Surface-Confined Ruthenium Complexes Bearing Benzimidazole Derivatives: Toward Functional Devices

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, 25, 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, 30, 31, 32, 33, 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 fluorine-doped SnO₂ (FTO) with immobilized Ru complexes that contain 2,2′-bipyridyl-4,4′-dicarboxylate and other bpy-derived ligands have been developed [35]. Interestingly, 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 two-dimensional 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 C_2v symmetry, no optical isomers do not exist. Hence, surface-confined Ru complexes that contain tridentate ligands with C_2v 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 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, 44, 45, 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 10exhibits a spherical-shaped structure around the central Ru ion, while the other dinuclear Ru complexes 8, 9, 11,and 12exhibit 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 11immobilized 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 8–12can 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.

3.1 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 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.

Here, Γ(t), k, C, and trefer to the surface coverage, rate constant, concentration of the Ru complex, and time, respectively [33, 46]. A typical kinetic plot for a Ru–XPcomplex with an acridine group at the top is shown in Figure 8. This surface-immobilized Ru–XPcomplex 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.

Here, Γi,Γs,CB,andΔGadsrefer 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 9was 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 8–12depends 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 9on 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 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 9using 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]. 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 13can be described by the square scheme in Figure 12, in which complex 13is abbreviated as Ru–LH₂. This reaction involves both electron transfer and proton-transfer 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 pK_avalues of 13were 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 14shows a lower Ru(II/III) oxidation potential than complex 13; the pK_avalues of 14also 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 13and 14, which show PCET with different potentials and pK_avalues in unbuffered aqueous solutions, have been used to construct two half-cells separated by a Nafion membrane [51]. The Pourbaix diagrams of complexes 13and 14are shown in Figure 14; the initial oxidation states of 13and 14were Ru(II) and Ru(III), respectively. Upon charging, the oxidation of 13from the Ru(II) to the Ru(III) state releases the proton(s), while the reduction of 14from Ru(III) to Ru(II) at the other half-cell captures the proton(s). As a result, 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 pH-tunable 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 15and 16are assembled on an ITO electrode, the surface-immobilized 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 15at different pH values; these responses arise from a well-defined 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 8–17immobilize 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.

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 15is 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 × 10⁻¹⁰ mol cm⁻².

Given that the spectral change due to the redox reactions of a monolayer film of 15on the ITO electrode can be observed by UV–vis spectroscopy, even surface-assembled monolayer films of Ru complexes on an ITO electrode can be detected. Upon the oxidation of 15under 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 self-assembly 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 two-dimensional zirconium–phosphonate framework structure via LbL growth [40].

Similarly, the rod-shaped Ru complexes 8–16with 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 11and 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, 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 pK_a 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 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

3.4.2 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 μA·cm⁻². 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·g⁻¹ was found at an applied current density of 10 μA·cm⁻², but the capacitance decreased to 63.3 F·g⁻¹ at the highest current density of 100 μA·cm⁻² (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 16and 17are basically formed by chemical modification of the mononuclear Ru complexes 13and 14via the C–C coupling of two 13or 14 units. Thus, the PCET behaviors of complexes 16and 17were almost the same as those of 13and 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 13and 14and the surface immobilization on an electrode. The Ru complexes 16and 17were 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 16and 17to evaluate the cell performance (Figure 21) [20]. Under galvanostatic conditions at a constant current density of 10 μA·cm⁻², stable charge–discharge behavior occurred, as shown in Figure 21.

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

Ru(II)2LNH2⟶Ru(III)2LN+2e−+2H+.E5

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

Ru(III)2LCH3+2e−+H+⟶Ru(II)2LcH4.E6

where 16and 17are represented as Ru₂L_NH_nand Ru₂L_CH_n, and L_NH_nand L_CH_nindicate 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 pH-indicator 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, 65, 66, 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 11and 12on 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 11and 12, heterolayer films made from combinations of 11and 12exhibited 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)_n(n = 1, 2, where n stands for the number of layers) and ITO|(12)_n/(11)_n(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 hetero-film exhibited four waves (Figure 22a), which correspond to the redox processes from both the inner 11and outer 12layers; the Ru(II)–Ru(III)/Ru(III)–Ru(III) process of 11and the Ru(III)–Ru(III)/Ru(III)–Ru(IV) process of 12overlap at +1.11 V. However, the peak separation of the outer layer of 12increased at higher scan rates, while the peak separation for the waves from the inner layer of 11remained unaffected. This result indicated a slower ET rate from the outer layer of 12due to the higher spatial separation relative to the inner 11film [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 11at +0.83 and + 1.04 V vs. Fc+/Fc were present; no waves from the outer 12layer 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⁺/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 12layers through ET mediation; that is, direct ET from the outer 12layer to the electrode was blocked by the bilayer of 11, and an avalanche ET from the outer 12layers to the inner holes in 11occurred through the potential gradient once a hole was generated in the inner 11layers at the onset of the first 11oxidation wave (Figure 23). Consequently, the large cathodic peak at −0.5 V was assigned to the catalytic reduction wave of the 12film mediated by the reduction process of the inner 11film. Furthermore, the sequence of the 11and 12layers 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 pnheterojunctions 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 pnjunction 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 spectrum of 11, which indicates the inner 11layers 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 cathodicto anodicafter 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 12were reduced by the potential, and this reduced state was maintained as a charge-trapping state until holes were generated on the inner 11layers 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 mixed-valence 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.

4.1 Rectification switching in Ru complex molecular junctions in response to external humidity

Conductive-probe atomic force microscopy (C-AFM) was employed to measure the I–Vcharacteristics of self-assembled monolayers of Ru complexes on an ITO electrode. An ITO-coated Pt probe was used as the C-AFM tip in order to employ the same material for the top and bottom electrode, and the I–Vcurves were measured via the two-terminal method [77].

Under dry (low humidity) conditions, the I–Vplots of both mononuclear 10and dinuclear complex 11became symmetric in the positive and negative potential range, indicating that the molecules at the junction behaved as a molecular wire or resistor (Figure 25). However, under the wet (high humidity) conditions, the I–Vcurves became asymmetric and showed diode-like behavior onlyfor dinuclear Ru complex 11, while they remained symmetric for 10. The rectification ratio R(= |I(-V)|/|I(V)| was found to have a high value of ∼1000, where |I(V)| represents the absolute value of the forward or reverse current density at a certain voltage. When the measurement conditions were changed from wet to dry, the I–Vplots for 11became symmetric again. This rectification switching via humidity change occurred repeatedly for 11(Figure 26); conversely, such switching was not observed for 10[77].

Several factors need to be considered to explain this behavior. The tip-radius affected the asymmetry of the I–Vcurves, whereby a small tip (50 nm radius) leading to a larger rectification ratio Ron account that water molecules were more strongly attracted to the smaller tip. However, this effect could not fully explain the absence of asymmetry for 10. Based on combined theoretical and experimental results, water molecules and counter-ion displacement have an important influence on two localized molecular orbitals in 11, and under an applied voltage bias, a large asymmetry was induced externally via water molecules screening the counter-ions. Thus, via these rectifying properties, the self-assembled Ru complex 11molecular junctions act as nanoscale sensors with ON/OFF switching in response to external humidity [77].

4.2 Protonic memristor devices based on Ru complexes with PCET

Recently, memristors have attracted substantial attention as the fourth passive element after resistors, capacitors, and inductors. The memristor was predicted by Chua in 1971 as a new electronic circuit element linking charge and magnetic flux, [80] and the first example of such a device was demonstrated experimentally in 2008. This first memristor device consisted of TiO_2-x sandwiched between two Pt metal terminals, in which the oxygen defects led to filament formation, and the defects acted as mobile charged dopants and drifted in the applied electric field [81]. As a result, the device exhibited a periodic pinched hysteresis loop in its I–Vcurves. Since then, not only metal oxides, but also many other materials such as organic polymers and metal chalcogenide films, have been sandwiched between the two terminals to fabricate devices with a non-linear hysteretic I–Vloop [82, 83]. In the two-terminal devices, non-linear changes in the current occur during the voltage sweep, and the associated resistance changes. This type of electric element is generally referred to as a memristor. In recent years, the memristor has become relevant in the context of the action of synapses in the neuromorphic systems of the brain. In biological synapses, ion/molecule migration is used for signal transduction. Inspired by the ion migration in the synapse system, we developed a system in which a proton serves as the charge carrier at the interface between Ru complexes with PCET properties and a proton-conducting polymer such as poly(4-vinylpyridine) (P4VP). In Section 4.4.3, a charge-storage system was constructed from two electrodes modified with films of the Ru complexes 16and 17, which show PCET properties in unbuffered 0.1 M NaClO₄ solution, and the resulting two-terminal cells showed a stable charging–discharging process via a proton rocking-chair-type mechanism. The unbuffered aqueous solution was replaced with proton-conducting P4VP (pK_a ∼ 4.0–5.2), and the resulting two-terminal heterolayer device, ITO|(16)₃/P4VP/(17)₃|ITO, was tested. Figure 27 shows the typical I–Vcharacteristics of the two-terminal device, which produced “8” shaped and non-linear I–Vloop curves [79].

To elucidate the coupling of the proton-transfer ability of P4VP and the PCET reactions of Ru complexes 16and 17at the hetero-interface, CV measurements were performed in 0.1 M NaClO₄ aqueous solution (Figure 28). A large negative potential shift was observed for the Ru(II/III) peak of the ITO|(16)₃/P4VP film compared to that of ITO|(16)₃ without P4VP. This negative potential shift arises from the hydrogen-bonding interactions between the N–H benzimidazolyl groups in 16and the pyridine groups in P4VP, based on a previous study of the hydrogen-bonding interactions between the N–H imino groups in Ru–benzimidazole-derivative complexes and N-heteroaromatics such as pyridine [84]. In this study, the magnitude of the shift was strongly correlated to the pK_a values of both components (i.e., the complex and the heteroaromatic component) [84]. On the other hand, only a small potential shift was observed between the peak of the ITO|(17)₃/P4VP film and that of ITO|(17)₃ without P4VP. The difference between these two systems was attributed to the pK_a difference between the Ru complexes in the Ru(II) and Ru(III) oxidation states.

In the initial stage, the 16site of the two terminal device ITO|(16)₃/P4VP/(17)₃|ITO is in the Ru(II) oxidation state with pK_a values in the range of 4.1–8.8, while the 17site on the other side is in the Ru(III) state with pK_a values in the range of 5.2–9.8. The pK_a value of the intervening P4VP polymer is 4.0–5.2. Thus, the proton gradient across the interfaces is small, and it is equilibrated through hydrogen bonding between the P4VP and both 16and 17(Figure 29). When a bias potential of +1.5 V is applied to the two-terminal ITO|(16)₃/P4VP/(17)₃|ITO device, redox processes occur on the immobilized Ru complexes, namely, the oxidation of 16from the Ru(II) state to Ru(III) takes place at the positive-potential side of the terminal, accompanied by the reduction of 17from the Ru(III) state to Ru(II) at the other terminal. The resulting redox reactions induce large changes in the pK_a of 16and 17relative to those of their initial Ru(II) or Ru(III) states, resulting in a large proton gradient across the immobilized Ru complex/P4VP interfaces (Figure 11). Specifically, the pK_a of 16drops to <3.8, which means that protons are easily released upon the oxidation of 16, while on the 17side, the pK_a increases to >8.4. Given the pK_a value of the intervening P4VP is 4.0–5.2, the (16)/P4VP proton transfer equilibrium is shifted toward the protonation of the P4VP side, while at the (17)/P4VP side, the equilibrium shifts toward the protonation of the film of 17upon reduction of the Ru center. The proton conductivity through the protonated P4VP layer is improved by the resulting large pK_a gradient, resulting in enhanced conduction until the opposite redox reactions take place at both terminals. When the bias potential was scanned in the negative direction toward −1.5 V, the pK_a gradient returned to the initial state, and the current decreased in the absence of a driving force for electron or proton transport between 16/P4VP and P4VP/17.

Therefore, the large pK_a difference in the Ru complexes 16and 17induced by the PCET redox reactions at the two terminals cause a proton gradient across the intervening proton-conducting P4VP, leading to high proton conductivity under an applied voltage. The change in the proton gradient due to the PCET redox reaction in the two-terminal device can be applied to use the proton-conducting switching devices as protonic memristors [79].