Ferrocenyl-Benzothienobenzothiophene (BTBT) Conjugate: Synthesis, Crystal Structure, Redox Behavior, and Intramolecular Charge Transfer Properties of One-Electron-Oxidized Species

Benzothienobenzothiophene (BTBT) has attracted increasing attention as an organic semiconducting molecular scaffold. This paper presents the synthesis and characterization of a covalent conjugate of BTBT and ferrocene (FcH). Target conjugate 1 was characterized through ¹H NMR and UV–vis absorption spectroscopies and compared with its parent components, FcH and BTBT. Compound 1 exhibited a highly reversible Fe(III)/Fe(II) redox couple, in accordance with the redox properties inherited from FcH. The Fc-based characteristics of 1 were consistent with DFT calculation data, indicating that the HOMO of 1 is primarily localized on the Fc moiety, with some distribution on the BTBT moiety. Single-crystal X-ray diffraction revealed that 1 possesses molecular packing associated with intermolecular homointeractions (Fc···Fc and BTBT···BTBT) and heterointeractions (Fc···BTBT) in the solid state. In the DFT-optimized structure of the one-electron-oxidized species 1⁺, both the positive charge and LUMO were predominantly localized on the Fc moiety. In solution, 1⁺ exhibited intramolecular charge transfer (ICT) transition absorption from the BTBT to the Fc⁺ moiety. These results provide insights into the functionalization of small-molecule organic semiconducting scaffolds with organometallic fragments, as well as the evaluation of the properties of the resulting conjugates toward molecular electronics.