1,3,6,8-Tetrakis(methylchalcogeno)pyrenes: Effects of Chalcogen Atoms on the Crystal Structure and Transport Properties

Molecular semiconductors that crystallize into structures
facilitating
efficient two-dimensional (2D) overlap of molecular orbitals, such
as herringbone, pitched π-stack, and brickwork (2D π-stack)
structures, are promising as active materials in high-mobility organic
field-effect transistors (OFETs). We have recently reported that 1,3,6,8-tetrakis­(methylthio)­pyrene
(MT-pyrene) that crystallizes into a new type of brickwork structure
shows “ultrahigh” mobility of up to 32 cm2 V–1 s–1 and band-like transport
in a single-crystal field-effect transistor (SC-FET). Its oxygen and
selenium analogues, that is, 1,3,6,8-tetramethoxypyrene (MO-pyrene)
and 1,3,6,8-tetrakis­(methylseleno)­pyrene (MS-pyrene), respectively,
are likewise interesting in terms of crystal structure and transport
properties. In the present work, MO-pyrene and MS-pyrene were synthesized
and characterized. They crystallized into brickwork structures that
seemed similar to that of MT-pyrene at first glance but were noticeably
different on close inspection. The brickwork structure of MO-pyrene
was more even in two π-stacking directions compared with that
of MT-pyrene, whereas that of MS-pyrene was highly anisotropic and
almost one-dimensional. The carrier transport properties of MO-pyrene
and MS-pyrene were evaluated by fabricating SC-FETs. The SC-FETs demonstrated
almost ideal transistor characteristics with sharp turn-on behavior
and negligible hysteresis. On the other hand, MO-pyrene and MS-pyrene
showed relatively low hole mobilities of up to 0.03 and 7.3 cm2 V–1 s–1, respectively.
These experimental mobilities are consistent with those estimated
by the hopping model, which is in sharp contrast to the band-like
transport of MT-pyrene. The results indicate that methylchalcogeno
groups not only contribute to the control of the crystal structure
but also have a significant impact on the molecular orbital overlaps
in the solid state and, therefore, the transport properties.

能够结晶为可促进分子轨道高效二维重叠的结构的分子半导体，诸如人字形、倾斜π堆叠与砖砌（二维π堆叠）结构，作为高迁移率有机场效应晶体管（organic field-effect transistors, OFETs）的活性材料极具应用前景。我们近期曾报道，结晶于新型砖砌结构的1,3,6,8-四（甲硫基）芘（MT-pyrene）在单晶场效应晶体管（single-crystal field-effect transistor, SC-FET）中展现出高达32 cm²·V⁻¹·s⁻¹的“超高”空穴迁移率与类能带输运特性。其氧代与硒代类似物——即1,3,6,8-四甲氧基芘（MO-pyrene）与1,3,6,8-四（甲硒基）芘（MS-pyrene）——在晶体结构与载流子输运性质方面同样具备研究价值。本研究中，我们合成并表征了MO-pyrene与MS-pyrene。二者结晶所得的砖砌结构乍看与MT-pyrene的结构相似，但经细致观测后存在显著差异。相较于MT-pyrene的砖砌结构，MO-pyrene的π堆叠在两个方向上更为均匀；而MS-pyrene的结构则呈现高度各向异性，近乎一维特征。我们通过制备单晶场效应晶体管（SC-FET）评估了MO-pyrene与MS-pyrene的载流子输运性质。所得SC-FET展现出近乎理想的晶体管特性：开启行为锐利，迟滞效应可忽略不计。与之相对，MO-pyrene与MS-pyrene的空穴迁移率相对较低，最大值分别为0.03与7.3 cm²·V⁻¹·s⁻¹。上述实验迁移率与跳跃模型的预测值相符，这与MT-pyrene的类能带输运特性形成鲜明对比。本研究结果表明，甲基硫属基团不仅可用于调控晶体结构，还对固态下的分子轨道重叠乃至载流子输运性质具有显著影响。