Long-Lived Polypyridyl Based Mononuclear Ruthenium Complexes: Synthesis, Structure, and Azo Dye Decomposition

Two mononuclear ruthenium complexes [(bpy)2:(RuL1)-L-II/L-2](ClO4)(2) ( [1](2+)/[2](2+)) (bpy-2,2' bipyridine, L-1 = 2,3-di(pyridin-2-yl)pyrazino [2,3-f] [1,10]-phenanthroline) and L-2 = 2,3-di(thiophen-2-yl)pyrazino [2,3-f] [1,10]phenanthroline have been synthesized. The complexes have been characteriied using various analytical techniques. The complex [1](2+) has further been characterized by its single crystal X-ray structure suggesting ruthenium is coordinating through the N donors of phenanthroline end. Theoretical investigation suggests that the HOMOs of both complexes are composed of pyridine and pyrazine unit of ligands L-1 and L-2 whereas the LUMOs are formed by the. contribution of bipyridine units. The low energy bands at similar to 480 nm of the complexes can be assigned as MLCT with partial contribution from ligand transitions, whereas the rest are ligand centered. The complexes have shown Ru-II/Ru-III oxidation couples at E-1/2 at 1.26 (70 mV) V and 1.28 (62 mV) V for [1](2+) and [2](2+) vs Ag/AgCl, respectively, suggesting no significant role of distal thiophene or pyridine units of the ligands. The complexes are emissive and display solvent dependent emission properties. Both complexes have shown highest emission quantum yield and lifetime in DMSO (phi = 0.05 and tau(avg) = 460 ns and lambda(em)(max) at 620 nm for [1](2+); phi = 0.043 and tau(avg) = 425 ns and lambda(em)(max) at 635 nm for [2](2+)). Further, the long-luminescent lifetime of these complexes has been utilized to generate reactive oxygen species for efficient azo dye decomposition.