Synthesis, structure and spectral properties of O,N,N coordinating ligands and their neutral Zn(ii) complexes: a combined experimental and theoretical study

Abstract

Mononuclear Zn(II) complexes with the general formula [Zn(L)₂] have been synthesized in good yields by reacting Zn(OAc)₂ with HL in a ratio of 1 : 2 in methanol solvent. Here L is the deprotonated form of 6-[(quinolin-8-ylamino)methylene]cyclohexa-2,4-dienone (HL¹), 4-chloro-6-[(quinolin-8-ylamino)methylene]cyclohexa-2,4-dienone (HL²), 4-methyl-6-[(quinolin-8-ylamino)methylene]cyclohexa-2,4-dienone (HL³), 2,4-dimethyl-6-[(quinolin-8-ylamino) methylene]cyclohexa-2,4-dienone (HL⁴), 2-methoxy-6-[(quinolin-8-ylamino) methylene]cyclohexa-2,4-dienone (HL⁵). The electronic structures and photophysical properties of the ligands were calculated by DFT and TDDFT methods. The X-ray structure of one complex is reported. The ligands have a strong binding ability [(0.75–15.37) × 10⁴] and ratiometric response to Zn²⁺ ions. With the addition of Zn²⁺ ions to the ligands in THF solution a sharp color change is observed visually, and as well a significant enhancement of the fluorescence intensity and the quantum yield for this series occurs. The introduction of other metal ions having biological and environmental effects results in either unaltered or quenched emission intensity. However we observed the sensing property of the ligands strongly depends on the substituent at the ortho position of the phenol group. DFT calculation reveals that the ICT process take place from the salicylaldehyde (donor moiety) to quinoline (acceptor moiety) which is responsible for the enhancement of the fluorescence intensity of ligands after complexation.