Exploiting Coordination Isomerism for Controlled Self-Assembly

pH-Dependent Reversible Self-Assembly of β-Lactoglobulin-Derived Reducing Peptides

Peptide-based self-assembled nanostructures are emerging vehicles for nutrient delivery and interface engineering. The present study screened eight β-lactoglobulin (β-Lg) derived peptides and found that two reducing peptides [EQSLVCQCLV (EV-10) and VCQCLVR (VR-7)] demonstrated pH-dependent reversible fibrilization. EV-10 formed fibrils at pH 2.0 but became unordered aggregates at pH 7.0. VR-7 showed the opposite trend. Both peptides could undergo repetitive transitions between fibrils and unordered aggregates during consecutive pH-cycling. Fibrilization of both peptides was dominated by charges carried by N- and C-terminals. Both fibrils were characterized by a cross-β sheet structure where the β-sheet was arranged in an antiparallel manner. Fe3+ was reduced by Cys and EV-10 (pH 5.0 and 7.0) simultaneously upon mixing. In contrast, EV-10 fibrils released Fe3+ reducing capacity progressively, which were beneficial to long-term protection Fe2+. The EV-10 fibrils remained intact after simulated gastric digestion and finally dissociated after intestinal digestion. The results shed light on the mechanisms of fibrilization of β-Lg derived peptides. This study was beneficial to the rational design of smart pH-responsive materials for drug delivery and antioxidants for nutrients susceptible to oxidation.

Effect of chirality in the supramolecular polymerization of N-annulated perylenediimides: Cancelling pathway complexity

Herein, the synthesis of two chiral NPBIs, (S)-1 and (R)-1, is reported and their self-assembling features investigated. The reported NPBIs form chiral supramolecular polymers with a rich dichroic pattern by the π-stacking of the aromatic backbones and the formation of an array of H-bonds between the amide functional groups. Furthermore, the peripheral 3,4,5-trialkoxy benzamide groups can form seven-membered pseudocycles by the intramolecular H-bonding interaction between the NH of the peripheral amides and one of the carbonyls of the imide units thus yielding a kinetically controlled self-assembly process. Unlike achiral NPBI 1, that has been reported to form up to four supramolecular polymorphs, the reported chiral NPBIs form only a J-type aggregated species. The results presented herein reveal how subtle changes exert an enormous influence on the supramolecular polymerization outcome.

Distinct seed topologies enable comparison of elongation and secondary nucleation pathways in seeded supramolecular polymerization

The influence of seed topologies on seeded supramolecular polymerization was examined using helicoidal and toroidal supramolecular polymer seeds. The addition of these seeds to a supersaturated solution of monomers led to distinct nucleation-growth kinetics, which were attributed to the significant difference between the elongation from helicoid termini and secondary nucleation catalyzed by the toroid surface.

Angularly fused diaza-dinaphthopyrenes: regio-selective synthesis, crystal structures and isomer-dependent mechanochromic fluorescent properties

We report a one-pot synthesis of a series of unprecedented angular-fused diaza-dinaphthopyrene isomers (1,8-DNPy and 1,6-DNPy) in high yields, which are enabled by regio-selective Bischler-Napieralski cyclization to fuse two quinolone rings either on the same or opposite faces of a pyrene core. Benefiting from the high reactivity of the 1- and 8-positions of the pyrene ring, steric effect from substitution and remarkably different dipole moments, high ring closure selectivity for the 1,8-form vs. the 1,6-form up to 6 : 1 is achieved with ease of separation. With differentiated molecular symmetry, conformation, intermolecular interactions and aromaticity, the two kinds of regio-isomers exhibit distinct single-crystal structures and optoelectronic properties. Impressively, isomer-dependent mechanochromic fluorescent properties of these 2D-azaacenes are identified, which are unique in their turn-on fluorescence feature and contrasting spectral shifts. These findings allow facile and modular access to regio-specific 2D-N-heteroarenes, which provide a way to create innovative optical sensors with improved sensitivity and fruitful fluorescent properties.

Luminescence and Length Control in Nonchelated d 8 ‐Metallosupramolecular Polymers through Metal‐Metal Interactions

Supramolecular polymers (SPs) of d⁸ transition metal complexes have received considerable attention by virtue of their rich photophysical properties arising from metal‐metal interactions. However, thus far, the molecular design is restricted to complexes with chelating ligands due to their advantageous preorganization and strong ligand fields. Herein, we demonstrate unique pathway‐controllable metal‐metal‐interactions and remarkable ³MMLCT luminescence in SPs of a non‐chelated Pt^II complex. Under kinetic control, self‐complementary bisamide H‐bonding motifs induce a rapid self‐assembly into non‐emissive H‐type aggregates (1A). However, under thermodynamic conditions, a more efficient ligand coplanarization leads to superiorly stabilized SP 1B with extended Pt⋅⋅⋅Pt interactions and remarkably long ³MMLCT luminescence (τ^77 K=0.26 ms). The metal‐metal interactions could be subsequently exploited to control the length of the emissive SPs using the seeded‐growth approach.

Acylhydrazone-based supramolecular assemblies undergoing a converse sol-to-gel transition on trans → cis photoisomerization

Photoisomeric supramolecular assemblies have drawn enormous attention in recent years. Although it is a general rule that photoisomerization from a less to a more distorted isomer causes the destruction of assemblies, this photoisomerization process inducing a converse transition from irregular aggregates to regular assemblies is still a great challenge. Here, we report a converse sol-to-gel transition derived from the planar to nonplanar photoisomer conversion, which is in sharp contrast to the conventional light-induced gel collapse. A well-designed acylhydrazone-linked monomer is exploited as a photoisomer to realize the above-mentioned phase transition. In the monomer, imine is responsible for trans–cis interconversion and amide generates intermolecular hydrogen bonds enabling the photoisomerization-driven self-assembly. The counterintuitive feature of the sol-to-gel transition is ascribed to the partial trans → cis photoisomerization of acylhydrazone causing changes in stacking mode of monomers. Furthermore, the reversible phase transition is applied in the valves formed in situ in microfluidic devices, providing fascinating potential for miniature materials.

A converse sol-to-gel transition system based on trans → cis photoisomerization of acylhydrazone-based supramolecular assemblies has been sucessfully established, which was applied in the gel-based microvalves that can in situ control flow by light.

Crystallization-Driven Controlled Two-Dimensional (2D) Assemblies from Chromophore-Appended Poly(L-lactide)s: Highly Efficient Energy Transfer on a 2D Surface

A rational approach towards precision two-dimensional (2D) assemblies by crystallization-driven self-assembly (CDSA) of poly(L-lactides) (PLLAs), end-capped with dipolar dyes like merocyanine (MC) or naphthalene monoimide (NMI) and hydrophobic pyrene (PY) or benzene (Bn) is described. PLLA chains crystallize into diamond-shaped platelets in isopropanol, which forces the terminal dyes to assemble into a 2D array on the platelet surface by either dipolar interactions or π-stacking and exhibit tunable emission. Dipolar dyes play a critical role in imparting colloidal stability and structural uniformity to the 2D crystals, which is partly compromised for hydrophobic ones. Co-crystallization between NMI- and PY-labeled PLLAs yields similar diamond-shaped co-platelets with highly efficient (≈80 %) Förster Resonance Energy Transfer on the 2D surface. Further, the "living" CDSA method confers enlarged, segmented block co-platelets using one of the homopolymers as "seed" and the other as "unimer".

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.

Construction and structural transformation of two coordination sphere supramolecular isomers based on Co(ii) and 4-(2-pyridyl)-NH-1,2,3-triazole via one-pot synthesis

Two coordination sphere supramolecular isomers based on Co(ii) have been obtained via one-pot synthesis. The structural transformation process has been investigated with the help of mass spectroscopy and theoretical calculations.

Structural Transformation of Copper Coordination Complexes with Inducing Supramolecular Chirality

Four copper coordination complexes [CuL(CH3OH)]n (1), [CuL(C2H5OH)]n (2), [(CuL)·0.5H2O]n (3), and [CuL(H2O)·0.5H2O]n (4) with a tridentated Schiff base ligand (H2L = (E)-2-((4-fluoro-2-hydroxybenzylidene)amino)acetate) were obtained in different solvents and reaction times....

Non-uniform Photoinduced Unfolding of Supramolecular Polymers Leading to Topological Block Nanofibers

Synthesis of one-dimensional nanofibers with distinct topological (higher-order structural) domains in the same main chain is one of the challenging topics in modern supramolecular polymer chemistry. Non-uniform structural transformation of supramolecular polymer chains by external stimuli may enable preparation of such nanofibers. To demonstrate feasibility of this post-polymerization strategy, we prepared a photoresponsive helically folded supramolecular polymers from a barbiturate monomer containing an azobenzene-embedded rigid π-conjugated scaffold. In contrast to previous helically folded supramolecular polymers composed of a more flexible azobenzene monomer, UV-light induced unfolding of the newly prepared helically folded supramolecular polymers occurred nonuniformly, affording topological block copolymers consisting of folded and unfolded domains. The formation of such blocky copolymers indicates that the photoinduced unfolding of the helically folded structures initiates from relatively flexible parts such as termini or defects. Spontaneous refolding of the unfolded domains was observed after visible-light irradiation followed by aging to restore fully folded structures.

Recent progress and future challenges in the supramolecular polymerization of metal-containing monomers

The self-assembly of discrete molecular entities into functional nanomaterials has become a major research area in the past decades. The library of investigated compounds has diversified significantly, while the field as a whole has matured. The incorporation of metal ions in the molecular design of the (supra-)molecular building blocks greatly expands the potential applications, while also offering a promising approach to control molecular recognition and attractive and/or repulsive intermolecular binding events. Hence, supramolecular polymerization of metal-containing monomers has emerged as a major research focus in the field. In this perspective article, we highlight recent significant advances in supramolecular polymerization of metal-containing monomers and discuss their implications for future research. Additionally, we also outline some major challenges that metallosupramolecular chemists (will) have to face to produce metallosupramolecular polymers (MSPs) with advanced applications and functionalities.

Unraveling Halogen Effects in Supramolecular Polymerization

Halogens play a crucial role in numerous natural processes and synthetic materials due to their unique physicochemical properties and the diverse interactions they can engage in. In the field of supramolecular polymerization, however, halogen effects remain poorly understood, and investigations have been restricted to halogen bonding or the inclusion of polyfluorinated side groups. Recent contributions from our group have revealed that chlorine ligands greatly influence molecular packing and pathway complexity phenomena of various metal complexes. These results prompted us to explore the role of the halogen nature on supramolecular polymerization, a phenomenon that has remained unexplored to date. To address this issue, we have designed a series of archetypal bispyridyldihalogen Pt^II complexes bearing chlorine (1), bromine (2), or iodine (3) and systematically compared their supramolecular polymerization in nonpolar media using various experimental methods and theory. Our studies reveal a remarkably different supramolecular polymerization for the three compounds, which can undergo two competing pathways with either slipped (kinetic) or parallel (thermodynamic) molecular packing. The halogen exerts an inverse effect on the energetic levels of the two self-assembled states, resulting in a single thermodynamic pathway for 3, a transient kinetic species for 2, and a hidden thermodynamic state for 1. This seesaw-like bias of the energy landscape can be traced back to the involvement of the halogens in weak N-H···X hydrogen-bonding interactions in the kinetic pathway, whereas in the thermodynamic pathway the halogens are not engaged in the stabilizing interaction motif but rather amplify solvophobic effects.

Tuning energy landscapes and metal-metal interactions in supramolecular polymers regulated by coordination geometry

Herein, we exploit coordination geometry as a new tool to regulate the non-covalent interactions, photophysical properties and energy landscape of supramolecular polymers. To this end, we have designed two self-assembled Pt(ii) complexes 1 and 2 that feature an identical aromatic surface, but differ in the coordination and molecular geometry (linear vs. V-shaped) as a result of judicious ligand choice (monodentate pyridine vs. bidentate bipyridine). Even though both complexes form cooperative supramolecular polymers in methylcyclohexane, their supramolecular and photophysical behaviour differ significantly: while the high preorganization of the bipyridine-based complex 1 enables an H-type 1D stacking with short Pt⋯Pt contacts via a two-step consecutive process, the existence of increased steric effects for the pyridyl-based derivative 2 hinders the formation of metal–metal contacts and induces a single aggregation process into large bundles of fibers. Ultimately, this fine control of Pt⋯Pt distances leads to tuneable luminescence—red for 1vs. blue for 2, which highlights the relevance of coordination geometry for the development of functional supramolecular materials.

In this article, we exploit coordination geometry as a new tool to control the energy landscape and photophysical properties (red vs. blue luminescence) of supramolecular polymers.

Influence of the Z/E Isomerism on the Pathway Complexity of a Squaramide-Based Macrocycle

The rising interest on pathway complexity in supramolecular polymerization has prompted the finding of novel monomer designs able to stabilize kinetically trapped species and generate supramolecular polymorphs. In the present work, the exploitation of the Z/E (geometrical) isomerism of squaramide (SQ) units to produce various self-assembled isoforms and complex supramolecular polymerization pathways in methylcyclohexane/CHCl₃ mixtures is reported for the first time. This is achieved by using a new bissquaramidic macrocycle (MSq) that self-assembles into two markedly different thermodynamic aggregates, AggA (discrete cyclic structures) and AggB (fibrillar structures), depending on the solvent composition and concentration. Remarkably, UV-vis, ¹ H NMR, and FT-IR experiments together with quantum-chemical calculations indicate that these two distinct aggregates are formed via two different hydrogen bonding patterns (side-to-side in AggA and head-to-tail in AggB) due to different conformations in the SQ units (Z,E in AggA and Z,Z in AggB). The ability of MSq to supramolecularly polymerize into two distinct aggregates is utilized to induce the kinetic-to-thermodynamic transformation from AggA to AggB, which occurs via an on-pathway mechanism. It is believed that this system provides new insights for the design of potential supramolecular polymorphic materials by using squaramide units.

Phototriggered Supramolecular Assembly

Most of the supramolecular structures lack accuracy at the macroscopic scale because of their spontaneous uncontrolled self-assembly. Although significant progress has been made in understanding the pros and cons of various directional noncovalent interactions and their specific molecular recognition ability, it is only in the recent past that the focus has been shifted toward controlling the dimension, dispersity, and other macroscopic properties of supramolecular assemblies. In this context, recent examples encompass various external stimuli to gain the structural precision in supramolecular assembly, among which light-triggered supramolecular assembly has gained significant interest as it is reagent free and endows the possibility of the controlled synthesis of supramolecular assemblies via manipulation of the light energy, irradiation time, or spatial control. This mini-review highlights representative recent examples of phototriggered supramolecular assemblies (from the monomer or another type of assembly) by removal of a photolabile protecting group, photoisomerization, or other methods of photoactivation of a dormant building block.

Solute-Solvent Interactions in Modern Physical Organic Chemistry: Supramolecular Polymers as a Muse

Interactions between solvents and solutes are a cornerstone of physical organic chemistry and have been the subject of investigations over the last century. In recent years, a renewed interest in fundamental aspects of solute-solvent interactions has been sparked in the field of supramolecular chemistry in general and that of supramolecular polymers in particular. Although solvent effects in supramolecular chemistry have been recognized for a long time, the unique opportunities that supramolecular polymers offer to gain insight into solute-solvent interactions have become clear relatively recently. The multiple interactions that hold the supramolecular polymeric structure together are similar in strength to those between solute and solvent. The cooperativity found in ordered supramolecular polymers leads to the possibility of amplifying these solute-solvent effects and will shed light on extremely subtle solvation phenomena. As a result, many exciting effects of solute-solvent interactions in modern physical organic chemistry can be studied using supramolecular polymers. Our aim is to put the recent progress into a historical context and provide avenues toward a more comprehensive understanding of solvents in multicomponent supramolecular systems.

Solvent-controlled E/Z isomerization vs. [2 + 2] photocycloaddition mediated by supramolecular polymerization

Control over the photochemical outcome of photochromic molecules in solution represents a major challenge, as photoexcitation often leads to multiple competing photochemical and/or supramolecular pathways resulting in complex product mixtures. Herein, we demonstrate precise and efficient control over the photochemical behaviour of cyanostilbenes in solution using a straightforward solvent-controlled approach based on supramolecular polymerization. To this end, we designed a π-extended cyanostilbene bolaamphiphile that exhibits tuneable solvent-dependent photochemical behaviour. Photoirradiation of the system in a monomeric state (in organic solvents) exclusively leads to a highly reversible and efficient E/Z photoisomerization, whereas a nearly quantitative [2 + 2] photocycloaddition into a single cyclobutane (anti head-to-tail) occurs in aqueous solutions. These results can be rationalized by a highly regular and preorganized antiparallel J-type arrangement of the cyanostilbene units that is driven by aqueous supramolecular polymerization. The presented concept demonstrates a novel approach towards solvent-selective and environmentally friendly photochemical transformations, which is expected to broaden the scope of supramolecular polymerization.

Morphology control in metallosupramolecular assemblies through solvent-induced steric demand

Controlling the supramolecular self-assembly of π-conjugated systems into defined morphologies is a prerequisite for the preparation of functional materials. In recent years, the development of sophisticated sample preparation protocols and modulation of various experimental conditions (solvent, concentration, temperature, etc.) have enabled precise control over aggregation pathways of different types of monomer units. A common method to achieve pathway control consists in the combination of two miscible solvents in defined proportions - a "poor" and "good" solvent. However, the role of solvents of opposed polarity in the self-assembly of a given building block still remains an open question. Herein, we unravel the effect of aggregation-inducing solvent systems of opposed polarity (aqueous vs. non-polar media) on the supramolecular assembly of a new bolaamphiphilic Pt(ii) complex. A number of experimental methods show a comparable molecular packing in both media driven by a synergy of solvophobic, aromatic and weak hydrogen-bonding interactions. However, morphological analysis of the respective aggregates in aqueous and non-polar media reveals a restricted aggregate growth in aqueous media into spherical nanoparticles and a non-restricted 2D-nanosheet formation in non-polar media. These findings are attributed to a considerably more efficient solvation and, in turn, increased steric demand of the hydrophilic chains in aqueous media than in nonpolar media, which can be explained by the entrapment of water molecules in the hydrophilic aggregate shell via hydrogen bonds. Our findings reveal that the different solvation of peripheral solubilizing groups in solvents of opposed polarity is an efficient method for morphology control in self-assembly.

Tuning Aqueous Supramolecular Polymerization by an Acid-Responsive Conformational Switch

Besides their widespread use in coordination chemistry, 2,2'-bipyridines are known for their ability to undergo cis-trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecular level. However, the exploitation of such conformational switching in self-assembly has remained unexplored. In this work, the use of 2,2'-bipyridines as acid-responsive conformational switches to tune supramolecular polymerization processes has been demonstrated. To achieve this goal, we have designed a bipyridine-based linear bolaamphiphile, 1, that forms ordered supramolecular polymers in aqueous media through cooperative aromatic and hydrophobic interactions. Interestingly, addition of acid (TFA) induces the monoprotonation of the 2,2'-bipyridine moiety, leading to a switch in the molecular conformation from a linear (trans) to a V-shaped (cis) state. This increase in molecular distortion along with electrostatic repulsions of the positively charged bipyridine-H+ units attenuate the aggregation tendency and induce a transformation from long fibers to shorter thinner fibers. Our findings may contribute to opening up new directions in molecular switches and stimuli-responsive supramolecular materials.

Consequences of hidden kinetic pathways on supramolecular polymerization

In recent years, the development of sophisticated analytical tools, kinetic models and sample preparation methods has significantly advanced the field of supramolecular polymerization, where the competition of kinetic vs. thermodynamic processes has become commonplace for a wide range of building blocks. Typically, the kinetic pathways are identified in thermally controlled assembly experiments before they ultimately evolve to the thermodynamic minimum. However, there might be cases where the identification and thus the assessment of the influence of kinetic aggregates is not trivial, making the analysis of the self-assembly processes a hard task. Herein, we demonstrate that "hidden" kinetic pathways can have drastic consequences on supramolecular polymerization processes, to the point that they can even overrule thermodynamic implications. To this end, we analyzed in detail the supramolecular polymerization of a chiral PdII complex 1 that forms two competing aggregates (Agg I and Agg II) of which kinetic Agg II is formed through a "hidden" pathway, i.e. this pathway is not accessible by common thermal polymerization protocols. The hidden pathway exhibits two consecutive steps: first, Agg II is formed in a cooperative process, which subsequently evolves to clustered superstructures driven by rapid kinetics. At standard conditions, Agg II displays an extraordinary kinetic stability (>6 months), which could be correlated to its cooperative mechanism suppressing nucleation of thermodynamic Agg I. Furthermore, the fast kinetics of cluster formation sequester monomers from the equilibria in solution and prevents the system from relaxing into the thermodynamic minimum, thus highlighting the key implications of hidden pathways in governing supramolecular polymerization processes.

Self-assembly of isomeric naphthalene appended glucono derivatives: nanofibers and nanotwists with circularly polarized luminescence emission

Two isomeric naphthalene appended glucono derivatives substituted at the 1 or 2-naphthyl positions (Nap-1 and Nap-2) were designed and their self-assembly behaviors and optical properties were investigated. Nap-1 and Nap-2 were found to self-assemble into nanofibers and nanotwists, respectively. While the molecular chirality of the glucono moiety could not be effectively transferred to the naphthalene moiety in the Nap-1 system, this was achieved in the Nap-2 assembly. Thus, the Nap-2 assembly showed obvious circular dichroism (CD) and circularly polarized luminescence (CPL) signals. From the XRD patterns and IR spectra of the supramolecular assemblies, it was found that Nap-2 packed in a more orderly fashion than Nap-1, leading to a hierarchical assembly forming nanotwist structures. Moreover, a light-harvesting system based on Nap-2 supramolecular gels and dyes was established, in which an efficient energy transfer was demonstrated from Nap-2 to an acceptor Eosin Y. It was further found that both chirality and energy transfer enhanced the dissymmetry factor of Eosin Y CPL emission.

Supramolecular materials based on AIE luminogens (AIEgens): construction and applications

The emergence of aggregation-induced emission luminogens (AIEgens) has significantly stimulated the development of luminescent supramolecular materials because their strong emissions in the aggregated state have resolved the notorious obstacle of the aggregation-caused quenching (ACQ) effect, thereby enabling AIEgen-based supramolecular materials to have a promising prospect in the fields of luminescent materials, sensors, bioimaging, drug delivery, and theranostics. Moreover, in contrast to conventional fluorescent molecules, the configuration of AIEgens is highly twisted in space. Investigating AIEgens and the corresponding supramolecular materials provides fundamental insights into the self-assembly of nonplanar molecules, drastically expands the building blocks of supramolecular materials, and pushes forward the frontiers of supramolecular chemistry. In this review, we will summarize the basic concepts, seminal studies, recent trends, and perspectives in the construction and applications of AIEgen-based supramolecular materials with the hope to inspire more interest and additional ideas from researchers and further advance the development of supramolecular chemistry.

Exploiting Coordination Isomerism for Controlled Self-Assembly

We exploited the inherent geometrical isomerism of a PtII complex as a new tool to control supramolecular assembly processes. UV irradiation and careful selection of solvent, temperature, and concentration leads to tunable coordination isomerism, which in turn allows fully reversible switching between two distinct aggregate species (1D fibers↔2D lamellae) with different photoresponsive behavior. Our findings not only broaden the scope of coordination isomerism, but also open up exciting possibilities for the development of novel stimuli-responsive nanomaterials.