Blocking the dark state as sensing mechanism of 3-nitro-1,8-naphthalimide derivatives for detection of carbon monoxide in the living cells

Fluorescent probes based on 3-nitro-1,8-naphthalimide derivatives have been developed and applied in the detection of carbon monoxide (CO) in the biological system. Although photoinduced electron transfer has been proposed as a sensing mechanism of 3-nitro-1,8-naphthalimide derivatives for detecting CO, the underlying mechanism for tuning the fluorescence of 3-nitro-1,8-naphthalimide derivatives remains unexplored theoretically. Herein, based on the calculation results from time-dependent density functional theory, we proposed that a dark state accounts for the fluorescence quenching of 3-nitro-1,8-naphthalimide-based fluorescent probes. The dark state can be a charge transfer state or n pi* state. After reacting with CO, the nitro group will be converted to the amino group. The amino group can significantly decrease the energy of the bright state or directly remove the dark state from the system, thereby blocking the nonradiative decay pathway from the dark state and opening the emissive pathway. Our findings can provide an instructive design strategy for further developing new 1,8-naphthalimide-based fluorescent probes.