Fluorescence enhancement mechanism of new 4-Cyanobiphenyl-based schiff base probe by coordination interaction with cadmium ion

In this research, we focus on the theoretical exploration of the fluorescent process and sensing mechanism for Cd2+ of a novel Schiff base probe BPC based on 4-Cyanobiphenyl [Journal of Luminescence, 249, (2022), 119,017.]. The proton transfer process of BPC is investigated by constructing potential energy curves of the ground (S0) and excited singlet (S1) states with the extended O1–H2 distance. In S1 state, the probe BPC, in addition to the excited state intramolecular proton transfer (ESIPT) process, is found to undergo a CN isomerization process by rotating the dihedral angle C2–C3N3–C4. Meanwhile, a non-emissive state in BPC is found in our work. The weak fluorescence phenomenon observed in experiment is caused by the CN isomerization product of BPC. Upon the addition of Cd2+, BPC can coordinate with Cd2+, which prevents CN isomerization and ESIPT processes from occurring and turns on the intense fluorescence emission. This study clarifies sensing mechanism of BPC for detecting Cd2+ based on the inhibition of ESIPT and CN isomerization processes, which paves the way for the further development of new fluorescent probes for metal ions in the future.