OPTOELECTRONIC PROPERTIES OF SELF-ASSEMBLED BINARY PORPHYRIN NANOSTRUCTURES
Borders, Bryan Clarke
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Porphyrins are structurally similar to light harvesting chromophores found in nature and to active sites in hemoglobins. As such, materials made of porphyrins are useful in a variety of applications such as photovoltaics, sensors, and photocatalysis. Furthermore, self-assembled aggregates of porphyrins can exhibit properties different from those of the constituent molecules, offering another way to tune their properties. Ultimately, the goal of this work is to to synthesize porphyrin nanostructures designed for specific applications. Using X-ray powder diffraction, UV-visible and luminescence spectroscopy, Kelvin Probe Force Microscopy, conductivity measurements, and computational methods, the optoelectronic properties of six binary porphyrin nanostructures were characterized. The application of a nucleation and growth model allowed for the production of XRD quality crystals from which the structures of several of these nanorods were determined. These crystals were composed of meso-tetra(N-methyl-4-pyridyl) porphyrin (H2TMPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (H2TSPP) or their metallated analogs. Having these crystal structures allowed for the use of computational methods to help correlate the crystal structure to the optoelectronic properties, particularly the photoconductive properties. It was found that the addition of a metal ion to the core of either the H2TSPP or H2TMPyP didn’t affect the crystal structure, but did result in a significant change in the optoelectronic properties of the crystals. The persistent photoconductivity observed in many of these systems, including crystals composed of meso-tetra(4-pyridyl)porphyrin (H2TPyP) and diacid meso-tetra(4-sulfonatophenyl)porphyrin (H4TSPP), was modeled. This behavior is attributed to the formation of metastable defects that allow for a Miller-Abrahams hopping mechanism. We demonstrated that crystals composed of meso-tetra(4-aminophenyl)porphyrin (H4TAPP) and H4TSPP could be grown as either sheaves or as rods and both morphologies have the same crystal structure. There are some differences in the properties of the sheaves and the rods which result from the large number of defects within the sheaves. Finally, it was observed that the photoconductivity of the metal-free porphyrin systems (H2TMPyP:H2TSPP, H2TPyP:H4TSPP, and H4TAPP:H4TSPP) decreases upon exposure to oxygen gas due to the formation of O2- causing the number of free electrons within the crystals to decrease. These findings have provided insight into the origin of optoelectronic properties of porphyrin nanostructures.