Synthesis of a Ru(II) Complex with a Naphthoquinone-Annelated Imidazole Ligand Exhibiting Proton-Responsive Redox and Luminescent Behavior

A mononuclear ruthenium complex, [RuII(L)(bpy)2](PF6), with a naphthoquinone-annelated imidazole ligand HL (2-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazole-4,9-dione) was synthesized and structurally characterized. Electrochemical study reveals that the Ru complex shows four reversible redox waves at +0.98 V, −1.13 −1.53 −1.71 V versus SCE in acetonitrile, which are assigned to Ru(II)/Ru(III), L−/L•2−, two bpy/bpy•− couples, respectively. The potential of Ru(II)/Ru(III) positively shifted upon addition trifluoromethanesulfonic acid due protonation L− moiety, leading stabilization 4d orbital. In UV-vis absorption measurements for metal-to-ligand charge transfer (MLCT) band observed 476 nm, 450 nm by protonation, might be decrease electron delocalization π orbitals L−. blue shift MLCT associated an emission from 774 620 could caused decreased electronic excited state. These electrochemical spectroscopic changes were protonation/deprotonation stimuli.