image: Donor-acceptor (D-A) molecule featuring a unique twisting D-A-A-D backbone achieves exceptional light-harvesting capacity and efficient singlet oxygen generation in both solution and crystalline state.
Credit: ©Science China Press
Organic photosensitizers play pivotal roles in diverse applications including photodynamic therapy, wastewater treatment, and aerobic organic reactions. However, creating efficient, non-toxic, and stable photosensitizers has been a long-standing challenge in materials science.Conventional organic photosensitizers often rely on the incorporation of heavy atoms, which leads to issues such as dark toxicity, short triplet-state lifetimes, and poor photostability. A more persistent problem arises when these molecules are concentrated into crystalline forms for practical use: the assembly of monomers often leads to significantly compromised singlet oxygen (¹O₂) generation. This is primarily due to intermolecular π–π stacking, which promotes non-radiative decay pathways that compete directly with intersystem crossing (ISC), severely limiting efficiency in the solid state.
A recent study published in “National Science Review” offers a novel strategy to overcome these limitations. A research team led by Yanke Che and Jin-Song Hu from the Institute of Chemistry, Chinese Academy of Sciences, in collaboration with Ling Zang from the University of Utah, revealed an innovative molecular design.
The team designed a new organic molecule featuring a "twisted" backbone—specifically, a donor-acceptor (D-A) molecule with a uniquely twisted dual-acceptor backbone (D-A-A-D). This specific topology drives the molecules to self-assemble into crystalline nanofibers via CH/π and electrostatic interactions while effectively suppressing π-π stacking. "This connection topology facilitates the formation of crystalline nanofibers... while effectively suppressing pi-pi stacking," the authors explain.
The performance of these crystalline nanofibers is remarkable. They exhibit a high molar absorptivity (the ability to absorb light) of 53,400 M-1cm-1 and a singlet oxygen quantum yield of 72%. These figures are significantly higher than typical solid-state organic materials and even surpass the molecule's performance when dissolved in solution.
To test the practical utility of these nanofibers, the researchers applied them to the photo-oxidation of organic substrates, such as benzylamines and sulfides. The nanofibers successfully catalyzed the reactions, achieving complete conversion within minutes. Crucially for industrial and environmental applications, the material is highly durable. The nanofibers retained their catalytic efficiency over at least five consecutive cycles. Because the nanofibers are solid and stable, they naturally settle to the bottom of the reaction vessel after use, making them easy to recover and recycle compared to liquid-based catalysts.
This work establishes a new paradigm for designing "heavy-atom-free" crystalline photosensitizers. By engineering the molecular shape to control how crystals grow, scientists can now combine the benefits of high light-harvesting efficiency with the robustness required for real-world applications.