Developing Red-Emissive Ruthenium(II) Complex-Based Luminescent Probes for Cellular Imaging

Ruthenium­(II) complexes have rich photophysical attributes, which enable novel design of responsive luminescence probes to selectively quantify biochemical analytes. In this work, we developed a systematic series of Ru­(II)-bipyrindine complex derivatives, [Ru­(bpy)_3‑n(DNP-bpy)_n]­(PF₆)₂ (n = 1, 2, 3; bpy, 2,2′-bipyridine; DNP-bpy, 4-(4-(2,4-dinitrophenoxy)­phenyl)-2,2′-bipyridine), as luminescent probes for highly selective and sensitive detection of thiophenol in aqueous solutions. The specific reaction between the probes and thiophenol triggers the cleavage of the electron acceptor group, 2,4-dinitrophenyl, eliminating the photoinduced electron transfer (PET) process, so that the luminescence of on-state complexes, [Ru­(bpy)_3‑n(HP-bpy)_n]²⁺ (n = 1, 2, 3; HP-bpy, 4-(4-hydroxyphenyl)-2,2′-bipyridine), is turned on. We found that the complex [Ru­(bpy)­(DNP-bpy)₂]²⁺ remarkably enhanced the on-to-off contrast ratio compared to the other two (37.8 compared to 21 and 18.7). This reveals a new strategy to obtain the best Ru­(II) complex luminescence probe via the most asymmetric structure. Moreover, we demonstrated the practical utility of the complex as a cell-membrane permeable probe for quantitative luminescence imaging of the dynamic intracellular process of thiophenol in living cells. The results suggest that the new probe could be a very useful tool for luminescence imaging analysis of the toxic thiophenol in intact cells.