Huang Yen-Chin, Cheng Yuan-Chung
Department of Chemistry , National Taiwan University , No. 1, Sec. 4, Roosevelt Rd. , Da-an District, Taipei City 106 , Taiwan.
J Phys Chem A. 2019 May 23;123(20):4333-4341. doi: 10.1021/acs.jpca.9b00856. Epub 2019 May 8.
We investigate energy transfer and electron transfer in a dimethylsilylene-spaced aminostyrene-stilbene donor-acceptor dimer using time-dependent density functional theory calculations. Our results confirm that the vertical S, S, and S excited states are, respectively, a local excitation on the aminostyrene, local excitation on the stilbene, and the charge-transferred (CT) excited state with electron transfer from aminostyrene to stilbene. In addition, an energy minimum with the C-N bond of the amino group twisted at about 90° is also identified on the S potential energy surface. This S state exhibits a twisted intramolecular charge transfer (TICT) character. A potential energy scan along the C-N bond torsional angle reveals a conical intersection between the S stilbene local excitation and the S CT/TICT state at a torsional angle of ∼60°. We thus propose that the conical intersection dominates the electron transfer dynamics in the donor-acceptor dimer and copolymers alike, and the energy barrier along the C-N bond rotation controls the efficiency of such a process. Moreover, we show that despite the zero oscillator strength of the S excited states in the CT and TICT minima, an emissive S state with a V-shaped conformational structure can be located. The energy of this V-shape CT structure is thermally accessible; therefore, it is expected to be responsible for the CT emission band of the dimer observed in polar solvents. Our data provide a clear explanation of the complex solvent-dependent dual emission and photoinduced electron transfer properties observed experimentally in the dimer and copolymer systems. More importantly, the identifications of the conical intersection and energy barrier along the C-N bond rotation provide a novel synthetic route for controlling emissive properties and electron transfer dynamics in similar systems, which might be useful in the design of novel organic optoelectronic materials.
我们使用含时密度泛函理论计算方法，研究了一个由二甲基硅亚甲基间隔的氨基苯乙烯 - 芪供体 - 受体二聚体中的能量转移和电子转移。我们的结果证实，垂直的S₁、S₂和S₃激发态分别是氨基苯乙烯上的局域激发、芪上的局域激发以及电荷转移（CT）激发态，其中电子从氨基苯乙烯转移到芪上。此外，在S₃势能面上还发现了一个能量最低点，此时氨基的C - N键扭转约90°。这个S₃态表现出扭曲的分子内电荷转移（TICT）特征。沿着C - N键扭转角进行的势能扫描显示，在扭转角约为60°时，芪的S₂局域激发态与S₃ CT/TICT态之间存在一个锥形交叉点。因此，我们提出锥形交叉点主导了供体 - 受体二聚体和共聚物中的电子转移动力学，并且沿着C - N键旋转的能垒控制了这一过程的效率。此外，我们表明，尽管在CT和TICT最低点的S₃激发态的振子强度为零，但可以找到一个具有V形构象结构的发光S₁态。这个V形CT结构的能量在热学上是可及的；因此，预计它是在极性溶剂中观察到的二聚体CT发射带的原因。我们的数据为在二聚体和共聚物体系中实验观察到的复杂的溶剂依赖型双发射和光致电子转移性质提供了清晰的解释。更重要的是，锥形交叉点和沿着C - N键旋转的能垒的确定为控制类似体系中的发光性质和电子转移动力学提供了一条新的合成途径，这可能对新型有机光电子材料的设计有用。
