The molecular triplet state has a long excited state lifetime and has important applications in the fields of photocatalysis, photodynamic therapy, photon upconversion and room temperature phosphorescence. Traditional triplet photosensitizers usually rely on heavy atoms to enhance spin-orbit coupling. However, pure organic molecules still face challenges in achieving efficient triplet generation due to weak spin-orbit coupling. In recent years, spin-orbit charge transfer intersystem crossing (SOCT-ISC) has provided a new way to solve this problem. In non-coplanar organic donor-acceptor molecules, after light-induced charge separation, a triplet state can be formed through a charge recombination process accompanied by changes in orbital angular momentum, thereby achieving efficient triplet generation without heavy atoms. The traditional view is that a nearly orthogonal configuration is conducive to the generation of triplet states, but at the same time it weakens the electronic coupling between the donor and the acceptor, which is not conducive to the formation of strong charge transfer (CT) absorption. On the contrary, although a smaller dihedral angle can enhance CT absorption, it may accelerate charge recombination back to the ground state. Therefore, how to balance long-wavelength CT absorption and efficient triplet generation in the same molecule is an important issue facing this field.
Recently, a research team from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has made progress in the study of triplet photochemistry of pure organic molecules. By regulating the configuration of the donor-acceptor molecules, the team revealed the competition mechanism between different charge recombination channels, and proposed a new idea of using the intermediate dihedral configuration to take into account strong charge transfer absorption and efficient triplet generation, providing a new physical image and control strategy for the design of pure organic triplet photosensitive molecules.
研究选取由四个咔唑给体和一个苝酰亚胺受体组成的PBI-4Cz分子为模型体系。该分子基态具有约66°的中间二面角，激发后的CT态接近正交构型。依托基态较强的给体—受体电子耦合，PBI-4Cz具有明显的CT吸收特性，吸收范围可延伸至约700nm，为利用长波长光直接激发CT态提供了条件。
团队进一步利用变温飞秒瞬态吸收光谱研究了分子构型对电荷复合和三线态生成的影响。结果显示，随着温度降低，PBI-4Cz的电荷分离态寿命明显延长，三线态生成效率提高。动力学分析表明，降温主要抑制了回到基态的单线态电荷复合，而对三线态生成通道的影响较小，从而提高了三线态生成的竞争优势。将分子固定在刚性聚合物基质后，团队也观察到类似的高三线态产率。这一结果进一步说明限制分子构型运动是调控SOCT-ISC效率的重要因素。
Research results show that in room temperature solutions, thermal motion will cause the donor-acceptor configuration to continuously fluctuate, and some of the transient configurations have stronger electronic coupling, which can accelerate charge recombination back to the ground state, causing triplet generation losses. When the temperature is lowered or the molecule is fixed in a rigid environment, such configurational fluctuations are suppressed, singlet recombination is slowed, and triplet generation is preserved. Therefore, the design of pure organic SOCT-ISC molecules is not simply to pursue a completely orthogonal donor-acceptor configuration, but to achieve a dynamic balance of electronic coupling, spin-orbit coupling and configuration stability. A moderate donor-acceptor dihedral angle is expected to simultaneously achieve strong CT absorption and efficient triplet generation.
Based on the characteristics of PBI-4Cz with both long-wavelength CT absorption and efficient triplet generation, the team used TIPS-anthracene as the triplet annihilation agent to achieve triplet-triplet annihilation photon upconversion driven by 637nm red light, obtaining obvious blue-green upconversion luminescence, and the normalized upconversion quantum yield reached 7.1±0.3%, demonstrating the application potential of this molecular design strategy in long-wavelength photochemistry and light energy conversion.
This study established a direct connection between "molecular configuration constraints - charge recombination competition - triplet generation", showing that by rationally regulating the donor-acceptor configuration and its dynamic fluctuations, both long-wavelength absorption and efficient triplet generation can be taken into account, providing new ideas for the design of pure organic triplet photosensitizers, photon upconversion and photocatalytic systems.
Relevant research results were published in the Journal of the American Chemical Society (Journal of the American Chemical Society)superior. The research work has been supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences, etc.
研究揭示有机给受体分子中电荷转移吸收与三线态生成的协同机制
Source: https://www.cas.cn/syky/202608/t20260827_5119138.shtml