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

ABSTRACT: Molecular semiconductors that crystallize into structures facilitating efficient two-dimensional (2D) overlap of molecular orbitals, such as herringbone, pitched pi-stack, and brickwork (2D pi-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 pi-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.