Reversible Photoswitching between Fluorescence and Room Temperature Phosphorescence by Manipulating Excited State Dynamics in Molecular Aggregates

Regulation of fluorescence–phosphorescence pathways in organic molecular aggregate remains a challenge due to the complicated singlet–triplet excited state dynamics process. Herein, we demonstrated a successful example (o‐BFT) to realize photoreversible fluorescence and room temperature phosphorescence (RTP) switching based on an effective strategy of integrating a phosphor (dibenzofuran) with a photoswitch (dithienylbenzothiophene). o‐BFT exhibited dual emission of fluorescence and RTP in both powder and doping polymer film. Notably, the long‐lived RTP of o‐BFT could be repeatedly erased and restored through reversible photocyclization and decyclization under alternate ultraviolet and visible photoirradiation. In‐depth theoretical and spectroscopic investigations revealed that the triplet inactivation was dominated by a photo‐controlled triplet‐to‐singlet Förster resonance energy transfer from light‐activated o‐BFT to photoisomer c‐BFT. Yet, the initial fluorescence could be preserved in this process to afford a photoreversible fluorescence‐RTP switching. Photoinduced reversible fluorescence (FL) and room temperature phosphorescence (RTP) switching in molecular aggregates was realized through repeatedly erasing and restoring the long‐lived RTP under alternate ultraviolet and visible light exposure. The triplet excitons can be quenched via a triplet‐to‐singlet Förster resonance energy transfer (TS‐FRET) process.