Synthesis and Characterization of Benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene (BTT) Oligomers

Two dimers (2 and 3), dendritic tetramer (4), hexamer (5), and decamer (6) of benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene (BTT), a potential π-core unit with C3h symmetry, were synthesized, characterized, and evaluated for possible use as organic semiconductors. Single crystal X-ray analyses of the dimers (2 and 3) revealed that they have planar molecular structures with dihedral angles of almost 180° between two BTT units. In accordance with the rigid and planar molecular structure, the unsubstituted dimer (2) is poorly soluble, whereas the octyl-substituted dimer (3) has improved solubility. Although the solubility of the dendritic tetramer (4) is decreased, further extended systems, i.e., the dendritic hexamer (5) and decamer (6), have solubilities better than that of 4. With increasing numbers of BTT units in the molecule, the experimentally determined energy levels of HOMO shift upward slightly and the HOMO–LUMO energy gaps become smaller, but the extent of HOMO destabilization and reduction of the HOMO–LUMO gap are not significant. Taking into account the energy levels of the frontier orbitals, 3–6 could be useful as p-channel organic semiconductors rather than n-channel. In fact, the spin-coated thin film of 3 with edge-on molecular orientation acted as an active channel of field-effect transistors that showed hole mobilities as high as 0.14 cm2 V–1 s–1, indicating that the BTT core is a useful π-conjugated system for application to organic semiconductors, although 4–6 gave FET characteristics rather inferior to those of 3, owing to their amorphous nature in the thin film state.本研究合成、表征并评估了具有C₃ₕ对称性的潜在π核心单元苯并[1,2-b:3,4-b′:5,6-b′′]三噻吩（benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene，简称BTT）的两种二聚体（2和3）、树枝状四聚体（4）、六聚体（5）以及十聚体（6）作为有机半导体的应用潜力。对二聚体2和3的单晶X射线衍射分析表明，二者的分子结构均呈平面性，两个BTT单元间的二面角接近180°。鉴于其分子结构刚性且平面化，未取代二聚体（2）的溶解性极差，而经辛基取代的二聚体（3）的溶解性得到改善。尽管树枝状四聚体（4）的溶解性有所下降，但进一步延伸的共轭体系即树枝状六聚体（5）与十聚体（6）的溶解性优于4。随着分子中BTT单元数量的增加，实验测定的最高占据分子轨道（Highest Occupied Molecular Orbital，简称HOMO）能级略微上移，最低未占据分子轨道（Lowest Unoccupied Molecular Orbital，简称LUMO）能隙逐渐减小，但HOMO轨道的去稳定化程度以及HOMO-LUMO能隙的缩减幅度均不显著。结合前沿轨道（Frontier Orbitals）能级，化合物3~6可作为p型沟道有机半导体而非n型沟道有机半导体。事实上，化合物3的旋涂薄膜中分子呈边缘取向排列，作为场效应晶体管（Field-Effect Transistor，简称FET）的有源沟道，其空穴迁移率高达0.14 cm²·V⁻¹·s⁻¹，表明BTT核是一种适用于有机半导体领域的优良π共轭体系。尽管4~6的FET性能远逊于3，这归因于其薄膜状态下的无定形特性。