Terazulene Isomers: Polarity Change of OFETs through Molecular Orbital Distribution Contrast

Intermolecular orbital coupling is fundamentally important to organic semiconductor performance. Recently, we reported that 2,6′:2′,6″-terazulene (TAz1) exhibited excellent performance as an n-type organic field-effect transistor (OFET) via molecular orbital distribution control. To validate and develop this concept, here we present three other terazulene regioisomers, which have three azulene molecules connected at the 2- or 6-position along the long axis of the azulene, thus constructing a linear expanded π-conjugation system: 2,2′:6′,2″-terazulene (TAz2), 2,2′:6′,6″-terazulene (TAz3), and 6,2′:6′,6″-terazulene (TAz4). TAz2 and TAz3 exhibit ambipolar characteristics; TAz4 exhibits clear n-type transistor behavior as an OFET. The lowest unoccupied molecular orbitals (LUMOs) of all terazulenes are fully delocalized over the entire molecule. In contrast, the highest occupied molecular orbitals (HOMOs) of TAz2 and TAz3 are delocalized over the 2,2′-biazulene units; the HOMOs of TAz4 are localized at one end of the azulene unit. These findings confirm that terazulene isomers which are simple hydrocarbon compounds are versatile materials with a tunable-polarity FET characteristic that depends on the direction of the azulene unit and the related contrast of the molecular orbital distribution in the terazulene backbone.分子间轨道耦合对有机半导体的性能具有至关重要的基础性意义。近期，我们通过调控分子轨道分布，报道了2,6′:2′,6″-三联薁（terazulene，TAz1）作为n型有机场效应晶体管（OFET）时展现出优异性能。为验证并拓展这一研究理念，本文报道了另外三种三联薁区域异构体：它们均通过薁长轴上的2位或6位连接三个薁单元，构建出线性扩展的π共轭体系，分别为2,2′:6′,2″-三联薁（TAz2）、2,2′:6′,6″-三联薁（TAz3）以及6,2′:6′,6″-三联薁（TAz4）。TAz2与TAz3表现出双极性传输特性；TAz4作为OFET则展现出清晰的n型晶体管行为。所有三联薁的最低未占据分子轨道（LUMO）均在整个分子范围内完全离域。与之相对，TAz2与TAz3的最高占据分子轨道（HOMO）仅在2,2′-联薁单元上发生离域，而TAz4的HOMO则定域于其中一个薁单元的末端。上述研究结果证实，作为简单烃类化合物的三联薁异构体是一类多功能材料，其场效应晶体管具备可调控的极性特性，调控依据为薁单元的连接方向以及三联薁骨架内分子轨道分布的相关差异。