分子间单重态激子裂变的动力学过程与电荷转移模型的实验研究

Experimental study on the dynamic process and chargetransfer model of intermolecular singlet exciton fission

电荷转移模型是解释分子间单重态激子裂变过程的一个重要理论机制,然而对此模型的合理性仍然存在争论,对模型中涉及的电子转移过程也缺乏研究.本实验以具有激子裂变特性的红荧烯分子为研究对象,通过双源共蒸发的方法制备了4个系列红荧烯掺杂的混合薄膜,并在室温下分别测量了混合薄膜的光致发光谱及其瞬态衰减曲线.理论上,基于"S_1+S_0?1^(TT)_i?T_1+T_1"三状态反应模型,采用耦合的速率方程组对所有的发光衰减曲线进行了拟合,得到了激子裂变过程中所涉及的重要速率常数.根据每种材料的摩尔质量与摩尔体积,计算了混合薄膜中红荧烯分子的平均间距.实验发现:红荧烯分子间的电子转移过程具有量子隧穿的特点,激子裂变过程的速率随分子间距的增大呈现出明显的指数下降规律.上述结果符合电荷转移模型的特征,可作为支持电荷转移模型的实验证据,这对于澄清激子裂变的微观图像具有重要价值.

The charge-transfer model is an important theoretical mechanism to explain the intermolecular singlet exciton fission process. However, the rationality of this model is still on debate, and there is also a lack of research on the electrontransfer process involved in this model. In this work, highly efficient fission molecule, i.e. rubrene, was selected to be the research object. By using co-deposition technique, we fabricated four series of rubrene-doped films in which rubrene molecules were uniformly mixed with other inert organic molecules, forming amorphous composite solid. For each sample, steady-state photoluminescence spectra and their time-resolved fluorescence decay curves were measured at room temperature. Theoretically, based on a traditional three-state reaction model of ＂S1＋S0？1（TT）i？T1＋T1＂, all measured fluorescence decay curves could be well fitted by using a set of coupled rate equations. Then the important rate constants involved in singlet exciton fission process were obtained by curve-fitting. By using the molar volume and molar mass of all materials, the averaged intermolecular distances between doped Rubrene molecules were calculated. It was found that the electron-transfer process between adjacent Rubrene molecules showed the character of quantum tunneling. And the obtained rate constants for state conversion of ＂S1＋S0→1（TT）I＂ exhibited a clear exponential attenuation with increasing intermolecular distance. These results were in line with the charge-transfer model and could be regarded as experimental confirmation. We believe that all the results present in this work are of significance for clarifying the microscopic picture and physical mechanism of exciton fission.