Authors: Shunto Arai, Kaede Morita, Jun'ya Tsutsumi, Satoru Inoue, Mutsuo Tanaka, Tatsuo Hasegawa
Published: 2019-11-04
Source: Full article
AbstractProduction and control of 2D self‐organized layers of π‐conjugated molecules is a fundamental issue in organic electronics devices. This study reports that an intralayer polymorphic transformation is apparently observed in abilayer‐type layered‐herringbonepacking of a series of 2‐mono‐alkylated‐benzothieno[3,2‐b][1]benzothiophenes (mono‐Cn‐BTBTs) with the change in the substituted alkyl‐chain length,n. Slight variation in the intermolecular arrangement in the single‐crystal films is investigated with reasonable sensitivity via polarized optical absorption spectroscopy through the excitonic interactions between neighboring π‐electron cores. A clear polymorphic transition is observed based on the features of the in‐plane optical anisotropy with the change in the chain length betweenn= 7 (or less) and 8 (or more). A unique phase separation of the polymorphs is also observed in the films produced using a mixed solution with molecules having differentn: the respective polymorphic phases appear separately, either as single molecular bilayers or multibilayer‐stacked films owing to geometrical frustration. The results indicate that the intralayer packing motif is predetermined by the competition and balance between the intermolecular core–core and chain–chain interactions. These findings pave the way for rational designing of organic molecules with considerably high layered crystallinity, and thus, of efficient carrier transport in organic thin‐film transistors.