Chemically Triggered Release of Singlet Oxygen from Bisphenalenyl Endoperoxides with a Brønsted Acid

Aromatic endoperoxides have emerged as intriguing stimulus-responsive materials for molecular oxygen (O₂) storage and delivery but are currently limited in their application because they require heat to trigger O₂ release. Here we present the first example of acid-triggered singlet oxygen (¹O₂) release that does not require external heating by treating bisphenalenyl endoperoxides (EPOs) with trifluoroacetic acid. Mechanistic studies reveal that diprotonation of EPOs leads to a >10-fold increase in cycloreversion rates by lowering the energy of activation (ΔE_a) by as much as 71.1 kJ mol^-1. Remarkably, acid-catalyzed ¹O₂ release is even demonstrated at room temperature. Chemical trapping experiments indicate that reactive ¹O₂ is present during acid-triggered release, which is promising for the development of these molecular materials for metal-free, on-demand ¹O₂ delivery.