Plants mastered the art of harvesting sunlight billions of years ago, using elegant rings of pigments in their leaves. Now, researchers from Osaka Metropolitan University have taken a major step toward mimicking that leafy architecture with human-made molecules that self-assemble into stacked rings where charge and energy can circulate freely — just like in photosynthesis. Their design paves the way for new approaches to light capture, energy transport and next-generation electronics.

In photosynthetic organisms, pigment molecules form ring-shaped antennae to efficiently absorb light. A key feature of this system is toroidal conjugation, a phenomenon in which energy and charge flow continuously around the ring. Mimicking this clever trick, for example, in solar cells ­— devices that convert sunlight directly into electricity ­— promises to boost performance significantly, but the path to success has been a long journey.

“Artificial versions of toroidal conjugation have been limited to single molecules,” said Daisuke Sakamaki, an associate professor at Osaka Metropolitan University’s Graduate School of Science and lead author of this study.

Taking a “the more, the merrier” approach, the team aimed to design structures in which multiple molecules are assembled in a ring, as seen in nature.

Using phthalocyanines — aromatic compounds common in dyes and solar cells — the researchers designed flat, dye-like molecules with eight upright, pillar-shaped extensions that readily transfer electrons. These molecules naturally self-assemble in pairs, with their pillars interlocking like gears to form a tightly packed ring of 16 stacked layers. This circular structure brings the flat surfaces close enough for electrons and energy to move freely around the loop, echoing the design of nature’s light-harvesting complexes.

X-ray crystallography confirmed the ring formation, while spectroscopic and theoretical studies revealed that energy and charge could circulate through the structure in both charged and excited states.

“This is the first clear evidence of intermolecular toroidal conjugation,” Sakamaki said. “Not only does this confirm that charge and energy can circulate in such assemblies, but it also reinvents how we think about using phthalocyanines — materials with more than a century of history.”

The team’s technique shows that complex natural energy systems can be engineered using relatively simple molecular building blocks and self-assembly principles. This may lead to a better understanding of mechanisms similar to photosynthesis and to the development of advanced energy generation.

“Our plan is to extend this approach to different types of molecules, aiming to design a wider variety of conjugated systems for future energy and optoelectronic applications,” Sakamaki said.

The study was published in Angewandte Chemie International Edition.

Funding

• Grant-in-Aid for Scientific Research (JP17H04874, JP18K05264, JP20H02726, JP21H04564, JP22H05377, JP24H01092, JP22K05069) from the Japan Society for the Promotion of Science (JSPS)
• Grant-in-Aid for Transformative Research Areas (A) “Condensed Conjugation” (JP20H05866, JP21H05494) from the Japan Society for the Promotion of Science (JSPS)
• Toshiaki Ogasawara Memorial Foundation
• Shorai Foundation for Science and Technology
• The Foundation for the Promotion of Ion Engineering
• Izumi Science and Technology Foundation

Conflict of Interests
The authors declare no conflict of interest.

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