Quantifying Environmental Effects on the Solution and Solid-State Stability of a Phenothiazine Radical Cation

Organic radical cations are important intermediates in a wide variety of chemical processes. To date, significant progress has been made to improve the stability of these charged materials for use in electrochemical energy storage applications, especially in redox flow batteries. Here, we report the synthesis and isolation of four radical cation salts of N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), synthesizing MEEPT-X where X is tetrafluoroborate (BF4–), hexafluorophosphate (PF6–), perchlorate (ClO4–), and bis(trifluoromethanesulfonyl)imide (TFSI–), and a comparison of their stability in solution and in the solid state. In the solution, UV–vis spectroscopy and rotating ring-disk electrode voltammetry show similar stability trends with respect to anion identity, with the TFSI– salt being the most stable. In the solid state, these compounds show remarkable stability in air and at elevated temperatures, with the ClO4– salt surviving after being heated at 90 °C overnight in air. The different trends in MEEPT-X stability with X highlight the importance of concentration and the environment on the overall stability.