Reactive Oxygen species (ROS) are well known for their oxidizing behaviour but it differs in fluorescein derivatives chemistry. With t-BuOOH, the probe F shows its unique turn-off sensing (A), while with NaOCl probe shows turn-on sensing property (B). In this study, it was observed that in situ generated species formed by the reaction of probe (F) with t-BuOOH is a unique molecule, having high turn-on behaviour in presence of sodium hypochlorite (≈500 times). Further, fluorescence microscopy imaging using adenocarcinoma cells, A549 cells revealed that in-situ generated molecule, could be used as an efficient fluorescent probe for detecting NaOCl in living cells.