Understanding the excited state intramolecular proton transfer phenomena of 2-hydroxy-3-naphthaldehyde thiosemicarbazone

The specific objective of this theoretical study is to explore the excited state intramolecular proton transfer (ESIPT) property of 2-hydroxy-3-naphthaldehyde thiosemicarbazone (2H3NS). Proton transfer takes place when the acidic or basic part of a molecule becomes stronger in the excited state which leads to the formation of a tautomer. This takes place via an intramolecular hydrogen bond formation between the two moieties that opens a door for the intramolecular proton transfer process. The present approach utilizes density functional theoretical (DFT) approach to determine the proper geometrical optimization of the molecule, determination of the intramolecular bond distance of O─H during the transition state between the enolic and tautomeric forms, and simulating potential energy curves (PECs) at both the ground and excited states. The excited states explore include the first two singlet states (S₁, S₂) and the first excited triplet state (T₁). The distances of the detachable H from the O atom and the N atom (to which it gets attached) have been used as the unique reaction coordinates to check the variation in the angle O···H···N. The ensuing results help to determine the progress of the prototropic process as both lead to the same species passing through the same transition state. The findings are further corroborated by FTIR spectroscopy through the formation of both O─H and ─N─H bond in the transition state. Frontier molecular orbitals of the tautomer's at the ground state reveals the shifting charge density from ─O─H to N─H region in the HOMO and certifies the stated mechanism.