**5. Miscellaneous devices based on Ru–benzimidazole complexes for solar-energy transduction**

Further important applications for ruthenium complexes include photoredox catalysts and dye-sensitized solar cells. In particular, Ru(bpy)3 or Ru(tpy)2 derivatives have been used as photosensitizer dyes on mesoporous TiO2 surfaces in Grätzel-type solar cells. Over the past two decades, many Ru complexes with phosphonate anchors have been reported, [33] and many Ru dyes derived from Ru (bpy)3 derivatives that contain phosphonate anchors (**Figure 30**). Additionally, Ru-2,6-bis(benzimidazole-2-yl)pyridine complexes have been employed as photoredox catalysts [85, 86]. The Ru–benzimidazole bond is known to be more stable than the Ru–pyridine bond in the photoexcited state, rendering such Ru-benzimidazole complexes promising candidates for photoelectrochemical redox catalysts.

#### **6. Conclusion**

Substitutionally inert ruthenium complexes bearing benzimidazole derivatives have unique electrochemical and photochemical properties. In particular, protoncoupled electron-transfer in ruthenium–benzimidazole complexes endows them with rich redox chemistry and makes them useful as a modular unit for redox mediators or reactive sites for switching by external stimuli. In this chapter, the role of PCET reactions on Ru–benzimidazole complexes in energy-storage applications and the tuning of metal–metal interactions in aqueous solution was emphasized first. Based on this knowledge acquired from solution chemistry, the chemistry of Ru complexes confined on an electrode surface via their self-assembling from solution for the fabrication of the surface functional molecular devices on electrodes was discussed. Indium-tin oxide (ITO) is often chosen as the electrode due to its transparency and wide use as a substrate in electronics. To immobilize the redoxactive Ru complexes on an ITO electrode, tetrapod phosphonic acid anchor groups

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

are often incorporated into tridentate 2,6-bis(benzimidazole-2-yl)pyridine or benzene ligands, which enables the construction of free-standing self-assembled monolayer structures on an ITO electrode. Starting from this monolayer as a primer layer, multilayer films can be constructed by the LbL layer growth method. The resulting multilayers using redox-active Ru complexes as a modular unit exhibited long-range electron transport even in films with over 60 layers (100 nm thick) through the "stepping-stone" mechanism. Furthermore, a combinatorial approach to LbL layer growth can be used to obtain bespoke functional heterolayer films via material design. Using this strategy, blocking of electron transfer or rectification can be made to occur in such Ru complex heterolayer films, which results in charge trapping; the trapped electrons can subsequently be released via photo-irradiation, which leads to the new concept of photo-responsive memory devices. The CV response of multilayer films of Ru complexes with PCET depends strongly on the pH value. By judicious selection of the redox potentials and p*K*<sup>a</sup> values of the two Ru complexes with PCET properties, two-electrode cells based on the Ru complex multilayer films that acted as proton rocking-chair-type redox capacitors can be obtained. Furthermore, by sandwiching a proton-conducting polymer between two Ru multilayer terminals, new type of protonic memristor device can be fabricated.

Therefore, surface-confined Ru complexes exhibit highly promising potential for the development of new functional molecular-based devices.
