Asymmetric Conjugated Molecules Based on [1]Benzothieno[3,2‑<i>b</i>][1]benzothiophene for High-Mobility Organic Thin-Film Transistors: Influence of Alkyl Chain Length

Herein, we report the synthesis and characterization of a series of [1]­benzothieno­[3,2-b]­[1]­benzothiophene (BTBT)-based asymmetric conjugated molecules, that is, 2-(5-alkylthiophen-2-yl)[1]­benzothieno­[3,2-b]­[1]­benzothiophene (BTBT-Tn, in which T and n represent thiophene and the number of carbons in the alkyl group, respectively). All of the molecules with n ≥ 4 show mesomorphism and display smectic A, smectic B (n = 4), or smectic E (n > 4) phases and then crystalline phases in succession upon cooling from the isotropic state. Alkyl chain length has a noticeable influence on the microstructures of vacuum-deposited films and therefore on the performance of the organic thin-film transistors (OTFTs). All molecules except for 2-(thiophen-2-yl)[1]­benzothieno­[3,2-b]­[1]­benzothiophene and 2-(5-ethylthiophen-2-yl)[1]­benzothieno­[3,2-b]­[1]­benzothiophene showed OTFT mobilities above 5 cm2 V–1 s–1. 2-(5-Hexylthiophen-2-yl)[1]­benzothieno­[3,2-b]­[1]­benzothiophene and 2-(5-heptylthiophen-2-yl)[1]­benzothieno­[3,2-b]­[1]­benzothiophene showed the greatest OTFT performance with reliable hole mobilities (μ) up to 10.5 cm2 V–1 s–1 because they formed highly ordered and homogeneous films with diminished grain boundaries.