Single-Chain Folding of Diblock Copolymers Driven by Orthogonal H‑Donor and Acceptor Units

We report the precision single-chain folding of narrow dispersity diblock copolymers via pairwise orthogonal multiple hydrogen bonding motifs and single chain selected point folding. Well-defined linear polystyrene (PS) and poly­(<i>n</i>-butyl acrylate) (P<i>n</i>BA) carrying complementary recognition units have been synthesized via activators regenerated by electron transfer/atom transfer radical polymerization (ARGET ATRP) utilizing functional initiators yielding molecular weights of <i>M</i>_n,SEC = 10900 Da, <i><i>Đ</i> =</i> 1.09 and <i>M</i>_n,SEC = 3900 Da, <i><i>Đ</i> =</i> 1.10, respectively. The orthogonal hydrogen bonding recognition motifs were incorporated into the polymer chain ends of the respective building blocks (to yield an eight shaped single chain folded polymers). Diblock copolymer formation was achieved via the Cu­(I) catalyzed azide–alkyne cycloaddition (CuAAC) reaction, while the single-chain folding of the prepared linear diblock copolymer–at low concentrations–was driven by orthogonal multiple hydrogen bonds via three-point thymine–diaminopyridine and six-point cyanuric acid–Hamilton wedge self-association. The self-folding process was followed by proton nuclear magnetic resonance (¹H NMR) spectroscopy focused on the respective recognition pairs at low temperature. In addition, the single-chain folding of the diblock copolymer was analyzed by dynamic light scattering (DLS) and concentration dependent diffusion ordered NMR spectroscopy (DOSY) as well as atomic force microscopy (AFM), providing a limiting concentration for self-folding (in dichloromethane at ambient temperature) of close to 10 mg mL^–1

Light-Induced Step-Growth Polymerization of AB-Type Photo-Monomers at Ambient Temperature

We introduce two AB-type monomers able to undergo a facile catalyst-free photoinduced polycycloaddition of photocaged dienes, enabling rapid Diels–Alder ligations under UV-irradiation (λ_max = 350 nm) at ambient temperature, closely adhering to Carother’s equation established by a careful kinetic study (17800 g mol^–1 < <i>M</i>_w < 24700 g mol^–1). The resulting macromolecules were in-depth analyzed via size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. Additionally, SEC hyphenated to high resolution-electrospray ionization-mass spectrometry (HR-ESI-MS) enabled the careful mapping of the end group structure of the generated polymers. Furthermore, we demonstrate that both monomer systems can be readily copolymerized. The study thus demonstrates that Diels–Alder ligation resting upon photocaged dienes is a powerful tool for accessing step-growth polymers

Surface-Anchored Metal–Organic Frameworks as Versatile Resists for Gas-Assisted E‑Beam Lithography: Fabrication of Sub-10 Nanometer Structures

We demonstrate that surface-anchored metal–organic frameworks (SURMOFs) are extraordinary well-suited as resists for high-resolution focused electron beam induced processing (FEBIP) techniques. The combination of such powerful lithographic protocols with the huge versatility of MOF materials are investigated in respect to their potential in nanostructures fabrication. The applied FEBIP methods rely on the local decomposition of Fe­(CO)₅ and Co­(CO)₃NO as precursors, either by the direct impact of the focused electron beam (electron beam induced deposition, EBID) or through the interaction of the precursor molecules with preirradiated/activated SURMOF areas (electron beam induced surface activation, EBISA). We demonstrate the huge potential of the approach for two different types of MOFs (HKUST-1 and Zn-DPDCPP). Our “surface science” approach to FEBIP, yields well-defined deposits with each investigated precursor/SURMOF combination. Local Auger electron spectroscopy reveals clean iron deposits from Fe­(CO)₅; deposits from Co­(CO)₃NO contain cobalt, nitrogen, and oxygen. EBISA experiments were successful with Fe­(CO)₅. Remarkably EBISA with Co­(CO)₃NO does not result in deposit formation on both resists, making the process chemically selective. Most importantly we demonstrate the fabrication of “nested-L” test structures with Fe­(CO)₅ on HKUST-1 with extremely narrow line widths of partially less than 8 nm, due to reduced electron proximity effects within the MOF-based resists. Considering that the actual diameter of the electron beam was larger than 6 nm, we see a huge potential for significant reduction of the structure sizes. In addition, the role and high potential of loading and transport of the precursor molecules within the porous SURMOF materials is discussed

Nanoporous Designer Solids with Huge Lattice Constant Gradients: Multiheteroepitaxy of Metal–Organic Frameworks

We demonstrate the realization of hierarchically organized MOF (metal–organic framework) multilayer systems with pronounced differences in the size of the nanoscale pores. Unusually large values for the lattice constant mismatch at the MOF–MOF heterojunctions are made possible by a particular liquid-phase epitaxy process. The multiheteroepitaxy is demonstrated for the isoreticular SURMOF-2 series [Liu et al. Sci. Rep. 2012, 2, 921] by fabricating trilayer systems with lattice constants of 1.12, 1.34, and 1.55 nm. Despite these large (20%) lattice mismatches, highly crystalline, oriented multilayers were obtained. A thorough theoretical analysis of the MOF-on-MOF heterojunction structure and energetics allows us to identify the two main reasons for this unexpected tolerance of large lattice mismatch: the healing of vacancies with acetate groups and the low elastic constant of MOF materials

Post-Synthetic Modification of Metal–Organic Framework Thin Films Using Click Chemistry: The Importance of Strained C–C Triple Bonds

In this work, we demonstrate that strain-promoted azide–alkyne cycloaddition (SPAAC) yields virtually complete conversion in the context of the post-synthetic modification (PSM) of metal–organic frameworks (MOFs). We use surface-anchored MOF (SURMOF) thin films, [Zn₂(N₃-bdc)₂(dabco)], grown on modified Au substrates using liquid-phase epitaxy (LPE) as a model system to first show that, with standard click chemistry, presently, the most popular method for rendering additional functionality to MOFs via PSM, quantitative conversion yields, cannot be reached. In addition, it is virtually impossible to avoid contaminations of the product by the cytotoxic Cu^I ions used as a catalyst, a substantial problem for applications in life sciences. Both problems could be overcome by SPAAC, where a metal catalyst is not needed. After optimization of reaction conditions, conversion yields of nearly 100% could be achieved. The consequences of these results for various applications of PSM-modified SURMOFs in the fields of membranes, optical coatings, catalysis, selective gas separation, and chemical sensing are briefly discussed

A Mild and Efficient Approach to Functional Single-Chain Polymeric Nanoparticles via Photoinduced Diels–Alder Ligation

We present a new ambient temperature synthetic approach for the preparation of single-chain polymeric nanoparticles (SCNPs) under mild conditions using a UV-light-triggered Diels–Alder (DA) reaction for the intramolecular cross-linking of single polymer chains. Well-defined random copolymers with varying contents of styrene (S) and 4-chloromethylstyrene (CMS) were synthesized employing a nitroxide-mediated radical polymerization (NMP) initiator functionalized with a terminal alkyne moiety. Postpolymerization modification with 4-hydroxy-2,5-dimethylbenzophenone (DMBP) and an <i>N</i>-maleimide (Mal) derivative led to the functional linear precursor copolymers. The intramolecular cross-linking was performed by activating the DMBP groups via irradiation with UV light of 320 nm for 30 min in diluted solution (<i>c</i>_Polymer = 0.017 mg mL^–1). The ensuing DA reaction between the activated DMBP and the Mal groups resulted in well-defined single-chain polymeric nanoparticles. To control the size of the SCNPs, random copolymers with varying CMS contents (i.e., different functional group densities (FGD)) were employed for the single-chain collapse. Additionally, monotethered nanoparticles were prepared via the copper-catalyzed azide–alkyne cycloaddition between the alkyne bearing copolymer with the highest FGD and an azide-terminated poly­(ethylene glycol) (PEG) prior to UV-induced cross-linking. The formation of SCNPs was followed by size exclusion chromatography (SEC), nuclear magnetic resonance (NMR) spectroscopy, dynamic light scattering (DLS), and atomic force microscopy (AFM)

New Approaches for Bottom-Up Assembly of Tobacco Mosaic Virus-Derived Nucleoprotein Tubes on Defined Patterns on Silica- and Polymer-Based Substrates

The capability of some natural molecular building blocks to self-organize into defined supramolecular architectures is a versatile tool for nanotechnological applications. Their site-selective integration into a technical context, however, still poses a major challenge. RNA-directed self-assembly of tobacco mosaic virus-derived coat protein on immobilized RNA scaffolds presents a possibility to grow nucleoprotein nanotubes in place. Two new methods for their site-selective, bottom-up assembly are introduced. For this purpose, isothiocyanate alkoxysilane was used to activate oxidic surfaces for the covalent immobilization of DNA oligomers, which served as linkers for assembly-directing RNA. Patterned silanization of surfaces was achieved (1) on oxidic surfaces via dip-pen nanolithography and (2) on polymer surfaces (poly­(dimethylsiloxane)) via selective oxidization by UV-light irradiation in air. Atomic force microscopy and X-ray photoelectron spectroscopy were used to characterize the surfaces. It is shown for the first time that the combination of the mentioned structuring methods and the isothiocyanate-based chemistry is appropriate (1) for the site-selective immobilization of nucleic acids and, thus, (2) for the formation of viral nanoparticles by bottom-up self-assembly after adding the corresponding coat proteins

Electric Transport Properties of Surface-Anchored Metal–Organic Frameworks and the Effect of Ferrocene Loading

Understanding of the electric transport through surface-anchored metal–organic frameworks (SURMOFs) is important both from a fundamental perspective as well as with regards to possible future applications in electronic devices. To address this mostly unexplored subject, we integrated a series of representative SURMOF thin films, formed by copper nodes and trimesic acid and known as HKUST-1, in a mercury-drop-based tunneling junction. Although the transport properties of these SURMOFs are analogous to those of hybrid metal–organic molecular wires, manifested by a very low value of the tunneling decay constant (β ≈ 0.006 Å^–1), they are at the same time found to be consistent with a linear increase of resistance with film thickness. Upon loading of SURMOF pores with ferrocene (Fc), a noticeable increase in transport current was observed. A transport model and ab initio electronic structure calculations were used to reveal a hopping transport mechanism and to relate the changes upon Fc loading to those of the electronic and vibrational structures of the SURMOF films

Hierarchically Functionalized Magnetic Core/Multishell Particles and Their Postsynthetic Conversion to Polymer Capsules

The controlled synthesis of hierarchically functionalized core/multishell particles is highly desirable for applications in medicine, catalysis, and separation. Here, we describe the synthesis of hierarchically structured metal–organic framework multishells around magnetic core particles (magMOFs) <i>via</i> layer-by-layer (LbL) synthesis. The LbL deposition enables the design of multishell systems, where each MOF shell can be modified to install different functions. Here, we used this approach to create controlled release capsules, in which the inner shell serves as a reservoir and the outer shell serves as a membrane after postsynthetic conversion of the MOF structure to a polymer network. These capsules enable the controlled release of loaded dye molecules, depending on the surrounding media

Monolithic High Performance Surface Anchored Metal−Organic Framework Bragg Reflector for Optical Sensing

We report the fabrication of monolithic dielectric mirrors by stacking layers of metal–organic frameworks (MOFs) and indium tin oxide (ITO). Such Hybrid Photonic Band-Gap (PBG) Materials exhibit high optical quality (reflectivities of 80%) and are color tunable over the whole visible range. While the ITO deposition is accomplished by using a conventional sputter process, the highly porous MOF layers are deposited using liquid-phase epitaxy (LPE), therefore yielding crystalline, continuous, and monolithic HKUST-1 SURMOF thin films with high optical performance. We demonstrate the optical sensing capabilities of these monolithic and porous Bragg stacks by investigating the chemo-responsive optical properties (PBG shift and modulation of the intensity of the PBG maximum) upon the exposure to different organic solvents