Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are a large emerging class of nanoporous crystalline materials consisting of metal ions (or clusters) spatially coordinated by multidentate organic ligands. Due to the nearly infinite variety of MOFs building blocks, they are characterized with enormous structural possibilities with tunable pore sizes and functionality. Therefore, MOFs have become one of the most interesting research areas in materials science with a broad range of applications, ranging from the gas adsorption, storage and separation up to optics, energy transfer materials, sensorics, medical diagnostics etc. In our group, we use quantum mechanics and molecular dynamics, within all atom and coarse grained ansatz, to understand the photophysical phenomena: energy transfer, photon upconversion, photoconduction, and tuning of the optical properties of the MOF thin films with the specific focus on the systematic MOF crystal structure engineering and in silico prediction of MOF building blocks with desired functionality. 

The projects related to studies of MOFs in our group are the part of the 3DMM2O Cluster of Excellence, SFB 1176 Collaborative Research Centre and COORNETs (SPP 1928).

Photoconductivity in Metal–Organic Framework (MOF) Thin Films

X. Liu, M. Kozlowska, T. Okkali, D. Wagner, T. Higashino, G. Brenner-Weiß, S. M. Marschner, Z. Fu, Q. Zhang, H. Imahori, S. Bräse, W. Wenzel, C. Wöll, L. Heinke
Electron and hole transport in the SURMOF

Surface‐mounted MOF thin films (SURMOFs), containing porphyrin in the framework backbone and C60 guests loaded in the pores, show two orders of magnitude increase of On–Off photocurrent ratio. By comparison with results obtained for reference MOF structures and based on DFT calculations, we prove that donor–acceptor interactions between the porphyrin of the host MOF and the C60 guest molecules give rise to a rapid charge separation. Subsequently, holes and electrons are transported through separate channels formed by porphyrin and by C60, respectively. The ability to tune the properties and energy levels of the porphyrin and fullerene, along with the controlled organization of donor–acceptor pairs in this regular framework offers potential to increase the photoconduction On–Off ratio.

Angew. Chem. Int. Ed. 2019, 58, 9590

A de novo strategy for predictive crystal engineering to tune excitonic coupling

R. Haldar, A. Mazel, M. Krstić, Q. Zhang, M. Jakoby, I. A. Howard, B. S. Richards, N. Jung, D. Jacquemin, S. Diring, W. Wenzel, F. Odobel, C. Wöll
Photoluminescence was improved by computational screening

We integrated the photoactive compounds into metal-organic frameworks (MOFs) and tuned the molecular alignment by introducing adjustable “steric control units” (SCUs). We determined the optimal alignment of core-substituted naphthalenediimides (cNDIs) to yield highly emissive J-aggregates by a computational analysis. Then, we created a large library of handle-equipped MOF chromophoric linkers and computationally screened for the best SCUs. A thorough photophysical characterization confirmed the formation of J-aggregates with bright green emission, with unprecedented photoluminescent quantum yields for crystalline NDI-based materials. This data demonstrates the viability of MOF-based crystal engineering approaches that can be universally applied to tailor the photophysical properties of organic semiconductor materials.

Nat. Commun. 2019, 10, 2048

Light-Switchable One-Dimensional Photonic Crystals Based on MOFs with Photomodulatable Refractive Index

Z. Zhang, K. Müller, S. Heidrich, M. Koenig, T. Hashem, T. Schlöder, D. Bléger, W. Wenzel, L. Heinke
Photomodulation of the refractive index of a MOF

We investigate the photomodulation of the refractive index of a crystalline material, made of TiO2 layers and nanoporous MOF with azobenzene side groups. Spectroscopic ellipsometry and precise DFT calculations show the optical-density change results from the different orbital localizations of the azobenzene isomers and their tremendously different oscillator strengths. The photomodulation of the MOF refractive index controls the optical properties of the quasi-one-dimensional photonic crystal with Bragg reflexes reversibly shifted by more than 4 nm.

J. Phys. Chem. Lett. 2019, 10, 6626

Bunching and Immobilization of Ionic Liquids in Nanoporous Metal–Organic Framework

A. B. Kanj, R. Verma, M. Liu, J. Helfferich, W. Wenzel, L. Heinke
Field-dependent blocking of MOF pores by an ionic liquid

The experimental data combined with molecular dynamics simulations unveils astonishing dynamic properties of room-temperature ionic liquids (ILs) embedded in well-defined pores of metal–organic frameworks (MOFs). At low IL loadings, the ions drift in the pores along the electric field, whereas at high IL loadings, collective field-induced interactions of the cations and anions lead to blocking the transport, thus suppressing the ionic mobility and tremendously decreasing the conductivity. The mutual pore blockage causes immobilized ions in the pores, resulting in a highly inhomogeneous IL density and bunched-up ions at the clogged pores. These results provide novel molecular-level insights into the dynamics of ILs in nanoconfinement, significantly enhancing the tunability of IL material properties.

Nano Lett. 2019, 19, 2114