DFT/TD‐DFT Based Study to Decipher the Proton Transfer Process in Anion Sensing Mechanism of NTS Molecule

Abstract In the present work, we have performed DFT/TD‐DFT studies on NTS [1‐(((4‐(benzo[d]thiazol‐2‐yl) phenyl) imino) methyl) naphthalen‐2‐ol, referred to as 1E] to investigate the cyanide and fluoride anion sensing mechanism via proton transfer process in the ground and excited state. The potential energy curve for the proton transfer in ground (GSIPT) and excited state (ESIPT) suggests that, in case of NTS molecule, keto and enol form co‐exist. Based on the pattern of addition of cyanide on NTS, nucleophilic addition supports GSIPT rather ESIPT. However, fluoride addition showed both GSIPT and barrier less ESIPT processes followed by the deprotonation. From the energy calculations, we have observed that the cyanide addition (nucleophilic) block ICT character due to similar geometries in the ground and excited state, along with the interruption in the conjugation of the molecule. But, in the case of fluoride added product, ICT character was observed owing to the change in geometry of the corresponding states (ground and excited). ESIPT process has also been explained in terms of HOMO‐LUMO electron density, potential energy surfaces and NBO analysis.