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

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 mu s 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.