Department of Physical Chemistry, Faculty of Science, University of Málaga, E-29071-Málaga, Spain.
Phys Chem Chem Phys. 2018 Aug 29;20(34):21864-21880. doi: 10.1039/c8cp03482f.
We simulate the vibronic profile of the electronic circular dichroism (ECD) spectra of oligothiophenes in cisoid and transoid chiral arrangements. We consider oligomers of different lengths, from two to fifteen units, and investigate extensively how the ECD spectral shapes depend on the inter-ring torsions. In general, the molecular structures we consider are not stationary points of the ground state potential energy surface. Therefore, in order to perform vibronic calculations, we present a new computational protocol able to define reduced-dimensionality models where the effect of the off-equilibrium modes is removed. This is done adopting a description of the vibrational motions in curvilinear internal coordinates, and vertical harmonic models coupled with an iterative application of projectors to define energy Hessians, and therefore effective normal modes, in the space complementary to the one of the off-equilibrium coordinates. Although we consider both Franck-Condon and Herzberg-Teller contributions, the results show that transoid twisted ribbons always give rise to monosignated ECD spectra, while bi-signated and multi-signated spectra are expected for cisoid helices. These findings are explained on the basis of the different transition strengths of the lowest electronic states imparted by the different spatial arrangement, that is almost linear for transoid structures and more globular for cisoid ones. We predicted the chiroptical response of a large number of possible molecular arrangements. These data are employed to critically discuss the experimental ECD of polythiophenes in different experimental conditions, forming either aggregates or host-guest complexes. The method here proposed to perform vibronic calculations in reduced-dimensionality models is of general applicability and its potential interest goes beyond the practical application presented here.
我们模拟了顺式和反式手性排列的寡聚噻吩的电子圆二色性（ECD）光谱的振子轮廓。我们考虑了不同长度的寡聚物，从两个到十五个单元，并广泛研究了ECD 光谱形状如何依赖于环间扭转。通常，我们考虑的分子结构不是基态势能面的稳定点。因此，为了进行振子计算，我们提出了一种新的计算方案，能够定义降维模型，其中消除了非平衡模式的影响。这是通过采用曲线内部坐标描述振动运动，并采用垂直谐波模型并结合迭代应用投影来定义能量 Hessian 来实现的，从而在非平衡坐标空间的补空间中定义有效正则模式。尽管我们考虑了 Franck-Condon 和 Herzberg-Teller 贡献，但结果表明，反式扭曲带总是产生单符号 ECD 光谱，而顺式螺旋则预期产生双符号和多符号光谱。这些发现基于不同空间排列赋予的最低电子态的不同跃迁强度来解释，即对于反式结构几乎是线性的，而对于顺式结构则更球形。我们预测了大量可能的分子排列的手性响应。这些数据用于在不同实验条件下对聚噻吩的实验 ECD 进行批判性讨论，形成聚集体或主体-客体配合物。这里提出的在降维模型中进行振子计算的方法具有普遍适用性，其潜在的兴趣超出了这里提出的实际应用。
