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

Abstract

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 (ΔEa) 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.