Electrochemical Switching of Phenol Acidity over 14 p K a Units in an Ambipolar Julolidine–Benzothiadiazole Scaffold

Electrochemical control over Brønsted acidity offers a powerful strategy for regulating proton-transfer reactivity, yet organic redox-switchable acids remain rare. Here we report a julolidine–benzothiadiazole-functionalized phenol that combines a donor–acceptor π system with an acidic O–H bond in a single molecular scaffold. The phenol exhibits a pKa of 23.6 in acetonitrile and undergoes reversible oxidation in both its protonated and deprotonated forms. Single-electron oxidation lowers the O–H pKa to 13.5. Electrochemical studies further suggest that single-electron reduction suppresses the O–H bond acidity, leading to a pKa of 27.7 and an overall electrochemically accessible acidity window of approximately 14 pKa units. Density functional theory indicates that oxidation and reduction are governed by electronically complementary julolidine donor and benzothiadiazole acceptor fragments, respectively. This provides a molecular basis for the ambipolar electrochemical behavior. Our results demonstrate that ambipolar donor–acceptor molecular design provides a strategy for achieving bidirectional electrochemical modulation of Brønsted acidity.