Merging Pyrrole with Boron into Versatile Di(2-pyrryl)borane Building Blocks: π-Extension, Polymerization, and
Daniel Göbel1, Andreas Helbig1, Alexandra Friedrich1
1Julius-Maximilians-Universität Würzburg, Institute of Inorganic Chemistry and Institute for Sustainable Chemistry & Catalysis with Boron (ICB), Am Hubland, 97074, Würzburg, Germany.
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Conjugated di(hetaryl)boranes-particularly those of thiophene, but also of furan-have recently proven to be extremely useful building blocks for organic optoelectronic materials. The importance of the combination of boron with pyrrole, on the other hand, becomes particularly apparent when considering the well-known, versatile BODIPY dyes, in which the boron atom is tetracoordinated. However, synergistic effects of combining pyrrole with tricoordinate boron have been hardly exploited to date, which is probably due to synthetic challenges associated with the special reactivity of this five-membered heterocycle. Herein, a high-yield, gram-scale synthesis of two di(2-pyrryl)boranes is presented, including the previously elusive unprotected NH derivative. Their versatility is demonstrated by their application as building blocks for π-extended tetrahetarene boranes, a polymer, and a zirconium(IV) complex. The crystallographically determined molecular structures of both di(2-pyrryl)boranes and the Zr complex show largely planar dipyrrylborane moieties. Compared to their thiophene and furan congeners, the new conjugated boranes show bathochromically shifted absorption bands and more intense fluorescence emission, with solvatochromic behavior, aggregation-induced emission enhancement (AIEE), and solid-state emission. Time-dependent DFT calculations reveal that this is due to the population of twisted intramolecular charge transfer (TICT) states, giving rise to dual emission in the unprotected triarylborane.
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