Synthesis and Luminescent Properties of Lewis Base-Appended Borafluorenes

A series of Lewis base adducts of
9-bromo-9-borafluorene (BrBFl–LB, LB = IPr, IPrCH2, PPh3, and PCy3), parent borafluorenes (HBFl–IPr
and HBFl–IPrCH2), and the bisadduct [(DMAP)2BFl]Br were prepared and structurally characterized (IPr =
[(HCNDipp)2C:], IPrCH2 = [(HCNDipp)2CCH2], Dipp = 2,6-i-Pr2C6H3, and DMAP = N,N-dimethylaminopyridine). The adducts BrBFl–IPr,
BrBFl–PPh3, BrBFl–PCy3, [(DMAP)2BFl]­Br, BrBFl–IPrCH2, and HBFl–IPrCH2 were found to exhibit bright blue luminescence with low to
moderately high quantum efficiencies (19 to 63%). Selective irradiation
at different excitation wavelengths revealed the presence of two distinct
emission processes in the adducts BrBFl–LB, leading to a ligand-independent,
presumably borafluorene-based, blue light emission at 435 nm and another
less intense emission band in the ultraviolet region (315–324
nm); [(DMAP)2BFl]Br exhibits an emission profile that tails
into the visible region. Time-dependent density functional theory
studies are also included for representative borafluorene adducts.
With a judicious choice of functional groups at boron, one can envisage
the future generation of a whole library of 4-coordinate borafluorene-based
luminogens that complement the efficient light-emitting behavior known
for the widely studied boron-dipyrromethene
analogues.