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 (O-2) and UV light. All compounds (FxTCNQ (x = 0, 2, 4)) were stable in a neutral state (FxTCNQ(0)) in toluene and chlorobenzene, even under both O-2 and UV light. On the other hand, in EtOH, the formation of FxTCNQ(.-)was monitored upon controlled exposure to O-2 or UV light. Especially in air-equilibrated ethanol upon the UV-illumination, efficient alpha,alpha-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 O-2 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 FxTCNQ(0) depends on the choice of solvent.