Synthesis and structure of α-substituted pentathienoacenes

Abstract

Recent progress in thienoacene development has produced numerous high performance, air-stable organic semiconductor materials. Pentathienoacene (T5) thin film devices have shown poor performance despite promising computational studies, likely due to high mobility anisotropy and poor film morphology. 2,6-Bis-alkyl-pentathienoacene has been synthesized to enable solution-processing routes to better microstructures of T5-based devices. Soluble side groups are introduced to thieno[3,2-b]thiophene precursors through deprotonation at α-positions. Introduction of the sulfur bridge was achieved by Pd-catalyzed coupling reaction with bis(tri-n-butyltin)sulfide (Bu₃SnSSnBu₃), followed by final ring closure through oxidative coupling with CuCl₂. This method achieves higher purity and higher yield than sulfide-quenched Li–Br exchange. UV-vis and fluorescence emission spectra show a bathochromic shift of ≈10 nm, indicating the introduction of alkyl chains decreases the HOMO–LUMO gap. X-ray analysis yields unit cells for 2,6-bis-octyl and 2,6-bis-dodecyl substituted T5s (C8-T5 and C12-T5, respectively). C8-T5 grows orthorhombic crystals with lattice parameters a = 1.15 nm, b = 0.43 nm and c = 3.05 nm; C12-T5 grows monoclinic crystals (c-unique) with unit cell with parameters a = 1.10 nm, b = 0.42 nm, c = 3.89 nm and γ = 92.9°. Both materials grow large (>50 μm), faceted, single crystals that are microscopically composed of alternating layers of semiconducting cores and insulating substitutions.