Achieving metal-free phosphorescence in dilute solutions for imaging hypoxia in cells and tumors

The imaging of hypoxia in tumors by metal-free organic phosphors has long been a challenge due to the lack of molecules capable of phosphorescing efficiently in their dilute solutions. Herein, a new strategy for designing phosphorescent molecules was provided by building selenium-based intramolecular charge transfer systems to address this issue. The results showed that molecules based on this strategy exhibited efficient phosphorescence even in solutions, showed very low cytotoxicity to cells and can be successfully applied to image hypoxia in cells and tumors. In addition, the mechanism responsible for the efficient phosphorescence was investigated by the combination use of transient absorption spectra and theoretical calculations.