Figure 1.

(a) Energy diagram of the coumarin photolysis mechanism after excitation, showing an unstable CIP intermediate resulting in a high rate of CIP recombination, a relatively low rate of the productive step (k₂), and consequentially a low photochemical QY. (b) A stabilized CIP intermediate, reducing the rate of CIP recombination and consequentially increasing the photochemical QY. (c) Coumarin PPGs bearing substituents on the α-carbon (black) stabilizing the cationic intermediate through delocalization and hyperconjugation, as described in this paper. All coumarins are caging an acetic acid payload (gray).