Macrocyclic chemosensors with anthraquinone signaling unit built into ionophore. Experimental and computational studies (part II) - metal ion complexation, spectroscopic response and structural insights.

This paper presents a study of metal ion complexation by macrocyclic chromoionophores (AQ-Ncrown and AQ-Ncrypt) containing an anthraquinone signaling unit integrated into the ionophore. UV-Vis spectroscopy and quantum chemical calculations were used to analyze the binding mechanisms and spectral response of the systems. The studied ligands exhibit the most pronounced spectroscopic responses toward Mg2+, Zn2+, Co2+, and Al3+ ions, leading to changes in absorption spectra associated with electronic reorganization of the anthraquinone chromophore. Spectrophotometric titrations reveal the formation of ML and ML₂ stoichiometry complexes, with stability constants dependent on the metal ion and ligand structure. The highest stability constants were obtained for Zn2+ and Al3+ complexes, whereas Mg2+ complexes showed the lowest values. Computational analysis indicates diverse coordination modes involving nitrogen and oxygen donor atoms, as well as a significant influence of the crown or cryptand macrocyclic architecture on electron density distribution. Contrary to expectations, increased preorganization of the macrocyclic ligand did not lead to enhanced complex stability. AQ-Ncrown appeared to form slightly more stable complexes with Mg2+. Although AQ-Ncrypt is more preorganized, if its geometry is not perfectly suited to binding a given ion, the entropy gain may be offset by less favorable enthalpic interactions. Despite its lower preorganization, AQ-Ncrown appears to adapt better to some metal ions. Our study revealed the effect of both flexibility and preorganization of the macrocyclic ligand.