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Light-harvesting at 100%

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(Diagram via Tokyo Metropolitan University and Shinsuke Takagi)
 
 
At the Tokyo Metropolitan University and the Japan Science and Technology Agency, researcher Shinsuke Takagi led a team in realizing an artificial light-harvesting system (LHS) that achieves up to 100% efficiency. The LHS, or "artificial leaf," works off the same principle of real leaves, by arranging molecules to collect the light and carry the energy to the system's "reaction center."
 
Two types of porphyrin dye molecules were used on a clay surface. It was a challenge to arrange the molecules to work properly. Takagi explained, "A molecular arrangement with an appropriate intermolecular distance is important to achieve nearly 100% energy transfer efficiency. If the intermolecular distance is too near, other reactions such as electron transfer and/or photochemical reactions would occur. If the intermolecular distance is too far, deactivation of excited dye surpasses the energy transfer reaction.” 
 
In what is called the "Size-Matching Rule," the team was able to arrange the molecules by matching distances between charged porphyrin molecules and between negatively charged anionic sites on the clay. This stops much of the "aggregation" and "segregation" of the porphyrin molecules, which create chaotic, non-uniform, patterns on the surface. To make get to the efficiencies needed, a precise uniform pattern is needed, like in real leaf structures.
 
Takagi spoke about his plans for future use of the technology, "At the present, our system includes only two dyes. As the next step, the combination of several dyes to adsorb all sunlight is necessary. One of the characteristic points of our system is that it is easy to use several dyes at once. Thus, our system is a promising candidate for a real light-harvesting system that can use all sunlight. We believe that even photochemical reaction parts can be combined on the same clay surface. If this system is realized and is combined with a photochemical reaction center, this system can be called an ‘inorganic leaf.’”
 
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