Synthesis and characterization of oligo(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene)s: effect of the chain length and end-groups on their optical and charge transport properties
作者:Li Wang、Xiaojie Zhang、Jidong Zhang、Hongkun Tian、Yunfeng Lu、Yanhou Geng、Fosong Wang
DOI:10.1039/c4tc01899k
日期:——
Oligo(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene)s Tn and Tn-2CN (n = 4, 8, 12, 20, n represents the number of thiophene rings), which have H and CN end-groups, respectively, were synthesized, and the effect of chain-length and end-groups on their properties was studied. For both Tn and Tn-2CN, a red-shift of absorption spectra was observed as the chain length increased. Introduction of electron-withdrawing CN end-groups resulted in a remarkable red-shift of absorption spectra in both solution and film states for shorter oligomers (n = 4 and 8). However, for oligomers with n = 12 and 20, obvious red-shift of absorption spectra was only observed in the film state. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were both lowered by introducing CN end-groups, and the level of reduction decreased with an increase of the chain length. Both the chain length and end-groups influenced the thin film microstructures of the oligomers as revealed by two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and atomic force microscopy (AFM) studies. The oligomers with n = 4 and 8 stood on the substrates with a tilt angle, and the films of T4-2CN and T8-2CN were characterized with 3D crystal structures. In contrast, the oligomers with n = 12 and 20 adopted an edge-on arrangement with the long axes of the molecules parallel to the substrates, like polymeric counterparts. Compared to the films of T12 and T20, those of T12-2CN and T20-2CN comprised larger and more oriented crystal-domains to the substrates along with shorter lamellar distances. Charge-transport properties of the oligomers were influenced by both the chain length and end-groups due to their remarkable effect on the film morphology and packing structures. The film of T8-2CN combined three features: large domain sizes, low surface-roughness and diminished grain boundaries, and therefore exhibited the highest field-effect mobility.
研究人员合成了分别具有 H 和 CN 端基的寡(2,5-双(3-十二烷基噻吩-2-基)噻吩并[3,2-b]噻吩)Tn 和 Tn-2CN(n = 4、8、12、20,n 代表噻吩环数),并研究了链长和端基对其性质的影响。对于 Tn 和 Tn-2CN 来说,随着链长的增加,吸收光谱会发生红移。在较短的低聚物(n = 4 和 8)中,引入抽电子的 CN 端基可导致溶液和薄膜状态下的吸收光谱发生显著的红移。然而,对于 n = 12 和 20 的低聚物,只有在薄膜状态下才能观察到明显的吸收光谱红移。通过引入 CN 端基,最高占位分子轨道(HOMO)和最低未占位分子轨道(LUMO)能级都降低了,而且降低程度随着链长的增加而降低。二维掠入射 X 射线衍射(2D-GIXD)和原子力显微镜(AFM)研究表明,链长和末端基团都会影响低聚物的薄膜微观结构。n = 4 和 n = 8 的低聚物以倾斜角度立在基底上,T4-2CN 和 T8-2CN 的薄膜具有三维晶体结构。与此相反,n=12 和 20 的低聚物采用边缘排列,分子长轴平行于基底,就像聚合物一样。与 T12 和 T20 薄膜相比,T12-2CN 和 T20-2CN 薄膜的晶体区域更大,与基底的取向性更强,片层距离更短。低聚物的电荷传输特性受到链长和端基的影响,因为它们对薄膜的形态和堆积结构有显著的影响。T8-2CN 薄膜结合了三个特征:畴尺寸大、表面通透度低和晶界减少,因此表现出最高的场效应迁移率。