Effects of electron withdrawing and donating groups on the efficiency of tris(2,2′-bipyridyl)ruthenium(ii)/tri-n-propylamine electrochemiluminescence

Abstract

The effects of electron withdrawing and electron donating groups on the electrochemiluminescent (ECL) properties of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺ where bpy = 2,2′-pyridine) are reported. The electrochemistry, photophysics and ECL of (bpy)₂Ru(DC-bpy)²⁺, and (bpy)₂Ru(DM-bpy)²⁺ (DC = 4,4′-dicarboxy-2,2′-bipyridine; DM = 4,4′-dimethyl-2,2′-bipyridine) have been studied relative to Ru(bpy)₃²⁺ in 50∶50 (v/v) acetonitrile(CH₃CN)∶H₂O (0.1 M KH₂PO₄), and aqueous solutions. Furthermore, the effects of Triton X-100 (polyethylene glycol tert-octylphenyl ether) on the electrochemical, spectroscopic and ECL properties of these compounds are reported. The anodic oxidation of Ru(bpy)₃²⁺, (bpy)₂Ru(DC-bpy)²⁺, and (bpy)₂Ru(DM-bpy )²⁺ produces ECL in the presence of tri-n-propylamine (TPrA) in all solvent systems. ECL efficiencies (ϕ_ecl, photons produced per redox event) of 0.73 and 0.84 for (bpy)₂Ru(DC-bpy)²⁺, and (bpy)₂Ru(DM-bpy)²⁺ were obtained in aqueous buffered solution, using Ru(bpy)₃²⁺ as a relative standard (ϕ_ecl = 1.0). Addition of 0.4 mM Triton X-100 results in a greater than 2-fold increase in ECL efficiences (i.e., 3.8, 2.4 and 2.3 for Ru(bpy)₃²⁺, (bpy)₂Ru(DC-bpy)²⁺, and (bpy)₂Ru(DM-bpy)²⁺, respectively) using aqueous Ru(bpy)₃²⁺ containing no surfactant as standard (ϕ_ecl = 1.0). ECL efficiencies of 27.4, 16.5 and 26.1 were found in 50∶50 (v/v) CH₃CN∶H₂O (0.1 M KH₂PO₄) for Ru(bpy)₃²⁺, (bpy)₂Ru(DC-bpy)²⁺, and (bpy)₂Ru(DM-bpy)²⁺, respectively, using aqueous Ru(bpy)₃²⁺ containing no surfactant as standard (ϕ_ecl = 1.0). Detailed studies support adsorption of surfactant on the electrode surface, thus facilitating TPrA and ruthenium oxidation.