Luminescent properties of chameleon-like metal-organic framework between zinc(II) dichloride and two...

Zinc compounds in the form of inorganic/organic hybrids containing both zinc halides and heterocyclic ligands show various interesting optical and physicochemical properties. Due to these properties, there is a potential for development of various innovative technologies and applications within the life sciences. In this study, experimental and theoretical results on the absorption and emission (steady state and time-resolved) properties of the hybrid ZnCl₂(QO)₂ complex formed between ZnCl₂ and quinoline N-oxide, have been reported. Single crystal X-ray analysis revealed the tetragonal cell (Z = 4) with P4₁2₁2 space group and a slight crystal distortion. Interestingly, experiments in aprotic solvents show that both absorption and emission spectra peak in the ultraviolet (UV) region suggesting a weak CT character of the emissive S₁ state, confirmed by a middle Stokes shift values. The results of the nanosecond time-resolved emission spectroscopy suggest two different structures of the complex described by the two different lifetimes and variable amplitudes dependent on the polarity of the medium. In the solid state, a relatively strong, bright blue luminescence appears at 413 nm (τ = 2.26 ns). Theoretical calculations (DFT and TD DFT) confirm experimental studies and reveal the solvent-dependent chameleon properties of ZnCl₂(QO)₂ by two different structures in two solvents of a contrast polarity. In apolar cyclohexane (CHX, μ = 5.612 D), the planes of both lateral quinoline N-oxide (QO) rings show to be nearly parallel each to another, resembling the crystal structure, while in a strongly polar acetonitrile (AN, μ = 9.328 D) they are nearly perpendicular. Such parallel arrangement of quinoline rings of ZnCl₂(QO)₂ complex in weakly polar methylcyclohexane can hinder the process of Photoinduced Electron Transfer, resulting in a stronger emission and significant quantum yield in comparison to more polar media.