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.