Selective Triplet–Singlet Förster‐Resonance Energy Transfer for Bright Red Afterglow Emission

Abstract Förster‐resonance energy transfer (FRET_T‐S) from the lowest excited triplet state (T₁) of a donating sensitizer to a fluorescence acceptor can be used to obtain bright room‐temperature afterglow emission at long wavelengths. However, the energy transfer from the lowest excited singlet state of the donor to the acceptor is an undesirable deactivation pathway that prevents FRET_T‐S. Herein, heteroatoms in chromophores are shown to allow selective and efficient FRET_T‐S for enhanced triplet emission for bright room‐temperature afterglow emission at long wavelengths. Different transition characteristics between the lowest singlet excited state and triplet states in heteroatom‐containing chromophores accelerate triplet generation, enabling near‐zero fluorescence yields. Out‐of‐plane vibrations of the heteroatoms in aromatic fused rings greatly enhance the radiative rate from T₁ by a factor of 88 relative to non‐heteroatom‐containing fused chromophore. The compatibility of the near‐zero fluorescence and the enhanced triplet emission in a heteroatom‐containing fused chromophore enable selective and efficient FRET_T‐S pathways, resulting in room‐temperature red afterglow emission with a yield of 17%. The bright emission at long‐wavelengths allows distinguishable, multiple spectral signals in ambient white light.