New fluorene–bithiophene-based trimers as stable materials for OFETs

We report the synthesis of five new 5,5′-bis(9,9′-dialkylfluorene-2-yl)-2,2′-bithiophenes 4a–e as active materials for the use in OFETs. Depending on the type of alkyl substituents crystalline or amorphous materials are obtained. Cyclovoltammetry shows that the materials are electrochemically stable and have a HOMO level at −5.3 eV. The different morphologies of thin films of 4a–c have great influence on the performance of the materials in OFETs. The field effect mobilities are in the range of 10−5 cm2/Vs in an amorphous film of 4c to 3 × 10−3 cm2/Vs in a polycrystalline film of 4a. This high mobility remains constant after 3 months at ambient conditions, which proofs the high environmental stability of this class of materials.