Natural photosynthetic systems (PSs) have garnered attention due to their ability to efficiently convert light energy into chemical energy. Despite attempts to mimic PSs in artificial photocatalytic systems, this field still faces challenges, such as slow photogenerated charge migration and the recombination of electron-hole pairs.

Most efforts to regulate the behavior of charge carriers in catalysts have focused on molecular design in previous studies, while the exploration of the regulation of charge carrier behavior through spatial arrangement remain relatively scarce. This is particularly true as the material size increases from the nanoscale to the microscale where the understanding of changes in electronic structure during this transition is still limited.

In a study published in Angewandte Chemie International Edition, Prof. LIU Tianfu from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences achieved the regulation of photogenerated charge carrier behavior and the enhancement of catalytic performance by modulating the degree of π-π stacking within hydrogen-bonded organic frameworks (HOFs), without altering the building blocks or network topology.

Researchers synthesized three PFC-1 materials with varying lengths along the π-π stacking direction, designated as PFC-1-S (approximately 800 nm), PFC-1-M (approximately 20 µm), and PFC-1-L (approximately 120 µm). 

Experimental results revealed that due to insufficient orbital overlap between adjacent molecules, hydrogen bonds within the HOFs do not facilitate significant electron transport. Consequently, electron transport predominantly occurs along the π-π stacking direction rather than the hydrogen bond direction. Under these conditions, controlling the extent of π-π stacking can significantly modulate the electronic structure and charge carrier behavior.

The HOF catalyst with long-range π-π stacking exhibited a higher electron density, resulting in not only excellent charge separation and transport efficiency but also an enhanced electron density in the Pd nanoparticles loaded on its surface. As a result, it demonstrated outstanding CO2 photoreduction activity without the need for a hole scavenger, achieving a CO production rate of up to 48.1 μmol/g/h.

This study offers insights into the regulation of charge carrier behavior within HOFs and presents a novel approach for designing highly efficient photocatalysts.

Modulating π-π stacking perpendicular to the π-conjugated plane in hydrogen-bonded organic frameworks significantly influences electron density, redox potential, and charge carrier transport and separation, leading to enhanced catalytic performance. (Image by Prof. LIU's group)