Theoretical investigation and reconsideration of intramolecular proton-transfer-induced the twisted charge-transfer for the fluorescent sensor to detect the aluminum ion.

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 Al3+ ion and successfully applied for selective recognition of Al3+ ion. In this work, the reconsideration of excited state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT) has been explored in detail based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. In the absence of Al3+, the lone pair electrons are transferred from –C = N to –OH forming a hydrogen-bonding configuration, and AHM shows weak fluorescence. When AHM is coordinated with metal ion, the TICT state is eliminated, and emission is significantly 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. [ABSTRACT FROM AUTHOR]