Boron-Doped α-Oligo- and Polyfurans: Highly Luminescent Hybrid Materials, Color-Tunable through the Doping Density

Doping of pi-conjugated polymers or molecular compounds with trivalent boron atoms has recently emerged as a viable strategy to produce new materials with intriguing properties and functions. The combination of boron with furan moieties has been only scarcely explored so far, although the resulting furan-based materials have several notable features, including favorable optoelectronic properties and improved sustainability. Herein, we investigate the doping of alpha-polyfurans with a varying number of boron atoms. A series of poly(oligofuran boranes) and oligofuran-bridged bisboranes have been prepared via microwave-assisted Stille-type catalytic cross-coupling protocols. In the solid-state structures of the molecular compounds, the furan and the borane moieties exhibit a strictly coplanar arrangement; the derivative with a pentafuran bridge forms a dimeric structure in the solid state. All new compounds show considerable absorption and emission features in the visible range that arise from pi-pi* transitions in the oligofurylborane backbone. They are highly luminescent with quantum yields between 89 and 97% for the bisboranes and up to 87% for the difuran-bridged polymer PB2F. Their emission colors can be effectively tuned in the visible range from blue to orange via the length of the oligofuran linker. Spectroelectrochemical investigations on the difuran-bridged bisborane BB2F and polymer PB2F revealed fully reversible stepwise reductions to the respective radical anion (polaron), with absorption features in the near-infrared (NIR) region, and subsequently to a dianion (dipolaron). Overall, the doping of alpha-oligofurans with boron leads to a decrease of the frontier orbital energies, a reduction of the electronic band gap, and the formation of very robust and oxidatively stable materials.

Automated summary

The doping of alpha-oligofurans with boron leads to the formation of new materials with favorable optoelectronic properties, tunable emission colors in the visible range, and robust optical features. Spectroelectrochemical investigations reveal the reversible reduction of these materials to radical anions and subsequently to dianions, demonstrating their oxidatively stable nature.