Electrochemistry of Polyfluorinated Organic Molecules in Nonaqueous Electrolyte Solutions
Authors: Sun Haoran 
Published: September 2023

Fluoroorganic molecules provide significant benefit for many applications including pharmaceuticals, agrichemicals, and advanced functional materials due to its highly stable carbon fluorine bonds. Fluorination can have profound effects upon tuning the electronic and optical properties of organic molecules. Thus, fluorine is found in many state-of-the-art materials. For example, fluorinated liquid crystals are used for active-matrix liquid crystal displays; fluorinated membranes are utilized as proton transfer elements necessary for the function of fuel cells; fluorinated electrolyte solutions are tested in lithium batteries to improve their long-term stability and cell voltage. Lately, fluorinated and perfluoroalkylated organic semiconductors were reported for its application in organic field effect transistors (OFETs). Fluorination can tune a p-type organic semiconductor into an n-type semiconductor because of the strong electron withdrawing effect of fluorine, though use of multiple fluorine substituents directly on the aromatics or use of trifluoromethyl and perfluoroalkyl substituents on the pi system is essential for such conversion.

The high electronegativity of fluorine results in exceptionally strong bonds to carbon (BDE = 110-126 kcal/mol). Moreover, fluorine substitution results in an increase in the C-C bond energy in fluorocarbons. Strong C-F and corresponding strong C-C bonds in fluorocarbons make them less polarizable than typical hydrocarbons. The highly polar, weakly polarizable, non-hydrogen bonding, non-coordinating properties of perfluoroalkylated materials and perfluoropolyether materials, underlie the motto “Nothing sticks to Teflon® and Teflon®-coated products”. Though all these profound properties of fluoroorganics paved the road for applications exemplified above, their superior stability creates significant challenges for their end-of-life recycling and repurposing, as well as remediation of environmental persistent fluoroorganics such as PFAS. Furthermore, the chemical stability of many these materials often refers to the stability under oxidizing conditions. For fluorinated n-type organic semiconductor materials and fluorinated solvents (e.g. fluoroethylene carbonate) in lithium battery applications, access the stability of these materials and understand its decomposition mechanism under reductive conditions are necessary for rationally optimize the conditions for such applications.

In this presentation, we will present and discuss our latest electrochemical study of several different types of polyfluorinated organic molecules involving both fluorinated and perfluoroalkylated aromatics, as well as inert hydrofluorocarbons where only sp3-hybridized carbon presents. As an example, our electrochemical and computational chemistry results indicate that aromatic trifluoromethlation is likely a more stable option against reduction than pentafluoroethylation of the same aromatic core, though the electronic effects of trifluoromethyl and pentafluoroethyl substituents on aromatics are almost identical. We hope that these electrochemical results will shine some lights on searching new materials for electronic and energy storage applications and repurposing inert hydrofluorocarbons, for example, pentafluoroethane, a refrigerant with high global warming potential to be phased out in the next 10-20 years.