Insights on the Synthesis, Structures, and Photophysical Properties of Step-Conjugated Bi(dithienophosphole)s

Step conjugation allows 3D electronic communication between two π-conjugated subunits connected through a σ-bond. A pioneering class of these materials are bi(dithienophosphole)s, first synthesized and reported by us via a Lewis-acid-mediated synthetic pathway in 2018. We have now extended this methodology toward the synthesis of a series of electronically modified bi(dithienophosphole)s with either trimethylsilyl or trimethylsilylethynyl substituents at their 2,6-positions, a TIPS-substituted thiazole species, as well as a species with a benzothiophene-extended core that revealed a few surprising outcomes. The first is the elimination of Lewis acid mediation when employing electron-donating scaffolds. This mediator-free synthetic route proceeds via a negatively charged phospholide intermediate. The mechanism for this reaction, with intermolecular Brønsted acid-base chemistry as the key step instead of Lewis acid activation, was successfully modeled computationally. The modification with bulky substituents leads to twisted conformations that minimize intermolecular interactions, thus improving the solution-processability of these materials. Additionally, the benzothiophene-extended species shows a 6-fold increase in molar absorptivity and quantum yield when compared to the parent system. This species also shows green emission, in contrast to the blue emission of the other species in this series. All these refinements are possible while concurrently maintaining step conjugation, highlighting the intriguing inherent features of this unique molecular scaffold.