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.