Li Na, Jia Ke, Wang Sufan, Xia Andong
The State Key Laboratory of Molecular Reaction Dynamics, and Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing-100 080, P. R. China.
J Phys Chem A. 2007 Sep 27;111(38):9393-8. doi: 10.1021/jp074013b. Epub 2007 Aug 21.
The optical spectra of the dimethoxy-p-phenylene-ethynylene oligomers (up to n = 10) are calculated by DFT and TD-DFT methods. It is found that the conformational rotations around the cylindrical triple-bonded carbon links impact significantly the optical spectrum. The effective conjugation length (ECL) of the oligomer is obtained by extrapolating the HOMO-LUMO gap to infinite chain length with an alternative exponential function. The spectral shift is mainly dependent on the high pi-conjugation segment of oligomers, resulting from the planarization of the backbone. Although the rotational barrier is very low, the calculated results further indicate that rotation about the cylindrical triple bond still interrupts the conjugation of rod-like oligomers to some extent, and leads to an angle-dependent HOMO-LUMO gap. The results are helpful to interpret the conformational-dependent spectroscopic phenomena of p-phenyleneethynylene oligomers and polymers (PPEs) observed in ensemble and single molecule spectroscopy.
采用密度泛函理论（DFT）和含时密度泛函理论（TD-DFT）方法计算了二甲氧基-对亚苯基乙炔低聚物（n≤10）的光谱。研究发现，围绕圆柱形三键碳链的构象旋转对光谱有显著影响。通过用交替指数函数将最高占据分子轨道（HOMO）-最低未占据分子轨道（LUMO）能隙外推到无限链长，得到了低聚物的有效共轭长度（ECL）。光谱位移主要取决于低聚物的高π共轭段，这是由主链的平面化引起的。尽管旋转势垒非常低，但计算结果进一步表明，围绕圆柱形三键的旋转仍会在一定程度上中断棒状低聚物的共轭，并导致HOMO-LUMO能隙与角度相关。这些结果有助于解释在系综和单分子光谱中观察到的对亚苯基乙炔低聚物和聚合物（PPEs）的构象相关光谱现象。
