Tuning the Photocatalytic Activity of Ti-Based Metal-Organic Frameworks through Modulator Defect-Engineered Functionalization.

Defect engineering is a valuable tool to tune the photocatalytic activity of metal-organic frameworks (MOFs). Inducing defects through the attachment of functionalized modulators can introduce cooperative units that can tune the bandgap of the material and enhance their chemical, thermal, and photostabilities among other properties. However, the majority of defect engineering studies for photocatalytic applications are limited to Zr-based MOFs, and there is still a lack of interrelation between synthetic variables, the resultant MOF properties, and their effect on their photocatalytic performance. We report a comprehensive study on the defect engineering of the titanium heterometallic MOF MUV-10 by fluoro- and hydroxy-isophthalic acid (Iso) modulators, rationalizing the effect of the materials' properties on their photocatalytic activity for hydrogen production. The Iso-OH modified MOFs present a volcano-type profile with a 2.3-fold increase in comparison to the pristine materials, whereas the Iso-F modified samples have a gradual increase with up to a 4.2-fold enhancement. It has been demonstrated that ∼9% of Iso-OH modulator incorporation produces ∼40% defects, inducing band gap reduction and longer excited states lifetime. Similar defect percentages have been generated upon near 40% Iso-F modulator incorporation; however, negligible band gap changes and shorter excited states lifetimes were determined. The higher photocatalytic activity in Iso-F modulator derived MOF has been attributed to the effect of the divergent defect-compensation modes on the materials' photostability and to the increase in the external surface area upon introduction of Iso-F modulator.

Linker depletion for missing cluster defects in non-UiO metal-organic frameworks.

Defect engineering is a valuable tool to tune the properties of metal-organic frameworks. However, defect chemistry remains still predominantly limited to UiO-type MOFs. We describe the preferential formation of missing cluster defects in heterometallic titanium-organic frameworks of the MUV-10 family when synthesised in sub-stoichiometric linker conditions. Our results show the value of integrating experimental work, computational modelling and thorough characterization in rationalizing the impact of defects over the porosity and structure of this family of materials. Correlation of experiment with computational models reveals the dominance of missing cluster vacancies in the pore size distribution of defective MUV-10. These same models were used to investigate the correlation of defects by synchrotron X-ray diffraction. The diffraction at low reflection angles is dominated by diffuse scattering that is indicative of short-range order and cannot be indexed to the defective structural models generated. In addition to the low atomic scattering factor of titanium, these results confirm the need for high-resolution electron microscopy methods for modelling nanoscale disorder in titanium MOFs.

Controlling the molecular diffusion in MOFs with the acidity of monocarboxylate modulators.

The catalytic performance of metal-organic frameworks (MOFs) is related to their physicochemical properties, such as particle size, defect chemistry and porosity, which can be potentially controlled by coordination modulation. By combining PXRD, 1HNMR, FT-IR, and N2 uptake measurements we have gained insights into the control of different types of defects (missing linker or missing cluster consequence of the spatial distribution of missing linkers, and a combination of both) by the type of modulator employed. We show that the molar percent of defects, either as missing linkers or as a part of missing cluster defects, is related to the acidity of a modulator and its subsequent incorporation into the UiO-66 structure. Modulators with strong acidity and small size result in a considerable defect induction that causes an increase in the external surface area and mesopore volume, which is beneficial for the ring-opening of epoxides with amines, using UiO-66 defect-modulated MOFs as heterogeneous catalysts.

The excellent biocompatibility and negligible immune response of the titanium heterometallic MOF MUV-10.

The Ti-Ca heterometallic MOF MUV-10 exhibits good dispersibility in phosphate buffer and low phosphate-induced degradation in comparison to other MOF systems. It induces no cytotoxicity towards cells of the immune system and no inmune response, making it an attractive candidate for biomedical applications and demonstrating its safe use for other applications.

Controlled Transdermal Release of Antioxidant Ferulate by a Porous Sc(III) MOF.

The Sc(III) MOF-type MFM-300(Sc) is demonstrated in this study to be stable under physiological conditions (PBS), biocompatible (to human skin cells), and an efficient drug carrier for the long-term controlled release (through human skin) of antioxidant ferulate. MFM-300(Sc) also preserves the antioxidant pharmacological effects of ferulate while enhancing the bio-preservation of dermal skin fibroblasts, during the delivery process. These discoveries pave the way toward the extended use of Sc(III)-based MOFs as drug delivery systems (DDSs).

Effect of modulator connectivity on promoting defectivity in titanium-organic frameworks.

The recognition of defect chemistry as a true synthetic tool for targeted creation of defects and controllable performance remains limited by the pool of frameworks explored. The value of defect engineering in controlling the properties of defective frameworks has been beautifully exemplified and largely demonstrated with UiO-type materials based on Zr(iv) nodes. However, titanium-organic frameworks remain largely unexplored in this context arguably due to the complex chemistry in solution of Ti(iv) and the difficulties in growing crystalline solids. We report a systematic study on the ability of mono- and dicarboxylic modulators (benzoic and isophthalic acid) to promote defect creation in the heterometallic Ti-MOF of the MUV-10 family. Our results indicate that both acids behave as capping modulators at high concentrations, but isophthalic acid is a more efficient defect promoter, yielding defective phases with nearly 40% of missing linkers. Our computational results suggest that this difference cannot be solely ascribed to relative changes in acidity but to the ability of this bidentate linker in compensating the structural distortion and energy penalty imposed by breaking the connectivity of the underlying framework.

Synthesis and characterisation of a mesocyclic tripodal triamine ligand.

Meso- and macrocyclic polydentate amine ligands have been widely explored in oxidation catalysis and for the stabilization of unstable metal-superoxide, -peroxide, and -oxo intermediates. Herein we report on the design and synthesis of a novel mesocyclic, tripodal, triamine ligand that we believe will be an excellent addition to this field. We explored a number of synthetic procedures towards the mesocyclic asymmetric tetraalkylated ligand 1. We expect that 1 will bind metals in a facially capping manner, yielding complexes that display pseudo-tetrahedral geometry, potentially providing access to unprecedented late transition metal-oxo complexes (metal = Co, Ni, Cu). We describe the preparation of a library of mesocyclic polyamine synthons (8, 16, 17, 18, 19) that are precursors in the synthesis of 1. These synthons will be used to tailor the electronic properties of metal complexes of 1 and derivatives thereof. The X-ray crystal structures of 19 and mono- and di-protonated forms of 1b show that the triamine crystalises in a boat­chair conformation which is undesirable for metal coordination. However, solution (1)H NMR studies show that in solution both 19 and the tetraalkylated derivative 1b are remarkably flexible. 1b reacted with [CuI(NCCH3)4](OTf) yielding a 1:1 copper(I) complex [CuI(NCCH3)(1b)](+).

Drug delivery and controlled release from biocompatible metal-organic frameworks using mechanical amorphization.

We have used a family of Zr-based metal-organic frameworks (MOFs) with different functionalized (bromo, nitro and amino) and extended linkers for drug delivery. We loaded the materials with the fluorescent model molecule calcein and the anticancer drug α-cyano-4-hydroxycinnamic acid (α-CHC), and consequently performed a mechanical amorphization process to attempt to control the delivery of guest molecules. Our analysis revealed that the loading values of both molecules were higher for the MOFs containing unfunctionalized linkers. Confocal microscopy showed that all the materials were able to penetrate into cells, and the therapeutic effect of α-CHC on HeLa cells was enhanced when loaded (20 wt%) into the MOF with the longest linker. On one hand, calcein release required up to 3 days from the crystalline form for all the materials. On the other hand, the amorphous counterparts containing the bromo and nitro functional groups released only a fraction of the total loaded amount, and in the case of the amino-MOF a slow and progressive release was successfully achieved for 15 days. In the case of the materials loaded with α-CHC, no difference was observed between the crystalline and amorphous form of the materials. These results highlight the necessity of a balance between the pore size of the materials and the size of the guest molecules to accomplish a successful and efficient sustained release using this mechanical ball-milling process. Additionally, the endocytic pathway used by cells to internalize these MOFs may lead to diverse final cellular locations and consequently, different therapeutic effects. Understanding these cellular mechanisms will drive the design of more effective MOFs for drug delivery applications.