Abstract
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 V, −1.53 V, and −1.71 V versus SCE in acetonitrile, which are assigned to Ru(II)/Ru(III), L−/L•2−, and two bpy/bpy•− redox couples, respectively. The redox potential of Ru(II)/Ru(III) was positively shifted upon the addition of trifluoromethanesulfonic acid due to protonation of the L− moiety, leading to stabilization of the Ru 4d orbital. In UV-vis absorption measurements for the Ru complex in acetonitrile, a metal-to-ligand charge transfer (MLCT) band was observed at 476 nm, which was shifted to 450 nm by protonation, which might be due to a decrease in the electron delocalization and stabilization of the π orbitals in L−. The blue shift of the MLCT band by protonation was associated with a shift of an emission band from 774 nm to 620 nm, which could be caused by the decreased electronic delocalization in the MLCT excited state. These electrochemical and spectroscopic changes were reversible for the protonation/deprotonation stimuli.
Highlights
Stimuli-responsive materials have attracted considerable attention for their potential application as smart materials in the field of optoelectronics and life science [1,2,3]
Protons play an important role in biological systems, in which redox behavior is significantly altered through the proton-coupled electron transfer (PCET) process [13,14], and the pH-dependent redox reactions of ruthenium complexes have been intensively studied to understand the interplay between electrons and protons [15,16,17,18]
1 was proven to exhibit a characteristic redox and emission behavior based on the naphthoquinone moiety as an electron acceptor
Summary
Stimuli-responsive materials have attracted considerable attention for their potential application as smart materials in the field of optoelectronics and life science [1,2,3]. Various external stimuli, such as temperature, pressure, and light, have been utilized to control the physical properties of metal complexes [4,5]. In the field of solid state chemistry, proton dynamics and modulation of hydrogen bonds have been extensively studied in various types of switching materials [19,20,21,22], and many interesting physical properties, such as protonation-affected spin transition behavior in metal complexes [23,24,25,26], solid state proton transfer induced by spin-crossover phenomena [27], and proton dynamics-directed phase transition in conducting organic molecules [28]
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