Development of Transient Attenuated Total Reflectance Spectroscopy and Investigation of Photoinduced Kinetics in Thin Films

Abstract

The efficiency of photoconversion systems such as organic photovoltaic cells and photocatalytic water-splitting cells is largely governed by the interfacial charge transfer processes. Understanding the structure-function relationship, specifically at the molecular thin film/transparent conducting oxide interface will allow for engineering these interfaces to promote charge transfer and reduce the rate of charge recombination. Important factors that are hypothesized to influence charge transfer are morphology, chemical characteristics, electronic properties and molecular orientation. As molecules are bound to a transparent conducting oxide or incorporated into a thin film, the local solid-state molecular environment greatly influences the excited state properties of the molecule. Pathways for quenching, radiative and nonradiative decay drastically limit the excited state lifetimes. In order to investigate the photoinduced kinetics of thin films and at interfaces a instrument was developed coupling transient absorbance spectroscopy to attenuated total reflectance spectroscopy. The photoinduced kinetics of a thin film of bacteriorhodopsin was used to evaluate the instrument performance, and it was determined that 1% of a close-packed monolayer could be detected with this geometry. The properties of a molecular thin film/transparent conducting oxide were investigated by tethering zinc porphyrin to ITO. The electrochemical properties were influenced by the functional group of the binding moiety. To improve our understanding of how the solid state molecular environment affects excited states lifetimes, zinc porphyrins were incorporated into mono- and multilayer thin films and measured with transient ATR spectroscopy. Finally, multilayer films related to photocatalytic water-splitting were investigated with the incorporation of inorganic nanosheets. The nanosheets helped to create a stratified assembly for multilayer films, spatially segregating electron donor (palladium porphyrin) and electron acceptor (poly(viologen)) molecules. The role of the nanosheets in the electron transfer between the donor and acceptor was studied by monitoring the triplet state lifetimes of a palladium porphyrin with transient ATR spectroscopy.