Breaking Kasha's rule for photoswitchable reactive oxygen species generation in carbonylated carbon nitride.

Photoswitchable catalysis enables the non-invasive regulation of light-to-chemical energy conversion, in which catalysts with wavelength-dependent photoexcitation behaviors are typically required. However, for semiconductors, the ultrafast hot-excited-species cooling, depicted as Kasha's rule, tends to induce the accumulation of photoinduced species on the lowest excited states, which brings about a detrimental effect on achieving photoswitchable catalysis. Herein, by taking polymeric carbon nitride as an example, we demonstrate photoswitchable molecular-oxygen activation that is achieved by incorporating carbonyl groups into the matrix: in the range of 300-400 nm, hydroxyl radical and singlet oxygen are identified as the dominant reactive oxygen species under high- and low-energy excitations, respectively. Photoluminescence and ultrafast transient absorption measurements confirm the breakdown of Kasha's rule in carbonylated carbon nitride in which the obstructed relaxation of hot excited species leads to nontrivial wavelength-dependent photophysical properties. This anti-Kasha behavior is also attributed to carbonyl-induced spin-orbit coupling in which the mixing of singlet and triplet states would lead to robust leaps/relaxations between these states, which in turn affect the cooling process of hot photoinduced species. This work deepens the understanding of modulating hot-excited-species relaxation for gaining versatile semiconductor-based photocatalysis.