Department of Chemistry, Center for Theoretical Sciences, National Taiwan University, Taipei, Taiwan, Republic of China 106.
J Phys Chem A. 2010 Mar 4;114(8):2885-92. doi: 10.1021/jp909181p.
A theoretical composite-molecule (CM) model is adopted for evaluating the electronic excited states and excitonic couplings of cofacial conjugated dimers where the contributions of charge-transfer (CT) exciton, unavailable by the commonly used supermolecular approach due to the inadequate basis set construction, can be unambiguously identified within this methodology. This method builds up with the basis set of individual molecules and then constructs combined electronic states for the dimer by considering intermolecular interactions including charge-transfer interactions. The dependences of the matrix elements on intermolecular distance and conjugation length are examined. At the short distance region between two of the polyene molecules in the dimer, the CT transitions are apparently mixing to both first and second excited states. Also, some of the matrix elements for the mixing of CT transitions with local transitions which related to the second excited state are found to be considerably larger than the exciton-type elements. An interesting finding is that with increasing the chain size the CT contribution to the second excited state reveals a minimum and indicates HOMO to LUMO charge transfer is not the major CT contribution to the second excited state in the face-to-face polyene dimer with larger chain size and interchain separation in the region of 3.6-4.0 A. A detail analysis reveals that HOMO-1 to LUMO and HOMO to LUMO+1 charge transfers are major CT contributions to the second excited state in the condition under study.
采用理论复合分子（CM）模型来评估共面共轭二聚体的电子激发态和激子耦合，由于基组构建不足，通常使用的超分子方法无法识别这种方法中不可缺少的电荷转移（CT）激子。该方法基于单个分子的基组构建，然后通过考虑包括电荷转移相互作用在内的分子间相互作用来构建二聚体的组合电子态。研究了矩阵元对分子间距离和共轭长度的依赖性。在二聚体中两个聚烯分子之间的短距离区域，CT 跃迁明显混合到第一和第二激发态。此外，还发现一些与第二激发态相关的 CT 跃迁与局域跃迁混合的矩阵元明显大于激子型元素。有趣的是，随着链长的增加，第二激发态的 CT 贡献呈现出最小值，这表明在具有较大链长和分子间分离的面对面聚烯二聚体中，HOMO 到 LUMO 的电荷转移不是第二激发态的主要 CT 贡献，分子间分离在 3.6-4.0Å 的区域内。详细分析表明，在研究条件下，HOMO-1 到 LUMO 和 HOMO 到 LUMO+1 的电荷转移是第二激发态的主要 CT 贡献。
