Unraveling the Chemosensing Mechanism by the 7-(Diethylamino)coumarin-hemicyanine Hybrid: A Ratiometric Fluorescent Probe for Hydrogen Peroxide.

The hemicyanine hybrid containing the 7-(diethylamino)coumarin (ACou) donor attached to the cationic indolenium (Ind) acceptor through a vinyl linkage (ACou-Ind) represents a classic ratiometric fluorescent probe for detecting nucleophilic analytes, such as cyanide and reactive sulfur species (RSS), through addition reactions that disrupt dye conjugation to turn off red internal charge transfer (ICT) fluorescence and turn on blue coumarin emission. The chemosensing mechanism for RSS detection by ACou-Ind suggested in the literature has now been revised. Our studies demonstrate that thiolates react with ACou-Ind through conjugate addition to afford C4-SR adducts that lack coumarin fluorescence due to photoinduced electron transfer quenching by the electron-rich enamine intermediate.