Yonsei University Team Identifies 'Dielectric Environment Switch' Governing Charge Transfer in Supramolecular Materials

A research team led by Professor Kim Woo-jae from Yonsei University's Department of Chemistry, in collaboration with Professor Kim Tae-yeon's team at Sungkyunkwan University, has achieved a groundbreaking discovery: identifying a "dielectric environment switch" that fundamentally alters how charges generated by light separate and recombine within supramolecular aggregates. This world-first finding offers crucial insights into the behavior of charge transfer mechanisms.

The research, published in the prestigious journal Nature Communications, is expected to provide new theoretical frameworks for designing charge and exciton transport in advanced optoelectronic devices, including artificial photosynthesis systems and organic solar cells. For decades, understanding the precise role of the dielectric environment in these complex systems, separate from structural complexities and competing processes, remained a significant challenge.

Traditional theories like Marcus theory, while foundational, often struggled to fully explain experimental results in intricate systems like organic molecular aggregates. The new study focused on a "D-PBI-D derivative," a molecule where electron donor (TDOP) and acceptor (perylene bisimide, PBI) units are linked. This molecule forms a unique 1D helical H-aggregate with remarkable structural robustness, maintaining its stacking structure and exciton coupling strength regardless of solvent polarity.

By systematically altering only the dielectric constant of the solvent while keeping the molecular framework and aggregate structure constant, the researchers observed a fundamental crossover in the electron transfer mechanism. In nonpolar environments, the charge transfer proceeded via quantum mechanical tunneling, mediated by high-frequency molecular vibrations within the PBI core, a semiclassical regime. Conversely, in polar environments, the collective fluctuations of solvent dipoles dominated the reaction, exhibiting classical regime behavior. This demonstrated that solvent polarity acts as a switch, controlling the reaction mechanism, particularly in the charge recombination process where a shift from Marcus inversion to the normal region was observed with increasing solvent polarity.

Professor Kim Woo-jae highlighted the significance of the findings, stating, "This study provides a clear criterion for distinguishing when electron transfer in organic aggregates operates quantum mechanically versus classically, by purely isolating the effect of the dielectric environment using a supramolecular platform with a fixed structure." He added, "The research offers a roadmap to enhance the efficiency of optoelectronic devices like artificial photosynthesis and organic solar cells by integrating molecular structure, aggregate form, and the surrounding dielectric environment in design strategies."

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This study provides a clear criterion for distinguishing when electron transfer in organic aggregates operates quantum mechanically versus classically, by purely isolating the effect of the dielectric environment using a supramolecular platform with a fixed structure. The research offers a roadmap to enhance the efficiency of optoelectronic devices like artificial photosynthesis and organic solar cells by integrating molecular structure, aggregate form, and the surrounding dielectric environment in design strategies.