The synthesis and optical characterization of 9-(2,6bis(methoxymethyl)phenyl) borafluorene (BMMP-BF) are reported. NMR spectroscopic data and single-crystal X-ray diffraction data of BMMP-BF show intramolecular chelation by the 2,6-bis(methoxymethyl)phenyl moiety via a boron−oxygen dative bond. The optical spectra of BMMP-BF are unusual in that absorption is entirely in the UV region (λ max = 284 nm), yet fluorescence occurs at 536 nm. This equates to a Stokes shift of 2.05 eV (16 500 cm −1 ) and is among the highest Stokes shifts ever reported for a small molecule. Density functional theory (DFT) calculations show that the boron−oxygen dative bond in BMMP-BF is ruptured in the excited state and that emission occurs from a trigonal planar boron geometry. This bond cleavage and the concurrent planarization of the boron center are responsible for the high Stokes shift. Two borafluorenes related to BMMP-BF were also examined: 9-(2,6-bis((methylthio)methyl)phenyl) borafluorene (BMTMP-BF) and 9-(2,6-bis(tert-butoxymethyl)phenyl) borafluorene (B t BuMP-BF). Both BMTMP-BF and B t BuMP-BF have optical properties similar to those of BMMP-BF with remarkably large Stokes shifts. Finally, BMMP-BF-(2T) 2 , which possesses bithiophene moieties on the 2 and 7 positions of a BMMP-BF core, was also synthesized and studied. The absorption spectrum of BMMP-BF-(2T) 2 is red-shifted compared to BMMP-BF. BMMP-BF-(2T) 2 was found to exhibit dual emissions rather than the single, high Stokes shift emission of BMMP-BF. DFT calculations suggest that the dual emissions of BMMP-BF-(2T) 2 arise due to radiative relaxation from two different structures in the excited state.

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 Ph 2 Si, Ph 2 Ge, and PhP(O), were synthesized. Each bridged difuran has Me 3 Si 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 phosphorusbridged 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.

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