Steric effect of halogen substitution in an unsymmetrical benzothienobenzothiophene organic semiconductor

We report a crystal structure analysis of an unsymmetrical organic semiconductor 2-bromo-7-butyl[1]benzothieno[3,2-b][1]benzothiophene (Br-BTBT-C4) and its transistor characteristics. Unlike the parent compound 2-butyl-[1]benzothieno[3,2-b][1]benzothiophene (BTBT-C4) forming a bilayer-type structure composed of head-to-head herringbone packings, Br-BTBT-C4 forms a dense head-to-tail herringbone packing arranged so as to avoid the steric hindrance between bromine atoms. The intermolecular transfer integrals in Br-BTBT-C4 suggest its advantage for carrier transport; however, the transistor characteristics of Br-BTBT-C4 and BTBT-C4 were found to be almost the same. This is because BTBT-C4 has a higher crystallinity than Br-BTBT-C4 in vacuum deposited thin films. [Display omitted] •A short contact exists between the bromine atom and the hydrogen atom of the butyl group. This interaction seems to make it easier for Br-BTBT-C4 to form a head-to-tail structure.•Halogen-substituted materials have enhanced transfer integrals in the herringbone structure.•The formation of a bi-layer type molecular structure is effective for the development of high-performance transistors as well as the enhancement of transfer integral.