Supramolecular Hydrogels and High-Aspect-Ratio Nanofibers through Charge-Transfer-Induced Alternate Coassembly

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.

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.

Coronenes with push-pull geometries from macrocycle-forming Perkin condensations

Although the Perkin reaction has been successful in producing ester-substituted conjugated macrocycles with four or six building blocks, macrocycles composed of only two elements remained elusive until now. Through the development of a building block derived from phenanthrene with two glyoxylic acid substituents in a pincer-like arrangement, formation of a two-block macrocycle was induced when paired with a complementary phenylenediacetic acid unit. The addition of ether functions to the phenanthrene building block not only improved the yields, but led to macrocycles with push-pull geometries. Photocyclisation of the resulting cyclophanes efficiently yield tetra- and hexasubstituted coronenes.

Thermo-Chemical Cues-Mediated Strategy to Control Peptide Self-Assembly and Charge Transfer Complexation

Peptide amphiphiles (PAs) are known for their remarkable ability to undergo molecular self-assembly, a process that is highly responsive to the local microenvironment. Herein, we design a pyrene tethered peptide amphiphile Py-VFFAKK, 1 that exhibits pathway-driven self-assembly from metastable nanoparticles to kinetically controlled nanofibers and thermodynamically stable twisted bundles upon modulations in pH, temperature, and chemical cues. The presence of the pyrene moiety ensures donation of the electron to an electron acceptor, namely, 7,7,8,8-tetracyanoquinodimethane (TCNQ), to form a supramolecular charge transfer complex in aqueous solution that was studied in detail with microscopic and spectroscopic techniques. Excitation of the donor species in its excimer state facilitates electron donation to the acceptor moiety, paving away a long-lived charge-separated state that persists for over a nanosecond, as ascertained through transient absorption spectroscopy. Finally, the self-assembled charge transfer complex is explored toward antimicrobial properties with Escherichia coli while maintaining biocompatibility toward L929 mice fibroblast cells.

Emulating Ebbinghaus forgetting behavior in a neuromorphic device based on 1D supramolecular nanofibres

Mimicking synaptic functions in hardware devices is a crucial step in realizing brain-like computing beyond the von Neumann architecture. 1D nanomaterials with spatial extensions of a few μm, similar to biological neurons, gain significance given the ease of electrical transport as well as directionality. Herein, we report a two-terminal optically active device based on 1D supramolecular nanofibres consisting of CS (coronene tetracarboxylate) and DMV (dimethyl viologen) forming alternating D-A (donor-acceptor) pairs, emulating synaptic functions such as the STP (short-term potentiation), LTP (long-term potentiation), PPF (paired-pulse facilitation), STDP (spike-time dependent plasticity) and learning-relearning behaviors. In addition, an extensive study on the less explored Ebbinghaus forgetting curve has been carried out. The supramolecular nanofibres being light sensitive, the potential of the device as a visual system is demonstrated using a 3 × 3 pixel array.

Fabrication of High-Performance Visible-Blind Ultraviolet Photodetectors Using Electro-ionic Conducting Supramolecular Nanofibers

The detection of ultraviolet (UV) light is vital for various applications, such as chemical-biological analysis, communications, astronomical studies, and also for its adverse effects on human health. Organic UV photodetectors are gaining much attention in this scenario because they possess properties such as high spectral selectivity and mechanical flexibility. However, the achieved performance parameters are much more inferior than the inorganic counterparts because of the lower mobility of charge carriers in organic systems. Here, we report the fabrication of a high-performance visible-blind UV photodetector, using 1D supramolecular nanofibers. The nanofibers are visibly inactive and exhibit highly responsive behavior mainly for UV wavelengths (275-375 nm), the highest response being at ∼275 nm. The fabricated photodetectors demonstrate desired features, such as high responsivity and detectivity, high selectivity, low power consumption, and good mechanical flexibility, because of their unique electro-ionic behavior and 1D structure. The device performance is shown to be improved by several orders through the tweaking of both electronic and ionic conduction pathways while optimizing the electrode material, external humidity, applied voltage bias, and by introducing additional ions. We have achieved optimum responsivity and detectivity values of around 6265 A W^-1 and 1.54 × 10¹⁴ Jones, respectively, which stand out compared with the previous organic UV photodetector reports. The present nanofiber system has great potential for integration in future generations of electronic gadgets.

Synthesis and Emergent Photophysical Properties of Diketopyrrolopyrrole-Based Supramolecular Self-Assembly

Diketopyrrolopyrrole (DPP)-based molecular semiconductors exhibit intriguing optical and charge transport properties. Herein, we rationally design a series of electronically identical but structurally distinct Hamilton receptor (HR)-based supramolecular assembly of DPP. The HR endows supramolecular assemblies via hydrogen bonding with enhanced structural ordering and excitonic couplings. The mechanism of supramolecular self-assembly was probed by diffusion ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) and solid-state IR spectroscopy studies. We investigated the morphology of self-assembly, photophysical and electrochemical properties and compared them with the identical DPP molecular structures without HRs. The microstructure of self-assembly was probed with atomic force microscopy in thin films. Subsequently, the influence of solid-state packing was studied by single-crystal X-ray diffraction. The single-crystal structure of HR-TDPP-C₂₀ reveals slipped stack arrangements between the two neighboring chromophores with π-π stacking distance and slip angle of 3.55 Å and 35.4°, respectively. Notably, the slight torsional angle of 1° between thiophene and lactam rings and small π-π stacking distance suggest a significant intermolecular coupling between thiophene (D) and lactam (A) rings. This intramolecular coupling between two π-π chromophore stacks manifests in their optical properties. In this manuscript, we report rational design and synthesis of supramolecular self-assembly of DPP with a collection of compelling structural and optical properties.

Immunoassay based on Au-Ag bimetallic nanoclusters for colorimetric/fluorescent double biosensing of dicofol

The high toxicity of dicofol (DICO) to nontarget organisms has resulted in the contamination of food materials and caused a threat to human health. Developing a rapid and sensitive detection method of DICO in food samples is essential and still pursued. Fluorescent nanomaterials have been widely applied in biosensors to improve the sensitivity of detection. Herein, glutathione-capped Au-Ag bimetallic nanoclusters (Au-Ag NCs) exhibited the outstanding fluorescence characteristic with the average fluorescence lifetime of 1971.08 ns and photoluminescence quantum yield of 9.84% when the molar ratio of Au to Ag was 5:1. Polyethyleneimine modified gold nanoparticles (PEI-Au NPs) with the positive charge were prepared to generate a strong colorimetric signal. A dual-model colorimetric/fluorescent immune probe based on the Au-Ag NCs and PEI-Au NPs was successfully constructed by electrostatic force, and could be applied in both ic-ELISA and LFIA methods for rapid and ultrasensitive detection of DICO. In the ic-ELISA method, the introduction of fluorescence signal significantly increased the sensitivity of detection with the limit of detection (LOD) of 0.62 ng/mL and exhibited an excellent linear relationship within the range of 1.36 ng/mL-19.92 ng/mL. In the LFIA method, the fluorescence signal of Au-Ag NCs was accumulated on the test line and control line for the fluorescence model detection with a quantitative LOD at the level of 1.59 ng/mL. Such a dual-model colorimetric/fluorescent immunoassay serves as a promising candidate to develop new approaches in field detection.

Elucidating the Supramolecular Copolymerization of N- and C-Centered Benzene-1,3,5-Tricarboxamides: The Role of Parallel and Antiparallel Packing of Amide Groups in the Copolymer Microstructure

An in‐depth study of the supramolecular copolymerization behavior of N‐ and C‐centered benzene‐1,3,5‐tricarboxamides (N‐ and C‐BTAs) has been conducted in methylcyclohexane and in the solid state. The connectivity of the amide groups in the BTAs differs, and mixing N‐ and C‐BTAs results in supramolecular copolymers with a blocky microstructure in solution. The blocky microstructure results from the formation of weaker and less organized, antiparallel hydrogen bonds between N‐ and C‐BTAs. In methylcyclohexane, the helical threefold hydrogen‐bonding network present in C‐ and N‐BTAs is retained in the mixtures. In the solid state, in contrast, the hydrogen bonds of pure BTAs as well as their mixtures organize in a sheet‐like pattern, and in the mixtures long‐range order is lost. Drop‐casting to kinetically trap the solution microstructures shows that C‐BTAs retain the helical hydrogen bonds, but N‐BTAs immediately adopt the sheet‐like pattern, a direct consequence of the lower stabilization energy of the helical hydrogen bonds. In the copolymers, the stability of the helical aggregates depends on the copolymer composition, and helical aggregates are only preserved when a high amount of C‐BTAs is present. The method outlined here is generally applicable to elucidate the copolymerization behavior of supramolecular monomers both in solution as well as in the solid state.

Tuning Light-Driven Motion and Bending in Macroscale-Flexible Molecular Crystals Based on a Cocrystal Approach

Flexible molecular crystals with stimuli-responsive properties are highly desirable; however, uncovering them is still a challenging goal. Herein, we report a cocrystal approach to obtain elastic molecular crystals that exhibit light-induced fluorescence changes and dynamic mechanical responses at the macroscale level. Cocrystals of naphthylvinylpyridine and tetrafluoroterephthalic acid were fabricated in different stoichiometry ratios (2:1 and 1:1), which present different shapes [two-dimensional (2D) and one-dimensional (1D) morphologies], photoemission, and mechanical properties (rigidity and flexibility). Moreover, obviously different photomechanical energy conversions (light-driven cracking/popping and bending/motion) occur for the 2D and 1D cocrystals, respectively. Nuclear magnetic resonance (NMR) spectra show the occurrence of photoinduced [2 + 2] cycloaddition in both cocrystals, which is the primary mechanism for their photoactuating behaviors. Crystal structure analysis and theoretical calculation further reveal that protonation and the hydrogen-bonding network play important roles in light-stimulus-bendable 1D cocrystal. Thus, the transformation from rigidity to flexibility based on cocrystallization with different stoichiometry may offer an effective means to tune the dynamic light-driven responses for smart crystalline materials.

Strategy for Constructing Shape-Memory Dynamic Networks through Charge-Transfer Interactions

Recently, charge transfer (CT) interactions have received attention for the fabrication of supramolecular architectures due to their inherent compatibilities, directional nature and solvent tolerance. In this study, we report a shape-memory dynamic network constructed by the CT interaction between π-electron-rich naphthalene embedded in poly(ethylene glycol) (PEG-Np) and π-electron-poor six-arm methyl-viologen-ended poly(ethylene glycol) (⁶PEG-MV), which was verified by ultraviolet-visible spectroscopy (UV-vis), fluorescence spectra and swelling tests. Interestingly, the mechanical properties of this CT complex were dramatically enhanced compared with the control without CT interaction. Moreover, the excellent shape-memory effect (SME) was realized due to the good crystallization of the PEG segment and stable netpoints based on the CT interaction. In addition, as we expected, this supramolecular polymer network is self-healable and reprocessable.

Squaramide-Based Supramolecular Materials for Three-Dimensional Cell Culture of Human Induced Pluripotent Stem Cells and Their Derivatives

Synthetic hydrogel materials can recapitulate the natural cell microenvironment; however, it is equally necessary that the gels maintain cell viability and phenotype while permitting reisolation without stress, especially for use in the stem cell field. Here, we describe a family of synthetically accessible, squaramide-based tripodal supramolecular monomers consisting of a flexible tris(2-aminoethyl)amine (TREN) core that self-assemble into supramolecular polymers and eventually into self-recovering hydrogels. Spectroscopic measurements revealed that monomer aggregation is mainly driven by a combination of hydrogen bonding and hydrophobicity. The self-recovering hydrogels were used to encapsulate NIH 3T3 fibroblasts as well as human-induced pluripotent stem cells (hiPSCs) and their derivatives in 3D. The materials reported here proved cytocompatible for these cell types with maintenance of hiPSCs in their undifferentiated state essential for their subsequent expansion or differentiation into a given cell type and potential for facile release by dilution due to their supramolecular nature.

Unprecedented Charge-Transfer Complex of Fused Diporphyrin as Near-Infrared Absorption-Induced High-Aspect-Ratio Nanorods

Charge-transfer (CT) complexes of near-infrared absorbing systems have been unknown until now. Consequently, structural similarities between donor and acceptor are rather important to achieve this phenomenon. Herein, we report electron donors such as non-fused diporphyrin-anthracene (DP), zinc diporphyrin-anthracene (ZnDP) and fused zinc diporphyrin-anthracene (FZnDP) in which FZnDP absorbs in NIR region and permits a CT complex with the electron acceptor, perylene diimide (PDI) in CHCl₃ exclusively. UV/Vis-NIR absorption, ¹ H NMR, NOESY and powder X-ray diffraction analysis demonstrated that the CT complex formation occurs by π-π stacking between perylene units in FZnDP and PDI upon mixing together in a 1:1 molar concentration in CHCl₃ , unlike non-fused ZnDP and DP. TEM and AFM images revealed that the CT complex initially forms nanospheres leading to nanorods by diffusion of CH₃ OH vapors into the CHCl₃ solution of FZnDP/PDI (1:1 molar ratio). Therefore, these CT nanorods could lead to significant advances in optical, biological and ferroelectric applications.

Carbon-Nanotube-Mediated Electrochemical Transition in a Redox-Active Supramolecular Hydrogel Derived from Viologen and an l-Alanine-Based Amphiphile

A two-component hydrogelator (16-A)2 -V(2+) , comprising an l-alanine-based amphiphile (16-A) and a redox-active viologen based partner (V(2+) ), is reported. The formation the hydrogel depended, not only on the acid-to-amine stoichiometric ratio, but on the choice of the l-amino acid group and also on the hydrocarbon chain length of the amphiphilic component. The redox responsive property and the electrochemical behavior of this two-component system were further examined by step-wise chemical and electrochemical reduction of the viologen nucleus (V(2+) /V(+) and V(+) /V(0) ). The half-wave reduction potentials (E1/2 ) associated with the viologen ring shifted to more negative values with increasing amine component. This indicates that higher extent of salt formation hinders reduction of the viologen moiety. Interestingly, the incorporation of single-walled carbon nanotubes in the electrochemically irreversible hydrogel (16-A)2 -V(2+) transformed it into a quasi-reversible electrochemical system.

UV gelation of single-component polyacrylates bearing dinitrobenzoate side groups

Polyacrylates bearing dinitrobenzoate side groups undergo sol–gel–sol transformations in DMF or THF solutions regulated by alternating UV light and dark conditions.

One-pot one-cluster synthesis of fluorescent and bio-compatible Ag14 nanoclusters for cancer cell imaging

Small-molecule-protected silver nanoclusters have smaller hydrodynamic diameter, and thus may hold greater potential in biomedicine application compared with the same core-sized, macromolecule (i.e. DNA)-protected silver nanoclusters. However, the live cell imaging labeled by small-molecule-protected silver nanoclusters has not been reported until now, and the synthesis and atom-precise characterization of silver nanoclusters have been challenging for a long time. We develop a one-pot one-cluster synthesis method to prepare silver nanoclusters capped with GSH which is bio-compatible. The as-prepared silver nanoclusters are identified to be Ag14(SG)11 (abbreviated as Ag14, SG: glutathione) by isotope-resolvable ESI-MS. The structure is probed by 1D NMR spectroscopy together with 2D COSY and HSQC. This cluster species is fluorescent and the fluorescence quantum yield is solvent-dependent. Very importantly, Ag14 was successfully applied to label lung cancer cells (A549) for imaging, and this work represents the first attempt to image live cells with small-molecule-protected silver nanoclusters. Furthermore, it is revealed that the Ag14 nanoclusters exhibit lower cytotoxicity compared with some other silver species (including silver salt, silver complex and large silver nanoparticles), and the explanation is also provided. The comparison of silver nanoclusters to state-of-the-art labeling materials in terms of cytotoxicity and photobleaching lifetime is also conducted.

Autoresolution of Segregated and Mixed p-n Stacks by Stereoselective Supramolecular Polymerization in Solution

A "chirality driven self-sorting" strategy is introduced for the controlled supramolecular organization of donor (D) and acceptor (A) molecules in multicomponent assemblies. The trans-1,2-bis(amido)cyclohexane (trans-BAC) has been identified as a supramolecular motif with strong homochiral recognition to direct this chirality controlled assembly process of enantiomers in solution. Stereoselective supramolecular polymerization of trans-BAC appended naphthalene diimide monomers (NDIs) has been probed in detail by spectroscopic and mechanistic investigations. This chirality-driven self-sorting design of enantiomeric components also offers to realize mixed and segregated D-A stacks by supramolecular co-assembly of the NDI acceptors with trans-BAC appended dialkoxynaphthalene (DAN) donor monomers. Such an unprecedented chirality control on D-A organization paves the way for the creation of supramolecular p-n nanostructures with controlled molecular-level organization.

Supramolecular clippers for controlling photophysical processes through preorganized chromophores

A novel supramolecular clipping design for influencing the photophysical properties of functional molecular assemblies, by the preorganization (clipping) of chromophores, is described. Several chromophores end functionalized with molecular recognition units were designed. These molecular recognition units serve as handles to appropriately position these systems upon noncovalent interactions with multivalent guest molecules (supramolecular clippers). Towards this goal, we have synthesized 1,5-dialkoxynaphthalene (DAN) and naphthalenediimide (NDI) functionalized with dipicolylethylenediamine (DPA) motifs. These molecules could preorganize upon noncovalent clipping with adenosine di- or triphosphates, which resulted in preassociated excimers and mixed (cofacial) charge-transfer (CT) assemblies. Chiral guest binding could also induce supramolecular chirality, not only into the individual chromophoric assembly but also into the heteromeric CT organization, as seen from the strong circular dichroism (CD) signal of the CT transition. The unique ability of this design to influence the intermolecular interactions by changing the binding strength of the clippers furthermore makes it very attractive for controlling the bimolecular photophysical processes.

Dual responsive supramolecular amphiphiles: guest molecules dictate the architecture of pyridinium-tailored anthracene assemblies

By introducing an electron-deficient guest molecule and a counter anion, the assembly morphology of 1-[11-(2-anthracenylmethoxy)-11-oxoundecyl]pyridinium bromide (2-AP) was transformed to microsheets and nanofibers from microtubes, respectively.

Charge-transfer interactions for the fabrication of multifunctional viral nanoparticles

In this paper, a facile strategy to fabricate multifunctional viral nanoparticles was described by introducing charge-transfer interactions between a pyrenyl motif with dinitrophenyl and pyridinium-contained guest molecules.

Luminescent noble metal nanoclusters as an emerging optical probe for sensor development

In the past few years, highly luminescent noble metal nanoclusters (e.g., Au and Ag NCs or Au/Ag NCs in short) have emerged as a class of promising optical probes for the construction of high-performance optical sensors because of their ultrasmall size (<2 nm), strong luminescence, good photostability, excellent biocompatibility, and unique metal-core@ligand-shell structure. In this Focus Review, we briefly summarize the common syntheses for water-soluble highly-luminescent thiolate- and protein-protected Au/Ag NCs and their interesting luminescence properties, highlight recent progress in their use as optical sensors with an emphasis on the mechanisms underlying their selectivity, and finally discuss approaches to improving their sensitivity. The scope of the works surveyed is confined to highly luminescent thiolate- and protein-protected Au/Ag NCs.