Excited-state proton transfer relieves antiaromaticity in molecules

Significance Excited-state proton transfer (ESPT) is universally recognized as a reaction that relaxes the energy of a photoexcited organic compound. It is commonly found in many light-driven processes. Here we identify decisive principles underlying why and when ESPT happens. Our computational investigation of prototypical ESPT reactions finds that the occurrence of ESPT can be explained by an electron-counting rule—Baird’s rule, which remains largely ignored despite having a near–50-y-old history. We emphasize that this surprising connection not only explains the mechanistic principle of ESPT reactions, but it also predicts whether hydrogen bonding interactions that form within and between organic compounds might strengthen or weaken when irradiated by light. Recognizing this relationship has tremendous interpretive merit for organic photochemistry.