Boron-Doped Alpha-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.