Benzo[1,2-b:6,5-b′]dithiophene(dithiazole)-4,5-dione derivatives: synthesis, electronic properties, crystal packing and charge transport
A series of dihalo- and bis-aroyl-substituted benzo[1,2-b:6,5-b′]dithiophene-4,5-diones were synthesized, and their electronic, electrochemical, and electrical properties investigated. Synthetic strategies to increase (i) the conjugation length of the base molecular structure – through introduction of thiophene units bearing electronically neutral substituents (hydrogen or alkyl groups) or strong electron-withdrawing pentafluorobenzoyl group(s) – and (ii) the electron affinity – by moving to a benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione structure – were developed. Molecular packing in the single crystal was studied by single-crystal X-ray structural analysis, and this information was subsequently used in the determination of the electronic band structures, densities of states (DOS), effective transfer integrals, and effective charge-carrier masses via density functional theory (DFT) methods. The charge-carrier transport properties of the benzo[1,2-b:6,5-b′]dithiophene-4,5-dione and benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione derivatives were investigated through the fabrication and characterization of organic field-effect transistors (OFETs) via both solution-processed and vacuum-deposited films. 2,7-Bis-pentafluorobenzoyl-benzo[1,2-b:6,5-b′]dithiophene-4,5-dione (10a) exhibited field-effect behavior with an average electron mobility μe = 4.4 (±1.7) × 10−4 cm2 V−1 s−1 when the active layer was vacuum-deposited, and a larger μe= 6.9 × 10−3 cm2 V−1 s−1 when the active layer was solution-processed. These results are in stark contrast with the DFT-determined electronic band structure and effective mass, which indicate that the material possesses good intrinsic charge-carrier transport characteristics. The combined results reveal the importance of thin-film processing and that further processing refinements could lead to improved device performance. Only one material with benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione core, 2,7-bis-(4-n-hexyl-thiophene-2-yl)-benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione (19d), showed average μe = 8.2 × 10−5 cm2 V−1 s−1 in OFET with solution-processed active layer. Unexpectedly, measurable hole transport was observed for 2,7-bis-(5-n-nonyl-thiophen-2-yl)-benzo[1,2-b:6,5-b′]dithiophene-4,5-dione (19b) (μh = 8.5 × 10−5 cm2 V−1 s−1) and 2,6-bis-(thiophen-2-yl)-3,5-di-n-hexyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-4-one (30a) (μh = 3.7 × 10−4 cm2 V−1 s−1).