Modeling of metal-organic frameworks for optical applications

Metal-organic frameworks (MOFs) are an important class of porous hybrid materials. They consist of metal ions or metal-oxo clusters forming the so-called inorganic building units connected by organic ligands acting as linker molecules leading to an intrinsic porosity of the framework. MOFs have been discussed intensively with regard to classical applications of porous materials like catalysis and gas separation. In this work, a different aspect is focused, namely the use of MOFs as optical materials. One fundamental property in this respect is the refractive index (RI). To ensure the reliable and precise calculation of the RI of MOFs a novel ab initio simulation protocol was developed in this work. By applying density functional theory (DFT), accurate models of MOF crystal structures were prepared and the optical properties were calculated precisely. Starting with the well-known UiO-66 MOF and its established nitro- and aminofunctionalized derivatives UiO-66-NO2 and UiO-66-NH2, this simulation protocol was used to calculate the electronic structures and the corresponding optical properties. Furthermore, the incorporation of “push–pull” linkers into the UiO-66 framework was studied to allow a further tuning of the RI of the parent UiO-66 MOF. In this context, a novel UiO-66 analogue denoted as UiO-66-(NH2,NO2) was presented. In addition, the use of halogenated linkers yielding the well-known monohalogenated UiO-66 derivatives denoted as UiO-66-X ( = F, Cl, Br, I) was studied. To obtain high RI values while preserving the transparency in the visible spectral region, a novel dihalogenated UiO-66 derivative denoted as UiO-66-I2 was introduced. The simulation protocol developed in the first part of this work allows a detailed study of the electronic structure of MOFs and a better understanding how the the various modular components of a MOF influence its RI. This protocol is computationally demanding. As a consequence, a second, more efficient simulation protocol was presented allowing the screening of MOFs with regard to their RI. This simulation protocol is based on a fragmentation scheme for MOFs allowing the separate calculation of the polarizability of the modular components of MOFs using DFT. These polarizabilities were used to compute the total polarizability of a MOF and subsequently the corresponding RI by applying the Lorenz-Lorentz equation.Metall-organische Gerüste (engl. metal-organic frameworks, MOFs) stellen eine bedeutende Gruppe innerhalb der porösen Hybridmaterialien dar. Sie bestehen aus Metallionen oder Metall- Oxo-Clustern, die als anorganische Baueinheiten bezeichnet werden und durch organische Liganden verknüpft sind, die als Linker bezeichnet werden. MOFs wurden intensiv im Hinblick auf klassische Anwendungsgebiete für poröse Materialien wie beispielsweise Katalyse und Gastrennung betrachtet. Im Gegensatz dazu werden in dieser Arbeit MOFs für die Verwendung als Materialien für optische Anwendungen untersucht. Hierbei ist insbesondere der Brechungsindex (refraktiver Index, RI) von besonderer Relevanz. In dieser Arbeit wurde ein ab initio Simulationsprotokoll entwickelt, das eine verlässliche und präzise Berechnung der elektronischen Struktur und des RIs von MOFs ermöglicht. Hierzu wird die Dichtefunktionaltheorie (DFT) verwendet, um ausgehend von Einkristallstrukturdaten Modelle von MOFs zu erstellen und die optischen Eigenschaften zu berechnen. Beginnend mit dem bekannten MOF UiO-66 und dessen etablierten nitro- und aminofunktionalisierten Derivaten UiO-66-NO2 und UiO-66-NH2 wurde dieses Simulationsprotokoll angewendet. Anschließend wurde die Verwendung von „push-pull“-Linkermolekülen zum Aufbau des UiO-66 Gerüsts untersucht und das neue UiO-66-(NH2,NO2) Derivat computer-chemisch charakterisiert. Weiterhin wurde die Verwendung von monohalogenierten Linkermolekülen untersucht, mit welchen die UiO-66-X ( = F, Cl, Br, I) Derivate erhalten werden können. Zusätzlich wurde das neue UiO-66-I2 Derivat vorgestellt, um im sichtbaren Spektralbereich einen hohen RI bei gleichzeitiger Transparenz zu erhalten. Das im ersten Teil dieser Arbeit vorgestellte Simulationsprotokoll erlaubt ein tieferes Verständnis des Einflusses der modularen Komponenten eines MOFs auf den RI, aber ist mit einem hohen Rechenaufwand verbunden. Daher wurde ein weiteres effizienteres Simulationsprotokoll zur Berechnung des RIs von MOFs entwickelt. Dieses Protokoll basiert auf der Fragmentierung von MOFs, so dass die Polarisierbarkeiten der modularen Komponenten eines MOFs separat mittels DFT berechnet werden konnten. Anschließend wurde die Polarisierbarkeit des MOFs und der entsprechende RI unter der Verwendung der Lorenz-Lorentz-Gleichung berechnet

Development of high refractive index UiO-66 framework derivatives via ligand halogenation

UiO-66 is a Zr-based metal-organic framework (MOF) with exceptional chemical and thermal stability. The modular design of a MOF allows the tuning of its electronic and optical properties to obtain tailored materials for optical applications. Making use of the halogenation of the 1,4-benzenedicarboxylate (bdc) linker, the well-known monohalogenated UiO-66 derivatives were examined. In addition, a novel diiodo bdc based UiO-66 analogue is introduced. The novel UiO-66-I2 MOF is fully characterized experimentally. By applying density functional theory (DFT), fully relaxed periodic structures of the halogenated UiO-66 derivatives are generated. Subsequently, the HSE06 hybrid DFT functional is used to calculate the electronic structures and optical properties. The obtained band gap energies are validated with UV-Vis measurements to assure a precise description of the optical properties. Finally, the calculated refractive index dispersion curves are evaluated underlining the capabilities to tailor the optical properties of MOFs by linker functionalization

Fragment-based approach for the efficient calculation of the refractive index of metal-organic frameworks

Increasing demands on materials in the field of optical applications require novel materials. Metal-organic frameworks (MOFs) are a prominent class of hybrid inorganic-organic materials with a modular layout. This allows the fine-tuning of their optical properties and the tailored design of optical systems. In the present theoretical study, an efficient method to calculate the refractive index (RI) of MOFs is introduced. For this purpose, the MOF is split into disjoint fragments, the linkers and the inorganic building units. The latter are disassembled until metal ions are obtained. The static polarizabilities are calculated individually using molecular density functional theory (DFT). From these, the MOF's RI is calculated. To obtain suitable polarizabilities, an exchange-correlation functional benchmark was performed first. Subsequently, this fragment-based approach was applied to a set of 24 MOFs including Zr-based MOFs and ZIFs. The calculated RI values were compared to the experimental values and validated using HSE06 hybrid functional DFT calculations with periodic boundary conditions. The examination of the MOF set revealed a speed up of the RI calculations by the fragment-based approach of up to 600 times with an estimated maximal deviation from the periodic DFT results below 4%

Tuning the optical properties of the metal-organic framework UiO-66 via ligand functionalization

Metal-organic frameworks (MOFs) are a promising class of materials for optical applications, especially due to their modular design which allows fine-tuning of the relevant properties. The present theoretical study examines the Zr-based UiO-66-MOF and derivatives of it with respect to their optical properties. Starting from the well-known monofunctional amino- and nitro-functionalized UiO-66 derivatives, we introduce novel UiO-66-type MOFs containing bifunctional push-pull 1,4-benzenedicarboxylate (bdc) linkers. The successful synthesis of such a novel UiO-66 derivative is also reported. It was carried out using a para-nitroaniline (PNA)-based bdc-analogue linker. Applying density functional theory (DFT), suitable models for all UiO-66-MOF analogues were generated by assessing different exchange-correlation functionals. Afterwards, HSE06 hybrid functional calculations were performed to obtain the electronic structures and optical properties. The detailed HSE06 electronic structure calculations were validated with UV-Vis measurements to ensure reliable results. Finally, the refractive index dispersion of the seven UiO-66-type materials is compared, showing the possibility to tailor the optical properties by the use of functionalized linker molecules. Specifically, the refractive index can be varied over a wide range from 1.37 to 1.78

Correction: Tuning the optical properties of the metal-organic framework UiO-66 via ligand functionalization

The authors apologise that the comparison of calculated (HSE06) and experimental band gaps shown in Fig. 3 were incorrect, the experimental values did not match the presented UV-Vis spectra and Tauc plots. The figure is corrected as follows: The corrections shown here do not affect the conclusions in the paper. (Figure Presented).The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers

Metal–Organic Framework Co-MOF-74-Based Host–Guest Composites for Resistive Gas Sensing

Increasing demands in thefield of sensing, especially for gas detectionapplications, require new approaches to chemical sensors. Metal−organic frameworks (MOFs) can play a decisive role owing to their outstanding performances regardinggas selectivity and sensitivity. The tetrathiafulvalene (TTF)-infiltrated MOF, Co-MOF-74, has been prepared following the host−guest concept and evaluated inresistive gas sensing. The Co-MOF-74-TTF crystal morphology has beencharacterized via X-ray diffraction and scanning electron microscopy, while thesuccessful incorporation of TTF into the MOF has been validated via X-rayphotoemission spectroscopy, thermogravimetric analysis, UV/vis, infrared (IR), andRaman investigations. We demonstrate a reduced yet ample uptake of CO2in thepores of the new material by IR imaging and adsorption isotherms. Thenanocomposite Co-MOF-74-TTF exhibits an increased electrical conductivity incomparison to Co-MOF-74 which can be influenced by gas adsorption from asurrounding atmosphere. This effect could be used for gas sensing