Boranes with Ultra-High Stokes Shift Fluorescence

Abstract

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.

Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9‐Borafluorenes

“…To our surprise, F Mes F Bf and F Xyl F Bf exhibit very long fluorescent lifetimes in solution ( F Mes F Bf : τ =224 ns (hexane); τ =151 ns (CH 2 Cl 2 ); F Xyl F Bf : τ =249 ns (hexane)) as well as in the solid state ( F Mes F Bf : τ =173 ns). Similar fluorescence lifetimes (116–150 ns) of borafluorenes with bulky exo ‐aryl moieties were previously observed by Rupar and co‐workers . This results in exceptionally long natural lifetimes, τ 0 , uncommon for organic molecules, for which fluorescence usually takes place on a nanosecond timescale.…”

“…Steric shielding, which works well for triarylboranes, only stabilizes boroles to a certain degree . Through benzannulation, the stability of boroles can be greatly increased, but the anti‐aromatic character is significantly decreased due to delocalization of π‐electron density over the biphenylene backbone. This leads to a stabilization of the HOMO as well as a destabilization of the LUMO resulting in a larger HOMO–LUMO gap and loss of the characteristic strong color of boroles.…”

Phenylpyridyl‐Fused Boroles: A Unique Coordination Mode and Weak B−N Coordination‐Induced Dual Fluorescence

“…F F = 0.1 and the lifetime is ca. 6nsi nb oth THF and hexane.I tw as previously reported by Marder and coworkers [15] and Rupar and co-workers [31] that dissociation of a4-coordinate borole to a3-coordinate borole in the excited state leads to av ery similar long lifetime as for the 3-coordinate borole.T hus,t he short lifetime of [TipPBB1] 4 suggests that the coordination persists in solution even in the excited state.…”

Phenylpyridyl‐Fused Boroles: A Unique Coordination Mode and Weak B−N Coordination‐Induced Dual Fluorescence

“…6 ns in both THF and hexane. It was previously reported by Marder and co‐workers [15] and Rupar and co‐workers [31] that dissociation of a 4‐coordinate borole to a 3‐coordinate borole in the excited state leads to a very similar long lifetime as for the 3‐coordinate borole. Thus, the short lifetime of [TipPBB1] 4 suggests that the coordination persists in solution even in the excited state.…”