Although a linear relationship between the optical activity (normally the CD signal) and the enantiomeric excess (ee) of chiral auxiliaries has been the most commonly observed dependence in dynamic ...supramolecular helical aggregates, positive nonlinear CD–ee dependence, known as the “majority‐rules effect” (MRE), indicative of chiral amplification, has also been well documented and to some extent understood. In sharp contrast, the negative nonlinear CD–ee dependence has been much less reported and is not well understood. Here, the state of the art of both the positive and negative nonlinear CD–ee dependence in noncovalently bound supramolecular helical aggregates is summarized, with the hope that the vast examples of supramolecular aggregates showing positive nonlinear dependence, in terms of the methods of investigations, variations in the structure of the building block (single species or multiple species), and theoretical modeling using the mismatch penalty energy and helix reversal penalty energy, would help to guide the design of building blocks to form aggregates showing negative nonlinear dependence, and thus to understand the mechanisms. The potential applications of those functional supramolecular aggregates are also discussed.
The research progress of nonlinear CD–ee dependence in the dynamic supramolecular aggregates, positive and negative, is summarized from the viewpoint of their discoveries, experimental observations, theoretical descriptions, and applications, in a hope to facilitate a better understanding of supramolecular chirality and the fabrication of functional supramolecular chiral materials.
A significant challenge in creating supramolecular materials is that conjugating molecular functionalities to building blocks often results in dissociation or undesired morphological transformation ...of their assemblies. Here we present a facile strategy to preserve structurally labile peptide assemblies after molecular modification of the self‐assembling peptides. Sheet‐forming peptides are designed to afford a staggered alignment with the segments bearing chemical modification sites protruding from the sheet surfaces. The staggered assembly allows for simultaneous separation of attached molecules from each other and from the underlying assembly motifs. Strikingly, using PEGs as the external molecules, PEG400‐ and PEG700‐peptide conjugates directly self‐associate into nanosheets with the PEG chains localized on the sheet surfaces. In contrast, the sheet formation based on in‐register lateral packing of peptides does not recur upon the peptide PEGylation. This strategy allows for fabrication of densely modified assemblies with a variety of molecules, as demonstrated using biotin (hydrophobic molecule), c(RGDfK) (cyclic pentapeptide), and nucleic acid aptamer (negatively charged ssDNA). The staggered co‐assembly also enables extended tunability of the amount/density of surface molecules, as exemplified by screening ligand‐appended assemblies for cell targeting. This study paves the way for functionalization of historically challenging fragile assemblies while maintaining their overall morphology.
We present a unique way to preserve structurally labile nanosheets after conjugation of a variety of molecules to the self‐assembling peptides. The sheet‐forming peptides afford a staggered alignment for simultaneous separation of external molecules from each other and from the underlying assembly motifs. This strategy allows the functionalization of historically challenging fragile assemblies while maintaining their overall morphology.
Optical sensors that respond to enantiomeric excess of chiral analytes are highly demanded in chirality related research fields and demonstrate their potential in many applications, for example, ...screening of asymmetric reaction products. Most sensors developed so far are small molecules. This
Tutorial Review
covers recent advances in chirality sensing systems that are different from the traditional small molecule-based sensors, by using macrocycles, synthetic oligomers/polymers, supramolecular polymers and nanoparticles as the sensors, in which supramolecular interactions operate.
Recent advances in four classes of non-small-molecule based chirality sensors are reviewed.
Oligomers of benzoylalanine-based amidothioureas containing four -turn structures spaced by
meta
-substituted benzenes were shown to undergo assembly in dilute CH
3
CN solution into supramolecular ...helices of enhanced supramolecular helicity, whereas those spaced by
para
-substituted benzene spacer(s) or those spaced by
meta
-substituted benzenes but with one or two -turns exhibit a substantially decreased tendency of assembling.
Oligomers of benzoylalanine-based amidothiourea, consisting of 4 β-turns spaced by
meta
-substituted benzenes, assembled into supramolecular helices in CH
3
CN, but not those containing less turns or 4 turns but spaced by
para
-substituted benzenes.
Spontaneous resolution leading to conglomerate crystals remains a significant challenge. Here we propose the formation of orthogonal homochiral supramolecular helices in at least two dimensions to ...allow spontaneous resolution. We suggest the design rationale that the chiral species is made into helical building blocks to allow the helix formation. As a proof‐of‐concept, acetylalanine was made into a helical short azapeptide, its N‐amidothiourea derivative containing a β‐turn structure, to which a halogen atom was further introduced at the phenylthiourea aromatic ring. The resultant folded species undergoes both intermolecular hydrogen and halogen bonding across the turn structure to form orthogonal intermolecular hydrogen‐bonded and halogen‐bonded supramolecular helices in two dimensions, and undergoes chiral resolution upon crystallization. Meanwhile, counterparts containing either an F‐substituent with weak halogen bonding or no halogen atom crystallize as racemic compounds.
Acetylalanine was made into a folded N‐amidothiorea derivative containing a β‐turn and further modified with a halogen substituent (I, Br, Cl). Intermolecular halogen and hydrogen bonding, both across the turn structure, independently lead to supramolecular helices of homochirality orthogonal in two dimensions and thereby spontaneous resolution. Derivatives containing no halogen atom or a fluorine atom crystallize as racemic compounds.
Supramolecular helices from helical building blocks represent an emerging analogue of the α-helix. In cases where the helicity of the helical building block is well propagated, the head-to-tail ...intermolecular interactions that lead to the helix could be enhanced to promote the formation and the stability of the supramolecular helix, wherein homochiral elongation dominates and functional helical channel structures could also be generated. This feature article outlines the supramolecular helices built from helical building blocks,
, helical aromatic foldamers and helical short peptides that are held together by intermolecular π-π stacking, hydrogen/halogen/chalcogen bonding, metal coordination, dynamic covalent bonding and solvophobic interactions, with emphasis on the influence of efficient propagation of helicity during assembly, favouring homochirality and channel functions.
The synthesis, structures, and properties of 4cyclonaphthodithiophene diimides (4C‐NDTIs) are described. NDTIs as important n‐type building blocks were catenated in the α‐positions of thiophene rings ...via an unusual electrochemical‐oxidation‐promoted macrocyclization route. The thiophene–thiophene junction in 4C‐NDTIs results in an ideal pillar shape. This interesting topology, along with appealing electronic and optical properties inherited from the NDTI units, endows the 4C‐NDTIs with both near‐infrared (NIR) light absorptions, strong excitonic coupling, and tight encapsulation of C60. Stable orientations of the NDTI units in the nanopillars lead to stable inherent chirality, which enables detailed circular dichroism studies on the impact of isomeric structures on π‐conjugation. Remarkably, the 4C‐NDTIs maintain the strong π–π stacking abilities of NDTI units and thus adopt two‐dimensional (2D) lattice arrays at the molecular level. These nanopillar molecules have great potential to mimic natural photosynthetic systems for the development of multifunctional organic materials.
Chiral π‐conjugated nanopillar molecules based on naphthodithiophene diimide feature rigid pillar shapes along with a wealth of unique optical and chiral properties, which render them ideal building blocks for constructing important two‐dimensional supramolecular architectures.
The formation of well‐defined finite‐sized aggregates represents an attractive goal in supramolecular chemistry. In particular, construction of discrete π‐stacked dye assemblies remains a challenge. ...Reported here is the design and synthesis of a novel type of discrete π‐stacked aggregate from two comparable perylenediimide (PDI) dyads (PEP and PBP). The criss‐cross PEP‐PBP dimers in solution and (PBP‐PEP)‐(PEP‐PBP) tetramers in the solid state are well elucidated using single‐crystal X‐ray diffraction, dynamic light scattering, and diffusion‐ordered NMR spectroscopy. Extensive π–π stacking between the PDI units of PEP and PBP as well as repulsive interactions of swallow‐tailed alkyl substituents are responsible for the selective formation of discrete dimer and tetramer stacks. Our results reveal a new approach to preparing discrete π stacks that are appealing for making assemblies with well‐defined optoelectronic properties.
Look‐alikes: Reported here is the design and synthesis of a discrete π‐stacked aggregate from two comparable perylenediimide (PDI) dyads (PEP and PBP). The criss‐cross PEP‐PBP dimers in solution and (PBP‐PEP)‐(PEP‐PBP) tetramers in the solid state are elucidated using single‐crystal X‐ray diffraction, dynamic light scattering, and diffusion‐ordered NMR spectroscopy. The chromophore look‐alikes uniformly embrace each other through π–π interactions and therefore form stable, discrete π‐stacks.
This critical review highlights recent advances in the structurally modified (thio)urea-based receptors for anion complexation and sensing. Modifications of the (thio)urea structure are aimed at a ...better anion binding in terms of higher binding constant, anion selectivity and feasibility. Major (thio)urea receptors are reviewed as N-alkyl, N-aryl and N-amido/N-amino (thio)ureas. Hints for designing (thio)urea-based receptors for anions are discussed (102 references).
The reversible boronic acid-diol interaction empowers boronic acid receptors' saccharide binding capacities, rendering them a class of lectin mimetic, termed "boronlectins". Boronic acids follow ...lectin functions not just in being able to bind saccharides, but in multivalent saccharide binding that enhances both affinity and selectivity. For almost a decade, efforts have been made to achieve and improve selectivity for given saccharide targets, most notably glucose, by using properly positioned boronic acids, offering multivalent interactions. Incorporation of several boronic acid groups into a covalent framework or non-covalent assembly of boronic acid are two general methods used to create such smart sensors, of which the latter resembles lectin oligomerisation that affords multivalent saccharide-binding architectures. In this review, we discuss supramolecular selective sensing of saccharides by using simple boronic acids in their aggregate forms, after a brief survey of the general aspects of boronic acid-based saccharide sensing.
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