A rational design based on the proton-coupled electron transfer (PCET) concept allows us to structurally characterize for the first time isolable, air- and moisture-stable semiquinone radicals in a zwitterionic neutral form.
The presence of an alkoxy and the bulky pyridinio substituents causes only a minor perturbation of either the redox potentials or the spectral UV-vis characteristics of the semiquinone core but significantly stabilizes the new radicals.

This work was carried out in the group of Prof. Silvio Decurtins under the guidance of Dr. Shi-Xia Liu.

References:

• C. Yi, C. Blum, S.-X. Liu, T. D. Keene, G. Frei, A. Neels and S. Decurtins;
  "Isolable Zwitterionic Pyridinio-semiquinone p-Radicals. Mild and Efficient Single-Step Access to Stable Radicals"
  Org. Lett., 11, 2261-2264, (2009); doi:10.1021/ol900559p.