Noncovalent Aqua Materials Based on Perylene Diimides

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Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

Noncovalent Aqua Materials Based on Perylene Diimides Elisha Krieg,*,†,§ Angelica Niazov-Elkan,‡ Erez Cohen,‡ Yael Tsarfati,‡ and Boris Rybtchinski*,‡ †

Leibniz-Institut für Polymerforschung Dresden e.V., 01069 Dresden, Germany Technische Universität Dresden, 01069 Dresden, Germany ‡ Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 7610001, Israel

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CONSPECTUS: Most robust functional organic materials are currently based on polymers. These materials exhibit high stability, but once formed they are difficult to modify, adapt to their environment, and recycle. Materials based on small molecules that are held together by noncovalent interactions can offer an alternative to conventional polymer materials for applications that require adaptive and stimuli-responsive features. However, it is challenging to engineer macroscopic noncovalent materials that are sufficiently robust for practical applications. This Account summarizes progress made by our group towards the development of noncovalent “aqua materials” based on well-defined organic molecules. These materials are uniquely assembled in aqueous media, where they harness the strength of hydrophobic and π−π interactions between large aromatic groups to achieve robustness. Despite their high stability, these supramolecular systems can dynamically respond to external stimuli. We discuss design principles, fundamental properties, and applications of two classes of aqua materials: (1) supramolecular gels and (2) nanocrystalline arrays. The materials were characterized by a combination of steady-state and time-resolved spectroscopic techniques, electrical measurements, molecular modeling, and high-resolution microscopic imaging, in particular cryogenic transmission electron microscopy (cryo-TEM) and cryogenic scanning electron microscopy (cryo-SEM). All investigated aqua materials are based on one key building block, perylene diimide (PDI). PDI exhibits remarkably stable intermolecular bonds, together with useful chemical and optoelectronic properties. PDI-based amphiphiles carrying poly(ethylene glycol) (PEG) were designed to form linear supramolecular polymers in aqueous media. These one-dimensional arrays of noncovalently linked molecules can entangle and form three-dimensional supramolecular networks, leading to soft gellike materials. Tuning the strength of interactions between fibers enables dynamic adjustment of viscoelastic properties and functional characteristics. Besides supramolecular gels, we show that simple PDI-based molecules can self-assemble in aqueous medium to form robust organic nanocrystals (ONCs). The mechanical and optoelectronic properties of ONCs are distinctly different from gel-phase materials. ONCs are excellent building blocks for macroscopic free-standing materials that can be used in dry state, unlike hydrogels. Being constructed from small molecules, ONC materials are easy to fabricate and recycle. High thermal robustness, good mechanical properties, and modular design render ONC materials versatile and suitable for a variety of applications. In the future, noncovalent aqua materials can become a sustainable alternative to conventional polymer materials. Examples from our research include stimuli-responsive and recyclable filtration membranes for preparative nanoparticle separation, water purification and catalysis, light-harvesting hydrogels for solar energy conversion, and nanocrystalline films for switchable surface coatings and electronic devices.

1. INTRODUCTION As the chemists’ toolbox to form and break covalent bonds has grown increasingly powerful over the last two centuries, so has our ability to produce high-performance materials for a wide range of applications. Due to the strength of covalent bonds, these materials are robust; however, they usually are not prone to reversible changes, making them difficult to process, recycle, and adapt to their environment. In contrast to covalent bonds, noncovalent interactions are reversible and sensitive to their environment, allowing for adaptability, stimuli-responsiveness, and facile assembly of complex arrays, unachievable via covalent chemistry.1−9 Is it © XXXX American Chemical Society

possible to use noncovalent interactions to build materials from well-defined small molecules, rather than classical polymers? How can one achieve thermal and mechanical robustness in noncovalent materials? And how does one fabricate and process such materials? The concept of adaptive and environmentally friendly supramolecular “aqua materials” (or “aqua plastics”), which contain water as a key component to impose very strong noncovalent interactions,10−14 has been initially promoted by Received: April 14, 2019

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DOI: 10.1021/acs.accounts.9b00188 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research Aida and co-workers.15 Robustness and adaptive properties emerge in such materials, as demonstrated in mechanically strong supramolecular hydrogels that are capable of selfhealing.15 Because of their noncovalent nature, aqua materials are quickly assembled from small molecules and disassembled on demand. A possible synergy of hydrophobic, van der Waals, and electrostatic interactions in aqueous media may lead to unique assembly modes and emergent robustness that may be on par with covalent materials. If sufficiently strong, aqua materials may represent a sustainable alternative to conventional polymer materials. We define an aqua material (aqua plastic) as a bulk material that is formed in an aqueous environment, driven by molecular self-assembly involving strong hydrophobic interactions. It may contain water as a major component or can be dried and used in a dry form. A macroscopic aqua material must be competitive with conventional polymer-based materials in terms of robustness and functional value. The added value of aqua materials stems from their easy fabrication, adaptability, and recyclability. Besides robustness, an additional challenge is related to rational design and synthesis of noncovalent arrays. It requires controlled manipulation of multiple noncovalent bonds,7,16,17 involving complex bonding modes, unlike the “one bond at a time” approach in covalent synthesis. Strong noncovalent interactions introduce further challenges. The generally practiced noncovalent self-assembly methodology employs a paradigm of rapidly equilibrating systems where the aggregates are thermodynamic products. In the regime of strong noncovalent interactions, however, more complex patterns involving (multiple) stable kinetic products may operate.8 All these challenges must be addressed in order to develop a molecule → nanostructure → bulk material synthetic paradigm, seeking efficient fabrication of robust yet adaptive bulk materials.17,18 Herein we describe our work on two quintessential types of aqua materials: (1) Strongly hydrated materials, swollen supramolecular hydrogels, in which water represents the major component. In these materials, the hydrophobic effect plays a key role for achieving robustness, complexity, and adaptability, as it does in biological systems.11 As a result, aqua materials of this type can be highly stable, yet they lose their advantageous properties when water is evaporated. A wide variety of supramolecular hydrogels have been reported, and some of these systems exhibit relatively high robustness.19−21 With respect to our systems, stability under high pressures is the most important characteristic enabling real life applications, proving competitiveness with polymeric materials. (2) Crystalline materials based on ONCs that are formed through hydrophobically driven self-assembly of π-conjugated molecules in aqueous solution. As opposed to supramolecular hydrogels, ONC-based materials are used in a dehydrated state. As crystals of π-conjugated molecules can be highly stable,22,23 materials of this type may operate at high temperatures, exhibit mechanical robustness, and yet enable easy fabrication and recycling.

assembled nanostructures in aqueous media.24−26The fused aromatic system of PDIs provides a flat and rigid surface that mediates strong π−π stacking and hydrophobic interactions. The latter involve a complex interplay of entropic and enthalpic contributions that stem from water−water and water−solute interactions, which are yet to be fully understood.8,11,13,27,28 Many PDI crystals are thermally robust (crystalline PDIs are employed as industrial pigments) and possess useful photonic and electronic properties. PDI-based supramolecular arrays possess advantageous photonic and electronic properties,24,29 and were employed as wires, sensors, and waveguides. We targeted a different set of applications, in particular, in size-selective membranes, which directly parallels applications of macroscopic polymeric materials. We employed PDI amphiphiles decorated with hydrophilic polyethylene glycol (PEG) chains to assemble gel-like materials, and simpler PDI molecules to create nanocrystalline materials. Both types of materials are uniquely assembled in aqueous media, resulting in noncovalent 3D connectivity. Yet, the amorphous versus crystalline nature of these materials leads to distinctly different bulk properties. Strong hydrophobic binding is key for the design of aqua materials. Hence, it is important to obtain quantitative insights into its thermodynamics. We addressed this aspect by investigating the relationship between molecular structure, solvent composition, and thermodynamic stability of PDI assemblies.27,28,30 To understand the effect of strongly hydrophobic perfluorinated groups on self-assembly thermodynamics, fluorinated compound 1-F (Figure 1) was studied in

Figure 1. Supramolecular polymerization aptitudes (bottom) of 1-F (top). Adapted with permission from ref 27. Copyright 2014 American Chemical Society.

comparison to a nonfluorinated analogue (1-H).27 Depending on the amount of organic cosolvent, 1-F undergoes cooperative or isodesmic aggregation. The switching between two polymerization mechanisms results from a change in polymer structure, as observed by cryo-TEM. 1-F showed exceptionally strong noncovalent binding, with the largest directly measured association constant of 1.7 × 109 M−1 in 75:25 water/THF mixture (v/v), two orders of magnitude higher than that of 1-H. Based on this trend, the association constant of 1-F in pure water was estimated to be at least on the order of 1015 M−1 (ΔG° ≈ 87 kJ/mol), one of the highest known binding constants for supramolecular polymers. The bonding strength showed a good linear correlation with the empirical solvent polarity parameter Et(30).31 The study

2. DESIGN AND FUNDAMENTAL PROPERTIES OF AQUA MATERIALS We employ derivatives of perylene diimide (PDI) as primary building blocks for aqua materials. PDIs are organic dyes that exhibit excellent stability against harsh chemical conditions and light. They provide diverse molecular constituents for selfB


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Accounts of Chemical Research

Figure 2. (left) Structure of 2. (right) Cryo-TEM images of 2-G in water (1 × 10−4 M, left) and 2-P in water (1 × 10−4 M, right). Insets: Photographs of water solutions of 2-G (left) and 2-P (right).

Figure 3. (a) Chemical structure of PP2b. (b) Scheme of the reversible switching between swollen gel, shrunken gel, and solution in response to temperature and redox reagents. (c−e) Cryo-SEM images and photographs (insets) of dilute solution (c), swollen gel (d), and shrunken gel (e).

highlights that hydrophobic interactions involved in aqueous supramolecular polymerization may in effect approach the strength of weak covalent bonds (ΔG° ≈ 150 kJ/mol). The difference in aggregation strength between 1-F and 1-H is an effect of the larger surface area of the fluorocarbon group, rather than a unique nature of fluorocarbon hydrophobicity. How far can we push the strength of combined hydrophobic and π-stacking interactions in supramolecular polymerization? Notably, amphiphiles carrying multiple PDI groups (see below) did not undergo disaggregation in water/THF mixtures containing up to 20% THF (by volume), even at very low (≤1 μM) concentration. This indicates that the strength of noncovalent interactions in these systems exceeds the one observed for 1-F, which has important implications for creating robust hydrogel materials.18,32 Strong and predictable noncovalent binding can be also achieved by harnessing the specificity of DNA hydrogen bonding. The thermodynamic driving force of Watson−Crick base pairs in DNA can be precisely controlled via the length and nucleobase composition of the DNA strands.33 Our group investigated PDI−DNA conjugates, where hydrophobic interactions, π−π stacking, and DNA base pairs work in concert, giving rise to a complex self-assembly behavior.30,34,35

Stacking of PDI groups resulted in formation of extended fibers, with association constants of up to 8.0 × 107 M−1 in 200 mM aqueous NaCl.34 In such systems DNA base pairing could be used as an orthogonal binding motif to control the bundling of fibers, yielding hierarchically ordered assemblies.30 We found that strong hydrophobic interactions lead to selfassembly processes that are pathway dependent, thus enabling access to kinetically trapped assembly intermediates.36 In covalent polymerization, a single monomer can give rise to different polymer structures due to positional, geometric, or stereoisomerism. We demonstrated that kinetically controlled self-assembly based on strong hydrophobic interactions leads to stable noncovalent polymer isomers (2-G and 2-P) that are based on the same covalent unit 2 (Figure 2).37 2-G solutions are green in color and comprise short supramolecular fibers. In contrast, 2-P forms purple solutions, comprising much longer fibers that tend to align. As a result of different π-stacking geometries, 2-G and 2-P exhibit significantly different electronic and photonic properties. Remarkably, both isomers remain stable even upon prolonged heating at 100 °C in water. Thus, pathway-dependent self-assembly of robust arrays enables the idea of supramolecular isomerism, enriching the methodology of noncovalent synthesis. C


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Figure 4. Supramolecular membrane. (a) Unmodified cellulose acetate support membrane. (b) PP2b supramolecular membrane. (c) High and (d) low magnification cryo-SEM images of the membrane cross-section. (e, f) Size-selective separation of (e) gold nanoparticles and (f) quantum dots. (g) Application as a membrane reactor for cross-flow biocatalytic reactions.

a partially hydrophobic surface and entangle efficiently. We discovered that a hierarchical assembly motif leads to such fibers32,41 (and sometimes crystalline systems42) that interact to give a uniform 3D network. To conclude the section devoted to design strategies, we would like to address the following question: how to construct 3D networks based on ONCs that are quite different from supramolecular polymers? Exploiting insights from our research on mechanisms of organic crystallization in aqueous media,43,44 we developed strategies to control morphology of ONCs by simple means, such as varying the nature and content of organic cosolvent.45,46 This methodology afforded fibrous and sheet-like PDI nanocrystals. Entanglement of these high-aspect-ratio ONCs creates cohesive macroscopic materials with unique properties (cf. section 4).

Our thermodynamic and kinetic studies suggest that due to strong hydrophobic interactions and π-stacking aqueous supramolecular polymers based on PDI can match covalent polymers with respect to their robustness. It is now feasible to rationally design such one-dimensional molecular arrays. For example, we have shown that directional hydrophobic interactions28 stemming from specifically designed covalent building blocks result in well-defined supramolecular polymers, and specific conditions applied to PDI bolaamphiphiles lead to tunable nanotubular motifs.38 To complete the transition from small molecules to a gelphase material, one-dimensional supramolecular polymers need to further interact and entangle (cf. section 3). The degree of entanglement critically determines the material properties. However, controlling entanglement of supramolecular fibers in solution is challenging. Many gels are metastable and tend to precipitate over time. Engineering gelphase materials is therefore usually based on empirical approaches.39 One important question is, how does one predict and control the emerging macroscopic material properties?18,40 Fibers with anisotropic structure may feature

3. FUNCTIONAL SUPRAMOLECULAR GELS 3.1. Robust Supramolecular Gels with Multiple Stimuli-Responsiveness

Supramolecular hydrogels represent an important class of soft materials. The porous 3D network of entangled fibers provides D


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Figure 5. Scheme of fabrication, use, and recycling of the supramolecular PP2b membranes.

intriguing thermoresponsive behavior is caused by dehydration of PEG at high temperature.47,48 The dehydrated PEG shell promotes stronger fiber−fiber interactions, as revealed by a more compact 3D network (Figure 3e). While strong hydrophobic interactions make it impossible to melt the gel at high temperature, a gel−sol phase transition is readily triggered by a reducing agent (such as sodium dithionite). The latter introduces a negative charge into the π-conjugated system, leading to rapid fiber fission. Oxidation of the disassembled PP2b radical anions in air restores the hydrophobic core and causes reassembly of the supramolecular fibers. Thus, despite the material’s high stability, specific stimuli can be used to switch its mechanical and optoelectronic properties (Figure 3b).

structural stability while allowing for perfusion of water and solute particles. These features make supramolecular hydrogels useful for a variety of applications in medicine, biotechnology, and solar energy conversion. Seeking highly stable supramolecular polymers, we synthesized PP2b, which comprises two coplanar PDI groups that are connected via a bis(ethynyl)bipyridyl linker (Figure 3a).32 The large π-conjugated core was expected to promote strong hydrophobic and π−π stacking interactions. Similar to its analog 2, self-assembly of PP2b in water/THF mixtures yields long, hierarchically organized supramolecular fibers with a segmented hydrophobic core and a hydrophilic PEG shell. However, unlike 2, PP2b supramolecular fibers are prone to further assemble into strong bundles, an effect that appears to be driven by incomplete shielding of its extended hydrophobic surface by its PEG shell.18 As a result of fiber bundling, PP2b forms an entangled 3D network, leading to gelation above a critical concentration of 1.7 wt % (Figure 3c−e). PP2b’s supramolecular fibers exhibit interesting optoelectronic properties that are pertinent to light-harvesting applications. The PDI chromophores strongly absorb light, while tight π−π stacking promotes exciton hopping. This property allows efficient capture and transport of excitation energy within the 3D supramolecular network. The assembled state of PP2b in aqueous media is robust. Unlike most supramolecular polymers, PP2b fibers do not disaggregate at high temperatures. Instead, close to 100 °C, the gel undergoes a rapid (and fully reversible) contraction. The

3.2. Adaptive Supramolecular Filtration Membranes

The combination of high robustness, porosity, and responsiveness toward multiple external stimuli led us to explore the use of PP2b-based gel as a membrane material.49,50 Filtration membranes play important roles in many industrial and laboratory-scale processes. High stability is crucial, as membranes need to withstand pressure-driven flux of fluid solution and solute particles. Stimuli-responsiveness, on the other hand, addresses the demand for membrane materials with switchable properties, enabling self-healing, recycling, or custom modification of retention properties.51,52 PP2b membranes were fabricated in a simple one-step procedure by filtration of assembled PP2b solutions through a cellulose acetate (CA) support (Figure 4a,b). The entangled E


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Figure 6. Hybrid supramolecular membrane. (a) Photographs of PP2b/CNT dispersions in chloroform after varying sonication times. The initial noncovalent CNT functionalization in organic solvent was crucial for subsequent fabrication of PP2b/CNT aqueous dispersions. (b) PP2b/CNT hybrid supramolecular membrane fabrication. Aqueous PP2b/CNT dispersion (1) is filtered at a transmembrane pressure of 0.4 bar (2) over a CA support (3), thus forming a supramolecular layer (4, 5). (c) Molecular arrangement within the aqueous dispersion. Cryo-TEM images reveal helical structures. The arrows indicate high contrast segments that are PP2b assemblies on the CNT scaffold, constructing a half-pitch of the helix. (d) Cryo-SEM cross-section image comprising the supramolecular membrane (marked in yellow borders) with a uniform 3−4 μm thickness on the CA support. Inset, magnified image of the hybrid layer’s cross-section. CNTs appear as white strings embedded within the porous PP2b network.

Figure 7. Hybrid PP2b/Nafion membranes. (a) Nafion chemical structure. (b) Cross-section cryo-SEM image of a PP2b supramolecular membrane deposited on a poly(ether sulfone) (PES) support without Nafion. (c) Cross-section cryo-SEM image of a hybrid PP2b/Nafion supramolecular membrane deposited on a PES support. (d−f) Energy dispersive X-ray spectroscopy (EDS) of the hybrid membrane cross section. Mapped areas containing cadmium (d), fluorine (e), and carbon (f). (g) Rhodamine 110 structure. (h) Solutions before and after filtration and (i) corresponding UV/vis spectra.

F


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Figure 8. (a) Illustration describing film fabrication from fibrous ONCs. (b) Molecular structure of compounds 3−5. (c) Cryo-TEM image of nanocrystals of 3 formed in acetone/water solution after aging at 20−21 °C for one month. Inset, fast Fourier transform (FFT)-filtered magnification of a marked region revealing crystalline fringes. (d) SEM images of a representative dry film, F3a, prepared from acetone/water solution (1:4, v/v) of 3 aged for 10 days. Inset, photograph of the free-standing film F3a. (e−g) SEM images of films prepared using sonication: F3b (e), F4 (f), F5 (g). Insets, photographs of free-standing films (diameters of 10 mm). (h) Illustration describing a film built from 2D ONCs nanosheets. (i) Molecular structure of 6. (j) SEM image of F6 (top view). Inset, photograph of F6. (k) SEM image of F6 (cross-section).

versatile and environmentally friendly alternative to conventional polymer materials.

3D network of PP2b (Figure 3c) becomes quantitatively retained on the support and forms a highly porous layer, a supramolecular membrane (Figure 4c,d). Such membranes are stable toward pressure-driven cross-flow of aqueous solutions for several hours. They were used for size-selective separations of metal and semiconductor nanoparticles (Figure 4e,f),50 as well as proteins,49 with cutoff sizes in the range of 5−8 nm. Moreover, when enzymes were retained in the supramolecular matrix, the membrane could be used as a flow reactor to carry out biocatalysis, with continuous conversion of substrate into product (Figure 4g).49 The membrane permeance (pressurenormalized flux) of 110 L h−1 m−2 bar−1 is comparable to commercial ultrafiltration membranes with similar rejection properties. Unlike covalently cross-linked systems, the membrane’s structural stability critically depends on the presence of water. Addition of ethanol (EtOH) or other organic solvents (e.g., THF, acetone) to the feed solution causes rapid disassembly of the membrane and simultaneous release of retained particles. The disassembled membrane material was easily purified, reassembled, and redeposited to fabricate new ultrafiltration membranes with reproducible performance (Figure 5). Easy fabrication, stimulus-triggered release, and recyclability of PP2b membranes demonstrate that aqua materials can be a

3.3. Carbon Nanotube-Reinforced Membranes

In a subsequent study, we aimed to further augment the performance of aqua materials by integration of carbon nanotubes (CNTs). Embedding CNTs into a hybrid material can confer unique properties to the material, among them electric conductivity and mechanical stability.53 Yet, the poor solubility of CNTs under mild conditions limits their range of applications. We have shown that a CNT-reinforced aqua material can be fabricated by using PP2b as a CNTsolubilizing agent, followed by aqueous self-assembly (Figure 6).54 PP2b’s aromatic core binds to CNTs via π−π stacking and hydrophobic interactions. Unlike covalent CNT functionalization, this noncovalent approach preserves the properties of CNTs, as it maintains the sp2-hybridized carbon framework. Cryo-TEM imaging of the PP2b/CNT aqueous dispersion revealed helical structures, created by PP2b assemblies twisting around the CNT scaffold. Fabrication of membranes was achieved by depositing PP2b/CNT aqueous dispersions (10−4 M PP2b; 5:1 PP2b/CNT (w/w), in 1:4 THF/H2O (v/v)) onto CA supports. Similar to CNT-free PP2b membranes, CNT-reinforced membranes were applicable to size-selective separation of nanoparticles. Due to their enhanced strength, 3−4 times thinner membranes (3−4 μm thickness) could be G


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Figure 9. (a) Molecular structure of PDI derivatives 4, 5, and 7. (b) Cryo-TEM images of 4/MWCNT (the arrows point to ONCs). Inset shows MWCNTs coiled around the ONC of 4. (c) Magnification of the area marked by yellow box, showing crystalline fringes. Inset, fast Fourier transform (FFT) of the ONCs. (d−g) SEM images of the 4/CNT hybrid films: 4/SWCNT, CCNT = 8 wt % (d). 4/SWCNT, CCNT = 40 wt % (e). 4/MWCNT, CCNT = 5 wt % (f). 4/MWCNT, CCNT = 67 wt % (g). (h) Representative I−V curve of 4/MWCNT films with CMWCNT = 5 wt % (blue) and CMWCNT = 67 wt % (red). (i) Electrical conductivity (σ) of 4/SWCNT (blue) and 4/MWCNT (red) versus CNT content.

In collaboration with BASF, we aimed to demonstrate that this aqua material could possess the robustness necessary to sustain the harsh conditions encountered during industrial filtrations, with special focus on water purification.56,57 Indeed, the hybrid membrane was capable of purifying water from organic molecules and heavy metals (lead, nickel, cobalt, and cadmium) in a wide range of concentrations. Heavy metals were retained in the upper Nafion layer (Figure 7d−f) almost quantitatively (98.96−99.98%). In addition to heavy metals, small organic molecules need to be removed from contaminated water and urban sewage.58−60 Filtration experiments with several dyes, small organic molecules, and pharmaceuticals showed quantitative removal (Figure 7g−i). Our results suggest that both charge and molecular size contribute to the retention of contaminants: the Nafion layer provides ion exchange properties, and the compacted PP2b layer retains less charged species, reflecting the synergetic function of both membrane components. Remarkably, PP2b/Nafion membranes withstand high pressures of up to 40 bar while sustaining their function.

fabricated. A comparison of cryo-SEM imaging of PP2b/CNT membranes versus control depositions reveals the importance of PP2b supramolecular fibers in providing a porous structural scaffold that prevents CNTs from forming a compact layer with irregular thickness. Overall this study provides a strategy for achieving easy fabrication of strong and adaptive CNTbased membranes. 3.4. Nafion Membrane

The combination of PP2b and colloidal Nafion (Figure 7) to create a robust hydrogel-like membrane was based on the idea that self-assembly and function of both systems may be synergetic.55 Colloidal Nafion particles (40−50 nm in size) should be retained by a PP2b membrane50 and may undergo self-assembly in water interacting via their highly hydrophobic fluorocarbon chains. Indeed, Nafion deposited on the supramolecular layer assembled into a highly porous structure. Its polyelectrolyte nature exerts significant osmotic pressures on the semipermeable gel-like PP2b layer, resulting in its compression (Figure 7b,c). This synergy is critical for stability and performance of the hybrid membrane, as described below. H


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Stable ONC/CNT dispersions enabled solution-processed fabrication of free-standing hybrid films with varying CNT content. Solid films were fabricated by depositing the aqueous ONC/CNT dispersions via filtration onto a poly(vinylidene fluoride) (PVDF) support. The deposit was detached from the support to yield a free-standing film (Figure 9d−g, insets). Scanning electron microscopy (SEM) images of 4/SWCNT and 4/MWCNT hybrid films with different concentrations of CNTs are shown in Figure 9d−g. The ONC/CNT hybrids showed ohmic behavior and high electrical conductivities (up to 1800 S/m)(Figure 9h,i), while ONCs without CNTs were significantly less conductive (e.g., σ(ONC 4) = 0.0002 S/m). The interconnected 3D CNT networks in the ONC/CNT hybrid films showed significant conductivity even at 3% CNT content (σ(ONC 4/MWCNT) = 5.8 S/m). ONC/CNT hybrids exhibit higher thermal stabilities than their polymer counterparts,68−70 while their electrical conductivity is comparable to conductivities observed in composites of conductive polymers with CNTs.71−73 The thermal stability of the hybrids stems from the thermal stability of PDI crystals, and the enhanced conductivity is due to a distinct morphology favoring high CNT connectivity (since excess dispersant crystallizes and does not cover CNTs), which leads to a low electrical percolation threshold. ONC/CNT films with low CNT concentrations contain simple hydrophobic organic dyes as a major component; hence these hybrids represent an advantageous new platform for conductive colorants based on common organic dyes as primary color-producing entities, enabling simple fabrication, thermal robustness, and high electrical conductivity. Finally, using a suitable organic solvent, PDI ONCs in the hybrids can be selectively dissolved and removed, yielding pristine freestanding CNT films (“buckypapers”). If sufficiently robust, materials that are built from small molecules can be suitable to create “molecular plastics” potentially cost-efficient, recyclable, and tunable materials that may compete with and/or complement polymer-based plastics. Addressing this goal, we fabricated binary composites of ONCs based on nitro PDI derivative (4, Figure 9), with graphene oxide (GO), bentonite nanoclay (NC), and hydroxyethyl cellulose (HEC) using solution-based deposition.67b These materials exhibited enhanced mechanical strength (they can be an order of magnitude stronger than the ONC materials described in section 4.1), tunable porosity, and recyclability, enabling utilization as separation membranes. Hybrids 4/HEC and 4/NC were used for ultrafiltration; 4/NC also efficiently removed heavy metals from water. Composite 4/GO (with (210% GO by weight) showed mechanical properties similar to covalent materials, and was capable of enzyme immobilization, supporting biocatalysis. These systems demonstrate the feasibility of modular molecular nanoplastics, a family of sustainable functional materials with tunable properties.

Moreover, they can be reversibly assembled and disassembled in order to treat possible fouling.61 The robustness, adaptive properties, and performance of the PP2b/Nafion membrane system demonstrate that aqua materials are suitable for demanding industrial applications.

4. MATERIALS BASED ON ORGANIC NANOCRYSTALS 4.1. Nanocrystalline Films

Unlike polymers, small molecules normally fail to establish an extended 3D network of strong van der Waals bonds required for fabricating free-standing materials. We envisaged that fibrous ONCs assembled from aromatic molecules, if sufficiently uniform, may entangle to enable 3D connectivity and strong van der Waals interactions akin to covalent polymers (Figure 8a).62 We have shown that simple monosubstituted PDIs assemble into various ONCs in aqueous solution.43,45,46 Compounds 3− 5 were chosen as primary building blocks (Figure 8b), since their self-assembly can be adjusted to form soluble elongated ONCs. Assembly of fibers from 3 and deposition by filtration yields a free-standing film F3a (Figure 8c,d). To expand our molecular toolbox and expedite fabrication, we developed a solution-based method for sonication-assisted rapid preparation of uniform ONCs that yield porous free-standing organic films F3b−F562 (Figure 8e−g). The scope of the ONC freestanding materials was further expanded by applying the sonication-assisted ONC growth method to 6, which forms 2D nanosheets with micrometric surface area, that were used to fabricate film F6 comprising stacks of nanosheets (Figure 8h− k). These nanoporous films can be easily fabricated, feature remarkable thermal stability, and can be recycled without significant loss of material. The films are free-standing but somewaht brittle; their Young’s moduli (100−150 MPa) are comparable to those of electrospun atactic polymers.63−65 The films cast from the ONCs of 3 and 4 display advantageous photonic properties, such as second harmonic generation (SGH), representing thermally robust ONC-based bulk nonlinear optical materials. All the films are emissive, which may be useful for sensing based on fluorescence quenching of the films by various analytes.66 The intrinsic porosity of the material based on 4 gave rise to the application of the ONC film as a recyclable ultrafiltration membrane with a cutoff of 60 nm. 4.2. ONC-based Hybrid Materials

ONCs can be integrated with a variety of (nano)materials via solution-based fabrication. This resulted in a modular construction of hybrid materials with enhanced properties.67 Thus, we found that simple mono-PDI derivatives 4, 5, and 7 (Figure 9a) were capable of efficiently exfoliating and dispersing single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs) in aqueous media.67a MWCNT contents of ∼3−67 wt % and SWCNT contents of ∼3−40 wt % were achieved. Partial charge transfer from CNTs to PDIs (observed by both experimental and theoretical studies) promotes exfoliation and interaction with the polar medium. The excess of the PDI dispersant crystallizes, affording ONC/CNT hybrid materials with homogeneous distribution of ONCs and CNTs. A high degree of CNT exfoliation and connectivity was revealed by imaging solution-phase structures in cryo-TEM (Figure 9b,c).

4.3. Crystalline Switching of Hydrophobicity

Small molecules give rise to weaker, less predictable interactions than block copolymers.74,75 Therefore, their utilization in the rational design of complex ordered polymer-like materials is challenging.8,76,77 In this respect, self-assembly in aqueous medium, imposing strong hydrophobic interactions between molecular moieties and enhanced self-sorting, can be advantageous.26,78 For this purpose, we designed a simplified molecular version of common triblock copolymers,79−81 PPF, in which two different hydrophobic I


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Figure 10. (a) Chemical structure of PPF. (b) Optical microscope image (under crossed-polarizers) of a self-assembled PPF film deposited on glass (cast from 2.5 × 10−3 M water/THF solution (7:3, v/v)). (c) X-ray diffraction patterns of the PPF films. Air-dried (black); after annealing at 60 °C for 10 min and subsequent cooling (red); amorphous film deposited from CHCl3 (blue); after annealing at 60 °C (green). (d) SAXS of PPF film (red), and the fit to a model of a lamellar structure (blue). (e) SEM image of a PPF film cross-section; yellow arrow indicates film axis. (f) Proposed molecular model of the PPF in the lamellar structure. (g) Side view of the lamellar structure based on the unit depicted in panel f. (h) Schematic illustration of the adaptive behavior explaining structural and wetting changes upon heating and cooling. Perfluoroalkyl chains are shown in green, PEG in gray, and PDI in red.

fit model, the long PEG chains are folded multiple times (Figure 10f,g).85,86 The interactions between PDI cores and between perfluoroalkyl chains are responsible for the formation of an ordered lamellar phase (Figure 10f,g). The wettability of freshly cast PPF films was typical of hydrophobic surfaces, as evidenced by a contact angle of 100°−105°. This observation was attributed to the enrichment of perfluoroalkyl chains at the surface,87 as supported by angle-resolved X-ray photoelectron spectroscopy (AR-XPS). When heated above 60 °C, the film changes from hydrophobic to hydrophilic, as indicated by the immediate spreading of water on its surface, consistent with loss of crystallinity at temperatures higher than 60 °C (Figure 10c). When heated up to 120 °C, the films regained hydrophobicity upon cooling. These findings suggest that PPF exhibits structural memory in a certain temperature range despite PEG melting. Irreversible transformation of the film to

molecular units are attached to the termini of a hydrophilic polymer (Figure 10a).82 The films exhibited long-range order following deposition from aqueous solution, as detected by optical microscopy under crossed polarizers, with the dimensions of ordered domains extending over hundreds of micrometers (Figure 10b). X-ray diffraction (XRD) revealed that the film is crystalline, exhibiting sharp peaks (Figure 10c) typical of crystalline PEG.83 Self-assembly in aqueous medium was essential for the formation of these ordered structures, since deposition from molecular solution in CHCl3 resulted in an amorphous film. Small-angle X-ray scattering (SAXS) study of self-assembled PPF films is consistent with the formation of a lamellar phase (Figure 10d),84 as confirmed by SEM (Figure 10e). Molecular modeling based on SAXS and XRD data reveals that in the best J


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Accounts of Chemical Research superhydrophilic can be achieved by successive heating (to 150 °C) and cooling (to room temperature) cycles (Figure 10h). Notably, polymeric PEG provides a switchable component, whereas the PDI and perfluoroalkyl chains create a stabilizing framework.88 PPF is thus capable of both reversible and irreversible transformations, inducing hydrophilic/hydrophobic switching. The long-range order, reversible crystallinity switching, and temperature-controlled wettability demonstrate the potential of simple polymer/small aromatic molecule conjugates.

Erez Cohen received his B.Sc. in Chemistry from Tel-Aviv University in 2010 and his M.Sc. (2013) and Ph.D. (2018) in Chemistry from the Weizmann Institute of Science. His research interests during his Ph.D. work in the Rybtchinski group included the rational design of π-conjugated functional materials self-assembled in aqueous media. At present, he is the Research Manager of the Energy Storage group at the Materials Department, University of Oxford. Yael Tsarfati graduated from Tel-Aviv university in 2011 with a B.Sc. degree in Chemistry, with a minor in life sciences. She then moved to the Weizmann Institute of Science and obtained her M.Sc. under the guidance of Prof. Boris Rybtchinski. She focused on the noncovalent solubilization of single-walled carbon nanotubes and their utilization for the fabrication of advanced materials. Her Ph.D. research in the Rybtchinski group has been devoted to mechanistic studies on the crystallization of organic molecules in aqueous solutions, and she recieved the Dov Elad Memorial Prize for excellence in PhD studies.

5. CONCLUSIONS AND OUTLOOK Macroscopic materials based on small molecules can be uniquely assembled in aqueous media. Supramolecular hydrogels and ONC materials based on PDI are robust and suitable for real-life applications, withstanding mechanical stresses and high temperatures. Facile fabrication and recycling (through reversible self-assembly and disassembly) makes noncovalent aqua materials a sustainable and cost-efficient alternative to classical polymer materials. These materials can be used in a hydrated form (gels) or in a dry state (ONC arrays), adding to their versatility. Importantly, aqua materials are readily functionalized with additional building blocks (e.g., CNTs, polymers, and active enzymes) via hydrophobic and π−π interactions or mechanical entrapment to further expand the structural and functional space. While our work has focused on PDI-based molecules, we believe that other π-conjugated molecules can be similarly utilized for engineering robust noncovalent aqua materials.

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Boris Rybtchinski received his B.Sc. in Chemistry from Kiev State University in Ukraine. He then moved to Israel where he earned his M.Sc. and Ph.D. at the Weizmann Institute of Science (with Prof. D. Milstein). He conducted postdoctoral research at the Northwestern University (with Prof. M. Wasielewski) and joined the Weizmann Institute as a faculty member in 2005. His research interests include supramolecular chemistry, sustainable materials, and crystallization mechanisms.

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ACKNOWLEDGMENTS We thank Israel Science Foundation, Minerva Foundation, Helen and Martin Kimmel Center for Molecular Design, USIsrael Binational Science Foundation, Israel Innovation Authority, and BASF for financial support. E.K. acknowledges support from the Open Topic Postdoc Program of TU Dresden. A.N.-E. is a recipient of an Adams Fellowship.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

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REFERENCES

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Boris Rybtchinski: 0000-0002-2071-8429 Notes

The authors declare no competing financial interest. Biographies Elisha Krieg studied Chemistry at the University of Cologne and at the Weizmann Institute of Science. He received his Ph.D. degree in Chemistry (Shimon Reich Memorial Prize) at the Weizmann Institute in 2013 under the supervision of Prof. Boris Rybtchinski. He then joined the lab of Prof. William M. Shih at Harvard University as a postdoctoral fellow. During his studies, he received fellowships from the Studienstiftung des Deutschen Volkes, the Minerva Foundation, and the Human Frontier Science Program. He is currently a research associate at the Technical University of Dresden and the Leibniz Institute for Polymer Research Dresden, where he is building a junior research group. He is interested in combining techniques of DNA nanotechnology, molecular biology, and material science to develop programmable nanomaterials that enable new applications in life science research and medical diagnostics. Angelica Niazov-Elkan received B.Sc. in Chemistry from Bar-Ilan University and M.Sc. from the Hebrew University in the field of biosensing and bioelectronics, with Prof. Itamar Wilner. She is currently pursuing her Ph.D. degree at the Weizmann Institute of Science in the Rybtchinski group. Her research is devoted to materials based on organic nanocrystals and their hybrids. K


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Noncovalent Aqua Materials Based on Perylene Diimides

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