AIE-active bis-heteroleptic ruthenium(II) complexes featuring aryl-substituted pyridylimidazopyridine ligands for highly efficient detection of picric acid in aqueous media.

In this paper, three bis-heteroleptic ruthenium(II) complexes, 1[PF6]2, 2[PF6]2 and 3[PF6]2, containing aryl-substituted pyridyl-imidazole pyridine ligands were successfully synthesized and fully characterized by various spectroscopic techniques. The crystal structures of complexes 2[PF6]2 and 3[PF6]2 were obtained by single-crystal X-ray diffraction. The photophysical and electrochemical properties of all complexes were examined by UV-vis spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. Complexes 1[PF6]2 and 2[PF6]2 containing phenyl and naphthyl substituted pyridyl-imidazole pyridine ligands show AIEE and are weakly emissive in dilute acetonitrile but show strong emission in the aggregated and solid states due to a restriction in intramolecular motion. The formation of the nano-aggregated particles was supported by UV-vis, DLS, SEM and TEM studies. 3[PF6]2 contains anthracenyl-substituted pyridyl-imidazole pyridine ligands and is non-luminescent in acetonitrile and the solid or aggregated state. This is due to the population of the low-energy 3LLCT state and non-radiative decay to the ground state. TD-DFT calculations were carried out to assign the electronic transitions of 1[PF6]2, 2[PF6]2 and 3[PF6]2, in both the ground and excited states. Additionally, to further exploit the AIEE properties, turn-off detection of the nitroaromatic explosive picric acid (PA) was performed in aqueous medium. Very low limits of detection (LOD) of 12.94 nM and 19.40 nM were obtained for 1[PF6]2 and 2[PF6]2, respectively. Stern-Volmer analysis and emission-lifetime studies revealed the presence of a dynamic quenching mechanism.