Steric Manipulation as a Mechanism for Tuning the Reduction and Oxidation Potentials of Phenothiazines

Synthetic chemists customarily tune the redox characteristics of π-conjugated molecules by introducing electron-donating or electron-withdrawing substituents onto the molecular core, or by modifying the length of the π-conjugated pathway. Any steric effects of such efforts on molecular geometry typically affect both the neutral and charged (oxidized or reduced) states indiscriminately. However, in electroactive systems that undergo significant conformational changes upon oxidation or reduction, we can leverage the steric and inductive effects of substitution to attain considerable control over individual redox potentials. Here, we make use of density functional theory to elucidate the interplay between electronic and geometric effects of peripheral substitution on the model system of phenothiazine. For instance, we introduce substituents at positions ortho to the nitrogen atom (positions 1 and 9) to induce steric strain in the radical-cation state without significant effect on the neutral molecule, thereby augmenting the overall ionization potential. Notably, this steric effect persists for electron-donating substituents; the resulting ionization potentials therefore deviate from outcomes foretold by Hammett constants. Moreover, the same procedure has limited effect on electron affinities because of differences in phenothiazines' relaxation process upon reduction compared to oxidation. Our results promote molecular design guidelines for manipulating redox potentials in classes of electroactive compounds that experience dramatic changes in geometry upon ionization.