Synthesis of self-threading bithiophenes and their structure-property relationships regarding cyclic side-chains with atomic precision

We have recently reported a self-threading polythiophene as a new family of insulated molecular wires. Herein, we focused on the structure-property relationships of the unique three-dimensional architecture of the monomer. We have synthesized nine self-threading bithiophene monomers that have cyclic side-chains of different size and flexibility: i.e., 21-, 22-, 23-, 24-, 26-, and 30-membered rings composed of paraffinic, olefinic, or alkynic chains. To investigate their structure-property relationships, ¹H NMR spectroscopy, UV absorption, and fluorescence spectroscopy measurements were conducted. We found that cyclic side-chains define the movable range of the dihedral angle of the bithiophene backbone, thereby affecting its photophysical properties. Therefore, the ability to design a structure with atomic precision as described herein would lead to the fine-tuning of the electronic properties of insulated molecular wires.