**4. Molecular-device applications using Ru complexes on ITO**

The idea of molecular devices is based on a next-generation paradigm to overcome the limitations associated with Moore's law, which states that the number of transistors per silicon chip doubles every year, and the use of individual molecules as active electronic components. The first single molecular device was the theoretical proposal of a molecular diode by Aviram and Ratner; [68] subsequently, the concept of molecular electronic devices was further developed by Carter [69]. Various molecules have demonstrated basic electronic functionality as switches, as diodes for rectification, and as optical devices, storage devices, and sensing devices for future information technologies [54, 70]. The recent experimental development of single-molecular conductance measurements using metal–molecule–metal junctions [71] has opened a new avenue for the realization of molecular electronic devices through the judicious selection of molecules [70, 72–76]. Ru complexes are substitutionally inert in both the Ru(II) and Ru(III) oxidation states, and also exhibit fast self-exchange ET rates due to the small reorganization energy of the Ru (II/III) couple. Therefore, the use of a mixed-valence Ru(II)–Ru(III) complex as a perturbing motif branching from a conducting molecular wire has been proposed by Carter. Here, two molecular devices based on Ru complexes are discussed; the first is a Ru-complex molecular junction that exhibits rectification switching in response to humidity, and the other is a two-terminal memristor device based on the PCET reactions of Ru complexes.

### **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–V* characteristics 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–V* curves were measured via the two-terminal method [77].

Under dry (low humidity) conditions, the *I–V* plots of both mononuclear **10** and dinuclear complex **11** became 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–V* curves became asymmetric and showed diode-like behavior *only* for 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–V* plots for **11** became symmetric again. This rectification switching via humidity change occurred repeatedly for **11** (**Figure 26**); conversely, such switching was not observed for **10** [77].

#### **Figure 25.**

*Structures of Ru complexes 10 (top) and 11 (bottom), log |*I*|–*V *curves of the 10 and 11 molecular junctions under low humidity (5%) and high humidity (60%) conditions, and 2D histograms of the rectification ratio (R) at high humidity [77].*

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

**Figure 26.** *Conceptual illustration of a humidity-switchable molecular diode [78].*

Several factors need to be considered to explain this behavior. The tip-radius affected the asymmetry of the *I–V* curves, whereby a small tip (50 nm radius) leading to a larger rectification ratio *R* on 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 **11** molecular 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, [79] and the first example of such a device was demonstrated experimentally in 2008. This first memristor device consisted of TiO2-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 [80]. As a result, the device exhibited a periodic pinched hysteresis loop in its *I–V* curves. 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–V* loop [81, 82]. 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 **16** and **17**, which show PCET properties in unbuffered 0.1 M NaClO4 solution, and the resulting twoterminal cells showed a stable charging–discharging process via a proton rockingchair-type mechanism. The unbuffered aqueous solution was replaced with

**Figure 27.**

I–V *plots of the two-terminal device ITO|(16)3/P4VP/(17)3|ITO, showing ten scans on the same device. The blue line indicates the average of the ten scans [83].*

proton-conducting P4VP (p*K*<sup>a</sup> 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–V* characteristics of the two-terminal device, which produced "8" shaped and non-linear *I–V* loop curves [83].

To elucidate the coupling of the proton-transfer ability of P4VP and the PCET reactions of Ru complexes **16** and **17** at 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 **16** and 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 p*K*<sup>a</sup> 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 p*K*<sup>a</sup> difference between the Ru complexes in the Ru(II) and Ru(III) oxidation states.

#### **Figure 28.**

*Cyclic voltammograms of ITO|(16)3/P4VP (blue solid lines) and ITO|(17)3/P4VP films (red solid lines), together with those of ITO|(16)3 (blue dotted lines) and ITO|(17)3 films (red dotted lines), in 0.1 M NaClO4 aqueous solution, as well as a schematic illustration of the hydrogen-bonding interactions between the Ru complex 16 and the P4VP polymer [83].*

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

In the initial stage, the **16** site of the two terminal device ITO|(**16**)3/P4VP/(**17**)3| ITO is in the Ru(II) oxidation state with p*K*<sup>a</sup> values in the range of 4.1–8.8, while the **17** site on the other side is in the Ru(III) state with *p*Ka values in the range of 5.2–9.8. The *pK*<sup>a</sup> 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 **16** and **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 **16** from the Ru (II) state to Ru(III) takes place at the positive-potential side of the terminal, accompanied by the reduction of **17** from the Ru(III) state to Ru(II) at the other terminal. The resulting redox reactions induce large changes in the p*K*<sup>a</sup> of **16** and **17** relative 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 p*K*<sup>a</sup> of **16** drops to <3.8, which means that protons are easily released upon the oxidation of **16**, while on the **17** side, the p*K*<sup>a</sup> increases to >8.4. Given the p*K*<sup>a</sup> 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 **17** upon reduction of the Ru center. The proton conductivity through the protonated P4VP layer is improved by the resulting large p*K*<sup>a</sup> 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 p*K*<sup>a</sup> 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 *p*Ka difference in the Ru complexes **16** and **17** induced 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 [83].

#### **Figure 29.**

*Schematic illustration of the proton-conduction switching in the two-terminal device ITO|(16)3/P4VP/(17)3| ITO and the p*K*<sup>a</sup> gradient under the applied positive bias potential. The numbers refer to the p*K*<sup>a</sup> values of the Ru(II) and Ru(III) complexes [83].*

**Figure 30.**

*Chemical structures of Ru(bpy)3 derivatives that contain phosphonic acid anchors and surface-confined Ru catalyst and chromophore–catalyst assemblies [85, 86].*
