Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemistry

Major Professor

Randy W. Larsen, Ph.D.

Committee Member

James Leahy, Ph.D.

Committee Member

Gloria Ferreira, Ph.D.

Committee Member

Brian Space, Ph.D.

Keywords

Peroxidase Activity, Photophysical Properties, Singlet Oxygen

Abstract

The Global energy supply relies mostly on fossil fuels and only 10% comes from renewable energy. These places humankind in an unsafe situation as the fossil fuel resources won’t last forever. More efficient use of renewable energy is required sooner than later to meet the growing energy need. A promising approach is to utilize light harvesting chromophores (e.g., porphyrin derivatives) to advance organic photovoltaics and increase its efficiency or to improve photocatalysts. Metal-organic frameworks (MOFs) can be employed to aid the self-assembly of the porphyrin chromophores in a large light-harvesting system comparable to the natural photosystem. The advantage of using MOFs in such systems are discussed intensely in this thesis with a focus on how topologically different structures can modify the physicochemical properties of the porphyrin chromophores with the aim to improve their light harvesting properties. Pore modulation and tunability give MOFs an advantage over all other materials as they can be tailored to offer a specific characteristic for a specified application.

The dissertation displayed within the following seven chapters is focused on the synthesis, design and characterization of metalloporphyrins encapsulated and incorporated within MOFs for light harvesting application to understand the metalloporphyrins photophysical properties and the possible ways of modifying it through tuning the host. The essential understanding of electron transfer processes in predesigned pathways. In the first chapter, a brief introduction of light harvesting, photophysical and photochemical process, metalloporphyrins and MOFs are explained. The second chapter describes the techniques used in this work. The third chapter discusses the ability of tuning the MOFs to modulate the photophysical properties of encapsulated guests. The photophysical properties of tetra (N-methyl-4-pyridyl)-21H,23H porphine (TMPyP) encapsulated within two Cd-based MOFs (MOM-11 and MOM-12) are reported. The results show that the pores of CdTMPyP@MOM-11 and CdTMPyP@MOM-12 fix the orientation of the porphyrin peripheral pyridinium groups relative to the porphyrin plane. These fixed orientations have a significant effect on the porphyrin S1-CT coupled excited state leading to differences in steady state emission, emission lifetimes, and absorption properties. The pores of CdTMPyP@MOM-11 and CdTMPyP@MOM-12 also restrict out of plane porphyrin ring distortions associated with sit-atop porphyrins resulting in extended triplet state lifetimes. The results demonstrate that specific pore structures can systematically modulate the excited state properties of TMPyP type porphyrins by regulating accessible guest conformations. The fourth and fifth chapters discuss new novel porphyrin-based metal organic frameworks RWLAA-3 and RWLAA-1 for light harvesting applications. A study of the photophysical properties of Zn(II)TMPyP-RWLAA-3, formed from cadmium ions and benzene1,3,5-tribenzoate (H3BTB) templated by zinc (II) tetramethyl pyridyl porphyrin, Zn(II)TMPyP was investigated. The results show that the dihedral angle rotation of substituted pyridinium groups compared to the porphyrin macrocycle is combined with the hydrophobicity inside the cavity of MOFs to explain the large bathochromic shift observed in the optical and emission spectra. The lifetime measurements suggested rapid singlet lifetime and longer triplet decay due to the encapsulation of porphyrin that causes a change in the vibrionic states and spin-orbital coupling between the involved states. The photophysical properties of Zn Tetra(4-pyridyl) porphyrin incorporated within RWLAA-1 was also studied. The structure of RWLAA-1 was confirmed through single-crystal X-ray diffraction. Incorporation of the ZnTPyP into the framework results in significant distortions within the macrocycle that have a profound effect on the photophysics of the material including bathochromic shifts in the optical spectrum (Soret and visible bands) and emission bands as well as a reduction in energy separation between the Q(0,0) and Q(0,1) emission bands. The corresponding triplet state lifetime of RWLAA-1 is slightly shorter than ZnTPyP (∼1 ms and  ∼1.4 ms respectively) that are likely due to changes in the vibrational density of states connecting the triplet excited state and singlet ground state.

In the six and seventh chapters, the applications of metalloporphyrin MOFs are explored. The use of Cd2+TMPyP@MOM-12 and Cd2+TMPyP@MOM-11 to generate singlet oxygen is discussed in chapter six. The absorbance quenching of these PDT-MOFs was compared to the Cd2+TMPyP in solution. The reaction is monitored by the photo-oxidation of anthracene carboxylic (H2ADC) chemical trapping agent via ultraviolet-visible absorption. The degradation rate order of H2ADC was observed as follows Cd2+TMPyP@MOM-12> Cd2+TMPyP@MOM-11>Cd2+TMPyP. The results from chapter six prove the advantages of using MOFs as a host for light harvesting chromophores.

In Chapter seven the peroxidase kinetic mechanism of Fe(III)4SP@HKUST-1 MOMzyme-1 is investigated by probing the reaction rate as a function of hydrogen peroxide and heme concentrations. The results are compared with Fe4SP in solution as well as the myoglobin. In chapter eight a summary of all chapters and recommendations for future work are presented.

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