Supramolecular Chirality in Solvent‐Promoted Aggregation of Amphiphilic Porphyrin Derivatives: Kinetic Studies and Comparison between Solution Behavior and Solid‐State Morphology by AFM Topography

The solvent‐promoted aggregation behavior of some amphiphilic porphyrin derivatives bearing chiral functionality in the form of a charged L ‐proline group has been investigated by UV/Vis, resonance light scattering, fluorescence and circular dichroism spectroscopy. The investigated macrocycles give rise to aggregates featuring supramolecular chirality with high ellipticity. Kinetic studies reveal peculiar differences in the fashion of aggregation, depending on the intimate nature of the chiral functionality, namely, cationic (nitrogen‐quaternized L ‐proline, 3H₂ ) or anionic (carboxylate residue, 6H₂ ) group. Formation of anionic 6H₂ aggregates shows a diffusion‐limited kinetic behavior. AFM topography studies show formation of tighter globular structures. On the other hand, the corresponding 3H₂ aggregates are formed by a cooperative, fractal‐type decay, and appear as long‐fibrous, looser structures. In the templated aggregation of 3H₂ over preformed 6H₂ aggregates, AFM images show formation of globular structures with reduced sizes, as a likely consequence of shorter interchromophore distances, due to favorable Coulombic interactions. The results obtained show an interesting parallelism between the solution behavior and the solid‐state aggregate structures, corroborating the sergeant–soldier effect observed in the templated aggregation. The results presented give important insights for understanding the complex mechanisms involved in these issues, which are of key importance for the development of chiral supramolecular materials and stereoselective sensors and devices.     

Kinetic Control of the Supramolecular Chirality of Porphyrin J‐Aggregates

The aggregation of achiral sulfonatophenyl‐ and phenyl‐meso‐substituted diprotonated porphyrins to chiral J‐aggregates is a hierarchical noncovalent polymerization process preceded by a critical nucleation stage. This allows significant enantiomeric excesses by the formation of a few primary nuclei and the control of their growth by the effect that flows (imperfect mixing) have on the secondary nucleation of the J‐aggregate particles. In addition, the results strongly suggest that when only one species of aggregate predominates, the CD signals of the three excitonic bands in the visible region (around 420, 490, and 700 nm) show the same sign. Thus, differences on their relative sign would be due to the presence of different species.     

Tuning the Supramolecular Chirality of Polyaniline by Methyl Substitution

A game of Twister : The induced helicity of polyaniline and its supramolecular structures could be tuned by the methyl substitution of one of the monomers. By copolymerization of aniline with m‐toluidine, the helicity of copolymer (PMANI) nanofibers was totally inverted compared to that of polyaniline (PANI), while copolymer nanofibers with o‐toluidine (POANI) had the same helicity as that of polyaniline (see picture).

     

Autoamplification of Molecular Chirality through the Induction of Supramolecular Chirality

The novel concept for the autoamplification of molecular chirality, wherein the amplification proceeds through the induction of supramolecular chirality, is presented. A solution of prochiral, ring‐open diarylethenes is doped with a small amount of their chiral, ring‐closed counterpart. The molecules co‐assemble into helical fibers through hydrogen bonding and the handedness of the fibers is biased by the chiral, ring‐closed diarylethene. Photochemical ring closure of the open diarylethene yields the ring‐closed product, which is enriched in the template enantiomer.     

Inducing Axial Chirality in a Supramolecular Catalyst

A new type of ligand, which is able to form axially chiral, supramolecular complexes was designed using DFT calculations. Two chiral monomers, each featuring a covalently bound chiral auxiliary, form a bidentate phosphine ligand with a twisted, hydrogen‐bonded backbone upon coordination to a transition metal center which results in two diastereomeric, tropos complexes. The ratio of the diastereomers in solution is very temperature‐ and solvent‐dependent. Rhodium and platinum complexes were analyzed through a combination of NMR studies, ESI‐MS measurements, as well as UV‐VIS and circular dichroism spectroscopy. The chiral self‐organized ligands were evaluated in the rhodium‐catalyzed asymmetric hydrogenation of α‐dehydrogenated amino acids and resulted in good conversion and high enantioselectivity. This research opens the way for new ligand designs based on stereocontrol of supramolecular assemblies through stereodirecting chiral centers.     

超分子组装体激发态手性的响应性

圆偏振发光主要是指手性发光体系激发态的性质。由于其在信息加密、高分辨3D显示和智能传感器等领域的潜在应用而备受关注。圆偏振光除了可以通过物理方法获得,即使用线偏振片和四分之一波片的组合,还可以直接从具有光致发光或电致发光性质的手性材料中获得。目前研究者们已经开发了多种圆偏振发光材料,主要包括手性有机分子、手性金属配合物等小分子发光体系以及手性超分子组装体等复合体系。通过将手性组分与响应性功能基团结合而构筑的响应性自组装发光体系对实现智能圆偏振发光材料的发展起着重要作用。在这篇文章里,我们对手性超分子自组装发光体系对各种外界刺激的响应性能进行了总结和归纳,如光照、pH值、溶剂、温度、金属离子等。本综述通过对各种外部刺激对手性组装体激发态性能影响的总结和讨论,旨在进一步推动智能圆偏振发光材料在多学科领域的应用。     

Spectroscopic, morphological, and mechanistic investigation of the solvent-promoted aggregation of porphyrins modified in meso-positions by glucosylated steroids

Solvent-driven aggregation of a series of porphyrin derivatives was studied by UV/Vis and circular dichroism spectroscopy. The porphyrins are characterised by the presence in the meso positions of steroidal moieties further conjugated with glucosyl groups. The presence of these groups makes the investigated macrocycles amphiphilic and soluble in aqueous solvent, namely, dimethyl acetamide/water. Aggregation of the macrocycles is triggered by a change in bulk solvent composition leading to formation of large architectures that express supramolecular chirality, steered by the presence of the stereogenic centres on the periphery of the macrocycles. The aggregation behaviour and chiroptical features of the aggregates are strongly dependent on the number of moieties decorating the periphery of the porphyrin framework. In particular, experimental evidence indicates that the structure of the steroid linker dictates the overall chirality of the supramolecular architectures. Moreover, the porphyrin concentration strongly affects the aggregation mechanism and the CD intensities of the spectra. Notably, AFM investigations reveal strong differences in aggregate morphology that are dependent on the nature of the appended functional groups, and closely in line with the changes in aggregation mechanism. The suprastructures formed at lower concentration show a network of long fibrous structures spanning over tens of micrometres, whereas the aggregates formed at higher concentration have smaller rod-shaped structures that can be recognised as the result of coalescence of smaller globular structures. The fully steroid substituted derivative forms globular structures over the whole concentration range explored. Finally, a rationale for the aggregation phenomena was given by semiempirical calculations at the PM6 level.     

Chirality in Dynamic Supramolecular Nanotubes Induced by a Chiral Solvent

Amplification of chirality has been reported in polymeric systems. It has also been shown that related effects can occur in polymer‐like dynamic supramolecular aggregates, if a subtle balance between noncovalent interactions allows the coupling between a chiral information and a cooperative aggregation process. In this context, we report a strong majority‐rules effect in the formation of chiral dynamic nanotubes from chiral bisurea monomers. Furthermore, similar helical nanotubes (with the same circular dichroism signature) can be obtained from racemic monomers in a chiral solvent. Competition experiments reveal the relative strength of the helical bias induced by the chiral monomer or by the chiral solvent. The nanotube handedness is imposed by the monomer chirality, whatever the solvent chirality. However, the chirality of the solvent has a significant effect on the degree of chiral induction.     

Self‐Discriminating Termination of Chiral Supramolecular Polymerization: Tuning the Length of Nanofibers

Directing the supramolecular polymerization towards a preferred type of organization is extremely important in the design of functional soft materials. Proposed herein is a simple methodology to tune the length and optical chirality of supramolecular polymers formed from a chiral bichromophoric binaphthalene by the control of enantiomeric excess (ee). The enantiopure compound gave thin fibers longer than a few microns, while the racemic mixture favored the formation of nanoparticles. The thermodynamic study unveils that the heterochiral assembly gets preference over the homochiral assembly. The stronger heterochiral binding over homochiral one terminated the elongation of fibrous assembly, thus leading to a control over the length of fibers in the nonracemic mixtures. The supramolecular polymerization driven by π–π interactions highlights the effect of the geometry of a twisted π‐core on this self‐sorting assembly.     

Chirality amplification of porphyrin assemblies exclusively constructed from achiral porphyrin derivatives.

Two achiral porphyrin derivatives, 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine (TPPOMe) and 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphine (TPPOH), were spread onto an air/water interface. The spreading films were transferred onto solid substrates by the Langmuir-Schaefer (LS) method. Although both of the porphyrin derivatives are achiral species, the transferred LS multilayer films shows macroscopic supramolecular chirality, which is suggested to be due to the spontaneous symmetry breaking that occurs at the air/water interface. A strong CD signal is observed from the as-deposited TPPOH LS film, while a relatively weak CD signal is detected from that of TPPOMe. Interestingly, when the TPPOMe LS film was annealed in high vacuum, a significant amplification of the supramolecular chirality is observed. Atomic force microscopy observations confirm that TPPOMe form more ordered aggregates upon annealing. It is suggested that the small amount of chiral assemblies formed in the as-deposited LS film grow into larger ones following the "sergeants and soldiers" principle during the annealing process.     

手性杯芳烃及其超分子手性

杯芳烃是由苯酚单元通过亚甲基连接而成的空腔型分子,具有衍生位点多,构象丰富等特点,被称为第三代主体分子。在分子层次,依手性因素的结构特点不同,可将手性杯芳烃分为具有手性亚单元的杯芳烃、固有手性杯芳烃和桥手性杯芳烃。在超分子层次,杯芳烃自身或杯芳烃与其他分子或离子在溶液中、晶态中或二维表面可通过非共价键力形成多种拓扑结构的纳米手性聚集体。研究手性杯芳烃和基于杯芳烃的超分子手性组装体的合成、结构和性能,不仅在理解手性起源、手性结构等方面具有理论意义,而且有望获得以分子识别为基础的手性传感器、手性催化剂、手性分离材料、手性载体和手性纳米材料。本文综述近十年来有代表性的分子手性杯芳烃和以杯芳烃为组分的超分子手性聚集体的设计、合成、结构和功能。着重展示杯芳烃骨架在形成新颖分子手性和超分子手性上的优势,以及杯芳烃单元在实现特定功能如手性识别时发挥的作用。相信随着杯芳烃合成技术和杯芳烃超分子设计的发展,必将进一步发挥杯芳烃的结构优势,涌现出更多性能优异的手性杯芳烃功能分子和超分子手性杯芳烃功能材料。     

Chirality Enhancement of Porphyrin Supramolecular Assembly Driven by a Template Preorganization Effect

Cationic polylysine promotes, under neutral conditions, the spontaneous aggregation of opposite charged ZnTPPS in water. Spectroscopic investigations evidence a different preorganization of ZnTPPS onto the polypeptide matrix depending on the chain length. Spinodal decomposition theory in confined geometry is used to model this mechanism by considering the time evolution of a homogeneous distribution of randomly adsorbed particles (porphyrins) onto a rodlike polyelectrolyte (polymer) of variable length L.     

Dynamic Modulation of Supramolecular Chirality Driven by Factors from Internal to External Levels

Supramolecular chirality, generated by the asymmetric assembly of chiral or achiral molecules, has attracted intense study owing to its potential to offer insights into natural biological structures and its crucial roles in advanced materials. The optical activity and stacking pathway of building molecules both greatly determine the chirality of the whole supramolecular structure. The flexibility of supramolecular structures makes their chirality easy to modulate through abundant means. Adjustment of the molecular structure or packing mode, or external stimuli that act like a finger gently pushing toy bricks, can greatly change the chirality of supramolecular assemblies. The dynamic regulation of chiral nanostructures on the intramolecular, intermolecular, and external levels could be regarded as the modulatory essence in numerous strategies, however, this perspective is ignored in most reviews in the literature. Herein, therefore, we focus on the ingenious dynamic modulation of chiral nanostructures by these factors. Through dynamic modulation with changes in chiroptical spectroscopy and electron microscopy, the mechanism of formation of supramolecular chirality is also elaborated.     

Inversion of the Supramolecular Chirality of Nanofibrous Structures through Co‐Assembly with Achiral Molecules

To understand the behavior of chiral nanostructures, it is of critical importance to study how achiral molecules regulate the chirality of such nanostructures and what the main driving forces for the regulation processes are. In this work, the supramolecular chirality of helical nanofibers consisting of phenylalanine‐based enantiomers is inverted by achiral bis(pyridinyl) derivatives through co‐assembly. This inversion is mainly mediated by intermolecular hydrogen bonding interactions between the achiral additives and the chiral molecules, which may induce stereoselective interactions and different reorientations for the assembled molecules, as confirmed by calculations. This work not only exemplifies a feasible method to invert the helicity of chiral nanostructures by the addition of achiral molecules, but also provides a method to explore their functions in environments where chiral and achiral molecules are in close proximity.     

Artificial Neural Networks for Guest Chirality Classification through Supramolecular Interactions

A novel strategy for classification of guest chirality based on the combination of artificial neural networks and anion‐receptor chemistry is reported. The receptor reported herein forms supramolecular complexes with a variety of biologically important carboxylates, in which the chemical shift changes during addition of anions result in complex guest‐stereochemistry‐dependent patterns as followed by ¹H NMR spectroscopy. The neural network had learnt these patterns from a training set of 12 anions, and successfully identified the “unknown” chirality of 14 guests present in the test set. Additionally, principal component analysis could discriminate most of the guests studied (26) and allowed for identification of the receptor protons, which are responsible for information transfer of guest chirality.     

Characterization of Supramolecular Hidden Chirality of Hydrogen‐Bonded Networks by Advanced Graph Set Analysis

Supramolecular hidden chirality of hydrogen‐bonded (HB) networks of primary ammonium carboxylates was exposed by advanced graph set analysis from a symmetric viewpoint in topology. The ring‐type HB (R‐HB) networks are topologically regarded as faces, and therefore exhibit prochirality and positional isomerism due to substituents attached on the faces. To describe the symmetric properties of the faces, additional symbols, Re (right‐handed or clockwise), Si (left‐handed or anticlockwise), and m (mirror), were proposed. According to the symbols, various kinds of faces were classified based on the symmetry. This symmetry consideration of the faces enables us to precisely evaluate supramolecular chirality, especially its handedness, of 0D‐cubic, 1D‐ladder and 2D‐sheet HB networks that are composed of the faces. The 1D‐ladder and 2D‐sheet HB networks generate chirality by accumulating the chiral faces in 1D and 2D manners, respectively, whereas 0D‐cubic HB networks generate chirality based on combinations of eight kinds of faces, similar to the chirality of dice.     

Metal‐Ion‐Mediated Supramolecular Chirality of l‐Phenylalanine Based Hydrogels

For chiral hydrogels and related applications, one of the critical issues is how to control the chirality of supramolecular systems in an efficient way, including easy operation, efficient transfer of chirality, and so on. Herein, supramolecular chirality of l ‐phenylalanine based hydrogels can be effectively controlled by using a broad range of metal ions. The degree of twisting (twist pitch) and the diameter of the chiral nanostructures can also be efficiently regulated. These are ascribed to the synergic effect of hydrogen bonding and metal ion coordination. This study may develop a method to design a new class of electronically, optically, and biologically active materials.