Twisting of conjugated oligomers and polymers: case study of oligo- and polythiophene.
Twisting of conjugated oligomers and polymers: case study of oligo- and polythiophene.
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DOI：
10.1002/chem.200600819
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发表时间：
2007-04
期刊：
影响因子：
--
通讯作者：
Sanjio S. Zade;M. Bendikov
中科院分区：
文献类型：
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作者：
Sanjio S. Zade;M. Bendikov
利用周期边界条件（PBC）在B3LYP/6-31G(d)水平上对聚噻吩的环间扭转（共轭环间二面角的变化）进行了理论研究。我们发现聚合物的带隙与交织扭角密切相关；然而，扭转只需要很少的能量。在聚噻吩中，30度的扭转使带隙增加0.75 eV，而每个单体单位只需要0.41千卡摩尔（-1）。如此小的能量值是晶体堆积力或范德华力的数量级。这些结果与模型低聚物的计算结果进行了比较。采用B3LYP/6-31G(d)方法，在0-180度的端对端扭转角（对应0-36度的相互二面角）下对硫代噻吩及其自由基阳离子及其构型进行了优化。理论结果表明，低聚物的HOMO-LUMO间隙、电离势和电荷分布强烈依赖于扭转，而与聚噻吩类似，中性低聚噻吩的扭转消耗很少的能量。在自由基阳离子的情况下，在扭转时，最低能量跃迁到较长的波长区域，而第二最低的能量跃迁到较短的波长区域。这意味着，与平面掺杂聚合物相比，扭曲的掺杂导电聚合物（在这里由低聚自由基阳离子建模）在一定的光学窗口内（在远可见光区域，约1.5 eV）应该是透明的。这一观察结果是根据前沿分子轨道的形状和重叠的变化来解释的。
Interring twisting (change in the dihedral angle between conjugated rings) of polythiophene was studied theoretically using periodic boundary conditions (PBC) at the B3LYP/6-31G(d) level. We find that the band gap of polymers is strongly dependent on the interring twist angle; yet twisting requires very little energy. A twist of 30 degrees increases the band gap by 0.75 eV in polythiophene, while requiring only 0.41 kcal mol(-1) per monomer unit. Such a small energetic value is of the order of crystal packing or van der Waals forces. These results are compared with calculations performed on model oligomers. Sexithiophene, its radical cations, and its dication are optimized at 0-180 degrees end-to-end twist angles (which correspond to 0-36 degrees interring dihedral angles) using the B3LYP/6-31G(d) method. The theoretical results suggest that the HOMO-LUMO gap, ionization potential, and charge distribution of oligomers are strongly dependent on twisting, whereas, similar to the case of polythiophene, twisting of neutral oligothiophenes costs very little energy. In the case of the radical cation, the lowest energy transition is shifted to a longer wavelength region on twisting, while the second-lowest energy transition is shifted to a shorter wavelength region. This implies that twisted, doped conducting polymers (modeled here by an oligomer radical cation), in contrast to planar, doped polymers, should be transparent within a certain optical window (in the far-visible region, at approximately 1.5 eV). This observation is explained on the basis of changes in the shape and overlap of the frontier molecular orbitals.