A theoretical insight on the rigid hydrogen-bonded network in the solid state structure of two zinc(ii) complexes and their strong fluorescence behaviors

Abstract

A reduced Schiff base has been synthesized and characterized and used as a fluorescence chemo-sensor for the selective detection of zinc(II). Fluorescence titrations have also been conducted for the ligand and the binding constant for the ligand (K = 1.056 × 10⁶ M⁻¹) has been evaluated using the Benesi–Hildebrand equation. Two mononuclear zinc(II) complexes have also been synthesized with the ligand and S₀ and S₁ of their electronic structures were calculated. The HOMO–LUMO energy difference in each complex is ca. 4.69 eV in S₀ states and the energy gap is reduced to ca. 4.3 eV in S₁ facilitating easier electronic transition. Their strong fluorescence behaviors may be correlated with the presence of a rigid hydrogen-bonded network in their solid state structure. Two types of intermolecular hydrogen bonding are noticed in the dimeric form of the complexes. The hydrogen bonding environment is well supported qualitatively and quantitatively with the help of NCI-RDG (noncovalent interaction reduced density gradient) and QTAIM. The physical nature of other weak non-covalent interactions in both complexes was also examined. Based on the optimized ground state geometry (S₀), the TDDFT/B3LYP method combined with the SMD solvation model in methanol media was used to calculate the absorption properties of the investigated complexes. Additionally, analysis on the electronic structure of the excited states employing NTO (natural transition orbital) representation showed that the S₁ state can be mainly characterized by an inter-ligand charge-transfer (ILCT) transition, populating the highest-occupied (HO) NTO and lowest-unoccupied (LU) NTO, which describe the hole and the excited electron state, respectively. The calculation indicates that the fluorescence originates from the charge transfer from N₃/NCS to the reduced Schiff base.