Metal–Organic Framework Based Thermally Activated Delayed Fluorescence Emitter with Oxygen‐Insensitivity for Cell Imaging

Abstract To achieve better resolution and contrast in fluorescence techniques, time‐resolved fluorophores are promising constituents for probes in imaging and sensing, allowing for the elimination of background signals from scattering and short‐lived autofluorescence. Here a metal–organic framework (MOF) named Spiro‐MOF‐1 is designed with high thermally activated delayed fluorescence (TADF) persistence using a strategy of high rigidity to achieve a lifetime more than two times longer than that of pure linker. The rigid structure originated from spiro‐bi‐acridine unit is unambiguously revealed through 3D electron diffraction tomography (3D‐EDT). Furthermore, nanocrystals of Spiro‐MOF‐1 are successfully developed and modified with polyethylene glycol (PEG) as a biocompatible, time‐resolved fluorescent fluorophore with high TADF persistence, which exhibits a lifetime around 1 μs with no obvious decay under various bio‐mimicking environments, which has not been achieved by any other MOF‐based TADF emitter and small organic molecules. In further cell experiments, PEG‐modified nanocrystals exhibit a fluorescent signal that is more than 30 times longer than that of autofluorescence background in BGC823 cell line.