**1. Introduction**

Ruthenium is a precious metal that belongs to the platinum-group elements [1]. As ruthenium can adopt various oxidation numbers, its coordination complexes adopt a wide variety of oxidation states from -II to VIII [2]. When nitrogen-based donor ligands are coordinated to a central ruthenium atom, the resulting ruthenium complexes generally prefer the +II and + III oxidation states, but occasionally also adopt the +IV and + V states. Arguably the most studied Ru complexes with nitrogen-based donor ligands are those that contain the hexaammineruthenium dication, [Ru(NH3)6] 2+, and the tris(2,2<sup>0</sup> -bipyridine)ruthenium(II) dication, [Ru (bpy)3] 2+(bpy = 2,2<sup>0</sup> -bipyridine) [3–6]. [Ru(bpy)3] 2+ was first reported by Burstall [7] and is easily obtained from the reaction of RuCl3 • nH2O with an excess amount of bpy in aqueous ethanol. [Ru(bpy)3] 2+ exhibits a stable low-spin t2g <sup>6</sup> electronic configuration as well as a reversible one-electron oxidation at +1.29 V (vs SCE) and successive one-electron reductions at �1.33 V, �1.52 V, �1.76 V, and � 2.4 V vs. SCE; the oxidation is a Ru(II/III) metal-center-based process, while the reductions occur on the bpy ligands. [Ru(bpy)3] 2+ exhibits a metal-to-ligand charge transfer (MLCT) band at 452 nm and bright luminescence at 612 nm (lifetime: 600 ns) under MLCT excitation. This luminescence arises from the triplet MLCT photoexcited state, which allows this complex to serve as a photosensitizer for a wide scope of photoenergy conversion processes and as a photocatalyst for organic transformations [8]. Therefore, it is hardly surprising that during the past five decades, numerous studies on photoactive [Ru(bpy)3] 2+ complexes have been reported [3]. The tuning of their physical properties, such as their absorption/emission maxima or redox potential, via ligand modification has been achieved by introducing substituents on bpy or by replacing the bpy ligand with other *N*-heteroaromatic ligands. This tuning has led to a wide range of functional materials based on [Ru(bpy)3] 2+.

For example, replacing one of the bpy ligands in [Ru(bpy)3] 2+ with an *N*heteroaromatic ligand comprising a benzimidazole and a pyridine group shifts the Ru(II/III) oxidation potential in negative direction because benzimidazole is a stronger σ-donor and weaker π-acceptor than pyridine [9]. Furthermore, the coordinated benzimidazole N–H imino group acts as a Brønsted acid, and the corresponding deprotonated benzimidazolate site can coordinate to another metal ion (**Figure 1**) [10]. Protonated or *N*-alkyl benzimidazolium ions can act as precursors for *N*-heterocyclic carbene (NHC) metal complexes [11, 12]. The representative pyridine-containing ligands 2,2<sup>0</sup> -bipyridine (bpy) and 2,2<sup>0</sup> ,2″-terpyridine (tpy) form chelate complexes with bidentate and tridentate coordination modes. When one or two pyridine groups are replaced with benzimidazole, the resulting bidentate- and tridentate-coordinating ligands are known as 2-(2-pyridyl)benzimidazole and 2,6-bis(benzimidazol-2-yl)pyridine, respectively (**Figure 2**) [13, 14].

**Figure 1.** *Coordination modes of pyridine and benzimidazole as ligands.*

*Surface-Confined Ruthenium Complexes Bearing Benzimidazole Derivatives: Toward… DOI: http://dx.doi.org/10.5772/intechopen.97071*

#### **Figure 2.**

*Chemical structures of bidentate and tridentate ligands that contain pyridine and benzimidazole group(s).*

**Figure 3.** *Schematic illustration of molecular devices based on electron/proton-responsive Ru complexes confined to a surface.*

This chapter focuses on the molecular design of ruthenium complexes with *N*heteroaromatic ligands, particularly benzimidazole derivatives. In Ru–benzimidazole complexes in aqueous solution, the solution pH strongly affects the Ru(II/III) oxidation potential, which is derived from the proton-coupled electron transfer (PCET) reaction in solution [15]. The proton-responsiveness of Ru–benzimidazole complexes has been exploited to obtain switching functionality in multinuclear Ru complexes [16, 17]. In addition, ligand modification via *N*-alkylation of the benzimidazole N–H imino group has been used to achieve the anchoring of redox-active Ru complexes [18, 19]. and the layer-by-layer (LbL) assembly of Ru complexes on indium-tin oxide (ITO) surfaces toward molecular electronic devices [20–22]. **Figure 3** presents a schematic illustration of our conceptual approach for molecular devices based on surface-confined Ru complexes with electron/protonresponsiveness.
