A Schiff base compound 6-amino-5-(((2-hydroxynaphthalen-1-yl)methylene)amino)-2-mercaptopyrimidin-4-ol (AHM) in acetonitrile solvent is found to show "OFF-ON type" mode upon addition of Al 3+ ion and successfully applied for selective recognition of Al 3+ ion. In this work, the reconsideration of excited state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT) have been explored in detail based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. In the absence of Al 3+ , the lone pair electrons are transferred from -C = N to -OH forming a hydrogen-bonding con guration, and AHM shows weak uorescence. When AHM is coordinate with metal ion, the TICT state is eliminated, and emission is signi cantly enhanced. Thus, in this paper, the origination of the non-emissive behavior of AHM has been explained in detail. The frontier molecular orbitals (MOs) and hole-electrons are used to analyze the charge distribution, providing strong evidence for the possibility of ESIPT and TICT processes occurring.