**Abstract**

Polypyridyl ruthenium(II) complexes have been widely researched as promising functional molecules. We have found unique photoisomerization reactions of polypyridyl ruthenium(II) aquo complexes. Recently we have attempted to provide insight into the mechanism of the photoisomerization of the complexes and distinguish between the *distal*�/*proximal*-isomers in their physicochemical properties and functions. Moreover, polypyridyl ruthenium(II) aquo complexes have been intensively studied as active water oxidation catalysts (WOCs) which are indispensable for artificial photosynthesis. The catalytic aspect and mechanism of water oxidation by the *distal-/proximal*-isomers of polypyridyl ruthenium(II) aquo complexes have been investigated to provide the guided thought to develop more efficient molecular catalysts for water oxidation. The recent progress on the photoisomerization and water oxidation of polypyridyl ruthenium(II) aquo complexes in our group are reviewed to understand the properties and functions of ruthenium complexes.

**Keywords:** Ruthenium aquo complexes, Photoisomerization, Water oxidation catalysis, Artificial photosynthesis

## **1. Introduction**

Polypyridyl ruthenium(II) complexes have been widely researched as promising functional molecules due to appealing photochemical [1–3] and photophysical [4–6] properties as well as redox properties [7, 8], which enable them to exhibit a number of functions such as electrochromism [9, 10], proton-coupled electron transfer [11–13] and photocatalysis [14, 15]. As a result, the polypyridyl ruthenium(II) complexes have been applied to a large variety of devices including sensors [16], photovoltaic cells [17], displays [18] and artificial photosynthesis [19, 20].

We presented irreversible and stoichiometric photoisomerization of *distal*-[Ru (tpy)(pynp)OH2] 2+ (*d*-**1**) (tpy = 2,2<sup>0</sup> ;6<sup>0</sup> ,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8 naphthyridine) to *proximal*-[Ru(tpy)(pynp)OH2] 2+ (*p*-**1**) as shown in **Figure 1** [21–23], which had not been characterized previously for polypyridyl ruthenium

**Figure 1.**

*Photoisomerization of* distal-*[Ru(Rtpy)(pynp)OH2] 2+ to* proximal-*isomers.*

(II) aquo complexes although various photochemical reactions of the ruthenium(II) complexes have been reported [24–33]. We have attempted to provide insight into the mechanism of the photoisomerization of polypyridyl ruthenium(II) aquo complexes and distinguish between the *distal*�/*proximal*-isomers in their physicochemical properties and functions [21–23, 34–36]. We have also developed new synthetic strategy to form dinuclear ruthenium(II) complexes utilizing the photoisomerization [37, 38]. Moreover, polypyridyl ruthenium(II) aquo complexes have been intensively studied as active WOCs [21, 22, 34, 35, 37, 39, 40] which are indispensable for artificial photosynthesis. The catalytic aspect and mechanism of water oxidation by the *distal*-/*proximal*-isomers of polypyridyl ruthenium(II) aquo complexes have been investigated to provide the guided thought to develop more efficient molecular catalysts for water oxidation. In this chapter, we review the recent progress on the photoisomerization and water oxidation of polypyridyl ruthenium(II) aquo complexes in our group.
