**2.1 Bridging ligands that contain benzimidazole groups**

Benzimidazole ligands can be synthesized by the Phillips condensation reaction between an organic carboxylic acid or nitrile and *o*-phenylenediamine [23]. **Figure 4** shows the structures of benzimidazole ligands bridging two Ru centers that have been used by our group [17, 24–26] and others [27, 28] for the preparation of dinuclear Ru complexes. Depending on the chemical structure of the linker moiety, significant electronic coupling between the two ruthenium ions is posible. In addition, the oxidation processes of the Ru centers in aqueous solution involve PCET reactions, resulting in switching of the metal–metal interaction via the change in electron density on the conjugated linker ligand.

#### **2.2 Introducing anchor groups in benzimidazole ligands**

Surface modification plays an important role in controlling the electron-transfer events and chemical reactivity in photocatalysis, as well as the charge-transport process in heterojunctions. Recently, several reviews of the applications of surface modification toward dye-sensitized solar cells and electrochemical catalysts for hydrogen/oxygen-evolution reactions have been published [21, 29–34], showing the importance of such interdisciplinary research. In the area of solar-energy conversion, Grätzel-type dye-sensitized solar cells composed of mesoporous TiO2 on fluorinedoped SnO2 (FTO) with immobilized Ru complexes that contain 2,2<sup>0</sup> -bipyridyl-4,4<sup>0</sup> dicarboxylate and other bpy-derived ligands have been developed [35]. Interestingly,

#### **Figure 4.**

*Chemical structures of benzimidazole-containing ligands that bridge two Ru centers in bis-bidentate or bis-tridentate coordination modes.*

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

the electron-injection efficiency from the photoexcited-state Ru complex to TiO2 was found to strongly dependent on the anchoring group [34, 36, 37].

Given that the adsorption strength of a Ru complex on a surface depends on the combination of the anchoring group and the surface material, judicious selection of both is necessary for effective surface functionalization. Recently, indium-tin oxide (ITO)-coated glass or polymer substrates have been employed in a wide variety of electronic display devices such as organic light-emitting diodes (OLEDs) [38]. Transparent ITO electrodes are also employed as cell windows for spectroelectrochemistry. Therefore, ITO is a suitable substrate for monitoring both the electrochemical and spectrochemical changes in redox-active Ru complexes immobilized on its surface. The phosphonic acid group, which is known to immobilize on ITO electrodes, has been employed to anchor the Ru complexes [39]. Furthermore, organic phosphonic acids are known to bind zirconium(IV) ions to form a solid twodimensional layer structure, [40]. which demonstrates their suitability for use in a layer-by-layer growth method based on redox-active metal complexes. Therefore, we developed several new tridentate benzimidazole ligands with phosphonic acid or phosphonate ester anchor groups (**Figure 5**). Alkylation of the imino N–H groups of the benzimidazole moieties using bromoalkyl-diethylphosphonate derivatives furnished chelating benzimidazolyl ligands with ethyl-protected phosphonates, which were used for the synthesis of Ru complexes [41]. After the ethyl-protected phosphonate Ru complexes had been purified, the diethyl phosphonate groups were deprotected to provide the corresponding Ru complexes with phosphonic acid groups. In particular, the tridentate ligand **XP** (**Figure 5**) contains several methylene groups on its side-arms; these methylene moieties are sterically hindered, thus fixing the conformation of the phosphonic acid anchor groups upon surface immobilization.

#### **2.3 Molecular design of redox-active Ru complexes with anchor groups**

Ru complexes are substitutionally inert, and the octahedral coordination geometry around the Ru ion is maintained throughout the Ru(II/III) redox reaction. Therefore, they can be immobilized on a surface to design redox-active molecular devices. When three bidentate ligands are coordinated to an octahedral Ru complex, the formation of Δ and Λ optical isomers is possible, but in the case of Ru complexes surrounded by two tridentate ligands with C2v symmetry, no optical isomers do not exist. Hence, surface-confined Ru complexes that contain tridentate ligands with C2v symmetry such as 2,6-bis(benzimidazolyl)pyridine with phosphonic acid anchors are often selected for surface immobilization [21, 42]. Furthermore, the molecular orientation of Ru complexes self-assembled on a surface is crucial to the construction of further

**Figure 5.**

*Chemical structures of benzimidazole-containing ligands with phosphonic acid anchor groups and their abbreviations.*

**Figure 6.**

*Chemical structures of free-standing multipodal phosphonic acid anchor groups on a surface [43–45].*

**Figure 7.** *Chemical structures of rod-shaped dinuclear Ru complexes that bear free-standing multipodal anchor groups at both ends.*

layered structures. To maintain the vertical orientation of the Ru complexes on a surface, free-standing multipodal anchor groups with phosphonic acid have been developed over the last two decades. Several examples are shown in **Figure 6** [21, 43–46]. together with our multipodal tridentate benzimidazole ligand with phosphonate anchors, **XP. Figure 7** shows a new series of rod-shaped Ru complexes with 2,6-bis (benzimidazolyl)pyridine moieties and bi- or tetrapodal phosphonic acid groups at both ends that have been reported by our group. Mononuclear Ru complex **10** exhibits a spherical-shaped structure around the central Ru ion, while the other dinuclear Ru complexes **8, 9, 11,** and **12** exhibit a rod-like structure with a Ru–Ru axis [21, 43]. When the dinuclear Ru complexes are immobilized on a surface, the Ru–Ru axis can be oriented vertically or horizontally relative to the surface plane. In AFM measurements, the obtained molecular heights of the dinuclear Ru complexes **9**, **10**, and **11** immobilized on an ITO surface were consistent with the predicted heights for the vertical orientation of the dinuclear Ru complexes at the ITO surface. Ru complexes **8–12** can be dissolved in aqueous solution by adjusting the solution pH, given that the phosphonic acid groups at both ends of these act as polybasic acids.

## **3. Characterization and functionality of surface-immobilized redox-active Ru complexes**

#### **3.1 Surface modification by Ru complexes bearing phosphonic acid anchors**

When Ru complexes bearing phosphonic acid anchors are immobilized on an ITO or mica surface by immersion of the substrate into a solution of the Ru

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

complex, the surface coverage of the Ru complex is dependent on the immersion time and the concentration of the complex in the solution. The temporal evolution of the surface coverage can be analyzed using the kinetic Langmuir equation (Eq. (1)), and curve fitting with a rate constant parameter, *k*.

$$
\Gamma\_t = \Gamma\_s(\mathbf{1} - \exp\left(-kC^\*t\right))\tag{1}
$$

Here, Γ(t*)*, *k*, *C*, and *t* refer to the surface coverage, rate constant, concentration of the Ru complex, and time, respectively [33, 46]. A typical kinetic plot for a Ru–**XP** complex with an acridine group at the top is shown in **Figure 8**. This surface-immobilized Ru–**XP** complex is able to capture double-stranded DNA from solution [46].

Another chemical approach to evaluate the surface coverage is using the thermodynamic Langmuir isotherm based upon the concentration dependence of the adsorption of the Ru complex.

Γ*i=*ð Þ <sup>Γ</sup>*s*�Γ*<sup>i</sup>* ¼ *exp* <sup>Δ</sup>*Gads<sup>o</sup>* ð Þ *=RT CB* (2)

Here, Γ*i*, Γ*s*,*CB*, and Δ*Gads* refer to the surface adsorption at a given concentration, the adsorption at saturation, the concentration of the bulk solution, and the free energy of adsorption, respectively. The adsorption of a Ru complex on a mica and ITO surface can be fitted with the typical Langmuir isotherm model. The free energy of adsorption for Ru complex **9** was found to be �33.4 kJ/mol.

In recent years, several binding modes for the absorption of phosphonic acid groups on metal oxides have been proposed based on intensive studies using polarization modulation infrared reflection adsorption spectroscopy (PM-IRRAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations in recent years, and the bidentate or tridentate binding modes are considered to be most probable (**Figure 9**) [30, 32, 34, 44, 47]. The optimum solution pH value for the adsorption of Ru complexes **8**–**12** depends on the complex*; i.e*., pH = 6 is optimal for complexes **8–11,** while pH = 4 is better for complex **12.**

Atomic force microscopy (AFM) measurements have been used to provide clear surface images depicting the adsorption of these Ru complexes, particularly the surface morphology (*e.g*. height and surface coverage). **Figure 10** shows AFM images of Ru complex **9** on a flattened ITO surface. The samples were prepared using two different immersion conditions, *i.e*., a dilute solution of the Ru complex with a short immersion time (1 μM; 10 min) and a higher concentration solution with a longer immersion time (25 μM; 3 h). As shown on the left in **Figure 10**, for the dilute

**Figure 8.**

*Chemisorption kinetics of the Ru–XP complex determined by the temporal evolution of the surface coverage of the Ru–XP complex (right) on a mica surface. (left) Plot of contact angle versus immersion time [46].*

#### **Figure 9.**

*Proposed surface binding modes of phosphonic acid groups on a metal oxide (MOx) surface (M = metal) [30].*

#### **Figure 10.**

*AFM images of dinuclear Ru complex 9 immobilized on a flat ITO surface using either a 1 μM solution of complex 9 with an immersion time of 10 min (left) or a 25 mM solution for 3 h (right). Plots show cross-sectional height profiles. In the schematic drawings of the surface images, 9 is shown as a cylinders.*

condition, scattered dots were observed on the surface. The average height of the dots was approximately 4 μm, which is consistent with the height that was predicted for vertically oriented complex **9** using a molecular model. The AFM image on the right in **Figure 10** shows that the surface was fully covered with spherical domains with a diameter of 20–50 μm when the concentrated conditions were used.

One advantage of ITO electrodes is that they enable the use of UV–vis spectroscopy to monitor the surface immobilization process. Ru(II) complexes bearing *N*-heteroaromatic ligands exhibit a relatively strong metal-to-ligand charge transfer (MLCT) band in the visible wavelength region. In particular, the temporal changes during the immobilization or multilayering processes of such complexes on modified ITO substrates are easily monitored via the absorbance changes in the UV–vis spectra. Furthermore, electrochemical methods such as cyclic voltammetry (CV) can be used to determine the surface coverage of redox-active Ru complexes immobilized on an ITO electrode [48].

#### **3.2 PCET reaction of Ru-benzimidazole complexes in solution and on ITO electrodes**

Ru complexes with benzimidazole derivative ligands act as Brønsted acids and exhibit proton-coupled electron transfer (PCET) reactions in aqueous solution [49, 50].

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

For example, [Ru(mbibzim)(bibzimH2)]2+ (**13**) (mbibzim = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine and bibzimH2 = 2,6-bis(benzimidazole-2-yl)pyridine) behaves as a dibasic acid, as shown in **Figure 11**. The Ru(II/III) oxidation potential of **13**, which results from a PCET reaction, strongly depends on the pH-value (2 < pH < 9) in a Britton–Robinson buffer/CH3CN(1:1 v/v) solution [14, 49, 50]. The PCET reaction of **13** can be described by the square scheme in **Figure 12**, in which complex **13** is abbreviated as Ru–LH2. This reaction involves both electron transfer and protontransfer equilibria. In the half-wave potential/pH plot of **13**, the so-called Pourbaix diagram, the half-wave potential is gradually shifted in the negative potential direction with increasing solution pH (**Figure 13**). Based on this diagram, the p*Ka* values of **13** were determined to be 6.31 and 7.94 for the Ru(II) oxidation state, and < 2 and 3.60 for the Ru(III) oxidation state. When the central pyridine group in the bibzimH2 ligand is replaced with a cyclometallated phenylene group, the resulting Ru complex **14** shows a lower Ru(II/III) oxidation potential than complex **13**; the p*Ka* values of **14** also increase compared to those of **13**, *i.e*., to 10.91 and > 12 for the Ru(II) oxidation state and to 6.46 and 9.15 for the Ru(III) oxidation state [51].

In biological systems, protons play a very important role in reactions and energy storage. Proton gradients are the driving force for the synthesis of ATP in biological membranes. Applications of proton gradients in energy storage in materials chemistry have shown that PCET chemical systems can be used for energy storage in redox batteries and capacitors. Ru complexes **13** and **14**, which show PCET with different potentials and p*Ka* values in unbuffered aqueous solutions, have been used to construct two half-cells separated by a Nafion membrane [51]. The Pourbaix diagrams of complexes **13** and **14** are shown in **Figure 14**; the initial oxidation states of **13** and **14** were Ru(II) and Ru(III), respectively. Upon charging, the oxidation of **13** from the Ru(II) to the Ru(III) state releases the proton(s), while the reduction of **14** from Ru(III) to Ru(II) at the other half-cell captures the proton(s). As a result,

**Figure 11.**

*Stepwise proton-transfer equilibria of Ru(II)(LMe)(LH2) complexes (13: X = N; 14: X = C) that act as Brønsted dibasic acids. The total charge of the Ru complex is omitted.*

#### **Figure 12.**

*Square scheme of the electron–proton equilibria of Ru complexes 13 or 14 as dibasic acids. Numbers in parentheses indicate the Ru species present in the Pourbaix diagram in Figure 13.*

**Figure 13.** *Pourbaix diagram of Ru complex 13 in CH3CN/BR buffer (1/1 v/v) [51].*

#### **Figure 14.**

*Pourbaix diagram for a solution redox battery based on a pair of Ru complexes, 13 and 14, which both exhibit PCET reactions in CH3CN/BR buffer (1/1 v/v).*

the electrical energy was stored as a pH gradient between the half-cells. The concept of using a PCET system in this way had been implemented with organic quinone derivatives, [52] but its scope was extended to redox-flow batteries by using a pHtunable Fe(III) azamacrocyclic complex as both the catholyte and anolyte based on the multiple protonated forms of the Fe complex [53].

The PCET chemistry of Ru complexes that contain benzimidazole derivatives in solution can be extended to surface-bound systems by attaching surface-anchoring groups to the Ru complexes as described in the previous section.

Dinuclear Ru complexes **15**, **16,** and **17** (**Figure 15**) have N–H sites that can be deprotonated on the bridging ligand and the phosphonic acid anchoring groups at both ends. When **15** and **16** are assembled on an ITO electrode, the surfaceimmobilized redox-active Ru(II) complexes exhibit a well-defined CV response with a typical shape derived from adsorbed chemical species [20]. **Figure 16** shows the CV responses that originate from the Ru(II/III) oxidation process of immobilized complex **15** at different pH values; these responses arise from a welldefined two-electron oxidation wave corresponding to the Ru(II)–Ru(II)/Ru(III)– Ru(III) PCET process in **15** [24]. Upon changing the pH of the aqueous solution was changed, Ru complexes **8**–**17** immobilize stably via the phosphonic acid groups on the ITO electrode at pH = 1.0–9.0, but at pH >9.5, the Ru complexes easily detached from the ITO surface.

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

#### **Figure 15.**

*Chemical structures of dinuclear Ru complexes bearing both a benzimidazole bridging ligand from which protons can dissociate and free-standing multipodal phosphonic acid anchor groups at both ends [24].*

#### **Figure 16.**

*Cyclic voltammograms of Ru complex 15 immobilized on an ITO electrode at different pH values in CH3CN-BR buffer (1/1 v/v) [24].*

The anodic peak current, *i*pa, is expressed as shown in Eq. (3), where *n*, *ν*, *A*, and Γ represent the number of electrons, scan rate, electrode area, and surface coverage, respectively. Therefore, the anodic peak current for complex **15** is linearly proportional to the scan rate *ν*, indicating that the Ru complex is a surface-bound species. The total area of the anodic wave is related to the surface coverage of the Ru complex, which was determined to be 0.80 � <sup>10</sup>�<sup>10</sup> mol cm�<sup>2</sup> .

$$i\_{\rm pt} = \frac{n^2 F^2 \nu}{4RT} A \Gamma \tag{3}$$

Given that the spectral change due to the redox reactions of a monolayer film of **15** on the ITO electrode can be observed by UV–vis spectroscopy, even surfaceassembled monolayer films of Ru complexes on an ITO electrode can be detected.

Upon the oxidation of **15** under an applied potential of +0.8 V vs. Ag–AgCl at pH = 6.5, the differential absorption spectrum demonstrates strong bleaching of the MLCT band at 550 nm and π-π\* transitions of the ligands at 364 nm, along with absorbance increases at 400 and 750 nm, which are characteristic spectral features of Ru(III)–L complexes [24].

## **3.3 Fabrication of multilayer films based on Ru complexes by layer-by-layer (LbL) growth**

Surface modification using a molecular monolayer film alone enables only a single set of functionalities to be incorporated, and the measurable physical quantities, such as optical absorption or current value, are very low due to the low molecular density on the surface. On the other hand, multi-layer modification has the advantages of allowing various molecular units to be deposited at the surface, achieving greater increases in physical quantities such as optical density and charge stored, and enables the integration of various functionalities at the interface [31]. Thus, the integration of functional metal complexes on an electrode holds great promise for applications in *e.g.* molecular-based devices for photochemical energy production/transduction, photocatalysis, and information storage, among other applications [54]. To achieve this integration, the LbL assembly method, in which multilayer structures are constructed via molecular interactions between two layers on a solid surface, is an appropriate technique [39]. LbL assembly using electrostatic interactions, [55]. hydrogen bonding, and coordination metal–ligand bonds has been reported [21, 56]. Among these, structures based on metal-coordination assembly are robust toward environmental changes, such as variations in pH or ionic strength, in aqueous solution.

The formation of well-ordered zirconium(IV) bisphosphonate multilayer films is a well-known method for LbL assemblies on a solid surface that has been developed by Mallouk and others [40]. This method is based on the reconstitution of a two-dimensional layered compound, Zr(HPO3)2, on a gold surface via selfassembly of molecular units with metal ions. Starting from self-assembled 4 mercaptobutylphosphonic acid on the gold surface as a primer layer, alternate immersion of the modified gold substrate into a zirconium(IV) oxychloride solution and a bisphosphonic acid solution leads to multilayer films composed of a twodimensional zirconium–phosphonate framework structure via LbL growth [40].

Similarly, the rod-shaped Ru complexes **8**–**16** with polyphosphonic acid groups at both ends were immobilized by self-assembly on an ITO electrode. The polyphosphonic acids at one side of the complexes were attached to the ITO surface, while the other side of the polyphosphonic acid groups remained free to interact with metal ions in solution. Multilayer films of the Ru complexes could thus be obtained by successive alternate immersion in a zirconium(IV) ion and a Ru complex solution (**Figure 17**) [57]. The immobilized Ru film on the ITO substrate was monitored via UV–vis spectroscopy, CV, and the AFM-surface-scratching method throughout each stage of the LbL growth. The use of these monitoring techniques during LbL growth is shown in **Figure 18** with the combination of Ru complex **11** and zirconium as an example.

In each physical measurement, the physical quantities, such as absorbance, amount of charge, and the height of the scratch increased linearly with increasing number of layers. Two types of growth models have been proposed for LbL growth from the surface-primer points via metal coordination on a solid surface (**Figure 19**) [59]. The first model involves dendritic divergent growth, which would result in an exponential increase in the physical quantities, while the second model involves linear growth of a layered structure. In the case of Ru complex **11**,

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

#### **Figure 17.**

*Illustration of the layer-by-layer (LbL) assembly by successive immersion of a solid substrate such as ITO, mica, or a Si wafer into (i) a solution of a given Ru complex with phosphonate anchors, and (ii) a solution of Zr(IV) ions. After washing, this immersion process was conducted repeated several times.*

#### **Figure 18.**

*Illustration of the LbL multilayer film of 11 and monitoring of its LbL growth using UV–vis spectroscopy, CV, and the AFM-scratching method, together with a plot of the height of the scratch vs. the number of layers.*

the linear increase in the physical quantities such as absorbance indicates the formation of well-ordered two-dimensional multilayer films on the solid surfaces.

The use of the LbL method on a solid surface makes it possible to accumulate various molecular units with different chemical functionalities by adjusting the number of layers and metals in the multilayer films. Furthermore, the sequential order of the various complex units can be varied; the units can be arranged in order of descending or ascending potential or p*K*<sup>a</sup> to produce hetero-multilayer films. Such gradients in the layers play an important role in energy transduction. Therefore, "coordination programming" at the surface is possible via judicious choice of molecular units [60]. For example, when two different complexes A and B are assembled into a four-layered film on a solid substrate and a primer layer A is fixed as the first immobilized layer, seven combinations can be obtained by varying the order of the successive layers (substrate|ABAB, |AAAB, etc.). As a result, various films with different potential gradients can be created, and their rectification of electron transfer can be evaluated. Finally, in the fabrication of heterolayers using

**Figure 19.** *Illustration of two-layer-growth models: Dendritic divergent growth (left) and linear growth (right) [58].*

metal-coordination via the LbL method by immersion in solutions of each component, it is important to ensure that each layer is segregated and that the components are not mixed with others within the layer. In particular, care should be taken using metal ions with substitutionally labile properties, such as Ni(II) and Co(II) ions.

#### **3.4 Functionality of LbL-multilayer films based on Ru complexes**

#### *3.4.1 Electron-transfer rate in homo-multilayer Ru complex films on ITO electrodes*

Measuring electron-transfer events in multilayer LbL films composed of redoxactive Ru complexes on an ITO electrode is fundamental to determine whether electrons can be transmitted through the multiple layers of Ru complexes as the number of layers is increased.

CV measurements of homo-multilayer films of **11** (X = N) to evaluate their electron-transfer rate showed two one-electron oxidation waves. With increasing number of layers, the peak current during CV increased almost linearly. The peakpotential separation between the anodic and cathodic waves was unchanged at a scan rate of 0.1 V�s �1 , but at 1 V�s �1 , the separation increased with an increasing number of layers. Using Laviron's procedure, [61] the heterogeneous electrontransfer rate between the Ru complexes in the layers and the ITO electrode was determined for different numbers of layers. The electron-transfer rates gradually decreased with increasing number of layers, and a small attenuation coefficient was obtained from the slope of the plots of ln(*k*app) as a function of the thickness of films (L). The dependence of the electron-transfer rate, *k*app, on the distance is expressed by the following equation:

$$k\_{app} = k^0 \exp\left\{-\beta L\right\} \tag{4}$$

In addition, potential-step chronoamperometry (PSCA) measurements furnished a relatively small value for the apparent electron-transfer rate, *k*app (*β* = 0.014 Å�<sup>1</sup> ) [62].

Multi-layer films of ruthenium complex **9** were grown on an ITO substrate using hydrogen-bonding interactions. In these films, free-standing ruthenium complexes of **9** adsorbed on an ITO electrode via its tetrapod phosphonic acid anchors interacts with complexes of **9** that are dissolved in the solution via multiple hydrogen bonds between the free phosphonic acid groups at its surface and those of the complex **9** in solution at pH = 6. Surprisingly, the film thickness of the resulting multiply hydrogen-bonded system was controlled by the immersion time and the solution

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

pH. The current*–*voltage (*I–V*) characteristics of the multilayered Ru film grown on ITO were measured with a sandwich-type two-terminal devices using the conductive polymer PEDOT:PSS as the top electrode. Based on the observed current, the value of the coefficient for the attenuation with increasing film thickness was relatively small (*β* = 0.012–0.021Å<sup>1</sup> ) [43]. This small *β* value for the multilayer Ru complex film system indicates that long-range electron transport can be expected to be possible even though the system exhibits low conductivity. To clarify the reason for the long-range electron transport in the Ru multilayer film despite its low conductivity, first-principles calculations using Green's non-equilibrium function technique and theoretical analysis of the experimental results were performed. The results indicated that the metal centers covered with the π electrons of the ligands become "stepping stone" sites that provide a resonant tunneling mechanism [43]. Small *β* values were also reported for terpyridine-metal-complex nanowires produced by the sequential metal-coordination method [63].

### *3.4.2 Redox-active LbL multilayer films in redox capacitors*

As portable electronic devices continue to proliferate, cost-effective cheap energy storage devices that are small, flexible, and low cost and provide high performance during the charging and discharging cycle are in high demand. Molecular-based supercapacitors are promising candidates in terms of these requirements. Redox-active Ru complexes are suitable for the fabrication of energy storage devices, since multilayer molecular Ru assemblies obtained via the LbL method can be scaled by increasing the number of layers, which leads to enhancement of the electrical capacitance in such films [57].

The charge–discharge properties of a sixty-five-layer film of Ru complex **11**, ITO|(Ru complex **11**)65, were examined under galvanostatic experimental conditions by applying various constant current densities from 10 to 100 <sup>μ</sup><sup>A</sup>cm<sup>2</sup> . The galvanostatic charge and discharge curves show two small plateaus at 0.12 V and 0.37 V vs. Pt, which correspond to two reversible Ru(II)/Ru(III) redox processes (**Figure 20**). A maximum capacitance of 92.2 F<sup>g</sup><sup>1</sup> was found at an applied current density of 10 <sup>μ</sup><sup>A</sup>cm<sup>2</sup> , but the capacitance decreased to 63.3 F<sup>g</sup><sup>1</sup> at the highest

#### **Figure 20.**

*Galvanostatic charge–discharge curves of ITO|(Ru complex 11)65 in CH3CN/0.1 M HClO4 at different current densities [57].*

#### **Figure 21.**

*Schematic illustration of two half-cells composed of multilayer ITO|(16)*<sup>n</sup> *and ITO|(17)*<sup>n</sup> *(top), galvanostatic charge–discharge curves for 16 and 17 with different layer numbers,* n *(bottom left), and a plot of the capacitance as a function of* n *(bottom right) [20].*

current density of 100 <sup>μ</sup>A�cm�<sup>2</sup> (**Figure 20**). Stability tests of the Ru multilayer films (>1000 galvanostatic charge–discharge cycles) showed a capacitance retention of 72% [57].

#### *3.4.3 PCET reactions in Ru-multilayer films for energy storage devices*

PCET reactions can be used in energy-storage devices such as redox-flow batteries or two half-cells in unbuffered aqueous solution as described in Section **4.2.** The frameworks of dinuclear Ru complexes **16** and **17** are basically formed by chemical modification of the mononuclear Ru complexes **13** and **14** via the C–C coupling of two **13** or **14** units. Thus, the PCET behaviors of complexes **16** and **17** were almost the same as those of **13** and **14,** except that the number of electrons involved in the PCET reaction was doubled to result in a two-electron process due to the C–C coupling of two molecular units of **13** and **14** and the surface immobilization on an electrode. The Ru complexes **16** and **17** were immobilized by the LbL method on an ITO electrode and applied in a redox capacitor device, in which an aqueous solution was sandwiched between two electrodes composed of multilayer films of **16** and **17** to evaluate the cell performance (**Figure 21**) [20]. Under galvanostatic conditions at a constant current density of 10 <sup>μ</sup>A�cm�<sup>2</sup> , stable charge–discharge behavior occurred, as shown in **Figure 21**.

During the charging process, the multilayer **16** acted as the anode and the following oxidation reaction proceeded, resulting in the release of protons:

$$\text{Ru(II)}\_{2}\text{L}\_{\text{N}}\text{H}\_{2} \rightarrow \text{Ru(III)}\_{2}\text{L}\_{\text{N}} + 2\text{e}^{-} + 2\text{H}^{+}.\tag{5}$$

At the same time, the multilayer electrode of **17** acted as a cathode, and the following reduction took place, resulting in the capture of protons:

$$\text{Ru(III)}\_{2}\text{L}\_{\text{C}}\text{H}\_{3} + 2\text{e}^{-} + \text{H}^{+} \rightarrow \text{Ru(II)}\_{2}\text{L}\_{\text{c}}\text{H}\_{4}.\tag{6}$$

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

where **16** and **17** are represented as Ru2LNH*<sup>n</sup>* and Ru2LCH*n*, and LNH*<sup>n</sup>* and LCH*<sup>n</sup>* indicate the bridging ligand bearing the indicated number of protons on the benzimidazole groups. In other words, the capacitance increased with an increasing number of layers [20].

On the other hand, when all four imino N–H protons on the benzimidazole moieties were protected by N–Me groups, the capacitance decreased by 77% compared to that of the original PCET-type capacitor. This result strongly suggests that the proton movement plays a more important role than the anion movement in the charge storage. Furthermore, the proton movement accompanying the redox reaction in the Ru multilayer films on the ITO electrode was confirmed using a pHindicator probe in aqueous solution. In this type of LbL films composed of Ru complexes that exhibit PCET-type redox reactions, the capacitance increases almost linearly with the number of layers (**Figure 21**) [20].

To obtain proton rocking-chair-type redox capacitors that use protons as the charge carriers, the quinone/hydroquinone couple is often used [52, 64–67]. However, at neutral pH, the electron–proton transfer rate of the quinone/hydroquinone couple is slower than that of the Ru(II/III) couple. The use of the LbL method to fabricate multilayered structures of redox-active Ru complexes that exhibit PCET is advantageous, as the storage capacity can be enhanced by increasing the number of redox-active modular units on demand.

#### **3.5 Sequentially assembled heterolayer films of Ru complexes**

One advantage of the LbL assembly method using metal coordination at the interface is that a combinatorial approach can be employed to construct sequentially ordered hetero-multilayer films(*cf.* Section **4.3)**.

Monolayer films of the Ru complexes **11** and **12** on ITO showed two well-defined CV waves at +0.83 and + 1.04 V and at 0.37 and + 0.09 V vs. Fc+/Fc, which were assigned to Ru(II)–Ru(II)/Ru(II)–Ru(III) and Ru(II)–Ru(III)/Ru(III)–Ru(III) processes, respectively. Due to the large differences between the potentials of **11** and **12**, heterolayer films made from combinations of **11** and **12** exhibited interesting electronic/photonic behaviors such as electron-transfer blocking, rectification, and vectorial photoelectron transfer. **Figure 22** shows the CV responses of the heterolayer films ITO|(**11**)*n*/(**12**)*<sup>n</sup>* (*n* = 1, 2, where n stands for the number of layers) and ITO|(**12**)*n*/(**11**)*<sup>n</sup>* (*n* = 1,3) and the alternating heterolayer films ITO|(**11**/**12**)4 and ITO|(**12**/**11**)4 in CH3CN (0.1 M HClO4) [22, 62]. The ITO|(**11**)1/(**12**)1 bilayer heterofilm exhibited four waves (**Figure 22a**), which correspond to the redox processes from both the inner **11** and outer **12** layers; the Ru(II)–Ru(III)/Ru(III)–Ru(III) process of **11** and the Ru(III)–Ru(III)/Ru(III)–Ru(IV) process of **12** overlap at +1.11 V. However, the peak separation of the outer layer of **12** increased at higher scan rates, while the peak separation for the waves from the inner layer of **11** remained unaffected. This result indicated a slower ET rate from the outer layer of **12** due to the higher spatial separation relative to the inner **11** film [22].

Conversely, for the ITO||(**11**)2|(**12**)2 hetero-film, in the first scan over the potential range of 0.5 V to +0.7 V vs. Fc+/Fc, only oxidative waves from **11** at +0.83 and + 1.04 V vs. Fc+/Fc were present; no waves from the outer **12** layer were observed (see blue line in **Figure 22a**). However, when the potential was scanned in the negative direction to 0.5 V, a large cathodic wave appeared at approximately 0.5 V vs. Fc<sup>+</sup> /Fc, and subsequently, a new anodic pre-peak at approximately +0.64 V (marked as x) was observed in the potential scan in the positive direction. The new pre-peak x was assigned as a catalytic oxidation wave derived from the ET of the outer **12** layers through ET mediation; that is, direct ET from the outer **12** layer to the electrode was blocked by the bilayer of **11**, and an avalanche ET from

**Figure 22.**

*Cyclic voltammograms of heterolayer films of Ru complexes 11 and 12 with varying sequences: (a) ITO|(11)*n*/ (12)*<sup>n</sup> *(*n *= 1, 2, where n stands for the number of layers), (b) ITO|(12)*n*/(11)*<sup>n</sup> *(*n *= 1, 3), (c) alternating ITO|(11/12)4 and (d) ITO|(12/11)4 in CH3CN (0.1 M HClO4) [22, 62].*

the outer **12** layers to the inner holes in **11** occurred through the potential gradient once a hole was generated in the inner **11** layers at the onset of the first **11** oxidation wave (**Figure 23**). Consequently, the large cathodic peak at 0.5 V was assigned to the catalytic reduction wave of the **12** film mediated by the reduction process of the inner **11** film. Furthermore, the sequence of the **11** and **12** layers on the ITO electrode leads to characteristic CV responses depending on the sequence and the number of layers, as shown in **Figure 22** [62].

Silicon-based *pn* heterojunctions play an important role in various types of electronic devices, such as diodes, transistors, solar cells and light-emitting diodes (LEDs). The potential difference at the *pn* junction causes blockage of charge transport, resulting in a rectification effect. Under photoirradiation, current flows through the external circuit, which acts as a silicon-based solar cell. Similarly, the photo-response of Ru-complex heterolayer films has been examined [22]. Photoirradiation of the ITO|(**11**)4/(**12**)4 hetero-multilayer film in CH3CN (0.1 M HClO4) at the open circuit potential of +0.45 V produced a *cathodic photocurrent.* The action spectrum of this film is nearly identical to the UV–vis absorption

**Figure 23.**

*Rectified ET mechanism for the CV response of the ITO||(11)2|(12)2 hetero-film shown in Figure 22a [22].*

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

**Figure 24.**

*Switching between the "0" and "1" states by applying potentials of 0.5 V and + 0.7 V in the ITO|(11)4/(12)4 hetero-film (top), and the corresponding photocurrent responses (bottom) [22].*

spectrum of **11**, which indicates the inner **11** layers are the main contributor to the photoexcitation, although little photo-response was observed for homo-Ru–complex multilayer films such as ITO|(**11**)n or ITO|(**12**)n. Interestingly, the direction of the photocurrent on the ITO|(**11**)4/(**12**)4 heterolayer film switched from *cathodic* to *anodic* after the application of a potential pulse at 0.5 V. This photo-response change arose from the generation of a charge trapping state after the application of the pulse; the outer layers of **12** were reduced by the potential, and this reduced state was maintained as a charge-trapping state until holes were generated on the inner **11** layers by electrochemical oxidation or photoexcitation. This charge trapping state was recognized from the differential Vis–NIR spectra, in which a new inter-valence charge-transfer (IVCT) band at 1140 nm appeared for the mixedvalence state of **12** (**Figure 24**). Thus, the heterolayer film ITO|(**11**)4/(**12**)4 represents a typical example of a photo-responsive memory device; the writing process is achieved by applying a potential of either 0.5 or + 0.7 V, and the readout process is achieved by measuring the direction of the photocurrent [22]. Accordingly, the judicious selection of both redox potentials and the sequential ordering of the Ru modular units on the ITO surface makes it possible to design functional electronic devices such as molecular diodes and memory devices.
