Twisting of diarylnitroxides: An efficient tool for redox tuning

The electrochemical behavior of a new type of stable diarylnitroxide radicals with bulky substituent in the ortho-position of one of the phenyl rings is described. The presence of the substituent results in broken conjugation of one of the phenyl rings with the NO moiety (the other ring stays conjugated) giving rise to so called “twisted nitroxides”. New diphenylnitroxides with t-Bu and/or CF3 substituents were synthesized and are the first examples of diarylnitroxides which are extremely stable in solution and form both stable oxidized and reduced forms (oxoammonium cations and aminoxyl anions). These compounds exhibit fast ET kinetics and reversible electrochemical oxidation and reduction. Varying the torsion angle θ shifts Eox and Ered values in the opposite directions thus increasing the electrochemical potential gap, contrary to the traditional effects of substituents that commonly shift both potential values in the same direction. The electronic effect of the substituents in the ortho-position as well as the total electronic effect of both rings on the electrochemical potential values was estimated using the linear fit for the relationship between E1/2Ox+E1/2Red vs. the sum of Hammett constants (σp + σo·cosθ). The ρ value (the slope in the linear fit) is 0.501 indicating significant sensitivity of the potential values to the electronic effects and wide possibilities for tuning. The results obtained shed light on the efficient principles of molecular design for stable electroactive nitroxide-based materials.