Superiority of imidazolium over triazole moiety in bis-heteroleptic ruthenium(II) complex toward selective sensing of phosphates

A new phosphorescent ruthenium-based receptor, designated as Ru-1, functionalized with imidazolium as an anion-binding moiety was synthesized and characterized for selective recognition and sensing of H₂PO₄⁻ and HP₂O₇³⁻. The binding affinity of Ru-1 towards numerous anions was assessed using UV–visible and phosphorescent spectroscopic techniques in CH₃CN solvents. Notably, phosphate and pyrophosphate exhibited superior binding constants compared to other anions, with measured binding constants (K_a) of 7.95 × 10⁴ and 4.78 × 10⁴ M⁻¹ respectively. Moreover, discernible blue shifts of 25 nm and 28 nm respectively, corresponding to H₂PO₄⁻ and HP₂O₇³⁻ and significant enhancements in photoluminescence intensities by ∼5-fold and ∼4-fold, respectively were observed with H₂PO₄⁻ and HP₂O₇³⁻. Despite the existence of excess other anions, Ru-1 demonstrated selectivity towards these ions, facilitating sensitive detection with a limit of detection (LOD) of 0.045 µM for H₂PO₄⁻ and 0.017 µM for HP₂O₇³⁻. In the presence of H₂PO₄⁻, the lifetime of Ru-1 was amplified noticeably from 0.2329 μs to 0.7914 μs, whereas exposure to HP₂O₇³⁻ resulted in an observed increase of 0.7465 μs. Furthermore, comparative photophysical studies revealed that the imidazolium moiety-based Ru-1 receptor outperformed the triazole-based Ru-2 receptor in terms of excited state lifetime, binding constant, and detection limit of the anions.