Computational Investigation of Electrical Conductivities and Magnetism of Phosphonate Metal-Organic Frameworks and Frontier Orbital Gaps, Structure, and Proton Transfer Mechanisms in Phosphonate Hydrogen-Bonded Organic Frameworks
Creators: Peeples, Craig
Publisher: name: University of Alberta Library
Year: 2024

<p>The study of metal-organic frameworks (MOFs) and hydrogen-bonded organic frameworks (HOFs) has gained much interest over the last decade.  MOFs and HOFs are constructed through the self-assembly of organic building units and metals (in the case of MOFs), giving rise to a diverse range of framework materials with varying properties.  Herein, we focus on an emerging class of MOFs/HOFs known as phosphonate MOFs/HOFs, which are generally known to be highly thermally and chemically stable.  We computationally study the electronic structures of several recently synthesized phosphonate MOFs/HOFs and the mechanisms of proton conduction in two phosphonate HOFs, to gain a deeper understanding of the microscopic origins of their macroscopic properties.  We use density functional theory (DFT) to study the structures and electronic factors contributing to their electrical conductivity and magnetic behaviour and Born-Oppenheimer molecular dynamics (BOMD) to study the proton conduction mechanisms in the HOFs.  In the DFT studies, we considered three copper phosphonate MOFs, namely, TUB75, TUB40, and TUB1.  Our work on TUB75 revealed a low frontier orbital gap, which paved the way for our further study of electrically conductive phosphonate MOFs.  Our DFT calculations of the highest occupied crystal orbital (HOCO) to lowest unoccupied crystal orbital (LUCO) gaps agree well with the experiment results.  An analysis of the orbitals revealed that the HOCOs lie on the organic linkers, while the location of the LUCOs is system-dependent.  We also confirmed that the magnetic behaviour of these MOFs is due to the unpaired electrons on the copper atoms.  In addition to the DFT studies on MOFs, we considered five porphyrin-based phosphonate HOFs, namely GTUB-5, Cu-, Ni-, Pd-, and Zn-GTUB-5, investigating their structures, HOCO-LUCO gaps, and relative hydrogen bond strengths.  From the experimental structure, Ni-GTUB-5 is twisted compared to the other GTUB-5 systems.  Our DFT results suggest that this twisting is due the d-orbital composition of the Ni HOCO, short Ni-N bonds in the porphyrin, and sharing of the electron density between the Ni and N atoms in the porphyrin.  We found that the HOCOs and LUCOs for GTUB-5, (α-spin) Cu-, Pd-, and Zn-GTUB-5 all lie on the porphyrin, while the LUCO for (β-spin) Cu-GTUB-5 is localized on the copper d-orbitals, its HOCO lies on the porphyrin, and the HOCO and LUCO for Ni-GTUB-5 are localized on the nickel d-orbitals.  A DFT-based vibrational analysis on Zn-GTUB-5 revealed that the O-H bond stretch gives rise to a similar peak structure/width for all the HOFs, suggesting that all four metalated HOFs have similar hydrogen bond strengths.In the BOMD study, we examined the proton transport mechanisms in two HOFs, namely GTUB-5 and UPC-H5a, under humid conditions.  These two HOFs have similar porphyrin-based building blocks, but UPC-H5a has a metal in its porphyrin core (while GTUB-5 does not) and GTUB-5 has an extra phenylphosphonate linker in its unit cell.  Furthermore, the experimental activation energies suggest a Grotthuss mechanism for proton transport in both cases.  However, despite their similarities, UPC-H5a has a much higher experimental proton conductivity than GTUB-5.  To study the proton transport mechanisms and gain insight into this difference, we inserted water molecules and excess protons into the pores of each HOF and simulated their dynamics using a combination of BOMD and metadynamics (MTD).  Radial distribution functions (calculated from unbiased BOMD trajectories) show that, on average, the O-O bond lengths in GTUB-5 are shorter than in UPC-H5a, whereas the O-H bond lengths are shorter in UPC-H5a than in GTUB-5, suggesting that GTUB-5 has stronger hydrogen bonds than UPC-H5a.  Based on the unbiased trajectories, we identified three proton transport pathways, which were used to define the MTD simulations: water-to-water (WtW), water-to-framework (WtF), and framework-to-framework (FtF).  The MTD results reveal that, for both GTUB-5 and UPC-H5a, the free energy barriers associated with proton transfers between hydrogen bonded oxygen atoms increase in going from WtF to WtW to FtF, and all pathways in UPC-H5a have lower barriers compared to their respective ones in GTUB-5.  Lastly, we observed three proton transport mechanisms in the pathways, viz., single-proton, stepwise, and concerted transport.  </p>