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.