Boranes with Ultra-High Stokes Shift Fluorescence

The synthesis and optical characterization of 9-(2,6-bis­(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 (λmax = 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 (BtBuMP-BF). Both BMTMP-BF and BtBuMP-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.