Time-Resolved Insight into the Photosensitized Generation of Singlet Oxygen in Endoperoxides

A synergistic approach combining high-level multiconfigurational static calculations and full-dimensional ab initio surface hopping dynamics has been employed to gain insight into the photochemistry of endoperoxides. Electronic excitation of endoperoxides triggers two competing pathways, cycloreversion and OO homolysis, that result in the generation of singlet oxygen and oxygen diradical rearrangement products. Our results reveal that cycloreversion or the rupture of the two CO bonds occurs via an asynchronous mechanism that can lead to the population of a ground-state intermediate showing a single CO bond. Furthermore, singlet oxygen is directly generated in its most stable excited electronic state (1)Delta(g). The triplet states do not intervene in this mechanism, as opposed to the OO homolysis where the exchange of population between the singlet and triplet manifolds is remarkable. In line with recent experiments performed on the larger anthracene-9,10-endoperoxide, upon excitation to the spectroscopic pp* electronic states, the primary photoreactive pathway that governs deactivation of endoperoxides is OO homolysis with a quantum yield of 65%.