The role of molecular oxygen (O₂) and UV light in the anion radical formation and stability of TCNQ and its fluorinated derivatives

We report the electronic absorption spectroscopy of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives (F2TCNQ and F4TCNQ), well-known electron-accepting molecules in common organic solvents (toluene, chlorobenzene, acetonitrile, and ethanol) under controlled exposure to air (O₂) and UV light. All compounds (FxTCNQ (x = 0, 2, 4)) were stable in a neutral state (FxTCNQ⁰) in toluene and chlorobenzene, even under both O₂ and UV light. On the other hand, in EtOH, the formation of FxTCNQ^·− was monitored upon controlled exposure to O₂ or UV light. Especially in air-equilibrated ethanol upon the UV-illumination, efficient α,α-dicyano-p-toluoylcyanide anion (DCTC⁻) and its fluorinated derivatives were generated evinced by the absorption peak near 480 nm, whereas the reaction was shut off by removing O₂ or blocking UV light, thereby keeping FxTCNQ^·− stable. However, even in deaerated ethanol, upon the UV-illumination, the anion formation of TCNQ and its fluorinated derivatives (FxTCNQ^·−, x = 0, 2, 4) was inevitable, showing the stability of FxTCNQ⁰ depends on the choice of solvent.