Stimuli-Responsive Thiele’s Hydrocarbon Derivatives: Potential Inversion, Strong Electronic Coupling, and Influence of Brønsted/Lewis Acids and Bases

Stimuli-responsive diradicaloid systems often display fascinating and tunable electrochemical, optical, and magnetic properties. Herein, we present the design and synthesis of a series of nitrogen-containing Thiele’s hydrocarbon derivatives (2Ph, 2Py, and 2Pz), based on phenyl, pyridinyl, and pyrazinyl spacers, with tunable electrochemical, electronic, and optical properties. By systematically increasing the number of nitrogen atoms in the molecular backbone, we observed consistent trends in both their absorption and their redox properties. A detailed investigation on the different oxidation states of 2Ph, which displays potential inversion, was also performed, including the intermediate radical cationic state (2PhRC). This intermediate represents an intriguing example of a mixed-valent species whose thermodynamic stability, as calculated by the comproportionation constant (Kc), is diminishingly small, whereas the electronic coupling is strong, making it a borderline class II–III mixed-valence compound. External modulation through coordination with Brønsted and Lewis acids reversibly altered the optical responses of 2Py and 2Pz. This work highlights both structural and stimuli-responsive strategies for tuning organic diradicaloid systems, thereby opening new avenues for their applications in functional materials and coordination chemistry.