Synchronizing the nπ* Transition Dipole in a Benzoyl-Linked N-Fused Ring for Highly Efficient Yellow Afterglow

The El-Sayed rule proposes that heteroatom-containing chromophores can enhance the intersystem crossing rate by inducing the nπ* transitions. However, extracting high radiation from the triplet state is often challenging due to the small nπ* transition dipole moment. Herein, we demonstrate that introducing the benzoyl group in a N-fused chromophore (BPM) increases the nπ* transition dipole. Connecting two heads with non-bonded electron pairs leads to an enhanced nπ* transition dipole that facilitates a multi-fold radiation rate enhancement compared to the native N-fused ring. The deuterated form of BPM provides a high room-temperature phosphorescence yield of 54.1% and a long lifetime of 0.32 s. As a result, a high triplet concentration of 1.7 × 10^-2 M for the yellow afterglow was achieved for the first time, enabling high-resolution afterglow imaging of nanoparticles in aqueous medium. Enhancing the nπ* transition dipole of the heavy-atom-free organic chromophores could be a practical strategy for harvesting the triplet excitons.