Probing electronic structure and reactivity in Ti-based metal organic frameworks

Metal organic Frameworks (MOFs) are a class of materials that consist of metal ion building units connected by organic linking moieties. These materials have high porosity and surface area, as well as convenient tunability due to the modularity of metal building unit and organic linker sites. These properties have made MOFs interesting candidates for applications such as gas storage and photocatalysis. MOF photocatalysis is an area of this research that is of particular interest. For example, Ti based MOFs have been identified as materials that may be useful to catalyze reactions previously facilitated by TiO2. Ti based MOFs can be of use here due to their increased porosity and surface area compared to traditional semiconductor materials. However, the understanding of how the electronic behavior of these materials is not widely understood. Traditional characterization methods are limited in their ability to probe MOF electronic states effectively. This dissertation aims to better understand the electronic structure of these Ti MOF materials through the use of X-ray spectroscopy methods. Following the introduction chapter, which provides a pertinent overview of metal organic frameworks, their modification strategies and the set of spectroscopy methods used to study them, the second chapter investigates how the introduction of a heterometallic trap site into a MOFs secondary building unit can affect the lifetime and reactivity of the photoinduced charge separated state. Fe doped MIL125-NH2 was synthesized as a function of heterometal doping level and the lifetime of the photoinduced ligand to metal charge transfer event was probed through Fe K-edge XTA. The photodegradation of Rhodamine B dye was carried out as a test reaction to measure the reactivity of these charge separated states. The third chapter uses RXES as a tool to measure the electronic environment of Ti sites within a range of MOF materials. Ti Kβ RXES was measured for anatase TiO2, MIL125-NH2 and MIL177 to show subtle changes in electron localization with resolution not possible from traditional 1D XAS techniques. The effect of doping MIL125-NH2 was also investigated. The RXES plane of Fe doped MIL125-NH2 was compared against the parent MOF to see if changes in electron localization around the Ti sites upon Fe doping would be reflected in the Ti kβ RXES planes. The fourth chapter looked to use established MOF modification techniques as tools to alter the band gap-like properties of the materials, in an attempt to tune their reactivities. A suite of first row transition metal doped MIL125-NH2 was synthesized, and their reactivity was probed through the RhB degradation test reaction seen in the first project. The amino tag of MIL125-NH2 was also post synthetically modified, introducing a chromophore tag that pushed light absorption of the framework further into the visible region. Again, these materials were then tested using the RhB degradation test reaction. MOFs that had both the heterometal dopant and the chromophore tag were generated and were screened using the RhB test reaction. Finally, solvent assisted ligand exchange procedures were carried out on MIL177 to promote visible light absorption in the framework. A perylene carboxylic acid chromophore was inserted into the framework post synthetically through an exchange with labile formate groups on the MOF’s SBU. The modified framework was characterized and suspended as nanoparticles before being measured by OTA. The presence of a perylene cation band shown in the OTA results was indicative of successful photoinitiated charge transfer from the perylene antenna onto the framework.