AIE-active cyclometalated iridium(iii) complexes for highly efficient picric acid detection in aqueous media

Abstract

Thiophenol-substituted 1,10-phenanthroline and three cyclometalating ligands 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (F₂ppy) and 2-phenylquinoline (pq) were employed to synthesize three organoiridium(III) complexes, 1[PF₆], 2[PF₆] and 3[PF₆]. These complexes exhibit efficient aggregation-induced emission (AIE) in the presence of the poor solvent water and highly dense polyethylene glycol which has been supported by DLS, SEM and TEM. The complexes can be self-assembled into nanoaggregates in aqueous solution facilitated by weak supramolecular interactions, and consequently, the efficient restriction of molecular motion leads to strong AIE. The amorphous solid and single crystal show very bright luminescence compared to the solution. In the solid state, 1[PF₆], 2[PF₆], and 3[PF₆] show very bright yellow, green, and red luminescence, which is due to the presence of three different cyclometalating ligands, ppy, F₂ppy, and pq, respectively. The triplet state TDDFT calculation supported the variant luminescence of these compounds. The single-crystal X-ray structure of these compounds shows that neighbouring complexes in the crystal are connected through both C–H⋯π and S⋯F interactions, C–H⋯F and hydrogen bonds, and the close crystal packing and aggregation in the solid state are the reasons for bright solid-state luminescence. These complexes were employed to selectively detect the nitroaromatic explosive picric acid (PA) in 90% aqueous (H₂O : CH₃CN; 9 : 1; v/v) solution. For 1[PF₆], 2[PF₆], and 3[PF₆], the Stern–Volmer quenching constants (K_SV) for the detection of PA were determined to be 8.39 × 10⁵, 5.67 × 10⁶ and 4.53 × 10⁵ M⁻¹ with a low limit of detection (LOD) of 0.60, 0.32, and 3.83 nM, respectively. Finally, a dynamic, photoinduced electron transfer-based (PET) PL quenching mechanism is established with the help of combined spectroscopic and theoretical studies.