Bridged Difurans: Stabilizing Furan with p‑Block Elements

Thiophene is one of the most commonly used moieties in conjugated materials, whereas its oxygen congener, furan, is much less frequently encountered due to the susceptibility of furan toward light- and oxygen-induced degradation. In an effort to stabilize furans, three difurans, bridged by Ph2Si, Ph2Ge, and PhP­(O), were synthesized. Each bridged difuran has Me3Si groups at the 2- and 6-positions. All of the bridged difurans were found to be stable under ambient conditions. Since it is known that electron-deficient furans exhibit improved stability, the stability of the bridged difurans is attributed, in part, to the p-block elements, which lower the LUMO levels of the molecules through σ*−π* conjugation. Single-crystal X-ray diffraction studies of the phosphorus-bridged species showed that the structure is similar to that of the known thiophene analogue, but with bond lengths consistent with the reduced aromaticity of furan. All of the p-block bridged difurans show strong absorption in the UV region and intense emissions with quantum yields ranging from 0.30 to 0.80. DFT calculations indicate that the frontier molecular orbitals of bridged difurans are slightly higher than those of their thiophene counterparts. In order to demonstrate the synthetic potential of bridged difurans, the phosphorus-bridged difuran was converted to its 2,6-dibromo derivative, which in turn was copolymerized with (9,9-dioctyl-9H-fluorene-2,7-diyl)­bis­(trimethylstannane) to form the stable conjugated polymer PDFP-F. The phosphorus-bridged difuran was also reduced from its phosphine oxide form and then quaternized with methyl triflate to produce the corresponding air-stable phosphonium salt.