Synthesis, characterization and applications of metal organic frameworks and coordination polymers using highly conjugated ligands
Creators: name: JACKSON, Sam
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340108
subject: Sensor technology (incl. chemical aspects)
classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)
classificationCode: 340307
subject: Structure and dynamics of materials

description: Phosphonate metal organic frameworks (MOFs) and coordination polymers (CPs) have attracted considerable interest due to their functionality and thermal stability. However, progress in the characterisation of such materials has been hampered by their poor crystallinity and complex coordination modes, and their propensity to form non-porous layered solids. A possible solution to these issues is the use of large, bulky ligands which may counteract these tendencies. The first section of the research presented in this thesis describes the synthesis of coordination polymers using a naphthalene diimide (NDI) platform functionalised with 2 or 4 phosphonate groups. These are the first NDI / phosphonate networks to be reported, where the predisposition of the phosphonate ligand to form ill-defined amorphous solids appears to have been overcome. A phosphonate monoester network, formulated as [Ba(H2O)3(Et2NDI-BP)] was fully characterised via Fourier transform infrared spectroscopy (FTIR), powder x-ray diffraction (PXRD), single crystal x-ray diffraction (SCXRD), thermogravimetric analysis (TGA)/ Gas Chromatography (GC), and fluorescence spectroscopy. This compound represents the first reported crystal structure for a metal NDI phosphonate. The network forms a layered system with alternating layers of chains of barium(II) ions, bridged by phosphonate oxygens and densely packed naphthalene diimide cores. Due to the dense packing of the naphthalene diimides, the network is non-porous and only weakly fluorescent as a result of the close packing of the NDI cores which facilitates fluorescence quenching via charge transfer between the NDI cores. The crystallinity and yield of the product were primarily controlled by utilising an in situ hydrolysis of the phosphonate ester precursor ligand during the synthesis, and varying the polarity of the medium, the reaction time, the ligand to metal ratio, and the pH of the starting solution. Increased yield was associated with an increase in copreciptation of the ligand precursor. A barium bromide tetraphosphonate compound was synthesised in a similar fashion, but the microcrystalline nature of the solid precluded the determination of a definitive crystal structure despite extensive effort. Instead, it was characterised via FTIR, XPS (X-ray Photoelectron Spectroscopy), PXRD, TGA and fluorescence spectroscopy. The presence of barium, phosphonate and bromide were confirmed by XPS, and FTIR confirmed the presence of the ligand and water. The network collapsed upon evacuation to give an amorphous powder. This work demonstrates the potential of selective deprotection to synthesise thermally- and chemically-robust phosphonate networks. The NDI core shows promise as a bulky ligand platform in the preparation of novel networks. Careful control of the synthetic conditions has enabled the synthesis of a crystalline product allowing, for the first time, a more complete characterisation of these phosphonate coordination polymers, and providing synthetic and structural guidelines for future work. The remainder of this thesis is devoted to exploring the potential of the luminophore Py4-TPE to form metal-organic networks and the potential applications of these networks. Three isostructural metal organic frameworks were synthesised using tetrapyridyltetraphenylethene (Py4-TPE) as an organic ligand, in combination with copper(II), manganese(II) and nickel(II), to yield compounds formulated as: [M2+(Py4-TPE)Cl2]•4TCE, TCE= tetrachloroethene. The overall structure is that of a 2D plane with large rhomboidal channels. These crystalline networks were characterised via elemental analysis, FTIR, PXRD, SCXRD, TGA, fluorescence spectroscopy and gas adsorption measurements. The MOF networks only formed in the presence of tetrachloroethene and 2,3-dimethyl-2-butene, demonstrating the role that the planar, tetra-substituted ethene core plays as a structure directing agent in the formation of the network. Hexachlorobenzene demonstrated a similar structural directing effect, but induced a different topology and interpenetration of the network. This represents a further degree of control over the topology of the resultant network by varying the structure and composition of the co-solvent used during the synthesis. After removal of the entrapped TCE, the air-stable manganese(II) and nickel(II) networks were investigated for their gas adsorption properties. Both MOFs were found to have a degree of CO2 and CH4 adsorption, but negligible N2 and H2 adsorption. This is mostly likely a function of the molecular sieving effect and/or preferential adsorption via the residual polarity in the CO2 and CH4 interacting with the functional groups surrounding each pore. The [M2+(Py4-TPE)Cl2]•4TCE networks preserved the strong fluorescence arising from the AIE (Aggregation Induced Emission) effect of the Py4-TPE, modulated according to the coordinating metal. The fluorescence of the Py4-TPE networks persisted even after removal of the TCE. To enable a contrast, comparable Py4-TPM networks (Ni, Mn) were synthesised and found to be non-emissive. This is attributed to the lack of conjugation at the single central C-atom preventing AIE. A novel coordination polymer, formulated as [Zn2(Py4-TPE)Cl4]•4TCE, was also synthesised using zinc(II) and the Py4-TPE ligand. It is structurally distinct from the other networks consisting of 1D chains and containing a tetrahedral metal centre, but with a similar porosity and thermal stability to the previously described networks. The sensing properties of all the TCE free Py4-TPE metal(II) networks were explored, but only the zinc(II) derivative demonstrated both a wavelength and intensity shift in response to volatile organic compounds. A systematic study of these fluorescence responses found that liquid samples of methyl-, chloro- and methoxy-substituted benzenes generated a “turn-on” (enhancement) in the fluorescent intensity of the MOF. In contrast, nitro-substituted benzenes exhibited a “turn-off” (suppression) of the fluorescent intensity attributed to electron transfer between the conduction band of the MOF and the LUMO of the nitro analytes. The kinetics and reproducibility of the vapour adsorption of nitrobenzene and dinitrotoluene was also explored, and although effective, required several hours to achieve the maximum response. The [Zn2(Py4-TPE)Cl4] coordination polymer represents a new sensor, demonstrating both “turn-on” and ”turn-off” responses combined with predictable wavelength shifts. The AIE mechanism observed here is distinct from the fluorescence observed in most other coordination polymers, and this compound represents one of only a few dual-response porous coordination polymer reported. It represents an expansion of the field of luminescence coordination polymers and demonstrates the potential of novel coordination polymers to perform as sensors with variable response emissions. The use of two dicarboxylate anions as pillaring agents was explored with the Zn(II) Py4-TPE network, employing either the planar, linear benzene-1,4-dicarboxylate(BDC) or the ‘bent’ camphorate(cam). In the camphorate network [Zn(cam)(HPy4-TPE)]+NO3-], cam = camphorate2-), each zinc(II) coordinates with three oxygens on two camphorates and two Py4-TPE ligands. The Py4-TPE ligands in turn coordinate with two zinc ions, a single carboxylate oxygen via a hydrogen bond with a protonated pyridyl group, and possess the unusual single non-coordinating pyridyl, which extends into the water occupied pores. This creates a crosslinked 1D chain that is bound to other chains by hydrogen bonds between the protonated Py4-TPE and the camphorate ion. This hydrogen bond requires a balancing anionic charge, and this is provided by a disordered nitrate ion within each pore in the framework. The network is less thermally stable than similar networks due to the protonated phenylpyridinium arm and the nitrate, leading to the facile formation of HNO3 when heated which subsequently reacts with the ligands. The [Zn4(BDC)4(Py4-TPE)] network consists of pairs of zinc(II) ions, each coordinating to a Py4-TPE molecule and three BDC dianion oxygens, arising from 2 monodentate carboxylates and one bridging. The overall framework is a 2D plane consisting of the Py4-TPE, two pairs of dimer zinc(II) ions and two BDC anions linked to other planes by the other two BDC anions. The network is porous with hexagonal channels running through it, and has two-fold interpenetration. Comparison of the BDC and camphorate networks clearly illustrates the influence of the two different pillaring agents that not only create different layering motifs but also alter the coordination of the Py4-TPE ligand. This work expands the known pillaring agents for Py4-TPE MOFs, and demonstrates a novel direction forward where it may be possible to use ‘bent’ co-ligands to deliberately introduce free, uncoordinated pyridyl groups. The strong fluorescence behaviour of the Py4-TPE ligand continues within these networks. The BDC network has a red shift of 41nm with an approximately four-fold increase in fluorescence, while the camphorate network has a larger red shift (60nm) with an approximately two-fold increase in fluorescence, suggesting that both networks are promising candidates for future luminescent applications.