Global warming and shortage of fossil resources are serious concerns for humanity. Artificial photosynthesis that converts sunlight energy to chemical energy is a promising research field for providing solutions to these problems. Prof. Osamu Ishitani has developed new methods to control visible-light absorption properties, photoreactivities, and catalyses of various metal complexes for creating artificial photosynthetic systems. His adopted methods and achieved results are as follows.

1. Improvement of metal-complex photocatalysis for CO₂ reduction
Photocatalytic CO₂ reduction producing high-energy carbon-containing materials has been receiving increasing attention because it might give a total solution to both global warming and the shortage of fossil resources. Prof. Ishitani has developed various visible-light driven photocatalysts for CO₂ reduction based on investigating details of the reaction mechanisms, and drastically improved photocatalysis by constructing new molecular architectures as photocatalysts. Since 1996, the most efficient photocatalysts for CO₂ reduction have been developed by Prof. Ishitani's group. He also constructed the molecular architecture of supramolecular photocatalysts for CO₂ reduction, of which photosensitizer and catalyst units are connected with a bridging ligand.

2. Development of Z-scheme photocatalysts: Emergent fusing of metal-complex photocatalysts and semiconductor materials
The supramolecular photocatalysts for CO₂ reduction, which Prof. Ishitani developed, especially work well on a solid surface. He applied this property of the supramolecular photocatalysts for constructing various hybrid photocatalytic systems with semiconductor materials. Hybrid systems of Ru(II)-Re(I) and Ru(II)-Ru(II) supramolecular photocatalysts with various semiconductor particles work as Z-scheme photocatalysts for CO₂ reduction. In these systems, both strong reduction and oxidation powers are achieved via the step-by-step absorption of two photons by the supramolecular photocatalyst and the semiconductor, i.e., the Z-scheme mechanism. The highest turnover number (~400,000) of CO₂ reduction was achieved by using such a system. Prof. Ishitani also successfully developed durable hybrid photoelectrochemical systems with dye-sensitized molecular photocathodes and semiconductor photoanodes that can reduce CO₂ using water as a reductant and visible light as energy.

3. Discovery of metal-complex catalysts with high CO₂-capture ability and their application to direct photocatalytic and electrocatalytic reduction of low-concentration CO₂
Prof. Ishitani successfully induced high CO₂ capturing ability into Re(I)- and Mn(I)-complex catalysts for CO₂ reduction by using a deprotonated triethanolamine ligand. These catalysts were applied to photocatalytic and electrocatalytic systems that could directly reduce low-concentration CO₂.

4. Development of selective hydride-transfer photocatalysts
In natural photosynthesis, electrons captured from water molecules are accumulated in the co-enzyme NADP as hydride, i.e., two electrons in each NADPH molecule. Prof. Ishitani found that photoexcitation of Ru(II) triethylamine complexes, which can be produced via photochemical ligand substitution reactions, selectively produces hydride Ru(II) complexes, and applied this new photochemical reaction to selective hydride reduction of NADP model compounds to their 1,4-dihydro forms.

5. Discovery of photochemical ligand substitution reactions of Re(I) diimine tricarbonyl complexes and application to synthesis of emissive Re(I)-complex polymers
The Re(I) diimine tricarbonyl complexes have been of interest because of their emissive properties and photocatalytic activity toward CO₂ reduction. However, substitution reactions of the CO ligands had not been reported. Prof. Ishitani found that Re(I) diimine tricarbonyl complexes with a phosphine ligand are photoactive and excitation of these complexes selectively gives the corresponding Re(I) biscarbonyl complexes. He applied this newly-found photoreaction to synthesizing various multinuclear Re(I) complexes. A linear Re(I) multinuclear complex with a emissive Ru(II) complex at the central moiety showed light harvesting ability, and this heteromultinuclear complex was combined with mesoporous organosilica (POM) to produce a two-step light harvesting system of which several hundreds of organic moieties in the POM absorb light and transfer energy to the linear Re(I) multinuclear complex which finally transfers accumulated energy to the Ru(II)-complex unit, i.e., this is a two-step light harvesting system. Ring-shaped Re(I) multinuclear complexes work as superior redox photosensitisers and were used in the most efficient photocatalytic system for CO₂ reduction.

6. Development of a new control method for photophysical properties of metal complexes
Prof. Ishitani found that photophysical properties and photoreactivities of metal complexes can be controlled by introducing π-π and/or CH-π interaction between ligands. For example, introduction of two triarylphosphine ligands to the cis-positions of the diimine ligand of Re(I) diimine biscarbonyl complexes induces shorter-wavelength shift of emission while longer-wavelength shift of visible absorption, and, in addition, longer emission lifetime and higher emission quantum yield are achieved. The oxidation power of the excited complex is also strengthened. All of these property changes of the Re(I) complexes drastically improved functions as redox photosensitizers.

In summary, Prof Ishitani found various new photochemical reactions and clarified their reaction mechanisms. Based on these revelations, he developed new molecules and materials with novel functions and photocatalysis. These many ingenious results opened up new fields related to photochemistry of metal complexes, photocatalysts, and photofunctions. Therefore, his achievements are recognized as worthy of the Chemical Society of Japan Award.