A PROOF-OF-CONCEPT EXPERIMENT TO INVESTIGATE FAST CHARGING TRANSIENTS BY SCANNING KELVIN PROBE MICROSCOPY AND 2) STUDIES ON BRIDGED RUTHENIUM COMPLEXES

Nathan, Sarah Ruth (2018) A PROOF-OF-CONCEPT EXPERIMENT TO INVESTIGATE FAST CHARGING TRANSIENTS BY SCANNING KELVIN PROBE MICROSCOPY AND 2) STUDIES ON BRIDGED RUTHENIUM COMPLEXES. Doctoral thesis, Cornell University.

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Abstract

Scanning kelvin probe microscopy has been to used to understand the operation of, and optimize the performance of both organic and inorganic photovolataic materials on the 10’s of nm scale. Most studies have only examined the spatial distribution of the electronic phenomena – including potential, photopotential, capacitance, photocapacitance, current, and photocurrent – however, there is much to be learned by measuring and mapping the time-dependence of these characteristics. In organic bulk heterojunction solar cells, Ginger and coworkers have shown by time-resolved electrostatic force microscopy (tr-EFM) that the microscopic photocapacitance charging rate in a film is directly proportional to the external quantum efficiency (EQE) of a photovoltaic device. Transients with sub-microsecond time resolution have been collected, but acquiring these transients required long signal-averaging times. Building on the previous work of Moore, Marohn, and co-workers, we developed a method to rapidly acquire transients of capacitance, frequency and phase indirectly, in a stepped-time, stepped-voltage experiment, by encoding and measuring the capacitance as a change in the phase of a vibrating cantilever. In a proof-of-concept experiment we show that this new method is viable for observing transients down to 100 μsec. This approach represents an exciting new route to understanding geminate recombination in photovoltaic materials. Mitochondrial calcium plays a critical role in regulating cell survival, apoptotic pathways, and cellular energetics. Acute overload of mitochondrial calcium will induce cell death and is implicated in the damage caused by lethal reperfusion injury. Inhibitors of mitochondrial calcium uptake minimize the harm caused by such conditions. The most well characterized mitochondrial calcium uptake inhibitor is Ru360, a μ-oxo-bridged dinuclear ruthenium complex. Its synthesis and purification is challenged by extremely low yielding reactions and purification by tedious cation exchange chromatography. Furthermore, this compound is not cell permeable, a feature that significantly limits its biological use. With the goal of preparing potent mitochondrial calcium uptake inhibitors that are biologically available, we investigated the synthesis and characterization of six dinuclear ruthenium species. One complex is a structural analog of Ru360, and five are structurally similar dinuclear ruthenium μ-nitrido complexes. These compounds were characterized by NMR spectroscopy, EPR spectroscopy, UV-vis spectroscopy, infrared spectroscopy, cyclic voltammetry, conductivity, and small-molecule crystallography. Mitochondrial calcium uptake inhibitory activity in permeabilized and unpermeabilized cells are described. Cellular uptake of the complexes and cytotoxicity are presented and discussed in the context of molecular structure. One of these complexes shows 10× better MCU-inhibitory activity, low cytotoxicity, and enhanced cellular uptake compared to Ru360.

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