From Monodisperse Thienyl- and Furylborane Oligomers to Polymers: Modulating the Optical Properties through the Hetarene Ratio

The application of our newly developed B-C coupling method by catalytic Si/B exchange is demonstrated for the synthesis of a series of triarylboranes (1), monodisperse thienyl- and furylborane dimers (2) and trimers (9), extended oligomers (3) and polymers (3), as well as mixed (oligo)thienyl-/furylboranes. The structures of 1aa(Tip), 1bb(Tip), and 2bbb(Mes*), determined by X-ray crystallography, reveal largely coplanar hetarene rings and BR3 environments, which are most pronounced in the furylborane species. Photophysical investigations, supported by TD-DFT calculations, revealed pronounced -electron delocalization over the hetarene backbones including the boron centers. With an extended series of derivatives of varying chain lengths available, we were able to determine the effective conjugation lengths (ECL) of poly(thienylborane)s and poly(furylborane)s, which have been reached with the highest-molecular-weight derivatives of our study. Through variation of the furan-to-thiophene ratio, the photophysical properties of these materials are effectively modulated. Significantly, higher furan contents lead to considerably increased fluorescence intensities. Compounds 1aa(Tip), 1bb(Tip), and 3a(Tip) showed the ability to bind fluoride anions. The binding process is signaled by a distinct change in their optical absorption characteristics, thus rendering these materials attractive targets for sensory applications.