Structural Control of Organic Molecules for Superior Functionality: A Photoreaction that Creates Two from One

When asked about substances that use organic molecules, many people probably think of pharmaceuticals, fibers, or plastics. With advancements in organic synthesis technology, organic molecules have recently been used as new functional materials. For example, they are used in electronic materials such as liquid crystals and organic light-emitting diodes (OLEDs) for displays in televisions and other devices. Furthermore, their application is being explored in light energy conversion materials like solar cells and photocatalysts. Because they can derive energy from sunlight, light energy conversion materials represent a low-environmental-impact energy technology. Materials constructed from organic molecules have advantages such as being lightweight, flexible, and inexpensive to manufacture. Therefore, organic light energy conversion materials can be considered one of the materials that will lead to clean and sustainable energy systems.

We are engaged in the design, synthesis, and evaluation of the physical properties of new organic molecules that can be used for these organic light energy conversion materials. Here, I will introduce a unique photoreaction that leads to improved light energy conversion efficiency, which is expressed in molecules we have recently designed and synthesized, as well as its potential applications.

Research on organic light energy conversion materials has been conducted since around the 1960s, but increasing their efficiency remains a challenge. Normally, an organic molecule can only generate one exciton from one photon, but if it were possible to generate two excitons from one photon, the potential for increased efficiency would be extremely high. This dream-like photophysical property, called "singlet fission," was predicted in 1965 to occur between two molecules, and its occurrence was indirectly proven in 1969 (Figure 1).

However, at the time, there were no means to directly observe singlet fission, which proceeds on an ultrafast picosecond timescale, so many aspects, such as the mechanism for expressing this property and the optimal molecular structure for its efficient expression, were unknown. With the recent development of ultrafast spectroscopic measurement techniques, it has become possible to directly observe singlet fission. To achieve the efficient expression of singlet fission, establish the optimal molecular structure, and elucidate the mechanism, we synthesized various molecules with different structures and performed ultrafast spectroscopic measurements. Through our investigations, we synthesized a molecule with an exciton generation quantum yield of approximately 200%—capable of generating two excitons from one photon almost quantitatively—and observed the change in electron spin during the reaction (Figure 2).

Next, we applied the molecule capable of exhibiting highly efficient singlet fission to a sequential reaction system. We were able to construct a system capable of highly efficient singlet fission after energy transfer from another molecule (Figure 3A), a system for a high-yield electron transfer reaction after highly efficient singlet fission (Figure 3B), and a system that can generate high-yield singlet oxygen (Figure 3C).

As shown, the physical properties of organic molecules change depending on the linkage type and the functional groups introduced into their framework. Sometimes, they can change dramatically just by adding or removing a single methylene (-CH ₂ -) group. Such a small change can lead to the expression of unimaginable functionality, and the molecules we develop may one day be used in products right before our eyes, like OLEDs. In this way, using organic molecules with their infinite possibilities, we conduct our daily research with the goal of developing new functional molecules that support and enrich society from perspectives such as "enriching people's lives" and "being environmentally friendly."