作者:Chi-Yang Chao、Chung-Hsiang Chao、Lung-Pin Chen、Ying-Chieh Hung、Shiang-Tai Lin、Wei-Fang Su、Ching-Fuh Lin
DOI:10.1039/c2jm12312f
日期:——
In this research, we demonstrated that the energy levels, including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and the optical absorptions of low band gap conjugated copolymers, consisting of 3-hexylthiophene (3HT) as electron-donating units and 2,3-diethylthieno[3,4-b]pyrazine (ETP) as electron-accepting units, could be systematically tuned by adjusting the composition and the geometric structure of the copolymers. Four new copolymers comprising of 3HT and ETP in different molar ratios (from 1 : 1 to 4 : 1), named P1–P4, were designed and synthesized via Suzuki coupling. The positions of the hexyl side chains on 3HT were varied to adjust the co-planarity of the copolymers. The LUMO, ranging from −2.94 eV to −3.11 eV, was lowered monotonically with increasing ETP content, and the break of co-planarity along the main chain showed a trivial effect on the LUMO. By contrast, the HOMO (−4.74 eV to −4.88 eV) was controlled by both the composition and the geometric structure of the copolymer. P2, having a twisted geometric structure, possessed a lower HOMO and a larger band gap compared to the planar P3. Bimodal optical absorptions with a relatively stronger absorption at long wavelengths were observed for all polymers which are due to intramolecular charge transfer. Optical band gaps in solution, ranging from 1.27 eV to 1.76 eV, decreased with increasing ETP content in the copolymer except P2 and the trend was consistent with the HOMO–LUMO gaps. The optical absorptions and the energy levels are further confirmed by theoretical calculations. Good co-planarity was also found to benefit the electrical conductivity of p-doped thin films. Our results suggested that tuning the composition and the geometric structure would be an effective molecular design strategy toward desired band structure for low band gap conjugated polymers based on thiophene and thienopyrazine derivatives.
在本研究中,我们展示了低带隙共轭共聚物的能级,包括最高占据分子轨道(HOMO)和最低未占据分子轨道(LUMO),以及它们的光学吸收特性,可以通过调整共聚物的组成和几何结构进行系统调节。这些共聚物由3-己基噻吩(3HT)作为给电子单元和2,3-二乙基噻吩[3,4-b]吡唑(ETP)作为受电子单元组成,设计并合成了四种不同摩尔比(从1:1到4:1)的新型共聚物,命名为P1-P4,采用铃木偶联反应合成。3HT上的己基侧链的位置有所变化,以调节共聚物的共平面性。随着ETP含量的增加,LUMO(范围从-2.94 eV到-3.11 eV)单调下降,主链的共平面性破坏对LUMO的影响微乎其微。相比之下,HOMO(-4.74 eV到-4.88 eV)则受到共聚物的组成和几何结构的共同控制。P2具有扭曲的几何结构,相较于平面结构的P3,其HOMO较低且带隙较大。所有聚合物均观察到双峰光学吸收,长波长处的吸收相对强烈,这源于分子内电荷转移。溶液中的光学带隙范围从1.27 eV到1.76 eV,随着共聚物中ETP含量的增加而减小,除了P2外,此趋势与HOMO-LUMO间隙一致。光学吸收和能级也得到了理论计算的进一步验证。良好的共平面性被发现有利于p掺杂薄膜的电导率。我们的结果表明,调节组成和几何结构将是一种有效的分子设计策略,以实现基于噻吩和噻吡唑衍生物的低带隙共轭聚合物所需的能带结构。