Structure−Property Relationships of Stable Free Radicals: Verdazyls with Electron-Rich Aryl Substituents

Substitution of the 3 position of 6-oxoverdazyl free radicals with electron-rich arylamines, phenols, and aryl ethers elicits changes in the UV−vis spectra and in the pKa of the aryl substituents consistent with the verdazyl being electron withdrawing. The pKa of substituents is decreased: in 80% methanol phenols 3a and 3b have pKa of 10.4 and 10.9, respectively, while the ammonium ion from protonation of 3j has pKa = 2.4. On the basis of these measurements, Hammett parameters for the verdazyl have been estimated: σp− = +0.48 and σm = +0.27. The longest wavelength band in the visible spectrum is red-shifted with increasingly electron-rich aromatic rings and with increasingly polar solvents, consistent with a transition from the highest fully occupied orbital to the radical SOMO. Exceptions occur when additional interactions occur between verdazyl and substituent; hydrogen bonding in the case of 3c and steric interference for 3f. Measurements such as ESR and electrochemistry that are dependent largely on the SOMO are relatively insensitive to changes in substituent.