Synthesis,Properties, and π‐Dimer Formation of Oligothiophenes Partially Bearing Orthogonally Fused Fluorene Units

A series of oligothiophenes that incorporate cyclopenta[c]thiophene‐based units bearing spiro‐substituted dialkylfluorene was synthesized. Photophysical measurements indicated that there was no interruption in the conjugation along the oligothiophene backbones, irrespective of the number or position of this unit. Electrochemical measurements showed that the thiophene 7‐mers and 11‐mer exhibit reversible multi‐oxidation waves. The formation of cationic species was clearly observed from UV/Vis/NIR measurements. Furthermore, the UV/Vis/NIR spectra at 223 K under one‐electron oxidation conditions revealed that the unsubstituted thiophene or bithiophene units remained in the absence of intermolecular π–π interactions, whereas the formation of π‐dimeric species was observed for the thiophene 7‐mer containing an unsubstituted terthiophene ( U₃ ) unit. Theoretical calculations indicated that the combination of the U₃ unit and the all‐trans conformation decreased the intermolecular steric repulsion between the fused cyclopentene ring and its facing thiophene, which may contribute to the formation of the dimeric structure.