Self-Assembly of Oligo(para-phenylenevinylene)s through Arene-Perfluoroarene Interactions: π Gels with Longitudinally Controlled Fiber Growth and Supramolecular Exciplex-Mediated Enhanced Emission

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.

The Key Role of Molecular Packing in Luminescence Property: From Adjacent Molecules to Molecular Aggregates

The luminescence materials act as the key components in many functional devices, as well as the detection and imaging systems, which can be permeated in each aspect of modern life, and attract more and more attention for the creative technology and applications. In addition to the diverse properties of organic luminogens, the multiple molecular packing at aggregated states frequently offers new and/or exciting performance. However, there still lacks comprehensive analysis of molecular packing in these organic materials, resulting in an increased gap between molecular design and practical applications. In this review, from the basic knowledge of organic compounds as single molecules, to the discernable property of excimer, charge transfer (CT) complex or self-assembly systems by adjacent molecules, and finally to the opto-electronic performance of molecular aggregates, the relevant factors to molecular packing and practical applications are discussed.

Pathway Control of π-Conjugated Supramolecular Polymers by Incorporating Donor-Acceptor Functionality

Controlling the nanoscale orientation of π-conjugated systems remains challenging due to the complexity of multiple energy landscapes involved in the supramolecular assembly process. In this study, we have developed an effective strategy for programming the pathways of π-conjugated supramolecular polymers, by incorporating both electron-rich methoxy- or methanthiol-benzene as donor unit and electron-poor cyano-vinylenes as acceptor units on the monomeric structure. It leads to the formation of parallel-stacked supramolecular polymers as the metastable species through homomeric donor/acceptor packing, which convert to slip-stacked supramolecular polymers as the thermodynamically stable species facilitated by heteromeric donor-acceptor packing. By further investigating the external seed-induced kinetic-to-thermodynamic transformation behaviors, our findings suggest that the donor-acceptor functionality on the seed structure is crucial for accelerating pathway conversion. This is achieved by eliminating the initial lag phase in the supramolecular polymerization process. Overall, this study provides valuable insights into designing molecular structures that control aggregation pathways of π-conjugated nanostructures.

Arene-Perfluoroarene Interactions Confer Enhanced Mechanical Properties to Synthetic Nanotubes

Supramolecular nanotubes prepared through macrocycle assembly offer unique properties that stem from their long-range order, structural predictability, and tunable microenvironments. However, assemblies that rely on weak non-covalent interactions often have limited aspect ratios and poor mechanical integrity, which diminish their utility. Here pentagonal imine-linked macrocycles are prepared by condensing a pyridine-containing diamine and either terephthalaldehyde or 2,3,5,6-tetrafluoroterephthalaldehyde. Atomic force microscopy and synchrotron in solvo X-ray diffraction demonstrate that protonation of the pyridine groups drives assembly into high-aspect ratio nanotube assemblies. A 1 : 1 mixture of each macrocycle yielded nanotubes with enhanced crystallinity upon protonation. UV-Vis and fluorescence spectroscopy indicate that nanotubes containing both arene and perfluoroarene subunits display spectroscopic signatures of arene–perfluoroarene interactions. Touch-spun polymeric fibers containing assembled nanotubes prepared from the perhydro- or perfluorinated macrocycles exhibited Young's moduli of 1.09 and 0.49 GPa, respectively. Fibers containing nanotube assemblies reinforced by arene–perfluoroarene interactions yielded a 93% increase in the Young's modulus over the perhydro derivative, up to 2.1 GPa. These findings demonstrate that tuning the chemical composition of the monomeric macrocycles can have profound effects on the mechanical strength of the resulting assemblies. More broadly, these results will inspire future studies into tuning orthogonal non-covalent interactions between macrocycles to yield nanotubes with emergent functions and technological potential.

Arene–perfluoroarene interactions resulted in enhanced crystallinity between analogous perhydro- and perfluoro macrocycles in a supramolecular nanotube assembly.

Improving the Overall Properties of Circularly Polarized Luminescent Materials Through Arene-Perfluoroarene Interactions

A major trade-off in the field of circularly polarized luminescence (CPL) of pure organic materials is that the large luminescence dissymmetry factor (g_lum ) usually gives rise to the suppression of luminescence efficiency (Φ_PL ). Here, a supramolecular self-assembled system, driven by arene-perfluoroarene (AP) interactions of chiral polycyclic aromatic hydrocarbons (PAHs) and octafluoronaphthalene (OFN), is reported to provide a solution to this problem. Two kinds of chiral PAHs based on pyrene and anthracene could co-assemble with OFN in hybrid solvents to form long-range-ordered AP assemblies. The detailed process of AP interaction driving self-assembly was verified by morphological measurements and fluorescence spectra. The AP assemblies exhibited chirality amplification not only in the excited state but also in the ground state. In addition, the AP assemblies showed an enhanced luminescence efficiency compared with the individual chiral PAHs due to the energy-barrier effect of OFN. The present strategy based on AP interactions could be applied to boost the development of highly efficient CPL-active materials.

Self-Assembled Extended π-Systems for Sensing and Security Applications

Molecules and materials derived from self-assembled extended π-systems have strong and reversible optical properties, which can be modulated with external stimuli such as temperature, mechanical stress, ions, the polarity of the medium, and so on. In many cases, absorption and emission responses of self-assembled supramolecular π-systems are manifested several times higher when compared with the individual molecular building blocks. These properties of molecular assemblies encourage scientists to have a deeper understanding of their design to explore them for suitable optoelectronic applications. Therefore, it is important to bring in highly responsive optical features in π-systems, for which it is necessary to modify their structures by varying the conjugation length and by introducing donor-acceptor functional groups. Using noncovalent forces, π-systems can be put together to form assemblies of different shapes and sizes with varied optical band gaps through controlling intermolecular electronic interactions. In addition, using directional forces, it is possible to bring anisotropy to the self-assembled nanostructures, facilitating efficient exciton migration, resulting in the modulation of optical and electron-transport properties. In this Account, we mainly summarize our findings with optically tunable self-assemblies of extended π-systems such as p-phenylenevinylenes (PVs), p-phenyleneethynylenes (PEs), and diketopyrrolopyrroles (DPPs) as different stimuli-responsive platforms to develop sensors and security materials. We start with how PV self-assemblies and their coassemblies with appropriate electron-deficient systems can be used for the sensing of analytes in contact mode or in the vapor phase. For example, whereas the PV having electron-deficient terminal groups has high sensitivity toward trinitrotoluene (TNT) in contact mode, the supercoiled fibers formed by the coassembly of self-sorted stacks of C₃-symmetrical PV and C₃-symmetrical electron-deficient perylene bisimide are capable of sensing vapors of nitrobenzene and o-toluidine. The power of different functional groups in combination with PVs has been further illustrated by attaching CO₂-sensitive tertiary amine moieties to a cyano-substituted PV, which allowed the bimodal detection of CO₂ using fluorescence and Raman spectroscopy. Interestingly, the functionalization of PVs with terminal amide groups and chiral alkoxy side chains provided a mechanochromic system that allows self-erasable imaging. Whereas PVs exhibit quenching of fluorescence in most cases during self-assembly, PE derivatives exhibit aggregation-induced emission. This property of PEs has been exploited for the development of stimuli-responsive security materials, especially for currency and documents. For instance, the blue fluorescence of a PE attached to hydrophilic oxyethylene side chains coated on a filter paper upon contact with water changes to cyan emission due to the change in the molecular packing. Interestingly, the molecular packing of a Bodipy-attached PE-based gelator allowed a stress-induced change in the emission behavior, resulting in strong near-infrared (NIR) emission upon the application of mechanical stress or gelation. Finally, the use of DPP-based π-systems for the development of NIR transparent optical filters that block UV-vis light and their security- and forensic-related applications are described. These selected examples of the π-system self-assemblies provide an idea of the current status and future opportunities for scientists interested in this field of self-assembly and soft materials research.

Perfluoroarene induces a pentapeptidic hydrotrope into a pH-tolerant hydrogel allowing naked eye sensing of Ca2+ ions

Self-assembly of a novel thermoresponsive, pyrene-appended oligopeptide sequence VPGKP (PyP) leads to the formation of spherical aggregates in water. The sizes of the globular aggregates of the peptide, PyP, strongly depend on the temperature of its suspension in water and decrease with the decrease in temperature showing a lower critical solution temperature (LCST) phenomenon. Furthermore, a pyrene-octafluoronaphthalene (OFN) 'pair' has been used as a supramolecular synthon to induce hydrogelation of PyP in the presence of an equimolar amount of OFN via complementary quadrupole-quadrupole interactions. The gel shows excellent pH tolerance and thixotropic behavior. Detailed studies suggest the existence of lamellar packing of the gelators in a right-handed helical fashion which yields globular aggregates. The globular aggregates are sticky in nature and form a gel via inter-globular interactions. Addition of Ca2+ ions reinforces the mechanical strength and also reduces the critical gelator concentration of the native gel through coordination with the free -COO- group of the gelator. Therefore, the present hydrogel system could further be used as a naked eye sensor of Ca2+ ions.

Synthesis and photophysical properties of water-soluble fluorinated poly(aryleneethynylene)s

Reported are the synthesis of water-soluble fluorinated conjugated polymers, and photophysical properties, and fluorescence quenching response to arylamines.

Bimodal detection of carbon dioxide using fluorescent molecular aggregates

Fluorescent aggregates of a cyano-substituted phenylenevinylene derivative (R-1) have been used as a bimodal probe for the easy and fast detection of CO₂.

Transforming a C 3-Symmetrical Liquid Crystal to a π-Gelator by Alkoxy Chain Variation

Rational understanding of the structural features involving different noncovalent interactions is necessary to design a liquid crystal (LC) or an organogelator. Herein, we report the effect of the number and positions of alkoxy chains on the self-assembly induced physical properties of a few π-conjugated molecules. For this purpose, we designed and synthesized three C ₃-symmetrical molecules based on oligo(p-phenylenevinylene), C ₃ OPV1-3. The self-assembly properties of these molecules are studied in the solid and solution states. All of the three molecules follow the isodesmic self-assembly pathway. Upon cooling from isotropic melt, C ₃ OPV1 having nine alkoxy chains (-OC₁₂H₂₅) formed a columnar phase with two-dimensional rectangular lattice and retained the LC phase even at room temperature. Interestingly, when one of the -OC₁₂H₂₅ groups from each of the end benzene rings is knocked out, the resultant molecule, C ₃ OPV2 lost the LC property, however, transformed as a gelator in toluene and n-decane. Surprisingly, when the -OC₁₂H₂₅ group from the middle position is removed, the resultant molecule C ₃ OPV3 failed to form either the LC or the gel phases.

Photophysical properties of a D–π-A Schiff base and its applications in the detection of metal ions

AIEE-active D–π-A Schiff-base H₂L is stimuli-sensitive and acts as a dual channel chemosensor for Cu²⁺ ions and naked-eye probes for Mn²⁺ ions.

Solvent-dependent gelation behaviour and liquid crystal properties of a bent-core dihydrazide derivative

Both liquid crystalline gel and crystalline gel are obtained by changing the solvent from non-polar to polar. Liquid crystalline gels exhibit better elastic property over crystalline gel from EtOH. And their corresponding xerogels show distinct mesophase behaviour.

Controlled self-organization of cyanostilbene: emission tuning and photo-responsiveness

Cyanostilbene modified with dimethylaniline (CMD) could self-assemble into vesicles and fibrous morphologies depending on the solvophobic properties. Furthermore, morphology of well-defined nanostructures could be changed with enhanced emission triggered by the photo-isomerization of cyanostilbene. The present system has potential for building luminescent color conversion materials.

4-Perfluoroalkylbutoxybenzene derivatives as liquid crystalline organogelators based on phase-selective gelators

4-Perfluoroalkylbutoxybenzene derivatives as a smart soft material show efficient and rapid phase-selective gelation ability from water at extremes of pH.

Topological and packing mode modification for solid-state emission enhancement of bis(perfluorostyryl)furan derivatives

An unsymmetrical bis(perfluorostyryl)furan was designed for enhancing solid-state emission property by multiplying the type and the quantity of the intermolecular contacts.

Halogen-bonding for visual chloride ion sensing: a case study using supramolecular poly(aryl ether) dendritic organogel systems

A convenient and straightforward method for the visual recognition of chloride ion has been established through a chloride-responsive dendritic organogel.

Signal enhancement of sensing nitroaromatics based on highly sensitive polymer dots

A new, rapid, sensitive, selective and portable fluorescence detection method for nitroaromatics based on polymer dots (Pdots) had been successfully developed not only in aqueous media but also in the solid state with test strips. The fluorescence quenching rates were proportional to the concentrations of 2,4,6-trinitrophenol (TNP) in the range of 0.2-20.0 μg mL(-1) and p-nitrophenol (PNP) in the range of 0.05-6.0 μg mL(-1), when Pdots were used as ratiometric fluorescent sensors in aqueous solution. The 3σ limit of detection of PNP reached 18.8 ng mL(-1). Compared with polymer-based detection for nitroaromatics in the organic phase, the signal enhancement effect was initially found when Pdots were used to detect nitroaromatics in the aqueous phase. The mechanism of the interaction between Pdots and nitroaromatics was revealed as an electron transfer phenomenon from the electron-rich chromophoric probe to the electron deficient nitroaromatics. The results indicated that Pdots-based detection was particularly suitable for on-site qualitative detection and quantitative analysis of nitroaromatics.

Methyl Cinnamate-Derived Fluorescent Rigid Organogels Based on Cooperative π-π Stacking and C═O···π Interactions Instead of H-Bonding and Alkyl Chains

A new class of rigid low-molecular-mass organic gelators (LMOGs) was synthesized by McMurry and Heck reactions, and their gels and photophysical properties were investigated. The LMOGs lacked alkyl chain and H-bonding units and produced good gelation ability in selected mixed organic solvents facilitated by cooperative π-π stacking and C═O···π interactions. Sensitive gel-sol transformation by molecular aggregation and disaggregation was easily achieved upon heating and cooling. H-H 2D NOESY and X-ray diffraction (XRD) patterns showed the π-π stacking and C═O···π interactions between tiny methyl acrylate groups as "tails". Importantly, this soft interaction model offers a useful tool for the future design and construction of supramolecular structures. At present, the LMOGs reported herein offer a sensitive gel-formation ability and aggregation-induced emission (AIE) property and thus have promising application potentials as functional soft matter in amorphous materials, photoelectric materials, and so on.

Redox-switchable copper(I) metallogel: a metal-organic material for selective and naked-eye sensing of picric acid

Thiourea (TU), a commercially available laboratory chemical, has been discovered to introduce metallogelation when reacted with copper(II) chloride in aqueous medium. The chemistry involves the reduction of Cu(II) to Cu(I) with concomitant oxidation of thiourea to dithiobisformamidinium dichloride. The gel formation is triggered through metal-ligand complexation, i.e., Cu(I)-TU coordination and extensive hydrogen bonding interactions involving thiourea, the disulfide product, water, and chloride ions. Entangled network morphology of the gel selectively develops in water, maybe for its superior hydrogen-bonding ability, as accounted from Kamlet-Taft solvent parameters. Complete and systematic chemical analyses demonstrate the importance of both Cu(I) and chloride ions as the key ingredients in the metal-organic coordination gel framework. The gel is highly fluorescent. Again, exclusive presence of Cu(I) metal centers in the gel structure makes the gel redox-responsive and therefore it shows reversible gel-sol phase transition. However, the reversibility does not cause any morphological change in the gel phase. The gel practically exhibits its multiresponsive nature and therefore the influences of different probable interfering parameters (pH, selective metal ions and anions, selective complexing agents, etc.) have been studied mechanistically and the results might be promising for different applications. Finally, the gel material shows a highly selective visual response to a commonly used nitroexplosive, picric acid among a set of 19 congeners and the preferred selectivity has been mechanistically interpreted with density functional theory-based calculations.

The Epitaxial Growth of Self-Assembled Ternaphthalene Fibers on Muscovite Mica

The morphology and structure of 2,2':6',2″-ternaphthalene (NNN) deposited on muscovite mica(001) substrates was investigated by scanning force microscopy (SFM) and specular X-ray diffraction measurements. Consistently, both methods reveal the coexistence of needle-like structures with a {111} contact plane and {001} orientated island-like crystallites, which are built up by almost upright standing NNN molecules. Both orientations are characterized by a well-defined azimuthal alignment relative to the substrate surface, which is analyzed by X-ray diffraction pole figure (XRD-PF) measurements. Based on XRD-PF and SFM analysis, the azimuthal alignment of {001} orientated crystallites is explained by ledge-directed epitaxy along the fibers' sidewalls. These fibers are found to orient along two dominant directions, which is verified and explained by a doubling of the energetically preferred molecular adsorption site by mirror symmetry of the substrate surface. The experimental findings are confirmed by force-field simulations and are discussed based on a recently reported growth model.

Twisting and piezochromism of phenylene-ethynylenes with aromatic interactions between side chains and main chains

This paper describes a series of three-ring phenylene-ethynylenes (PEs) in which specific, non-covalent arene–arene interactions control conformation in the solid-state.

Exciplex formation and energy transfer in a self-assembled metal-organic hybrid system

Exciting assemblies: A metal-organic self-assembly of pyrenebutyric acid (PBA), 1,10-phenanthroline (o-phen), and Mg(II) shows solid-state fluorescence originating from a 1:1 PBA-o-phen exciplex. This exciplex fluorescence is sensitized by another residual PBA chromophore through an excited-state energy-transfer process. The solvent polarity modulates the self-assembly and the corresponding exciplex as well as the energy transfer, resulting in tunable emission of the hybrid (see figure).

π-Conjugated cyanostilbene derivatives: a unique self-assembly motif for molecular nanostructures with enhanced emission and transport

π-Conjugated organic molecules represent an attractive platform for the design and fabrication of a wide range of nano- and microstructures for use in organic optoelectronics. The desirable optical and electrical properties of π-conjugated molecules for these applications depend on their primary molecular structure and their intermolecular interactions such as molecular packing or ordering in the condensed states. Because of the difficulty in satisfying these rigorous structural requirements for photoluminescence and charge transport, the development of novel high-performance π-conjugated systems for nano-optoelectronics has remained a challenge. This Account describes our recent discovery of a novel class of self-assembling π-conjugated organic molecules with a built-in molecular elastic twist. These molecules consist of a cyano-substituted stilbenic π-conjugated backbone and various terminal functional groups, and they offer excellent optical, electrical, and self-assembly properties for use in various nano-optoelectronic devices. The characteristic "twist elasticity" behavior of these molecules occurs in response to molecular interactions. These large torsional or conformational changes in the cyanostilbene backbone play an important role in achieving favorable intermolecular interactions that lead to both high photoluminescence and good charge carrier mobility in self-assembled nanostructures. Conventional π-conjugated molecules in the solid state typically show concentration (aggregation) fluorescence quenching. Initially, we describe the unique photoluminescence properties, aggregation-induced enhanced emission (AIEE), of these new cyanostilbene derivatives that elegantly circumvent these problems. These elastic twist π-conjugated backbones serve as versatile scaffolds for the preparation of well-defined patterned nanosized architectures through facile self-assembly processes. We discuss in particular detail the preparation of 1D nanowire structures through programmed self-assembly. This Account describes the importance of utilizing AIEE effects to explore optical device applications, such as organic semiconducting lasers (OSLs), optical memory, and sensors. We demonstrate the rich electronic properties, including the electrical conductivity, field-effect carrier mobility, and electroluminescence of highly crystalline 1D nanowire and coaxial donor-acceptor nanocable structures composed of elastic twist π-conjugated molecules. The electronic properties were measured using various techniques, including current-voltage (I-V), conducting-probe atomic force microscopy (CP-AFM), and space-charge-limited-current (SCLC) measurements. We prepared and characterized several electronic device structures, including organic field-effect transistors (OFETs) and organic light-emitting field-effect transistors (OLETs).

Synthesis of fluorinated oxadiazoles with gelation and oxygen storage ability

A new family of fluorinated low molecular weight (LMW) gelators has been synthesized through SNAr substitution of 5-polyfluoroaryl-3-perfluoroheptyl-1,2,4-oxadiazoles with glycine ester. The obtained compounds give thermal and pH-sensitive hydrogels or thermo-reversible organogels in DMSO. Oxygen solubility studies showed the ability to maintain high oxygen levels in solution and in gel blend with plate counter agar (PCA).