Catalytic Si/B Exchange Condensation Enables Modular Synthesis of Alkynyl-Spaced Conjugated Oligo- and Polyboranes

Cyclolinear oligo- and polyboranes are an emerging class of π-conjugated materials with potential use in organic (opto)-electronic devices and sensors. The range of boron-linked π-building blocks that can be effectively incorporated, however, is quite limited, since even a slight increase in their steric bulk leads to significant deviations from the planar molecular backbone required for effective charge delocalization. This can be mitigated by inserting an alkynyl spacer between the boron atom and one of these groups. However, such alkynylboranes have been scarcely studied to date, which is due to a lack of efficient synthetic methods. Here, we report on a catalytic silicon/boron exchange condensation process as a versatile synthetic route to alkynyl-spaced monoboranes 5, bisboranes 9, and polyboranes 13. This modular approach enables the variation of both the boron- and alkynyl-bound aryl substituent, while introduction of 2,6-bis(trifluoromethyl)phenyl (FXyl) as the third substituent on boron renders the resulting compounds air- and moisture-stable. The obtained conjugated boranes display a high degree of coplanarity of the central BC3 plane(s) with both the boron- and alkynyl-bound aryl or arylene substituents, respectively. This architecture allows effective conjugation over the entire molecular backbone, which is supported by single-crystal X-ray diffraction analysis, photophysical and electrochemical studies, and time-dependent density functional theory (TD-DFT) calculations. The obtained compounds generally exhibit favorable photophysical properties; some of them show very high quantum yields both in solution and as thin films in a polymer matrix, as well as solid-state emission.