**4. Conclusions**

Recent progress on the aspects and mechanistic insights of photoisomerization of Ru(II) aquo complexes in our group was reviewed to unveil the photoisomerization reactions and its mechanism comprehensively. The controls of properties and functions of mononuclear Ru(II) aquo complexes by the photoisomerization were exemplified in terms of their water oxidation catalyses. An example of application of the photoisomerization is demonstrated; the employment of the photoisomerization enabled to synthesize the dinuclear Ru(II) complexes that have been difficult to be synthesized by conventional thermochemical processes. The synthesized dinuclear complexes also serve as efficient catalysts for water oxidation. New design and development of variety types of Ru complexes are desired to explore unique reactions, functions and application based on the photoisomerization for future molecular systems such as artificial photosynthetic devices.

*Ruthenium - An Element Loved by Researchers*
