Matthew Addicoat

Conjugated organic framework with three-dimensionally ordered stable structure and delocalized π clouds

Covalent organic frameworks are a class of crystalline organic porous materials that can utilize π–π-stacking interactions as a driving force for the crystallization of polygonal sheets to form layered frameworks and ordered pores. However, typical examples are chemically unstable and lack intrasheet π-conjugation, thereby significantly limiting their applications. Here we report a chemically stable, electronically conjugated organic framework with topologically designed wire frameworks and open nanochannels, in which the π conjugation-spans the two-dimensional sheets. Our framework permits inborn periodic ordering of conjugated chains in all three dimensions and exhibits a striking combination of properties: chemical stability, extended π-delocalization, ability to host guest molecules and hole mobility. We show that the π-conjugated organic framework is useful for high on-off ratio photoswitches and photovoltaic cells. Therefore, this strategy may constitute a step towards realizing ordered semiconducting porous materials for innovations based on two-dimensionally extended π systems.

Charge Dynamics in A Donor–Acceptor Covalent Organic Framework with Periodically Ordered Bicontinuous Heterojunctions

The donor–acceptor heterojunction is a key structure in current technologies, including transistors, light-emitting diodes, and photovoltaics, because it controls the charge dynamics in the devices. Covalent organic frameworks (COFs) are crystalline molecular skeletons that allow atomically precise integration of building blocks into periodic array structures. In this regard, we have demonstrated arene, porphyrin, and phthalocyanine COFs that provide periodically ordered columnar arrays of p-components and show outstanding semiconducting and photoconductive properties. We recently synthesized a donor–acceptor COF that gives rise to a periodically ordered bicontinuous heterojunction structure and self-sorted donor and acceptor columnar arrays separated at nanometer-scale intervals. This nanoscopic segregation morphology forms a broad interface for charge separation, provides ambipolar pathways for charge collection, and would be ideal for the current semiconducting devices that involve photoenergy transformations; however, the charge dynamics, which is a key mechanism that controls the energy transformation, remains unclear. Here, we report the charge dynamics of a donor–acceptor COF, which were determined using time-resolved spectroscopy to elucidate the photochemical processes of the free charges from their generation to delocalization and retention. In the COF, the heterojunctions allow an ultrafast electron transfer from the donor to the acceptor columns. Consequently, the light absorption is directly coupled with charge dissociation to generate free charges in the donor and acceptor p-columns within 2 ps. On the other hand, the stacked p-columns delocalize the charges, suppress charge recombination, and retain the charges for a prolonged period of time. We show that both solvated and solid-state COFs enable rapid charge separation and exceptional long-term charge retention, thereby providing a key mechanistic basis to envisage the high potential of donor–acceptor COFs for photoelectric applications. The donor–acceptor COF (Scheme 1a, DZnPc-ANDI-COF) is a tetragonal, mesoporous 2D framework that is composed of zinc phthalocyanine as an electron donor and naphthalene diimide as an acceptor. In the COF, the two p-units are alternately linked within an electron-transfer distance and at a dihedral angle of approximately 428. The COF provides selfsorted, bicontinuous columnar arrays and constitutes periodically structured heterojunctions in which each donor column is interfaced with four acceptor columns that are equally active in capturing photo-generated electrons (Scheme 1b). The DZnPc-ANDI-COF absorbs light over a broad visible and near-infrared region up to 1100 nm (Figure S1 in the Supporting Information). Elemental analysis, infrared spectroscopy, nuclear magnetic resonance spectroscopy, and electron microscopy confirmed the formation of the COF (Figure S2–S4 and Table S1). The same COF has been reported as a thin film. The DZnPc-ANDI-COF exhibited a type IV nitrogen sorption curve that is characteristic of mesoporous frameworks (Figure 1a). The Brunauer–Emmett–Teller surface area and pore volume were calculated as 1410 mg 1 and 1.25 cmg , respectively. The pore-size distribution profile with a range up

Highly Emissive Covalent Organic Frameworks

Highly luminescent covalent organic frameworks (COFs) are rarely achieved because of the aggregation-caused quenching (ACQ) of π-π stacked layers. Here, we report a general strategy to design highly emissive COFs by introducing an aggregation-induced emission (AIE) mechanism. The integration of AIE-active units into the polygon vertices yields crystalline porous COFs with periodic π-stacked columnar AIE arrays. These columnar AIE π-arrays dominate the luminescence of the COFs, achieve exceptional quantum yield via a synergistic structural locking effect of intralayer covalent bonding and interlayer noncovalent π-π interactions and serve as a highly sensitive sensor to report ammonia down to sub ppm level. Our strategy breaks through the ACQ-based mechanistic limitations of COFs and opens a way to explore highly emissive COF materials.

Control of Crystallinity and Porosity of Covalent Organic Frameworks by Managing Interlayer Interactions Based on Self-Complementary π-Electronic Force

Crystallinity and porosity are crucial for crystalline porous covalent organic frameworks (COFs). Here we report synthetic control over the crystallinity and porosity of COFs by managing interlayer interactions based on self-complementary π-electronic forces. Fluoro-substituted and nonsubstituted aromatic units at different molar ratios were integrated into the edge units that stack to trigger self-complementary π-electronic interactions in the COFs. The interactions improve the crystallinity and enhance the porosity by maximizing the total crystal stacking energy and minimizing the unit cell size. Consequently, the COF consisting of equimolar amounts of fluoro-substituted and nonsubstituted units showed the largest effect. These results suggest a new approach to the design of COFs by managing the interlayer interactions.

Locking Covalent Organic Frameworks with Hydrogen Bonds: General and Remarkable Effects on Crystalline Structure, Physical Properties, and Photochemical Activity

A series of two-dimensional covalent organic frameworks (2D COFs) locked with intralayer hydrogen-bonding (H-bonding) interactions were synthesized. The H-bonding interaction sites were located on the edge units of the imine-linked tetragonal porphyrin COFs, and the contents of the H-bonding sites in the COFs were synthetically tuned using a three-component condensation system. The intralayer H-bonding interactions suppress the torsion of the edge units and lock the tetragonal sheets in a planar conformation. This planarization enhances the interlayer interactions and triggers extended π-cloud delocalization over the 2D sheets. Upon AA stacking, the resulting COFs with layered 2D sheets amplify these effects and strongly affect the physical properties of the material, including improving their crystallinity, enhancing their porosity, increasing their light-harvesting capability, reducing their band gap, and enhancing their photocatalytic activity toward the generation of singlet oxygen. These remarkable effects on the structure and properties of the material were observed for both freebase and metalloporphyin COFs. These results imply that exploration of supramolecular ensembles would open a new approach to the structural and functional design of COFs.

Extension of the Universal Force Field to Metal–Organic Frameworks

The Universal Force Field (UFF) (Rappé et al., J. Am. Chem. Soc. 1992) provides a general approach to molecular mechanics for molecules and materials composed of elements throughout the periodic table. Though the method is tunable by the specification of bond orders and the introduction of effective charges, the presently available list of atom types is insufficient to treat various systems containing transition metals, including metal-organic frameworks (MOFs). As MOFs are composite materials built of a combination of individually stable building blocks, a plethora of MOF structures are possible, and the prediction of their structure with a low-cost method is important. We have extended the UFF parameter set to include transition metal elements Zn, Cu, Ni, Co, Fe, Mn, Cr, V, Ti, Sc, and Al, as they occur in MOFs, and have proposed additional O parameters that provide reliable structures of the metal oxide clusters of the connectors. We have benchmarked the performance of the MOF extension to UFF (UFF4MOF) with respect to experimentally available data and to DFT calculations. The parameters are available in various well-spread programs, including GULP, deMonNano, and ADF, and all information is provided to include them in other molecular mechanics codes.

Rational design of crystalline supermicroporous covalent organic frameworks with triangular topologies

Covalent organic frameworks (COFs) are an emerging class of highly ordered porous polymers with many potential applications. They are currently designed and synthesized through hexagonal and tetragonal topologies, limiting the access to and exploration of new structures and properties. Here, we report that a triangular topology can be developed for the rational design and synthesis of a new class of COFs. The triangular topology features small pore sizes down to 12 Å, which is among the smallest pores for COFs reported to date, and high π-column densities of up to 0.25 nm−2, which exceeds those of supramolecular columnar π-arrays and other COF materials. These crystalline COFs facilitate π-cloud delocalization and are highly conductive, with a hole mobility that is among the highest reported for COFs and polygraphitic ensembles.

Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?

For inorganic semiconductors crystalline order leads to a band structure which gives rise to drastic differences to the disordered material. An example is the presence of an indirect band gap. For organic semiconductors such effects are typically not considered, since the bands are normally flat, and the band-gap therefore is direct. Herein we show results from electronic structure calculations demonstrating that ordered arrays of porphyrins reveal a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap. We demonstrate herein that such ordered structures can be fabricated by liquid-phase epitaxy and that the corresponding crystalline organic semiconductors exhibit superior photophysical properties, including large charge-carrier mobility and an unusually large charge-carrier generation efficiency. We have fabricated a prototype organic photovoltaic device based on this novel material exhibiting a remarkable efficiency.

Fabrication of highly uniform gel coatings by the conversion of surface-anchored metal-organic frameworks

We report the fabrication of 3D, highly porous, covalently bound polymer films of homogeneous thickness. These surface-bound gels combine the advantages of metal-organic framework (MOF) materials, namely, the enormous flexibility and the large size of the maximum pore structures and, in particular, the possibility to grow them epitaxially on modified substrates, with those of covalently connected gel materials, namely, the absence of metal ions in the deposited material, a robust framework consisting of covalent bonds, and, most importantly, pronounced stability under biological conditions. The conversion of a SURMOF (surface-mounted MOF) yields a surface-grafted gel. These SURGELs can be loaded with bioactive compounds and applied as bioactive coatings and provide a drug-release platform in in vitro cell culture studies.

Accurate treatment of nonbonded interactions within systematic molecular fragmentation

The accuracy of the systematic fragment approach to the estimation of molecular electronic energies is enhanced by a significantly improved treatment of nonbonded interactions between molecular fragments. Distributed electrostatic interactions, pairwise dispersion interactions, and many-body induction are evaluated from ab initio calculations of small molecular fragments. The accuracy of the complete approach is reported for a large sample of typical neutral organic molecules.