New Organic Nanostructures for Flexible Optoelectronic Devices

Jeremy Ray Eskelsen

Whenever a new material is considered for application in any type of electronic device, the first question is: Does this material possess electronic and physical properties that are desirable for this application? Organic materials essentially present a limitless pool to work with, due to their tunability by molecular design. One particularly useful organic material is porphyrins. Structurally similar to the light-harvesting chromophores in chlorophyll and active sites in hemoglobin, porphyrins have a wide potential for applications in various optoelectronic devices such as photovoltaics, sensors, and transistors. Additionally porphyrins can be self-assembled into nanostructures that possess new and enhanced properties compared to the starting tectons. The molecular driving forces behind the nanostructure formation and their physical properties is not well understood. A combinatorial approach using UV-Visible and X-ray photoelectron spectroscopy, atomic force and electron microscopy, electron and X-ray powder diffraction, and computational calculations were employed to provide insight into the structural and elastic properties of these nanostructures. Included is a discussion of nanostructures formed by the ionic self-assembly of a single porphyrin meso-tetra(4-carboxyphenyl)porphyrin (TCPP) and binary systems of a cationic and an anionic porphyrins. There are two main binary porphyrin systems studied. The first consists of meso-tetra(4-pyridyl)porphyrin (TPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (TSPP). The second consists of meso-tetra(4-aminophenyl)porphyrin (TAPP) and TSPP. Throughout these investigations there is great insight into the role of protonation of the individual porphyrin tectons on the overall structure and elastic properties of these materials. For example, from chemical composition and elemental analysis studies what protonation is expected in solution is not always what results in the solid state. Additionally, models of the internal structure of the nanostructures were generated from diffraction studies and the chemical composition studies. The first mechanical measurements of porphyrin based nanostructures using an atomic force microscope (AFM) as a nanoindenter were also performed, with Young’s modulus values observed similar to most polymers. The TAPP and TSPP system is the first ionic organic nanocrystal to have a sheaf-like structure. These finding show that porphyrin nanostructures are promising materials for use in optoelectronic devices, particularly in flexible electronics.

New Organic Nanostructures for Flexible Optoelectronic Devices

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