Control and prediction of crystal structures of molecular semiconductors are considered challenging, yet they are crucial for rational design of superior molecular semiconductors. It is here reported that through methylthiolation, one can rationally control the crystal structure of pyrene derivatives as molecular semiconductors; 1,6‐bis(methylthio)pyrene keeps a similar sandwich herringbone structure to that of parent pyrene, whereas 1,3,6,8‐tetrakis(methylthio)pyrene (MT‐pyrene) takes a new type of brickwork structure. Such changes in these crystal structures are explained by the alteration of intermolecular interactions that are efficiently controlled by methylthiolation. Single crystals of MT‐pyrene are evaluated as the active semiconducting material in single‐crystal field‐effect transistors (SC‐FETs), which show extremely high mobility (32 cm² V⁻¹ s⁻¹ on average) operating at the drain and gate voltages of −5 V. Moreover, the band‐like transport and very low trap density are experimentally confirmed for the MT‐pyrene SC‐FETs, testifying that the MT‐pyrene is among the best molecular semiconductors for the SC‐FET devices.

We have investigated the electronic states of highly oriented pyrolitic graphite and single-walled carbon nanotubes using x-ray absorption spectroscopy (XAS) before and after annealing treatment in ultrahigh vacuum, and observed that the small peak between ${\pi }^{*}$ and ${\sigma }^{*}$ features, which has been previously assigned to free-electron-like interlayer states, disappears after in situ annealing treatment, suggesting that the signal may be assigned to a surface contamination, especially oxygen contamination introduced by chemical processing or gas adsorption. Additional experiments by photoelectron spectroscopy as well as XAS methods, performed after aging in air, fully support this interpretation.