Nanocrystalline Precursors for the Co-Assembly of Crack-Free Metal Oxide Inverse Opals.

Inorganic microstructured materials are ubiquitous in nature. However, their formation in artificial self-assembly systems is challenging as it involves a complex interplay of competing forces during and after assembly. For example, colloidal assembly requires fine-tuning of factors such as the size and surface charge of the particles and electrolyte strength of the solvent to enable successful self-assembly and minimize crack formation. Co-assembly of templating colloidal particles together with a sol-gel matrix precursor material helps to release stresses that accumulate during drying and solidification, as previously shown for the formation of high-quality inverse opal (IO) films out of amorphous silica. Expanding this methodology to crystalline materials would result in microscale architectures with enhanced photonic, electronic, and catalytic properties. This work describes tailoring the crystallinity of metal oxide precursors that enable the formation of highly ordered, large-area (mm2 ) crack-free titania, zirconia, and alumina IO films. The same bioinspired approach can be applied to other crystalline materials as well as structures beyond IOs.

New Architectures for Designed Catalysts: Selective Oxidation using AgAu Nanoparticles on Colloid-Templated Silica.

A highly modular synthesis of designed catalysts with controlled bimetallic nanoparticle size and composition and a well-defined structural hierarchy is demonstrated. Exemplary catalysts-bimetallic dilute Ag-in-Au nanoparticles partially embedded in a porous SiO2 matrix (SiO2 -Agx Auy )-were synthesized by the decoration of polymeric colloids with the bimetallic nanoparticles followed by assembly into a colloidal crystal backfilled with the matrix precursor and subsequent removal of the polymeric template. This work reports that these new catalyst architectures are significantly better than nanoporous dilute AgAu alloy catalysts (nanoporous Ag3 Au97 ) while retaining a clear predictive relationship between their surface reactivity with that of single-crystal Au surfaces. This paves the way for broadening the range of new catalyst architectures required for translating the designed principles developed under controlled conditions to designed catalysts under operating conditions for highly selective coupling of alcohols to form esters. Excellent catalytic performance of the porous SiO2 -Agx Auy structure for selective oxidation of both methanol and ethanol to produce esters with high conversion efficiency, selectivity, and stability was demonstrated, illustrating the ability to translate design principles developed for support-free materials to the colloid-templated structures. The synthetic methodology reported is customizable for the design of a wide range of robust catalytic systems inspired by design principles derived from model studies. Fine control over the composition, morphology, size, distribution, and availability of the supported nanoparticles was demonstrated.

The Optical Janus Effect: Asymmetric Structural Color Reflection Materials.

Structurally colored materials are often used for their resistance to photobleaching and their complex viewing-direction-dependent optical properties. Frequently, absorption has been added to these types of materials in order to improve the color saturation by mitigating the effects of nonspecific scattering that is present in most samples due to imperfect manufacturing procedures. The combination of absorbing elements and structural coloration often yields emergent optical properties. Here, a new hybrid architecture is introduced that leads to an interesting, highly directional optical effect. By localizing absorption in a thin layer within a transparent, structurally colored multilayer material, an optical Janus effect is created, wherein the observed reflected color is different on one side of the sample than on the other. A systematic characterization of the optical properties of these structures as a function of their geometry and composition is performed. The experimental studies are coupled with a theoretical analysis that enables a precise, rational design of various optical Janus structures with highly controlled color, pattern, and fabrication approaches. These asymmetrically colored materials will open applications in art, architecture, semitransparent solar cells, and security features in anticounterfeiting materials.

Programming function into mechanical forms by directed assembly of silk bulk materials.

We report simple, water-based fabrication methods based on protein self-assembly to generate 3D silk fibroin bulk materials that can be easily hybridized with water-soluble molecules to obtain multiple solid formats with predesigned functions. Controlling self-assembly leads to robust, machinable formats that exhibit thermoplastic behavior consenting material reshaping at the nanoscale, microscale, and macroscale. We illustrate the versatility of the approach by realizing demonstrator devices where large silk monoliths can be generated, polished, and reshaped into functional mechanical components that can be nanopatterned, embed optical function, heated on demand in response to infrared light, or can visualize mechanical failure through colorimetric chemistries embedded in the assembled (bulk) protein matrix. Finally, we show an enzyme-loaded solid mechanical part, illustrating the ability to incorporate biological function within the bulk material with possible utility for sustained release in robust, programmably shapeable mechanical formats.

A colloidoscope of colloid-based porous materials and their uses.

Nature evolved a variety of hierarchical structures that produce sophisticated functions. Inspired by these natural materials, colloidal self-assembly provides a convenient way to produce structures from simple building blocks with a variety of complex functions beyond those found in nature. In particular, colloid-based porous materials (CBPM) can be made from a wide variety of materials. The internal structure of CBPM also has several key attributes, namely porosity on a sub-micrometer length scale, interconnectivity of these pores, and a controllable degree of order. The combination of structure and composition allow CBPM to attain properties important for modern applications such as photonic inks, colorimetric sensors, self-cleaning surfaces, water purification systems, or batteries. This review summarizes recent developments in the field of CBPM, including principles for their design, fabrication, and applications, with a particular focus on structural features and materials' properties that enable these applications. We begin with a short introduction to the wide variety of patterns that can be generated by colloidal self-assembly and templating processes. We then discuss different applications of such structures, focusing on optics, wetting, sensing, catalysis, and electrodes. Different fields of applications require different properties, yet the modularity of the assembly process of CBPM provides a high degree of tunability and tailorability in composition and structure. We examine the significance of properties such as structure, composition, and degree of order on the materials' functions and use, as well as trends in and future directions for the development of CBPM.

Self-Assembled Organic Nanocrystals with Strong Nonlinear Optical Response.

Facile molecular self-assembly affords a new family of organic nanocrystals that, unintuitively, exhibit a significant nonlinear optical response (second harmonic generation, SHG) despite the relatively small molecular dipole moment of the constituent molecules. The nanocrystals are self-assembled in aqueous media from simple monosubstituted perylenediimide (PDI) molecular building blocks. Control over the crystal dimensions can be achieved via modification of the assembly conditions. The combination of a simple fabrication process with the ability to generate soluble SHG nanocrystals with tunable sizes may open new avenues in the area of organic SHG materials.

Supramolecular polymers in aqueous medium: rational design based on directional hydrophobic interactions.

Self-assembly in aqueous medium is of primary importance and widely employs hydrophobic interactions. Yet, unlike directional hydrogen bonds, hydrophobic interactions lack directionality, making difficult rational self-assembly design. Directional hydrophobic motif would significantly enhance rational design in aqueous self-assembly, yet general approaches to such interactions are currently lacking. Here, we show that pairwise directional hydrophobic/π-stacking interactions can be designed using well-defined sterics and supramolecular multivalency. Our system utilizes a hexasubstituted benzene scaffold decorated with 3 (compound 1) or 6 (compound 2) amphiphilc perylene diimides. It imposes a pairwise self-assembly mode, leading to well-defined supramolecular polymers in aqueous medium. the assemblies were characterized using cryogenic electron microscopy, small-angle X-ray scattering, optical spectroscopy, and EPR. Supramolecular polymerization studies in the case of 2 revealed association constants in 10(8) M(-1) range, and significant enthalpic contribution to the polymerization free energy. The pairwise PDI motif enables exciton confinement and localized emission in the polymers based on 1 and 2's unique photonic behavior, untypical of the extended π-stacked systems. Directional pairwise hydrophobic interactions introduce a novel strategy for rational design of noncovalent assemblies in aqueous medium, and bring about a unique photofunction.

A recyclable supramolecular membrane for size-selective separation of nanoparticles.

Most practical materials are held together by covalent bonds, which are irreversible. Materials based on noncovalent interactions can undergo reversible self-assembly, which offers advantages in terms of fabrication, processing and recyclability, but the majority of noncovalent systems are too fragile to be competitive with covalent materials for practical applications, despite significant attempts to develop robust noncovalent arrays. Here, we report nanostructured supramolecular membranes prepared from fibrous assemblies in water. The membranes are robust due to strong hydrophobic interactions, allowing their application in the size-selective separation of both metal and semiconductor nanoparticles. A thin (12 µm) membrane is used for filtration (∼5 nm cutoff), and a thicker (45 µm) membrane allows for size-selective chromatography in the sub-5 nm domain. Unlike conventional membranes, our supramolecular membranes can be disassembled using organic solvent, cleaned, reassembled and reused multiple times.

Photofunctional self-assembled nanostructures formed by perylene diimide-gold nanoparticle hybrids.

We report on the synthesis of organic dye-metal nanoparticle hybrids from two thiol-derivatized perylenediimide (PDI) ligands and 1.5 nm gold nanoparticles. The hybrids form spherical nanostructures when cast from 40% methanol/chloroform solution and toluene. The spherical aggregates are in the size range 50-230 nm in 40% MeOH/CHCl(3) mixture and 100-400 nm in toluene solution, as evidenced by transmission electron microscopy (TEM). Scanning electron microscopy (SEM) measurements show that these spherical aggregates are vesicles with a hollow interior. The π-π interactions of the perylenediimides are the predominant driving force leading to the aggregation of the hybrids, whereby the sizes of the nanospheres can be regulated via the PDI linker moiety and solvent choice. Femtosecond transient absorption studies of the hybrids reveal complex photophysical behavior involving electron transfer from the gold nanoparticles to the PDI moieties. This study shows that the formation of well-defined hybrid nanostructures as well as tuning their sizes can be achieved through employing a combination of the capping ligand choice and regulating the solvophobic interactions between the ligands.

Supramolecular gel based on a perylene diimide dye: multiple stimuli responsiveness, robustness, and photofunction.

Design of an extensive supramolecular three-dimensional network that is both robust and adaptive represents a significant challenge. The molecular system PP2b based on a perylene diimide chromophore (PDI) decorated with polyethylene glycol groups self-assembles in aqueous media into extended supramolecular fibers that form a robust three-dimensional network resulting in gelation. The self-assembled systems were characterized by cryo-TEM, cryo-SEM, and rheological measurements. The gel possesses exceptional robustness and multiple stimuli-responsiveness. Reversible charging of PP2b allows for switching between the gel state and fluid solution that is accompanied by switching on and off the material's birefringence. Temperature triggered deswelling of the gel leads to the (reversible) expulsion of a large fraction of the aqueous solvent. The dual sensibility toward chemical reduction and temperature with a distinct and interrelated response to each of these stimuli is pertinent to applications in the area of adaptive functional materials. The gel also shows strong absorption of visible light and good exciton mobility (elucidated using femtosecond transient absorption), representing an advantageous light harvesting system.

Control over self-assembly through reversible charging of the aromatic building blocks in photofunctional supramolecular fibers.

Self-assembling systems, whose structure and function can be reversibly controlled in situ are of primary importance for creating multifunctional supramolecular arrays and mimicking the complexity of natural systems. Herein we report on photofunctional fibers self-assembled from perylene diimide cromophores, in which interactions between aromatic monomers can be attenuated through their reduction to anionic species that causes fiber fission. Oxidation with air restores the fibers. The sequence represents reversible supramolecular depolymerization-polymerization in situ and is accompanied by a reversible switching of photofunction.

Stable aromatic dianion in water.

Perylene diimide (PDI) bearing polyethylene glycol substituents at the imide positions was reduced in water with sodium dithionite to produce an aromatic dianion. The latter is stable for months in deoxygenated aqueous solutions, in contrast to all known aromatic dianions which readily react with water. Such stability is due to extensive electron delocalization and the aromatic character of the dianion, as evidenced by spectroscopic and theoretical studies. The dianion reacts with oxygen to restore the parent neutral compound, which can be reduced again in an inert atmosphere with sodium dithionite to give the dianion. Such reversible charging renders PDIs useful for controlled electron storage and release in aqueous media. Simple preparation of the dianion, reversible charging, high photoredox power, and stability in water can lead to development of new photofunctional and electron transfer systems in the aqueous phase.

Selective bromination of perylene diimides under mild conditions.

A novel method for the bromination of perylene diimides, PDI (1), under mild conditions is reported. Variation of the reaction conditions allows mono- and dibromination of PDIs to afford 2 and 3 (these can be separated through standard procedures) or exclusive dibromination to afford 3. Pure 1,7 regioisomers are obtained through repetitive crystallization. The structure of 1,7-3b was elucidated by a single-crystal X-ray analysis. The facility of the bromination reaction, which decreases in the order 1a > 1b > 1c, depends on PDI aggregation propensities. Monobrominated PDIs were utilized for the syntheses of novel unsymmetrical piperidinyl (4a and 4b) and trimethylsilylethynyl derivatives (5a and 5b). Computational studies (DFT) on imide substituent rotation in PDIs reveal that in the case of bulky groups there is a restricted rotation leading to isomers, in agreement with our experimental results. An aromatic core twist in PDIs bearing one and two bromine substituents was also investigated by DFT.

Palladium complexes of perylene diimides: strong fluorescence despite direct attachment of late transition metals to organic dyes.

We prepared the first sigma-bonded metal complexes of widely utilized organic dyes, perylene tetracarboxylic acid diimides (PDIs). These 1,7-dipalladium PDI complexes were synthesized by C-Br oxidative addition of 1,7-dibromo-N,N'-dicyclohexyl PDI (Br2PDI) to Pd(0) phosphine complexes bearing triphenylphosphine and bischelating 1,2-bis(diphenylphosphino)ethane (dppe). The structures of Pd-PDI complexes were elucidated by single-crystal X-ray analysis. Surprisingly, despite direct attachement of two late transition metal centers, Pd-PDI systems are highly fluorescent (Phi=0.65 and 0.22 for triphenylphosphine and dppe systems, respectively). This is rationalized in terms of weak electronic interactions between the metal centers and PDI pi-system, as revealed by TD-DFT calculations.

Formation of a carbon-carbon triple bond by coupling reactions in aqueous solution.

Formation of a carbon-carbon triple bond by coupling reactions usually takes place at high temperatures, in anhydrous media and anaerobic conditions. We describe the formation of a carbon-carbon triple bond at room temperature in an aqueous solution exposed to the atmosphere. Two ethylidyne ligands of a trimolybdenum cluster coupled spontaneously to form 2-butyne. This unexpected result demonstrates the plausibility of alkylidyne chain lengthening and metathesis processes under ambient, environmentally friendly conditions.