Boric Acid Matrix‐Activated nπ* Transition of Guest Chromophores: from Pure Fluorescence to Efficient Afterglow.

El‐Sayed rule highlights the important role of heteroatoms (e.g., N, O, and S) and the corresponding nπ* transition in designing efficient organic phosphors. Nevertheless, for some heteroatom‐rich fluorophores, their phosphorescence is quite weak (e.g., fluorescein), since the nπ* components are absent in the whole transitions. Here, these chromophores (mainly with n electron‐containing twisted structure) are found doping into the boric acid (BA) matrix can activate the nπ* transition for efficient afterglow luminescence. For example, doping purely green‐emitting fluorescein into BA yielded a high afterglow quantum yield (≈24%) and a long‐lasting, blue‐shifted cyan afterglow (>10 s). The covalent/non‐covalent interactions between the BA matrix and the guests resulted in twisting the n electron‐containing structure to promote the spin‐orbit coupling process, leading to the generation of new excited state triplet transition pathways. Since the guest chromophores feature visible light absorption, a series of multi‐color afterglow phosphors with visible or white light excitation are successfully constructed. [ABSTRACT FROM AUTHOR]