Asymmetric Conjugated Molecules Based on [1]Benzothieno[3,2-b][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, smectia 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-y1) [1] benzothieno [3,2-b] [1] benzothiophene and 2-(5-ethylthioph en-2-yl) [1] benzothieno[3,2-b] [1]benzothiophene showed OTFT mobilities above 5 cm(2) 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 (mu) up to 10.5 cm(2) V-1 s(-1) because they formed highly ordered and homogeneous films with diminished grain boundaries.