Changing Molecular Lengths Alters Light Pathways... Clues Found for Next-Generation Solar Cells [Reading Science]

Published in Nature Chemistry: Quantum Yield Increased from 47% to 75%
A New Paradigm for Designing Optoelectronic Materials

A research team has found that simply connecting and arranging molecules in an extended fashion allows for precise design of the migration path of light energy and the efficiency of luminescence. This achievement opens the door to controlling exciton behavior—a key factor in next-generation solar cells, high-efficiency light-emitting devices, and artificial photosynthesis technologies—at the molecular level.

Yonsei University announced on March 30 that a research team led by Dongho Kim, Honorary Adjunct Professor of Chemistry, in collaboration with Professor Frank Wurthner's group at the University of Wuerzburg in Germany, has revealed that the transfer mode of light energy varies depending on the length and arrangement of organic molecules. The results were published in the latest issue of the international journal Nature Chemistry.

Schematic of exciton behavior showing changes in light energy transfer as molecular length increases. Provided by the research team.

The research team newly developed a 'foldamer' structure, in which dye molecules are precisely connected in sequence, and systematically increased the molecular length from two up to fourteen units to analyze how their interaction with light changes as the length increases.

As a result, they found that the fluorescence properties changed dramatically once about four to six molecules were linked. The quantum yield, indicating the efficiency of light emission, was about 47% in conventional dimers, but increased to as much as 75% in the structure with fourteen linked units.

The team analyzed that this phenomenon was not simply due to the increased number of molecules, but rather the formation of a multiexciton state in which multiple excited states interact simultaneously. Once the molecule exceeds a certain length, both the generation and migration paths of excitons fundamentally change, meaning that the flow of light energy is essentially redesigned.

Dongho Kim, Honorary Adjunct Professor of the Department of Chemistry at Yonsei University (corresponding author, left), and Yongseok Hong, Postdoctoral Researcher at Columbia University (first author). Provided by Yonsei University

In particular, using resonance Raman spectroscopy and ultrafast time-resolved spectroscopy, the researchers directly observed the causal relationship between secondary structural arrangement and exciton dynamics as the molecular length increased. They also confirmed that the widely used simple dimer model cannot adequately explain the complex energy transfer processes that occur in actual solid-state optoelectronic materials.

This achievement is considered the first experimental proof of a new concept: that the flow of light energy can be pre-designed by controlling only the length and arrangement of molecules. It is expected to be widely applicable across the future landscape of optoelectronic technologies, including solar cells, artificial photosynthesis, high-efficiency light-emitting devices, and molecule-based electronic circuits.

Professor Kim stated, "This research demonstrates that the flow of light energy can be designed in advance depending on how molecules are connected and arranged," adding, "It will serve as an important clue in developing artificial photonic devices capable of sophisticated energy transfer, much like the photosynthetic processes found in nature."

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