Thiele’s Fluorocarbons: Stable Diradicaloids with Efficient Vis-NIR Fluorescence from a Zwitterionic Excited State

Thiele’s hydrocarbon was the first synthesized diradicaloid in the search for stable open-shell structures but itself remains sensitive to oxygen and light. We here report the synthesis of Thiele’s fluorocarbon (TFC) and its derivatives exhibiting exceptional thermal, oxidative, and photostability. TFCs have remarkable luminescent properties with yellow to NIR fluorescence and up to 100% quantum yields. X-ray crystallography and ESR spectroscopy confirm their closed-shell quinoidal ground state. As expected from their uniform HOMO/LUMO distributions, TFCs’ absorption spectra show no solvent effect, but their emission reveals an extraordinarily large Stokes shift which increases with solvent polarity (from 0.9 eV in cyclohexane to 1.5 eV in acetonitrile). We show that this behavior is a result of sudden polarization leading to a zwitterionic excited state.