in solution and crystals. Density functional theory calculations suggested that this difference was largely due to enhanced planarity between two consecutive aromatic rings.

Biasing the Hierarchy Motifs of Nanotoroids: from 1D Nanotubes to 2D Porous Networks

Hierarchical organization of self-assembled structures into superstructures is omnipresent in Nature but has been rarely achieved in synthetic molecular assembly due to the absence of clear structural rules. We herein report on the self-assembly of scissor-shaped azobenzene dyads which form discrete nanotoroids that further organize into 2D porous networks. The steric demand of the peripheral aliphatic units diminishes the trend of the azobenzene dyad to constitute stackable nanotoroids in solution, thus affording isolated (unstackable) nanotoroids upon cooling. Upon drying, these nanotoroids organize at graphite surface to form well-defined 2D porous networks. The photoirradiation with UV and visible light enabled reversible dissociation and reconstruction of nanotoroids through the efficient trans↔cis isomerization of azobenzene moieties in solution.

One-Minute Synthesis of a Supramolecular Hydrogel from Suspension-Gel Transition and the Derived Crystalline, Elastic, and Photoactive Aerogels

Solutions or sols are commonly employed as the starting materials for the preparation of supramolecular hydrogels; however, suspension-based synthesis has been much less reported because of inhomogeneity and quick sedimentation of large particles in a suspension. Further, it remains a technical challenge to derive supramolecular aerogels directly from the parental hydrogels owing to the ease of structural collapse during water removal. Herein, we report a suspension-gel transition for the ultrafast synthesis of a new supramolecular hydrogel simply by adding AgNO₃ into the aqueous suspension of cyanuric acid-melamine (CAM) aggregates. With the activation of preadded ammonia, Ag ions instantly reacted with the CAM particles, transforming into N-Ag(I)-N coordinating bonded supramolecular nanofibers; simultaneously, the suspension converted to a hydrogel without the use of polymer cross-linkers or external stimulus. Upon simple freeze-drying, a highly crystalline fibrous aerogel with a cellular network was obtained, which possessed a porosity up to 99.7% and a density as low as 4.8 mg/cm³, enabling remarkable oil uptake capacities (100 times of its weight). The supramolecular aerogel demonstrated intrinsic elasticity, which should arise from the cellular structure and elastic character of the nanofiber skeletons. Notably, the aerogel showed high compatibility to incorporate a range of external substances for further functionalization exemplified by polymeric carbon nitride (PCN, a metal-free semiconductor) included gels. The loaded PCN resulted in enhanced mechanical strength and endowed the aerogel unique photoactivity, i.e., in situ reducing Ag(I) into Ag nanoparticles upon light illumination and thus forming a plasmonic shell over the aerogel with potential applications in sensing and catalysis.

Interplay between H-Bonding and Preorganization in the Evolution of Self-Assembled Systems

Cooperative π-π interactions and H-bonding are frequently exploited in supramolecular polymerization; however, close scrutiny of their mutual interplay has been largely unexplored. Herein, we compare the self-assembly behavior of a series of C2 - and C3 -symmetrical oligophenyleneethynylenes differing in their amide topology (N- or C-centered). This subtle structural modification brings about drastic changes in their photophysical and supramolecular properties, highlighting the reciprocal impact of H-bonding vs. preorganization on the evolution and final outcome of supramolecular systems.

Photoresponsive Circular Supramolecular Polymers: A Topological Trap and Photoinduced Ring-Opening Elongation

Topological features of one-dimensional macromolecular chains govern the properties and functionality of natural and synthetic polymers. To address this issue in supramolecular polymers, we synthesized two topologically distinct supramolecular polymers with intrinsic curvature, circular and helically folded nanofibers, from azobenzene-functionalized supramolecular rosettes. When a mixture of circular and helically folded nanofibers was exposed to UV light, selective unfolding of the latter open-ended supramolecular polymers was observed as a result of the curvature-impairing internal force produced by the trans-to-cis photoisomerization of the azobenzene. This distinct sensitivity suggests that the topological features of supramolecular polymers define their mechanical stability. Furthermore, the exposure of circular supramolecular polymers in more polar media to UV irradiation resulted in ring opening followed by chain elongation, thus demonstrating that the circular supramolecular polymer can function as a topological kinetic trap.

Designing Self-Assembled Rosettes: Why Ammeline is a Superior Building Block to Melamine

In supramolecular chemistry, the rational design of self-assembled systems remains a challenge. Herein, hydrogen-bonded rosettes of melamine and ammeline have been theoretically examined by using dispersion-corrected density functional theory (DFT-D). Our bonding analyses, based on quantitative Kohn-Sham molecular orbital theory and corresponding energy decomposition analyses (EDA), show that ammeline is a much better building block than melamine for the fabrication of cyclic complexes based on hydrogen bonds. This superior capacity is explained by both stronger hydrogen bonding and the occurrence of a strong synergy.

Self-organizing p-quinquephenyl building blocks incorporating lateral hydroxyl and methoxyl groups into supramolecular nano-assemblies

The self-assembling behavior of coil-rod-coil molecules 1a, 1b, and 2a, 2b was investigated using DSC, POM, SAXS, and AFM in bulk and aqueous solutions. These molecules contain p-quinquephenyl groups as rod segments incorporating lateral hydroxyl or methoxyl groups in the center positions and oligo(ethylene oxide)s as the coil segments. Molecules 1a and 1b, with lateral methoxyl groups in the rod segments, self-assemble into oblique columnar structures in the crystalline phase and transform into nematic phases. On the other hand, molecules 2a and 2b, with hydroxyl groups in the center of their rod segments, self-organize into hexagonal perforated lamellar and oblique columnar nano-structures in the crystalline and liquid crystalline phase, respectively. In aqueous solutions, these molecules aggregate into nano-ribbons and vesicles, depending on their lateral groups and oligo(ethylene oxide) chain lengths. These results imply that the lateral methoxyl or hydroxyl groups, present in the center of the rod segments, significantly influence the formation of various supramolecular nano-structures in the bulk state and in aqueous solution. This is achieved via tuning of the non-covalent interactions of the rod building blocks.

Self-assembly of DNA-oligo(p-phenylene-ethynylene) hybrid amphiphiles into surface-engineered vesicles with enhanced emission

Surface-addressable nanostructures of linearly π-conjugated molecules play a crucial role in the emerging field of nanoelectronics. Herein, by using DNA as the hydrophilic segment, we demonstrate a solid-phase "click" chemistry approach for the synthesis of a series of DNA-chromophore hybrid amphiphiles and report their reversible self-assembly into surface-engineered vesicles with enhanced emission. DNA-directed surface addressability of the vesicles was demonstrated through the integration of gold nanoparticles onto the surface of the vesicles by sequence-specific DNA hybridization. This system could be converted to a supramolecular light-harvesting antenna by integrating suitable FRET acceptors onto the surface of the nanostructures. The general nature of the synthesis, surface addressability, and biocompatibility of the resulting nanostructures offer great promises for nanoelectronics, energy, and biomedical applications.

Supramolecular polymerization of C3-symmetric organogelators: cooperativity, solvent, and gelation relationship

A systematic study of the influence of solvent and the size of C(3)-symmetric discotics on their supramolecular polymerization mechanism is presented. The cooperativity of the self-assembly of the reported compounds is directly related to their gelation ability. The two series of C(3)-symmetric discotics investigated herein are based on benzene-1,3,5-tricarboxamides (BTAs) and oligo(phenylene ethynylene)-based tricarboxamides (OPETAs) that are peripherally decorated with achiral (1a and 2a) or chiral N-(2-aminoethyl)-3,4,5-trialkoxybenzamide units (1b and 2b). The supramolecular polymerization of compounds 1a,b and 2a,b has been exhaustively investigated in a number of solvents and by using various techniques: variable-temperature circular dichroism (VT-CD) spectroscopy, concentration-dependent (1)H NMR spectroscopy, and isothermal titration calorimetry (ITC). The supramolecular polymerization mechanism of compounds 2 is highly cooperative in solvents such as methylcyclohexane and toluene and is isodesmic in CHCl(3). Unexpectedly, chiral compound 1b is practically CD-silent, in contrast with previously reported BTAs. ITC measurements in CHCl(3) demonstrated that the supramolecular polymerization of BTA 1a is isodesmic. These results confirm the strong influence of the π-surface of the central aromatic core of the studied discotic and the branched nature of the peripheral side chains on the supramolecular polymerization. The gelation ability of these organogelators is negated in CHCl(3), in which the supramolecular polymerization mechanism is isodesmic.

Narcissistic versus social self-sorting of oligophenyleneethynylene derivatives: from isodesmic self-assembly to cooperative co-assembly

Narcissistic versus social! The self-assembly of two structurally related oligophenyleneethynylene derivatives featuring polar or nonpolar peripheral chains is reported. Their remarkable narcissistic versus social self-sorting behaviour in aqueous media can be controlled by concentration and solvent changes.

Oligo(p-phenylene-ethynylene)-derived super-π-gelators with tunable emission and self-assembled polymorphic structures

Linear π-conjugated oligomers are known to form organogels through noncovalent interactions. Herein, we report the effect of π-repeat units on the gelation and morphological properties of three different oligo(p-phenylene-ethynylene)s: OPE3, OPE5, and OPE7. All of these molecules form fluorescent gels in nonpolar solvents at low critical gel concentrations, thereby resulting in a blue gel for OPE3, a green gel for OPE5, and a greenish yellow gel for OPE7. The molecule-molecule and molecule-substrate interactions in these OPEs are strongly influenced by the conjugation length of the molecules. Silicon wafer suppresses substrate-molecule interactions whereas a mica surface facilitates such interactions. At lower concentrations, OPE3 formed vesicular assemblies and OPE5 gave entangled fibers, whereas OPE7 resulted in spiral assemblies on a mica surface. At higher concentrations, OPE3 and OPE5 resulted in super-bundles of fibers and flowerlike short-fiber agglomerates when different conditions were applied. The number of polymorphic structures increases on increasing the conjugation length, as seen in the case of OPE7 with n=5, which resulted in a variety of exotic structures, the formation of which could be controlled by varying the substrate, concentration, and humidity.

Rational construction of perylene bisimide columnar superstructures with a biased helical sense

Discotic supramolecular complexes bearing six perylene bisimide (PBI) chromophores were prepared by mixing monotopically triple-hydrogen-bonding melamines equipped with two PBI chromophores and two 3,7-dimethyloctyl chiral handles with tritopically triple-hydrogen-bonding cyanuric acid (CA). UV/Vis and fluorescence titration experiments demonstrated that the discotic complexes were formed in methylcyclohexane by the 3:1 complexation between the melamines and CA. TEM and AFM studies revealed that the complexes hierarchically organize into fibrous columnar assemblies, which eventually results in the formation of organogels. Circular dichroism (CD) and flash-photolysis time resolved microwave conductivity measurements revealed the presence of extended chiral stacks of PBI chromophores within the columns. The anisotropy factors of the columnar assemblies are remarkably high (g=1.5×10(-3)) when considering the presence of only one 3,7-dimethyloctyl chiral handle per perylene chromophore, suggesting that the columnar structures have a biased helical sense. The fact that the chiral centers are located inside the discotic complexes rather than at their peripheries might be unique structural property responsible for the rather strong optical activities for the assemblies of this chromophore. The effective transcription of the molecular chirality to the extended columnar assemblies through the formation of unique discotic complexes enables the expression of "majority-rules" chiral amplification effect, which is unprecedented for the supramolecular assemblies of PBIs.

Modulated morphology in the self-organization of a rectangular amphiphile

The rectangular oligo(phenylene ethynylene) amphiphile 1 has been synthesized to investigate its self-assembling features in solution and onto surfaces. Concentration-dependent and variable-temperature NMR experiments firstly demonstrate the influence of the solvent in the stabilization of the non-covalent forces involved in the association of 1, namely, pi-pi stacking interactions between the aromatic fragments and van der Waals, hydrogen-bonding and/or solvophobic forces between the triethyleneglycol chains. This subtle balance of non-covalent interactions also conditions the thermodynamics of the self-assembly process and concentration-dependent UV/Vis investigations show a linear correlation between the polarity of the solvent and the K(a) values (K(a) approximately 5.2 x 10(5) M(-1) for CH3CN/H2O mixtures and 4.4 x 10(4) M(-1) for benzene). Moreover, these UV/Vis studies prove the organization of this compound following the indefinite self-association model. Microscopy techniques reveal that the morphology and dimensionality of the assemblies formed from 1 can be finely modulated. Although polar solvents yield hollow vesicles or toroidal 3D objects, depending upon concentration, the utilization of non-polar benzene results in the formation of unimolecular wires that can grow to form networks upon increasing concentration. These findings support the direct relationship existing between the self-assembling features of this amphiphile in solution and onto surfaces.