A Ratiometric Fluorescent Probe Based on a Through‐Bond Energy Transfer (TBET) System for Imaging HOCl in Living Cells

A simple ratiometric probe (Naph‐Rh) has been designed and synthesized based on a through‐bond energy transfer (TBET) system for sensing HOCl. In this probe, rhodamine thiohydrazide and naphthalene formyl were connected by simple synthesis methods to construct a structure of monothio‐bishydrazide. Free probe Naph‐Rh showed only the emission of naphthalene. When probe Naph‐Rh reacted with HOCl, monothio‐bishydrazide could be converted into 1,2,4‐oxadiazole, which not only ensured that the donor and the acceptor were connected with electronically conjugated bonds, but also resulted in the spiro‐ring opening and the emission of rhodamine. Therefore, a typical TBET process took place. The probe possessed high‐energy transfer efficiency and large pseudo‐Stokes shifts. As the first TBET probe for HOCl, Naph‐Rh showed excellent selectivity and sensitivity toward HOCl over other reactive oxygen species (ROS)/reactive nitrogen species (RNS), and could respond fast to a low concentration of HOCl in the real sample. In addition, the probe was suitable for imaging HOCl in living cells due to its real‐time response, excellent resolution, and reduced cytotoxicity.