Self‐Trapped Excitons Activate Pseudo‐Inert Basal Planes of 2D Organic Semiconductors for Improved Photocatalysis

Abstract 2D organic semiconductors are widely considered superior photocatalysts due to their large basal planes, which host abundant and tunable reaction sites. However, here, it is discovered that these basal planes can be pseudo‐inert, fundamentally challenging conventional design strategies that assume uniform activity on the surface of 2D organic semiconductors. Using 2D potassium‐poly (heptazine imide) (KPHI) for hydrogen peroxide photocatalysis as a model, it is demonstrated that the pseudo‐inertness of basal planes stems from preferential exciton transport to edges, instead of interlayer transport in highly ordered structures. Thus, their dimension reduction enables controlled localization of exciton due to the self‐trapping mechanism, whereby the basal planes can transform from pseudo‐inert state into active catalytic sites. With this knowledge, a modified 2D KPHI capable of generating 35 mmol g −1 h −1 of H 2 O 2 , which is over 350% increase compared to pristine KPHI, is reported. More interestingly, the activated basal planes promote H 2 O 2 production through a reaction pathway distinct from that of pseudo‐inert basal planes. These findings establish fundamental principles connecting crystal structure, exciton dynamics, and reactive site distribution, providing new insights into the design of high‐performance photocatalysts.