Kevin Hendrickx

Understanding Intrinsic Light Absorption Properties of UiO-66 Frameworks: A Combined Theoretical and Experimental Study

A combined theoretical and experimental study is performed in order to elucidate the effects of linker functional groups on the photoabsorption properties of UiO-66-X materials. This study, in which both mono- and difunctionalized linkers (with X = OH, NH2, or SH) are investigated, aims to obtain a more complete picture of the choice of functionalization. Static time-dependent density functional theory calculations combined with molecular dynamics simulations are performed on the linkers, and the results are compared to experimental UV/vis spectra in order to understand the electronic effects governing the absorption spectra. The disubstituted linkers show larger shifts than the monosubstituted variants, making them promising candidates for further study as photocatalysts. Next, the interaction between the linker and the inorganic part of the framework is theoretically investigated using a cluster model. The proposed ligand-to-metal-charge transfer is theoretically observed and is influenced by the differences in functionalization. Finally, the computed electronic properties of the periodic UiO-66 materials reveal that the band gap can be altered by linker functionalization and ranges from 4.0 down to 2.2 eV. Study of the periodic density of states allows the band gap modulations of the framework to be explained in terms of a functionalization-induced band in the band gap of the original UiO-66 host.

Missing Linkers: An Alternative Pathway to UiO-66 Electronic Structure Engineering

UiO-66 is a promising metal–organic framework for photocatalytic applications. However, the ligand-to-metal charge transfer of an excited electron is inefficient in the pristine material. Herein, we assess the influence of missing linker defects on the electronic structure of UiO-66 and discuss their ability to improve ligand-to-metal charge transfer. Using a new defect classification system, which is transparent and easily extendable, we identify the most promising photocatalysts by considering both relative stability and electronic structure. We find that the properties of UiO-66 defect structures largely depend on the coordination of the constituent nodes and that the nodes with the strongest local distortions alter the electronic structure most. Defects hence provide an alternative pathway to tune UiO-66 for photocatalytic purposes, besides linker modification and node metal substitution. In addition, the decomposition of MOF properties into node- and linker-based behavior is more generally valid, so we propose orthogonal electronic structure tuning as a paradigm in MOF design.

Vibrational fingerprint of the absorption properties of UiO-type MOF materials

The absorption properties of UiO-type metal–organic frameworks are computed using TD-DFT simulations on the organic linkers. A set of nine isoreticular structures, including the UiO-66 and UiO-67 materials and functionalized variants, are examined. The excitation energies from a static geometry optimization are compared with dynamic averages obtained from sampling the ground-state potential energy surface using molecular dynamics. The vibrational modes that impact the excitation energy are identified. This analysis is done using a recently proposed tool based on power spectra of the velocities and the excitation energies. The applied procedure allows including important factors influencing the absorption spectra, such as the periodic framework, linker variation and dynamical effects including harmonic and anharmonic nuclear motions. This methodology allows investigating in detail the vibrational fingerprint of the excitation energy of advanced materials such as MOFs and gives perspectives to tailor materials toward new light-based applications.

Multicenter Bonding in Ditetracyanoethylene Dianion: A Simple Aromatic Picture in Terms of Three-Electron Bonds.

The nature of the multicenter, long bond in ditetracyanoethylene dianion complex [TCNE]2(2-) is elucidated using high level ab initio Valence Bond (VB) theory coupled with Quantum Monte Carlo (QMC) methods. This dimer is the prototype of the general family of pancake-bonded dimers with large interplanar separations. Quantitative results obtained with a compact wave function in terms of only six VB structures match the reference CCSD(T) bonding energies. Analysis of the VB wave function shows that the weights of the VB structures are not compatible with a covalent bond between the π* orbitals of the fragments. On the other hand, these weights are consistent with a simple picture in terms of two resonating bonding schemes, one displaying a pair of interfragment three-electron σ bonds and the other displaying intrafragment three-electron π bonds. This simple picture explains at once (1) the long interfragment bond length, which is independent of the countercations but typical of three-electron (3-e) CC σ bonds, (2) the interfragment orbital overlaps which are very close to the theoretical optimal overlap of 1/6 for a 3-e σ bond, and (3) the unusual importance of dynamic correlation, which is precisely the main bonding component of 3-e bonds. Moreover, it is shown that the [TCNE]2(2-) system is topologically equivalent to the square C4H4(2-) dianion, a well-established aromatic system. To better understand the role of the cyano substituents, the unsubstituted diethylenic Na(+)2[C2H4]2(2-) complex is studied and shown to be only metastable and topologically equivalent to a rectangular C4H4(2-) dianion, devoid of aromaticity.

Heterogeneous Ru(III) oxidation catalysts via ‘click’ bidentate ligands on a periodic mesoporous organosilica support

A 100% monoallyl ring-type Periodic Mesoporous Organosilica (PMO) is prepared as a novel, versatile and exceptionally stable catalytic support with a high internal surface area and 5.0 nm pores. Thiol–ene ‘click’ chemistry allows straightforward attachment of bifunctional thiols (–NH2, –OH, –SH) which, exploiting the thioether functionality formed, give rise to ‘solid’ bidentate ligands. [Ru(acac)2(CH3CN)2]PF6 is attached and complex formation on the solid is studied via density functional theory. All resulting solid catalysts show high activity and selectivity in alcohol oxidation reactions performed in green conditions (25 °C/water). The PMO catalysts do not leach Ru during reaction and are thus easily recuperated and re-used for several runs. Furthermore, oxidation of poorly water-soluble (±)-menthol illustrates the benefits of using hydrophobic PMOs as catalytic supports.

Exploring lanthanide doping in UiO-66 : a combined experimental and computational study of the electronic structure

Lanthanide-based metal-organic frameworks show very limited stabilities, which impedes their use in applications exploiting their extraordinary electronic properties, such as luminescence and photocatalysis. This study demonstrates a fast and easy microwave procedure to dope UiO-66, an exceptionally stable and tunable Zr-based metal-organic framework. The generally applicable synthesis methodology is used to incorporate different transition metal and lanthanide ions. Selected experiments on these newly synthesized materials allow us to construct an energy scheme of lanthanide energy levels with respect to the UiO-66 host. The model is confirmed via absolute intensity measurements and provides an intuitive way to understand charge transfer mechanisms in these doped UiO-66 materials. Density functional theory calculations on a subset of materials moreover improve our understanding of the electronic changes in doped UiO-66 and corroborate our empirical model.