Water stable fluorescent organotin(iv) compounds: aggregation induced emission enhancement and recognition of lead ions in an aqueous system

Abstract

Herein, synthesis, spectroscopic studies, single-crystal X-ray diffraction, aggregation-induced emission enhancement (AIEE) and sensing application of water-stable organotin(IV) compounds (4a–6a and 4b–6b) obtained from 3-hydroxy-4H-chromen-4-one ligands are reported. All the synthesized organotin(IV) compounds were characterized using elemental analysis, FT-IR spectroscopy, multi-nuclei NMR (¹H, ¹³C, and ¹¹⁹Sn) spectroscopy, UV–VIS, fluorescence spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. The ¹¹⁹Sn NMR signal of compounds in the range of δ −144.92 to −190.68 ppm indicated the formation of hexacoordinated organotin species. The spectroscopic and single-crystal X-ray diffraction studies confirmed the formation of [L₂SnR₂] type compounds (where L is the bidentate ligand and R is an alkyl group) with a ‘skew-trapezoidal bipyramidal’ geometry. Furthermore, DFT calculations of compound 4b based on the DGDZVP basis set fully supported the stability of the structure where two short bonds Sn–O_(C–O) acquire the cis position rather than the trans position. Single-crystal X-ray diffraction analysis of the crystals grown in the presence of water confirmed the stability of 4a in water. Moreover, the water stability of a test compound 4a was established by ¹¹⁹Sn NMR data and spectrofluorimetric data. The spectrofluorimetric scan at different time intervals revealed the stability and constant emission response up to 24 h. The compounds were found to be fluorescent and exhibited aggregation-induced emission enhancement in MeOH/H₂O mixtures, which was confirmed by HRTEM analysis. The test compound 4a showed selective spectrofluorimetric recognition of Pb²⁺ ions in an aqueous medium by displaying an enhanced emission signal at 478 nm and enabled detection up to 22.66 μM. A mechanism of interaction is also proposed by spectroscopic experiments, spectrofluorimetric experiments and computational studies.