Department of Chemistry, University of California, Riverside, 92521, United States.
J Am Chem Soc. 2012 May 23;134(20):8597-607. doi: 10.1021/ja301683w. Epub 2012 May 8.
A detailed analysis of the oscillations seen in the delayed fluorescence of crystalline tetracene is presented in order to study the mechanism of singlet fission. Three quantum beat frequencies of 1.06 ± 0.05, 1.82 ± 0.05, and 2.92 ± 0.06 GHz are resolved, which are damped on a time scale of 20 ns. The effects of sample morphology, excitation wavelength, and temperature are examined. A density matrix model for singlet fission is developed that quantitatively describes the frequencies, amplitudes, and damping of the oscillations. The model assumes a direct coupling of the initially excited singlet exciton to the triplet pair manifold. There is no electronic coherence between the singlet and triplet pair states, but the rapid singlet decay time of ∼200 ps in solution-grown single crystals provides the impulsive population transfer necessary to create a coherent superposition of three zero-field triplet pair states |xx>, |yy>, and |zz> with overall singlet character. This superposition of the three states gives rise to the three quantum beat frequencies seen in the experiment. Damping of the quantum beats results from both population exchange between triplet and singlet manifolds and pure dephasing between the triplet pair states. By lowering the temperature and slowing the SF rate, the visibility of the oscillations decreases. There is no evidence of magnetic dipole-dipole coupling between the product triplets. Our model provides good overall agreement with the data, supporting the conclusion that singlet fission in tetracene proceeds through the "direct" mechanism without strong electronic coupling between the singlet and triplet pair states.
为了研究单重态裂变的机制，对晶体并五苯延迟荧光中观察到的振荡进行了详细分析。解析出三个量子拍频，分别为 1.06±0.05GHz、1.82±0.05GHz 和 2.92±0.06GHz，其衰减时间尺度为 20ns。考察了样品形态、激发波长和温度的影响。提出了一个用于单重态裂变的密度矩阵模型，该模型定量描述了振荡的频率、幅度和衰减。该模型假设初始激发的单重激子与三重态对垒直接耦合。单重态和三重态对垒态之间没有电子相干性，但在溶液生长的单晶中，单重态的快速衰减时间（~200ps）提供了必要的脉冲群体转移，从而产生具有整体单重态特征的三个零场三重态对垒态|xx>、|yy>和|zz>的相干叠加。这种三个态的相干叠加导致了实验中观察到的三个量子拍频。量子拍频的衰减是由于三重态和单重态垒之间的群体交换以及三重态对垒态之间的纯相位失谐造成的。通过降低温度和减缓 SF 速率，振荡的可见度降低。没有证据表明产物三重态之间存在磁偶极子-偶极子耦合。我们的模型与数据吻合良好，支持了并五苯中单重态裂变通过“直接”机制进行且单重态和三重态对垒态之间没有强电子耦合的结论。
