Theoretical investigation of a "turn-on" fluorescent sensor induced by complexation of mercury(II) ion.

A selective Hg²⁺/Cu²⁺-induced OFF-ON-OFF type of signaling pattern was calculated by density functional theory and time-dependent density functional theory method. Geometries, molecular orbitals, and interaction thermal energies were evaluated at the restricted hybrid Becke's three-parameter exchange functional using 6-31+G(d) basis set and relativistic effective core potentials. The optimized geometric structures are used to perform natural bond orbital (NBO) analysis. The main type of driving force is the electrostatic interactions between the metal ions and the electron-donating oxygen and nitrogen atoms. Maximum absorption spectrums of the complexes are mainly contributed by the HOMO → LOMO transition. The high selectivity of ligand H₂NB for Hg²⁺ results in a "turn-on" fluorescent phenomenon.