Developing red-emissive ruthenium(II) complex-based luminescent probes for cellular imaging

(Figure Presented) 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](PF ₆) 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.