Florian Auras

A photoconductive thienothiophene-based covalent organic framework showing charge transfer towards included fullerene

Organic bulk heterojunctions combining electron donor and acceptor phases are of great interest for designing organic photovoltaic devices. While impressive advances have been achieved with these systems, so far a deterministic control of their nanoscale morphology has been elusive. It would be a major breakthrough to be able to create model systems with periodic, interpenetrating networks of electron donor and acceptor phases providing maximum control over all structural and electronic features. Herein we report a significant step towards this goal on the basis of the recently discovered class of crystalline covalent organic frameworks (COFs) which are created by condensation of molecular building blocks. Specifically, the stacked layers of two-dimensional COFs permit charge migration through the framework, and several semiconducting structures with high carrier mobilities have been described. We have created a COF containing stacked thieno[2,3-b]thiophene-based building blocks serving as electron donors (TT-COF), with high surface area and a 3 nm open pore system. This open framework takes up the wellknown fullerene electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), thus forming a novel structurally ordered donor–acceptor network. Spectroscopic results demonstrate light-induced charge transfer from the photoconductive TT-COF donor network to the encapsulated PCBM phase in the pore system. Moreover, we have created the first working COF-based photovoltaic device with the above components. The organization of the molecular building blocks into a crystalline framework with defined conduction paths provides a promising model system for ordered and interpenetrated networks of donors and acceptors at the nanoscale. The most prominent hole-conducting material used in organic solar cells is poly(3-hexylthiophene) (P3HT), a thiophene-containing polymer with high charge-carrier mobilities. The soluble fullerene derivative PCBM is often used as an electron acceptor in organic photovoltaics. Because of the lack of structural order in the respective bulk heterojunctions it is very difficult to assess the impact of molecular building blocks, bonding motifs, and energy levels on the microscopic processes involving light-induced exciton formation, charge separation, and transport in such systems. Hence ordered charge-transporting networks with a periodicity of several nanometers are of great interest to understand the mechanistic details of the light-induced processes and ultimately to obtain design rules for the creation of efficient and stable organic photovoltaic devices. The new TT-COF was synthesized under solvothermal conditions by co-condensation of thieno[3,2-b]thiophene-2,5diyldiboronic acid (TTBA) and the polyol 2,3,6,7,10,11hexahydroxytriphenylene (HHTP; Figure 1a). Reaction parameters are described in the Supporting Information. As described in the following, the thienothiophene-based COF forms stacks in an AA arrangement, as confirmed by N2 sorption and powder X-ray diffraction. Powder X-ray diffraction (PXRD) confirms the formation of a highly crystalline COF. Identification of the new structure was conducted by comparison of structures modeled with MS Studio (see Figures S1–S5 in the Supporting Information). Corresponding powder patterns were simulated and compared to the experimentally obtained data. For previous COF structures different stacking types of the hexagonal planar sheets were reported. Hence calculations were carried out simulating an eclipsed AA arrangement and a staggered AB arrangement. The experimental PXRD pattern for TT-COF agrees very well with the simulated pattern for an eclipsed AA arrangement (Figure 1b) with a hexagonal P6m symmetry. Moreover, unit-cell parameters determined from the experimental X-ray patterns match very well with those obtained from the structure simulations (peak broadening included). FT-IR spectroscopy can confirm the presence of the newly formed boronate ester functionality. As previously reported, the attenuation of the OH stretching band resulting from the ester formation is apparent, and furthermore the most characteristic modes of the C-B and C-O functionalities can be assigned to the bands at 1395 cm 1 and 1353 cm 1 (see Figure S8 in the Supporting Information). The B MAS NMR spectrum (see Figure S9 in the Supporting Information) shows a trigonal-planar boron atom with a chemical shift of d= 21 ppm, which can be distinguished from the starting material (TTBA: d= 15 ppm). Transmission electron microscopy (TEM) images show the nanoscale morphology of the crystals. A slightly tilted side view shows the long ordered channels with distinct pore sizes (see Figure S12 in the Supporting Information). A top view [*] Dr. M. Dogru, M. Handloser, F. Auras, Dr. T. Kunz, Dr. D. Medina, Prof. Dr. A. Hartschuh, Prof. Dr. P. Knochel, Prof. Dr. T. Bein Department of Chemistry and Center for NanoScience (CeNS), Ludwig-Maximilians-Universit t Munich (LMU) Butenandtstrase 5–13 (E), 81377 Munich (Germany) E-mail: knoch@cup.uni-muenchen.de bein@lmu.de Homepage: http://www.bein.cup.uni-muenchen.de

Extraction of photogenerated electrons and holes from a covalent organic framework integrated heterojunction.

Covalent organic frameworks (COFs) offer a strategy to position molecular semiconductors within a rigid network in a highly controlled and predictable manner. The π-stacked columns of layered two-dimensional COFs enable electronic interactions between the COF sheets, thereby providing a path for exciton and charge carrier migration. Frameworks comprising two electronically separated subunits can form highly defined interdigitated donor–acceptor heterojunctions, which can drive the photogeneration of free charge carriers. Here we report the first example of a photovoltaic device that utilizes exclusively a crystalline organic framework with an inherent type II heterojunction as the active layer. The newly developed triphenylene–porphyrin COF was grown as an oriented thin film with the donor and acceptor units forming one-dimensional stacks that extend along the substrate normal, thus providing an optimal geometry for charge carrier transport. As a result of the degree of morphological precision that can be achieved with COFs and the enormous diversity of functional molecular building blocks that can be used to construct the frameworks, these materials show great potential as model systems for organic heterojunctions and might ultimately provide an alternative to the current disordered bulk heterojunctions.

Oriented Thin Films of a Benzodithiophene Covalent Organic Framework

A mesoporous electron-donor covalent organic framework based on a benzodithiophene core, BDT-COF, was obtained through condensation of a benzodithiophene-containing diboronic acid and hexahydroxytriphenylene (HHTP). BDT-COF is a highly porous, crystalline, and thermally stable material, which can be handled in air. Highly porous, crystalline oriented thin BDT-COF films were synthesized from solution on different polycrystalline surfaces, indicating the generality of the synthetic strategy. The favorable orientation, crystallinity, porosity, and the growth mode of the thin BDT-COF films were studied by means of X-ray diffraction (XRD), 2D grazing incidence diffraction (GID), transmission and scanning electron microscopy (TEM, SEM), and krypton sorption. The highly porous thin BDT-COF films were infiltrated with soluble fullerene derivatives, such as [6,6]-phenyl C61 butyric acid methyl ester (PCBM), to obtain an interpenetrated electron-donor/acceptor host–guest system. Light-induced charge transfer from the BDT-framework to PCBM acceptor molecules was indicated by efficient photoluminescence quenching. Moreover, we monitored the dynamics of photogenerated hole-polarons via transient absorption spectroscopy. This work represents a combined study of the structural and optical properties of highly oriented mesoporous thin COF films serving as host for the generation of periodic interpenetrated electron-donor and electron-acceptor systems.

Room Temperature Synthesis of Covalent–Organic Framework Films through Vapor-Assisted Conversion

We describe the facile synthesis of several two-dimensional covalent–organic frameworks (2D COFs) as films by vapor-assisted conversion at room temperature. High-quality films of benzodithiophene-containing BDT-COF and COF-5 with tunable thickness were synthesized under different conditions on various substrates. BDT-COF films of several micrometer thickness exhibit mesoporosity as well as textural porosity, whereas thinner BDT-COF films materialize as a cohesive dense layer. In addition, we studied the formation of COF-5 films with different solvent mixture compositions serving as vapor source. Room temperature vapor-assisted conversion is an excellent method to form COF films of fragile precursors and on sensitive substrates.

Synthesis and Stability of Homoleptic Metal(III) Tetramethylaluminates

Whereas a number of homoleptic metal(III) tetramethylaluminates M(AlMe(4))(3) of the rare earth metals have proven accessible, the stability of these compounds varies strongly among the metals, with some even escaping preparation altogether. The differences in stability may seem puzzling given that this class of metals usually is considered to be relatively uniform with respect to properties. On the basis of quantum chemically obtained relative energies and atomic and molecular descriptors of homoleptic tris(tetramethylaluminate) and related compounds of rare earth metals, transition metals, p-block metals, and actinides, multivariate modeling has identified the importance of ionic metal-methylaluminate bonding and small steric repulsion between the methylaluminate ligands for obtaining stable homoleptic compounds. Low electronegativity and a sufficiently large ionic radius are thus essential properties for the central metal atom. Whereas scandium and many transition metals are too small and too electronegative for this task, all lanthanides and actinides covered in this study are predicted to give homoleptic compounds stable toward loss of trimethylaluminum, the expected main decomposition reaction. Three of the predicted lanthanide-based compounds Ln(AlMe(4))(3) (Ln = Ce, Tm, Yb) have been prepared and fully characterized in the present work, in addition to Ln(OCH(2)tBu)(3)(AlMe(3))(3) (Ln = Sc, Nd) and [Eu(AlEt(4))(2)](n). At ambient temperature, donor-free hexane solutions of Ln(AlMe(4))(3) of the Ln(3+)/Ln(2+) redox-active metal centers display enhanced reduction to [Ln(AlMe(4))(2)](n) with decreasing negative redox potential, in the order Eu ≫ Yb ≫ Sm. Whereas Eu(AlMe(4))(3) could not be identified, Yb(AlMe(4))(3) turned out to be isolable in low yield. All attempts to prepare the putative Sc(AlMe(4))(3), featuring the smallest rare earth metal center, failed.

Length‐Dependent Charge Generation from Vertical Arrays of High‐Aspect‐Ratio ZnO Nanowires

Aqueous chemical growth of zinc oxide nanowires is a flexible and effective approach to obtain dense arrays of vertically oriented nanostructures with high aspect ratio. Herein we present a systematic study of the different synthesis parameters that influence the ZnO seed layer and thus the resulting morphological features of the free-standing vertically oriented ZnO nanowires. We obtained a homogeneous coverage of transparent conductive substrates with high-aspect-ratio nanowire arrays (length/diameter ratio of up to 52). Such nanostructured vertical arrays were examined to assess their electric and piezoelectric properties, and showed an electric charge generation upon mechanical compressive stress. The principle of energy harvesting with these nanostructured ZnO arrays was demonstrated by connecting them to an electronic charge amplifier and storing the generated charge in a series of capacitors. We found that the generated charge and the electrical behavior of the ZnO nanowires are strictly dependent on the nanowire length. We have shown the importance of controlling the morphological properties of such ZnO nanostructures for optimizing a nanogenerator device.

From Highly Crystalline to Outer Surface-Functionalized Covalent Organic Frameworks—A Modulation Approach

Crystallinity and porosity are of central importance for many properties of covalent organic frameworks (COFs), including adsorption, diffusion, and electronic transport. We have developed a new method for strongly enhancing both aspects through the introduction of a modulating agent in the synthesis. This modulator competes with one of the building blocks during the solvothermal COF growth, resulting in highly crystalline frameworks with greatly increased domain sizes reaching several hundreds of nanometers. The obtained materials feature fully accessible pores with an internal surface area of over 2000 m(2) g(-1). Compositional analysis via NMR spectroscopy revealed that the COF-5 structure can form over a wide range of boronic acid-to-catechol ratios, thus producing frameworks with compositions ranging from highly boronic acid-deficient to networks with catechol voids. Visualization of an -SH-functionalized modulating agent via iridium staining revealed that the COF domains are terminated by the modulator. Using functionalized modulators, this synthetic approach thus also provides a new and facile method for the external surface functionalization of COF domains, providing accessible sites for post-synthetic modification reactions. We demonstrate the feasibility of this concept by covalently attaching fluorescent dyes and hydrophilic polymers to the COF surface. We anticipate that the realization of highly crystalline COFs with the option of additional surface functionality will render the modulation concept beneficial for a range of applications, including gas separations, catalysis, and optoelectronics.

A Photoactive Porphyrin-Based Periodic Mesoporous Organosilica Thin Film

A novel optoelectroactive system based on an oriented periodic mesoporous organosilica (PMO) film has been developed. A tetra-substituted porphyrin silsesquioxane was designed as a precursor, and the porphyrin macrocycles were covalently incorporated into the organosilica framework without adding additional silica sources, using an evaporation-induced self-assembly process. The synthesized PMO film has a face-centered orthorhombic porous structure with a 15 nm pore diameter. This large pore size enables the inclusion of electron-conducting species such as [6,6]-phenyl C61 butyric acid methyl ester in the periodic mesopores. Optoelectronic measurements on the resulting interpenetrating donor-acceptor systems demonstrate the light-induced charge generation capability and hole-conducting property of the novel porphyrin-based PMO film, indicating the potential of PMO materials as a basis for optoelectroactive systems.

Exploration of MOF nanoparticle sizes using various physical characterization methods – is what you measure what you get?

While the size of nanoparticles (NPs) seems to be a concept established in the field of NPs and is commonly used to characterize them, its definition is not that trivial as different “sizes” have to be distinguished depending on the physical characterization technique performed to measure them. Metal–organic frameworks (MOFs) are known for their crystallinity, their large variety of compositions due to a huge number of inorganic building blocks that can be combined with almost endless organic linkers, their tunable pore structure, their ultrahigh porosity, and the different ways their backbones can be functionalised. The combination of these features with the nanoworld offers manifold perspectives for the synthesis of well-defined MOF nanoparticles (NPs), whose size attribute should be accurately determined as it strongly influences their physicochemical properties (at this length scale). In order to elucidate size determination, we synthesised zirconium fumarate metal–organic framework nanoparticles (Zr-fum MOF NPs) and characterized them using various common characterization methods. Herein, we compare the results of different solid-state methods, including powder X-ray diffraction (PXRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to data obtained from dispersion-based methods, such as fluorescence correlation spectroscopy (FCS) and dynamic light scattering (DLS). In doing so, we illustrate the challenge of finding the appropriate method for obtaining a MOF NP size that is meaningful in the context of the desired application. Moreover, we demonstrate the importance of applying multiple complementary techniques as soon as the MOF NP size is considered. Throughout this paper, we highlight and define some reasonable recommendations of how the MOF NP size should be explored.

Synchronized Offset Stacking: A Concept for Growing Large-Domain and Highly Crystalline 2D Covalent Organic Frameworks

Covalent organic frameworks (COFs), formed by reversible condensation of rigid organic building blocks, are crystalline and porous materials of great potential for catalysis and organic electronics. Particularly with a view of organic electronics, achieving a maximum degree of crystallinity and large domain sizes while allowing for a tightly π-stacked topology would be highly desirable. We present a design concept that uses the 3D geometry of the building blocks to generate a lattice of uniquely defined docking sites for the attachment of consecutive layers, thus allowing us to achieve a greatly improved degree of order within a given average number of attachment and detachment cycles during COF growth. Synchronization of the molecular geometry across several hundred nanometers promotes the growth of highly crystalline frameworks with unprecedented domain sizes. Spectroscopic data indicate considerable delocalization of excitations along the π-stacked columns and the feasibility of donor–acceptor excitations across the imine bonds. The frameworks developed in this study can serve as a blueprint for the design of a broad range of tailor-made 2D COFs with extended π-conjugated building blocks for applications in photocatalysis and optoelectronics.