Tetrazine Linkers as Plug-and-Play Tags for General Metal-Organic Framework Functionalization and C60 Conjugation

The value of covalent post-synthetic modification in expanding the chemistry and pore versatility of reticular solids is well documented. Here we use mesoporous crystals of the metal-organic framework (MOF) UiO-68-TZDC to demonstrate the value of tetrazine connectors for all-purpose inverse electron-demand Diels-Alder ligation chemistry. Our results suggest a positive effect of tetrazine reticulation over its reactivity for quantitative one-step functionalization with a broad scope of alkene or alkyne dienophiles into pyridazine and dihydropyridazine frameworks. This permits generating multiple pore environments with diverse chemical functionalities and the expected accessible porosities, that is also extended to the synthesis of crystalline fulleretic materials by covalent conjugation of fullerene molecules.

Permanent Porosity in Hydroxamate Titanium-Organic Polyhedra

Following the synthesis of hydroxamate titanium-organic frameworks, we now extend these siderophore-type linkers to the assembly of the first titanium-organic polyhedra displaying permanent porosity. Mixed-linker versions of this molecular cage (cMUV-11) are also used to demonstrate the effect of pore chemistry in accessing high surface areas of near 1200 m²·g^-1.

Selective Implantation of Diamines for Cooperative Catalysis in Isoreticular Heterometallic Titanium-Organic Frameworks

We introduce the first example of isoreticular titanium-organic frameworks, MUV-10 and MUV-12, to show how the different affinity of hard Ti(IV) and soft Ca(II) metal sites can be used to direct selective grafting of amines. This enables the combination of Lewis acid titanium centers and available -NH₂ sites in two sizeable pores for cooperative cycloaddition of CO₂ to epoxides at room temperature and atmospheric pressure. The selective grafting of molecules to heterometallic clusters adds up to the pool of methodologies available for controlling the positioning and distribution of chemical functions in precise positions of the framework required for definitive control of pore chemistry.

Effect of Linker Distribution in the Photocatalytic Activity of Multivariate Mesoporous Crystals

The use of Metal-Organic Frameworks as crystalline matrices for the synthesis of multiple component or multivariate solids by the combination of different linkers into a single material has emerged as a versatile route to tailor the properties of single-component phases or even access new functions. This approach is particularly relevant for Zr₆-MOFs due to the synthetic flexibility of this inorganic node. However, the majority of materials are isolated as polycrystalline solids, which are not ideal to decipher the spatial arrangement of parent and exchanged linkers for the formation of homogeneous structures or heterogeneous domains across the solid. Here we use high-throughput methodologies to optimize the synthesis of single crystals of UiO-68 and UiO-68-TZDC, a photoactive analogue based on a tetrazine dicarboxylic derivative. The analysis of the single linker phases reveals the necessity of combining both linkers to produce multivariate frameworks that combine efficient light sensitization, chemical stability, and porosity, all relevant to photocatalysis. We use solvent-assisted linker exchange reactions to produce a family of UiO-68-TZDC_% binary frameworks, which respect the integrity and morphology of the original crystals. Our results suggest that the concentration of TZDC in solution and the reaction time control the distribution of this linker in the sibling crystals for a uniform mixture or the formation of core-shell domains. We also demonstrate how the possibility of generating an asymmetric distribution of both linkers has a negligible effect on the electronic structure and optical band gap of the solids but controls their performance for drastic changes in the photocatalytic activity toward proton or methyl viologen reduction.

Variation of Interfacial Interactions in PC61BM-like Electron-Transporting Compounds for Perovskite Solar Cells

The synthesis, characterization, and incorporation of phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM)-like derivatives as electron transporting materials (ETMs) in inverted perovskite solar cells (PSCs) are reported. These compounds have the same structure except for the ester substituent, which was varied from methyl to phenyl to thienyl and to pyridyl. The three latter derivatives performed better than PC₆₁BM in PSCs, mainly attributed to the specific interactions of the fullerenes with the perovskite layer, as evidenced by X-ray photoelectron spectroscopy (XPS) and steady-state and time-resolved photoluminescence (SS- and TRPL) measurements. The experimental results were fully supported by density functional theory (DFT) calculations, which showed that the strongest interactions were exhibited by the compound possessing the pyridyl substituent.

Optimizing CuO p-type dye-sensitized solar cells by using a comprehensive electrochemical impedance spectroscopic study

We introduce a novel and comprehensive approach for the evaluation and interpretation of electrochemical impedance spectroscopy (EIS) measurements in p-type DSSCs. In detail, we correlate both the device performance and EIS figures-of-merit of a series of devices in which, the calcination temperature, film thickness, and electrolyte concentration have been systematically modified. This new approach enables the separation of the different processes across the dye/semiconductor/electrolyte interface, namely the unfavorable charge recombination and the favorable electron injection/regeneration processes. In addition, studies on non-sensitized CuO and NiO electrodes provide insights into their affinity towards a reaction with the electrolyte - CuO is far less reactive towards the polyiodide species. Overall, this work underlines the superior features of CuO with respect to NiO for p-DSSCs and demonstrates a comprehensive optimization of the CuO-based DSSCs with respect to the device architecture by the aid of EIS analysis.

Synthesis and characterization of high molecular weight phthalocyanine-PPV copolymers through post-polymerization functionalization

High molecular weight poly(p-phenylenenevinylene) PPV copolymers laterally substituted with zinc (II) phthalocyanines ( Zn(II)Pc -PPV 1 and 2) have been synthesized by means of post-polymerization functionalization reactions through DCC-mediated esterifications between hydroxy-phthalocyanines 4 and 6 and carboxy-bearer PPV copolymers, comprising 2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene (MDMO-PPV) and 1,4-(2-(5-carboxypentyloxy)-5-methoxyphenylenevinylene (CPM-PPV) units in a 9:1 (3) and 1:1 (5) ratio, respectively. The resulting copolymer 1 contains a 7 mol % of Pc molecules, while copolymer 2, which is isolated as the soluble fraction of the reaction with the starting 1:1 copolymer (namely having around 50% of COOH -containing monomeric units), holds a 9 mol % of zinc(II) phthalocyanines. 1 and 2 were fully characterized by 1H NMR, UV-vis and FT-IR spectroscopies.

Broadening the absorption of conjugated polymers by "click" functionalization with phthalocyanines

Conjugated copolymer derivatives of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT) containing 10% of alkyne functionalities in the side chains have been prepared using the sulfinyl precursor route and the Rieke method, respectively. With the aim of expanding the absorption range of these conjugated polymers for their use in bulk heterojunction (BHJ) polymer:fullerene solar cells, appropriate phthalocyanine (Pc) molecules have been covalently bound through a post-polymerization "click chemistry" reaction between the alkyne functionalities in the side chains of the copolymers and a Pc functionalized with an azide moiety. The resulting poly(p-phenylenevinylene)-Pc (PPV-Pc) material holds a 9 mol% content of Pcs, while the polythiophene-Pc material (PT-Pc) contains a 8 mol% of Pc-functionalization in the side chains. As expected, the presence of the Pc contributes to the extension of the absorption up to 700 nm. BHJ solar cells have been prepared using PPV-Pc and PT-Pc materials in combination with PCBM. Although the Pc absorption contributes to the generation of photocurrent, the overall power conversion efficiencies (PCE) obtained from these cells are lower than those obtained with BHJ P3HT:PCBM (1:1) and MDMO-PPV:PCBM (1:4) solar cells. A plausible explanation could be the moderate solubility of the PPV-Pc and PT-Pc materials that limits the processing into thin films.