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.