Department of Chemistry and Chemical Biology, Cornell University , Ithaca, New York, 14853, United States.
J Am Chem Soc. 2014 Apr 16;136(15):5755-64. doi: 10.1021/ja500887a. Epub 2014 Apr 3.
We present a detailed study of pentacene monomer and dimer that serves to reconcile extant views of its singlet fission. We obtain the correct ordering of singlet excited-state energy levels in a pentacene molecule (E (S1) < E (D)) from multireference calculations with an appropriate active orbital space and dynamical correlation being incorporated. In order to understand the mechanism of singlet fission in pentacene, we use a well-developed diabatization scheme to characterize the six low-lying singlet states of a pentacene dimer that approximates the unit cell structure of crystalline pentacene. The local, single-excitonic diabats are not directly coupled with the important multiexcitonic state but rather mix through their mutual couplings with one of the charge-transfer configurations. We analyze the mixing of diabats as a function of monomer separation and pentacene rotation. By defining an oscillator strength measure of the coherent population of the multiexcitonic diabat, essential to singlet fission, we find this population can, in principle, be increased by small compression along a specific crystal direction.
我们对并五苯单体和二聚体进行了详细研究，旨在调和其单重态裂变的现有观点。我们通过多参考计算获得了并五苯分子中单重激发态能级的正确顺序（E（S1）<E（D）），其中包含了适当的活性轨道空间和动力学相关性。为了理解并五苯中单重态裂变的机制，我们使用了一种成熟的双原子化方案来描述一个近似于结晶并五苯单元晶格结构的并五苯二聚体的六个低能单重态。局部的、单激子双原子轨道并不直接与重要的多激子态耦合，而是通过与一个电荷转移构型的相互耦合而混合。我们分析了双原子轨道的混合作为单体分离和并五苯旋转的函数。通过定义一个振子强度来衡量多激子双原子轨道的相干布居，这对单重态裂变至关重要，我们发现原则上可以通过沿着特定晶向的小压缩来增加这个布居。
