Electrochemiluminescent Peptide Nucleic Acid-Like Monomers Containing Ru(II)–Dipyridoquinoxaline and Ru(II)–Dipyridophenazine Complexes

A series of Ru­(II)–peptide nucleic acid (PNA)-like monomers, [Ru­(bpy)₂(dpq-L-PNA–OH)]²⁺ (M1), [Ru­(phen)₂(dpq-L-PNA–OH)]²⁺ (M2), [Ru­(bpy)₂(dppz-L-PNA–OH)]²⁺ (M3), and [Ru­(phen)₂(dppz-L-PNA–OH)]²⁺ (M4) (bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline, dpq-L-PNA–OH = 2-(N-(2-(((9H-fluoren-9-yl)­methoxy)­carbonylamino)­ethyl)-6-(dipyrido­[3,2-a:2′,3′-c]­phenazine-11-carboxamido)­hexanamido)­acetic acid, dppz-L-PNA–OH = 2-(N-(2-(((9H-fluoren-9-yl) methoxy)­carbonylamino)­ethyl)-6-(dipyrido­[3,2-f:2′,3′-h]­quinoxaline-2-carboxamido)­acetic acid) have been synthesized and characterized by IR and ¹H NMR spectroscopy, mass spectrometry, and elemental analysis. As is typical for Ru­(II)–tris­(diimine) complexes, acetonitrile solutions of these complexes (M1–M4) show MLCT transitions in the 443–455 nm region and emission maxima at 618, 613, 658, and 660 nm, respectively, upon photoexcitation at 450 nm. Changes in the ligand environment around the Ru­(II) center are reflected in the luminescence and electrochemical response obtained from these monomers. The emission intensity and quantum yield for M1 and M2 were found to be higher than for M3 and M4. Electrochemical studies in acetonitrile show the Ru­(II)–PNA monomers to undergo a one-electron redox process associated with Ru^II to Ru^III oxidation. A positive shift was observed in the reversible redox potentials for M1–M4 (962, 951, 936, and 938 mV, respectively, vs Fc^0/+ (Fc = ferrocene)) in comparison with [Ru­(bpy)₃]²⁺ (888 mV vs Fc^0/+). The ability of the Ru­(II)–PNA monomers to generate electrochemiluminescence (ECL) was assessed in acetonitrile solutions containing tripropylamine (TPA) as a coreactant. Intense ECL signals were observed with emission maxima for M1–M4 at 622, 616, 673, and 675 nm, respectively. At an applied potential sufficiently positive to oxidize the ruthenium center, the integrated intensity for ECL from the PNA monomers was found to vary in the order M1 (62%) > M3 (60%) > M4 (46%) > M2 (44%) with respect to [Ru­(bpy)₃]²⁺ (100%). These findings indicate that such Ru­(II)–PNA bioconjugates could be investigated as multimodal labels for biosensing applications.