Structure and Vibrational Spectra of 2,5-Diiodothiophene: A Model for Polythiophene

The structure and vibrational spectroscopy of 2,5-diiodothiophene have been described for the first time. The structure is remarkable in that, despite the presence of eight molecules in the unit cell, there is almost no evidence for any interaction between them. This is reflected in the spectra that show coincident modes in the infrared, Raman, and inelastic neutron scattering (INS) spectra and almost resolution-limited line widths in the INS spectrum, showing that both the factor group splitting and the phonon dispersion are very small. This property has led to the use of the compound as a calibrant in INS spectroscopy. The deductions from the spectra are confirmed by a periodic density functional theory (DFT) calculation that allows a complete spectral assignment to be made. Comparison to the INS and Raman spectra of polythiophene (for which 2,5-diiodothiophene may be used as a monomer) shows a marked similarity for the former and a marked difference to the latter. The INS spectrum of polythiophene is dominated by the C–H bending modes, which are unchanged on polymerization. In contrast, the Raman spectrum of polythiophene is dominated by the changed electronic environment that results in strong electron–phonon coupling and a significant upshift of the CC stretch modes.本研究首次报道了2,5-二碘噻吩（2,5-diiodothiophene）的晶体结构与振动光谱。该晶体结构的独特之处在于：尽管晶胞中包含8个分子，但几乎未观测到分子间存在任何相互作用。这一特征在光谱中得到充分体现：红外、拉曼及非弹性中子散射（inelastic neutron scattering, INS）光谱中的振动模式重合度极高，且INS光谱的谱线宽度几乎达到分辨率极限，表明因子群分裂（factor group splitting）与声子色散（phonon dispersion）均极微弱。该特性使得该化合物被用作INS光谱分析的校准物质。基于光谱数据推导的结论通过周期性密度泛函理论（periodic density functional theory, DFT）计算得到验证，由此可完成全部振动光谱的指认。将该化合物的INS与拉曼光谱与聚噻吩（polythiophene，其单体可为2,5-二碘噻吩）的对应光谱进行对比，结果显示其与聚噻吩的INS光谱具有显著相似性，而与聚噻吩的拉曼光谱则存在明显差异。聚噻吩的INS光谱以C-H弯曲振动模式为主，该振动模式在聚合过程中未发生变化。与之相反，聚噻吩的拉曼光谱则受电子环境改变主导：该变化会引发强烈的电子-声子耦合（electron-phonon coupling），并使C=C伸缩振动模式发生显著的高移。