Very large silacylic substituent effects on response in silole-based polymer transistors

Understanding the interrelationships between molecular structure and organic thin film transistor performance is key to the realization of novel organic semiconductors achieving superior device characteristics. Herein we report the synthesis, characterization, and charge-transporting properties in organic field-effect transistors (OFETs) of dithieno silole-based oligomers and copolymers having silacycloalkyl substituents. Silacyclization of the alkyl substituents on the silole silicon atom reduces steric encumbrance, contracts solid state intermolecular π-π contacts, and enhances the charge-transport capacity of the oligomers. Oligomer 3,3′-dihexylsilylene-2,2′:5, 2′′:5′,2′′′:5′′, 2′′′′:5′′′,2′′′ ′′-sexithiophene (SM5) with two Si-n-hexyl substituents is not FET-active, while the mobilities of 3,3′-cyclopentanylsilylene-2,2′: 5,2′′:5′,2′′′:5′′, 2′′′′:5′′′,2′′′ ′′′-sexithiophene (SM4) and 3,3′-cyclobutysilylene-2, 2′:5,2′′:5′,2′′′:5′′, 2′′′′:5′′′,2′′′ ′′-sexithiophene (SM3) FETs are 2.6 × 10^-4 and 3.4 × 10^-4 cm²/(V s), respectively. Single crystal structural data and melting point derived intermolecular packing trends parallel these FET results. Copolymers P1-P4 based on the same dithienosilole cycloalkyl cores exhibit optimized hole mobilities of 2 × 10^-5, 6 × 10^-4, 3 × 10^-4, and 2 × 10^-3 cm²/V•s, respectively, lower than that of analogous silole-containing polymers with long Si-alkyl substituents, implying that the solubilizing and self-assembly functions of Si-alkyl substituents are important for optimizing the mobility. Interestingly, copolymer [poly{[N,N′-bis(2- octyl-dodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(3, 3′-cyclopentanylsilylene-2,2′-bithiophene (P5) films are the most ordered and exhibit a good electron mobility of 4 × 10^-3 cm²/V•s after thermal annealing. All of these OFETs exhibit good ambient-stability, which is attributed to their low-lying HOMOs (>0.2 eV lower than that of P3HT), a consequence of introducing silole cores into polythiophene backbones.