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 (T 1 ) 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 roomtemperature 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 T 1 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.

Role of Carbonyl Distortions Facilitating Persistent Room-Temperature Phosphorescence

“…Therefore, CO out-of-plane vibrations contributed to a selective enhancement of k p compared with k nr (RT) in VA . A recent report reveals an enhanced k p in addition to k nr (RT) depending on the vibrations of an unsubstituted π-structure with heteroatoms that exhibit phosphorescence . This reveals photophysical insight that vibrations of appropriate substituents to π-fused structures generating phosphorescence play a role in selectively enhancing k p with negligible increases in k nr (RT).…”

Förster resonance energy transfer involving the triplet state

“…In the study, R T-S at RT was carefully determined using F T r at RT without any assumptions by measuring F isc for the first time, which was the fourth contribution from our group. 76 Thus, a platform to discuss the capability of an afterglow sensitizer as the D could be obtained through investigation from the viewpoint of the D. Through introduction of the chemical structures of the D-A pair and a general explanation of e A and the spectral overlap between the emission of the D and the absorption of the A, the knowledge obtained through the detailed research regarding RT FRET T-S from heavy-atom-free chromophores inspired us to provide an overview of FRET involving triplet states from the viewpoint of the D in this feature article.…”

“…By using the original calculation procedures to enhance Φ T r at RT, the third contribution of our group was selective and efficient FRET T-S to overcome the above problem. 76 The molecular distortion effect using a conjugated backbone containing heteroatoms as the D to selectively enhance k T r without facilitating k S r allows selective and efficient FRET T-S from the D. It should be noted that the concept of selective FRET T-S logically allows efficient delayed emission from an A with relatively long emission wavelength because long-wavelength emitters often also contain large amounts of ε A ( λ ) in the visible wavelength. In the study, R T-S at RT was carefully determined using Φ T r at RT without any assumptions by measuring Φ isc for the first time, which was the fourth contribution from our group.…”

“…8(a)). 76 To enhance FRET T-S , it is recommended to use a D with large Φ T r . One way to enhance k T r without an increase of ( k T nr + k q ) and achieve larger Φ T r is to use a large transition dipole moment from the singlet states (Fig.…”

“…(f) Steady-state and afterglow RT emission spectra at 380 nm-excitation of an amorphous PPT film doped with 1 wt% DBPz-d 12 and 0.35 wt% DBP. Reproduced with permission from Wiley VCH 76.…”