Ruthenium Complexes

Ruthenium Complexes

Introduction

Ruthenium complexes are becoming the focus of research in photoelectrochemistry due to their unique chemical stability, redox properties, luminescence properties, and suitable excited state lifetimes. Ruthenium complexes are used in the field of electrochemiluminescence research. A series of novel luminescent ruthenium compounds are synthesized by introducing electron-withdrawing groups at different positions of the ruthenium complex ligands.

Electrochemiluminescence (ECL) is a process in which substances generated at electrodes undergo high-energy electron-transfer reactions to form excited states that emit light. It is a novel analytical method that combines electrochemical and chemiluminescence technology. Ru(bpy)₃²⁺[tris(2, 2'-bipyridine)ruthenium(II)] is widely used in electrochemiluminescence analysis due to its superior properties including high ECL efficiency, good sensitivity and strong electrochemical stability under moderate conditions in aqueous solution. Introduction of different alkyl substituents on the bipyridine ligands of Ru(bpy)₃²⁺ can effectively suppress the self-quenching phenomenon during luminescence.

ECL Mechanism of Ru(bpy)₃²⁺

Many reagents can be used as coreactants to form chemiluminescence systems with ruthenium bipyridine complexes, which can significantly enhance the luminescence intensity of the system. The most commonly used and effective reagent for ECL applications is tripropylamine (TPrA), forming the Ru(bpy)₃²⁺/TPrA system that exhibits the highest ECL efficiency. Most ECL reactions of ruthenium complexes with co-reactants follow the classic "oxidative-reduction" coreactant mechanism, where oxidation of TPrA generates a strongly reducing species TPrA• and the oxidation can be via the "catalytic pathway"^[1].

Fig. 1 The "catalytic route" in Ru(bpy)₃²⁺ / TPrA system

Applications

Ruthenium complexes are widely used in light-emitting electrochemical cells and luminescence-functionalized nanomaterials.

• Light-emitting electrochemical cells

The application of Ru(II) complexes to solid-state light-emitting electrochemical cells (LECs) is through spin coating or self-assembly techniques. The Ru(bpy)₃²⁺ was used as the light-emitting layer in the solid-state organic light-emitting device, which showed good photoluminescence and electroluminescence. By chemically modifying the Ru(bpy)₃²⁺ complex, blending the Ru(bpy)₃²⁺ layer with polymethyl methacrylate (PMMA), light-emitting devices with the three different Ru(bpy)₃²⁺ complexes were prepared in a indium tin oxide (ITO)/Ru(bpy)₃²⁺+ PMMA/Ag sandwich configuration. The performance of this light-emitting device has been significantly improved which shows excellent storage stability, good operational stability and high external quantum efficiency, leading to a wide range of applications for light-emitting electrochemical cells based on Ru(bpy)₃²⁺ complexes^[2].

Fig.2 Chemical structures of three different Ru(bpy)₃²⁺ complexes and the device structure of a Ru(bpy) ₃²⁺ LEC

• Luminescence-functionalized nanomaterials

Luminescence-functionalized nanomaterials refer to loading a large number of signal molecules that can generate chemiluminescence on nanomaterials. This material not only has the excellent characteristics of nanomaterials, but also has advantages in chemiluminescence biological analysis. Ruthenium complexes are used in light-emitting functional nanomaterials. A method of immobilization of nanoparticles on the surfaces has developed for effective immobilization of Ru(bpy)₃²⁺ on an electrode surface for solid-state ECL detection. The citrate-capped AuNPs and Ru(bpy)₃Cl₂ were mixed in an aqueous medium to form Ru(bpy)₃²⁺ - gold nanoparticles (AuNPs) aggregates (Ru-AuNPs) by electrostatic interaction, followed by the attachment of as-formed Ru-AuNPs on a sulfhydryl-derivated indium tin oxide (ITO) electrode surface via Au-S interactions. The Ru-AuNP-modified ITO electrode is quite stable, exhibits excellent ECL behavior^[3].

Fig.3 The formation of Ru-AuNPs and the immobilization of Ru-AuNPs on a sulfhydryl-derivated ITO electrode surface.

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References

• Miao, W. Electrogenerated chemiluminescence and its biorelated applications. Chem Rev. 2008, 108: 2506-2553
• Rudmann, H.; Shimada, S.; Rubner, M. F. Solid-state light-emitting devices based on the tris-chelated Ruthenium(II) Complex. 4. High-efficiency light-emitting devices based on derivatives of the tris (2,2'-bipyridyl) Ruthenium(II) complex. J Am Chem Soc. 2002, 124: 4918-4921
• Sun, X. et al. Method for effective immobilization of Ru(bpy)₃²⁺ on an electrode surface for solid-State electrochemiluminescene detection. Anal Chem. 2005, 77: 8166-8169