Synthesis and Characterization of Viologen Derivative-CdSe Quantum Dot Charge Transfer Pairs
Kamm, Judith M.
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As the quest for viable alternative energy sources continues, semiconductor quantum dots (QDs) and their ability to act as charge transfer partners are receiving a great deal of attention. This experiment investigated the effect that varying the alkyl chain length on viologen derivatives bound to cadmium selenide QDs has on surface interactions, specifically charge separation and recombination rates. Disubstituted carboxylic acid derivatives of the methyl viologen cation containing one, two, and three carbon linking chains were synthesized and electron transfer rates determined through transient absorption laser spectroscopy. It was seen that these molecules did in fact coordinate at the carboxylic acid site and that, as this linker length increased, the rates of both charge separation and recombination also increased. This is most likely because the longer carbon chain allows the molecule to conformationally orient itself closer to the surface of the QD allowing for a through-space and not a through-bond electron transfer. Continued work in synthesizing asymmetric molecules containing both the carboxylic acid group and a long alkyl chain will allow for the control of the orientation of the molecules at the QD surface and test this hypothesis. In this case, increasing linker length should slow down the rate of electron transfer. This will be of great use to future researchers investigating QD-ligand systems and their applications in photovoltaic cells as the rate and method of charge transfer could both be controlled.