Amyloid-like Behavior in Abiotic, Amphiphilic Foldamers

An Appraisal for Providing Charge Transfer (CT) Through Synthetic Porous Frameworks for their Semiconductor Applications

In recent years, there has been considerable focus on the development of charge transfer (CT) complex formation as a means to modify the band gaps of organic materials. In particular, CT complexes alternate layers of aromatic molecules with donor (D) and acceptor (A) properties to provide inherent electrical conductivity. In particular, the synthetic porous frameworks as attractive D-A components have been extensively studied in recent years in comparison to existing D-A materials. Therefore, in this work, the synthetic porous frameworks are classified into conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) and compare high-quality materials for CT in semiconductors. This work updates the overview of the above porous frameworks for CT, starting with their early history regarding their semiconductor applications, and lists CT concepts and selected key developments in their CT complexes and CT composites. In addition, the network formation methods and their functionalization are discussed to provide access to a variety of potential applications. Furthermore, several theoretical investigations, efficiency improvement techniques, and a discussion of the electrical conductivity of the porous frameworks are also highlighted. Finally, a perspective of synthetic porous framework studies on CT performance is provided along with some comparisons.

Self-Assembly of Discrete Oligomers of Naphthalenediimides in Bulk and on Surfaces

Here, we report on the synthesis of discrete oligomers of alkyl-bridged naphthalenediimides (NDIs) and study their molecular nanostructures both in bulk, in solution, and at the liquid-solid interface. Via an iterative synthesis method, multiple NDI cores were bridged with short and saturated alkyl-diamines (C3 and C12 ) or long and unsaturated alkyl-diamines (u2 C33 to u8 C100 ) at their imide termini. The strong intermolecular interaction between the NDI cores was observed by probing their photophysical properties in solution. In bulk, the discrete NDI oligomers preferentially ordered in lamellar morphologies, irrespective of whether a saturated or unsaturated spacer was employed. Moreover, both the molecular architecture as well as the crystallization conditions play a significant role in the nanoscale ordering. The long unsaturated alkyl chains lead preferably to folded-chain conformations while their saturated analogues form stretched arrangements. At the solution-solid interface, well-defined lamellar regions were observed. These results show that precision in chemical structure alone is not sufficient to reach well-defined structures of discrete oligomers, but that it must be combined with precision in processing conditions.

Supramolecular Recognition of Phosphodiester-Based Donor and Acceptor Oligomers Forming Gels in Water

Inspired by automated DNA synthesis, electron-rich dialkoxynaphthalene (DAN) donor and electron-deficient naphthalene-tetracarboxylic diimide (NDI) acceptor phosphodiester-linked homohexamers were synthesized by the phosphoramidite method. Two types of hexamers were prepared, one with only one phosphodiester between the aromatics (i.e., DAN or NDI) and a second with two phosphodiesters around a propanediol between the aromatics, leading to the latter more flexible and more hydrophilic hexamers. The folding properties of these homohexamers alone or mixed together, in water only, were studied by UV-visible absorption spectroscopy and atomic force microscopy (AFM). AFM imaging revealed that a 1:1 mixture of hexaDAN and hexaNDI formed fibers by charge transfer donor-acceptor recognition leading to a hydrogel after drying. The organization of the resulting structures is strongly dependent on the nature of the complementary partner, leading to the formation of mono- or multilayer hydrogel networks with different compactness.

General Principles Underpinning Amyloid Structure

Amyloid fibrils are a pathologically and functionally relevant state of protein folding, which is generally accessible to polypeptide chains and differs fundamentally from the globular state in terms of molecular symmetry, long-range conformational order, and supramolecular scale. Although amyloid structures are challenging to study, recent developments in techniques such as cryo-EM, solid-state NMR, and AFM have led to an explosion of information about the molecular and supramolecular organization of these assemblies. With these rapid advances, it is now possible to assess the prevalence and significance of proposed general structural features in the context of a diverse body of high-resolution models, and develop a unified view of the principles that control amyloid formation and give rise to their unique properties. Here, we show that, despite system-specific differences, there is a remarkable degree of commonality in both the structural motifs that amyloids adopt and the underlying principles responsible for them. We argue that the inherent geometric differences between amyloids and globular proteins shift the balance of stabilizing forces, predisposing amyloids to distinct molecular interaction motifs with a particular tendency for massive, lattice-like networks of mutually supporting interactions. This general property unites previously characterized structural features such as steric and polar zippers, and contributes to the long-range molecular order that gives amyloids many of their unique properties. The shared features of amyloid structures support the existence of shared structure-activity principles that explain their self-assembly, function, and pathogenesis, and instill hope in efforts to develop broad-spectrum modifiers of amyloid function and pathology.

Molecular Alignment of Alkynylplatinum(II) 2,6-Bis(benzimidazol-2-yl)pyridine Double Complex Salts and the Formation of Well-Ordered Nanostructures Directed by Pt···Pt and Donor-Acceptor Interactions

A new class of alkynylplatinum(II) bzimpy (bzimpy = bis(benzimidazol-2-yl)pyridine) double complex salts (DCSs) containing dialkoxynaphthalene or pyromellitic diimide moieties on the alkynyl ligand has been reported to display distinct morphological properties compared to their precursor alkynylplatinum(II) complexes, with the capability of being aligned by the directional Pt···Pt and/or π-π stacking interactions. The incorporation of donor and acceptor units on the alkynyl ligands has been found to significantly perturb the alignment of the oppositely charged complex ions in the DCSs to stack in a twisted head-to-head manner, attributed to the additional driving forces of electrostatic and donor-acceptor interactions. The modulation of the Pt···Pt distances and the extent of aggregate formation have been demonstrated by altering the charge matching between the platinum(II) bzimpy moieties and the donor or acceptor moieties on the alkynyl ligand.

Room temperature charge-transfer phosphorescence from organic donor–acceptor Co-crystals

Engineering the electronic excited state manifolds of organic molecules can give rise to various functional outcomes, including ambient triplet harvesting, that has received prodigious attention in the recent past. Herein, we introduce a modular, non-covalent approach to bias the entire excited state landscape of an organic molecule using tunable ‘through-space charge-transfer’ interactions with appropriate donors. Although charge-transfer (CT) donor–acceptor complexes have been extensively explored as functional and supramolecular motifs in the realm of soft organic materials, they could not imprint their potentiality in the field of luminescent materials, and it still remains as a challenge. Thus, in the present study, we investigate the modulation of the excited state emission characteristics of a simple pyromellitic diimide derivative on complexation with appropriate donor molecules of varying electronic characteristics to demonstrate the selective harvesting of emission from its locally excited (LE) and CT singlet and triplet states. Remarkably, co-crystallization of the pyromellitic diimide with heavy-atom substituted and electron-rich aromatic donors leads to an unprecedented ambient CT phosphorescence with impressive efficiency and notable lifetime. Further, gradual minimizing of the electron-donating strength of the donors from 1,4-diiodo-2,3,5,6-tetramethylbenzene (or 1,2-diiodo-3,4,5,6-tetramethylbenzene) to 1,2-diiodo-4,5-dimethylbenzene and 1-bromo-4-iodobenzene modulates the source of ambient phosphorescence emission from the ³CT excited state to ³LE excited state. Through comprehensive spectroscopic, theoretical studies, and single-crystal analyses, we elucidate the unparalleled role of intermolecular donor–acceptor interactions to toggle between the emissive excited states and stabilize the triplet excitons. We envisage that the present study will be able to provide new and innovative dimensions to the existing molecular designs employed for triplet harvesting.

A modular, non-covalent donor–acceptor strategy is proposed to bias the excited-state manifold of organic systems and to realize unprecedented charge-transfer phosphorescence.

Folding of aromatic polyamides into a rare intrachain β-sheet type structure and further reinforcement of the secondary structure through host–guest interactions

We report the design, synthesis, and folding of aromatic polyamides into an intrachain β-sheet-like structure. Additionally, the effect of a guest molecule in stabilizing the β-sheet structure has also been demonstrated here.

Folding of phosphodiester-linked donor–acceptor oligomers into supramolecular nanotubes in water

Water soluble foldamers, synthesized by DNA synthesis with dialkoxynaphthalene and naphthalene-tetracarboxylic diimide blocks, formed supramolecular nanotubes in water.

Co-assembly of functionalized donor-acceptor molecules within block copolymer microdomains via the supramolecular assembly approach with an improved charge carrier mobility

Here, we demonstrate the three-component self-assembly of functionalized small molecules (donor and acceptor) and a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer using the supramolecular approach. The introduction of functional groups on both the donor (1-pyrenebutyric acid, PBA) and acceptor (functionalized naphthalene diimide, FNDI) molecules can form stable charge-transfer (CT) complexes within the block copolymer domains and these supramolecules exhibited a charge carrier mobility of around 1.01 × 10^-4 cm² (V s)^-1. In this case, both the molecules can form H-bonding with P4VP chains, and as well as π-π stacking between the PBA and FNDI molecules is also possible within the block copolymer domains. These noncovalent interactions lead to the formation of stable hierarchical structures and CT complexes between PBA and FNDI, where bilayer donor-acceptor (D-A) stacks formed within the block copolymer microdomains. Overall, the organization of both functionalized donor and acceptor molecules within the block copolymer domain exhibits an enhanced charge carrier mobility, which is potentially useful in the fabrication of organic photovoltaic cells and organic light-emitting diodes.

The Diverse World of Foldamers: Endless Possibilities of Self-Assembly

Different classes of foldamers, which are synthetic oligomers that adopt well-defined conformations in solution, have been the subject of extensive studies devoted to the elucidation of the forces driving their secondary structures and their potential as bioactive molecules. Regardless of the backbone type (peptidic or abiotic), the most important features of foldamers are the high stability, easy predictability and tunability of their folding, as well as the possibility to endow them with enhanced biological functions, with respect to their natural counterparts, by the correct choice of monomers. Foldamers have also recently started playing a starring role in the self-assembly of higher-order structures. In this review, selected articles will be analyzed to show the striking number of self-assemblies obtained for foldamers with different backbones, which will be analyzed in order of increasing complexity. Starting from the simplest self-associations in solution (e.g., dimers of β-strands or helices, bundles, interpenetrating double and multiple helices), the formation of monolayers, vesicles, fibers, and eventually nanostructured solid tridimensional morphologies will be subsequently described. The experimental techniques used in the structural investigation, and in the determination of the driving forces and mechanisms underlying the self-assemblies, will be systematically reported. Where applicable, examples of biomimetic self-assembled foldamers and their interactions with biological components will be described.

Multifunctional Antimicrobial Biometallohydrogels Based on Amino Acid Coordinated Self-Assembly

There is a real need for new antibiotics against self-evolving bacteria. One option is to use biofriendly broad-spectrum and mechanically tunable antimicrobial hydrogels that can combat multidrug-resistant microbes. Whilst appealing, there are currently limited options. Herein, broad-spectrum antimicrobial biometallohydrogels based on the self-assembly and local mineralization of Ag⁺ -coordinated Fmoc-amino acids are reported. Such biometallohydrogels have the advantages of localized delivery and sustained release, reduced drug dosage and toxicity yet improved bioavailability, prolonged drug effect, and tunable mechanical strength. Furthermore, they can directly interact with the cell walls and membrane, resulting in the detachment of the plasma membrane and leakage of the cytoplasm. This leads to cell death, triggering a significant antibacterial effect against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria in cells and mice. This study paves the way for developing a multifunctional integration platform based on simple biomolecules coordinated self-assembly toward a broad range of biomedical applications.

Exploiting the interactions of aromatic units for folding and assembly in aqueous environments

A variety of non-covalent interactions (including hydrogen bonding, ionic interactions, metal coordination and desolvation/solvation) have been utilized to organize oligomers into well-defined structures. Herein is described a survey of aromatic foldamers that capitalize on electrostatic complementarity of substituted aromatic units to drive folding and assembly in aqueous environments. A brief description of recent advances in the understanding of aromatic interactions is provided, followed by examples of foldamers that exploit interactions between aromatic units to drive their assembly in predictable fashion. The history of our aromatic foldamers is traced from the first structure designed to fold into a pleated structure in an aqueous environment to a heteroduplex system more related to nucleic acids. Taken together, the results demonstrate that electrostatic complementarity of aromatic units provides a versatile framework for driving predictable folding and assembly in aqueous environments.

Temporal switching of an amphiphilic self-assembly by a chemical fuel-driven conformational response

The spatial and temporal control of self-assemblies is the latest scientific hurdle in supramolecular chemistry which is inspired by the functioning of biological systems fueled by chemical signals. In this study, we work towards alleviating this scenario by employing a unique amphiphilic foldamer that operates under the effect of a chemical fuel. The conformational changes in the foldamer amplify into observable morphological changes in its amphiphilic assembly that are controlled by external molecular cues (fuel). We take advantage of this redox responsive foldamer to affect its conformation in a temporal manner by an enzymatic pathway. The temporal characteristics of the transient conformation/assembly can be modulated by varying the concentrations of the fuel and enzyme. We believe that such a design strategy can have positive consequences in designing molecular and supramolecular systems for future active, adaptive and autonomous materials.

Molecular Architectonics of Naphthalenediimides for Efficient Structure-Property Correlation

We present a bioinspired design strategy to effectively tailor the assembly of naphthalenediimides (NDIs) into a wide variety of architectures by functionalizing with amino acid derivatives. This bioinspired process of custom designing and engineering molecular assemblies is termed "bioinspired architectonics". By employing minute structural mutations in the form of α-substituents of amino acids, we successfully engineered molecular assembly of NDIs into zero-dimensional (0D, spheres), one-dimensional (1D, fibers), and two-dimensional (2D, sheets) architectures. The 2D sheets of phenylalanine methylester appended NDI 1 showed remarkable bulk electron mobility of up to 1 cm(2) V(-1)s(-1). With the aid of photophysical, diffraction, and microscopy techniques we rationalize the effect of molecular structure with their ordering and electronic properties in an effort to find structure-property correlations via a bioinspired modular approach.

Energy transfer within responsive pi-conjugated coassembled peptide-based nanostructures in aqueous environments

Steady-state and time-resolved photophysical measurements demonstrate energy transfer within π-conjugated peptide nanostructures composed of oligo-(p-phenylenevinylene)-based donor units and quaterthiophene-based acceptor units in completely aqueous environments. These peptide-based assemblies encourage energy migration along the stacking axis, thus resulting in the quenching of donor emission peaks along with the development of new spectral features reminiscent of acceptor emission. These spectral changes were observed even at minute amounts of the acceptor (starting at 1 mol%), suggesting that exciton migration is involved in energy transport and supporting a funnel-like energy transduction mechanism. The reversibility of nanostructure formation and the associated photophysical responses under different conditions (pH, temperature) were also studied. This unique material design incorporates two different semiconducting units coassembled within peptide nanostructures and offers a new platform for the engineering of energy migration through bioelectronic materials in aqueous environments.

Direct core functionalisation of naphthalenediimides by iridium catalysed C–H borylation

We report the first boron-substituted naphthalenediimides (NDIs), prepared by iridium catalysed C–H activation. Both mono- and diborylated products are available, which have been further elaborated by Suzuki–Miyaura coupling.

Halophilic polysulfabetaines – synthesis and study of gelation and thermoresponsive behavior

Polysulfabetaines (PSBs) derived from zwitterionic sulfates (contrary to commonly used polysulfobetaines which are derived from zwitterionic sulfonates) were synthesized for the first time.

More than meets the eye: conformational switching of a stacked dialkoxynaphthalene-naphthalenetetracarboxylic diimide (DAN-NDI) foldamer to an NDI-NDI fibril aggregate

The thermally induced conformational switching of a stacked dialkxoynaphthalene-naphthalenetetracarboxylic diimide (DAN-NDI) amphiphilic foldamer to an NDI-NDI fibril aggregate is described. The aggregated fibril structures were explored by UV/Vis, circular dichroism (CD), atomic-force microscopy (AFM), and TEM techniques. Our findings indicate that the aromatic DAN-NDI interactions of the original foldamer undergoes transformation to a fibrillar assembly with aromatic NDI-NDI stacked interactions. These structural insights could help inform new molecular designs and increase our understanding of fibrillar assembly and aggregation process in aqueous solution.

Amino acid derivatized arylenediimides: a versatile modular approach for functional molecular materials

Nature's elegant molecular designs and their assemblies with specific structure-property correlations have inspired researchers to design and develop bio-mimics for advanced functional applications. To realize such advanced molecular materials, naturally evolved amino acids are arguably the ideal auxiliaries due to their remarkable molecular/chiral recognition and distinctive sequence specific self-assembling properties. Over the years, this modular approach of derivatizing naphthalenediimides (NDIs) and perylenediimides (PDIs) with amino acids and peptides have resulted in several hitherto unknown molecular assemblies with phenomenal impact on their performance. Derivatization with versatile arylenediimides is especially interesting due to their wide spread applications in fields ranging from biomedicine to electronics. Herein some of these seminal reports of this rapidly emerging field and the design principles embraced are discussed.

Adaptive supramolecular nanomaterials based on strong noncovalent interactions

Noncovalent systems are adaptive and allow facile processing and recycling. Can they be at the same time robust? How can one rationally design such systems? Can they compete with high-performance covalent materials? The recent literature reveals that noncovalent systems can be robust yet adaptive, self-healing, and recyclable, featuring complex nanoscale structures and unique functions. We review such systems, focusing on the rational design of strong noncovalent interactions, kinetically controlled pathway-dependent processes, complexity, and function. The overview of the recent examples points at the emergent field of noncovalent nanomaterials that can represent a versatile, multifunctional, and environmentally friendly alternative to conventional covalent systems.