Toward Development of Radical Materials for Charge Storage: Synthesis and Electrochemistry of Benzotriazinyl Radical Derivatives

Date

2013-09-27

Authors

Oakley, Nicholas Alfred

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Abstract

The benzotriazinyl radical is a highly stable organic radical that is known to possess fast and reversible oxidation and reduction electrochemical processes. Such properties make it an ideal candidate for use as an anodic or cathodic charge storage material in a new class of high-power secondary batteries known as organic radical batteries. Towards this application, several new benzotriazinyl radical derivatives were synthesized and fully characterized using electronic absorption, EPR, and IR spectroscopy as well as elemental analysis and mass spectrometry. The electrochemical properties of the radicals were studied using cyclic voltammetry. The introduction of electron donating groups onto the structure of the radical was found to result in cathodic shifts in both of the electrochemical processes, without loss of reversibility. It was also found that in some cases functional groups led to the destabilization of the radical to a known chemical oxidation pathway that resulted in the formation of closed-shell iminoquinone compounds. These materials demonstrated good multi-electron accepting properties, undergoing two reversible one-electron reduction processes. Synthetic methodologies were developed for the preparation of two new classes of benzotriazinyl biradicals. One class used an expansion of a known benzotriazinyl radical synthesis to prepare a m-phenylene-bridged biradical, while the other class used microwave-assisted synthesis to prepare biradicals bridged by electron accepting aromatic diimides. Spectroscopic studies of both classes of biradical showed electronic isolation of the two radicals within each molecule, consistent with computational predictions. This resulted in minimal perturbation of the electrochemistry of these compounds from that of typical benzotriazinyl radicals. The solid state properties of a selection of benzotriazinyl radical derivatives were studied. Structural information obtained through single crystal X-ray diffraction studies showed significant intermolecular π-π and hydrogen bonding interactions. These solid state interactions were found to provide pathways for magnetic exchange, as determined using SQUID magnetometry. Additionally, preliminary conductivity studies indicated semiconducting behaviour in the compounds that were studied, warranting further studies. Anionic polymerization of a vinyl-functionalized benzotriazinyl radical was investigated as a method for the synthesis of a pendant benzotriazinyl polyradical with a saturated backbone. The electrochemistry of the putative polymer was identical to the monomer, maintaining reversibility of both the oxidation and reduction processes and verifying that the polymer could be used as an anodic or cathodic charge storage material. SQUID magnetometry was used to estimate a polymer spin content to be ~ 44 %.

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Keywords

organic radical, batteries, biradicals

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