Impact of the Linker on the Electronic and Luminescent Properties of Diboryl Compounds: Molecules with Two BMes2 Groups and the Peculiar Behavior of 1,6-(BMes2)2pyrene

To investigate the impact of the linker on the electronic and photophysical properties of diboryl compounds, three new diboryl compounds that contain two BMes(2) groups (Mes = mesityl) have been synthesized, including a planar 1,6-(BMes(2))(2)pyrene (1), a V-shaped bis(p-BMes(2)phenyl)diphenylsilane (4), and a U-shaped 1,8-bis(p-BMes(2)phenyl)naphthalene (5). For comparison, two previously known compounds, p-(BMes(2))(2)benzene (3) and 1,8-bis(p-BMes(2)-biphenyl)naphthalene (6), were also investigated. The aromatic linkers in these molecules have been found to have a dramatic impact on the electron-accepting ability and Lewis acidity of the diboryl compounds through their distinct steric and electronic properties. Compound 1 has the most positive reduction potential (E-1/2(red1) = -1.81 V, relative to FeCp20/+), while 5 has the most negative reduction potential (E-1/2(red1) = -2.34 V). All compounds are blue emit ters with considerable variation of emission energy and efficiencies (e.g., lambda(em) = 446, 402, 395 nm, Phi = similar to 1.0, 0.17, similar to 1.0 for 1, 4, and 5, respectively), and each displays a distinct and selective response toward fluoride ions. Upon addition of fluoride ions, compound 1 displays an unusual red shift and an on-off response in both absorption and fluorescent spectra. By comparing the behavior of 1 to that of the monoboryl compound 1-BMes(2)pyrene (2) and 3, and with TD-DFT computations on 1 and its fluoride adducts 1F and 1F(2), it has been found that the peculiar response of 1 toward fluoride ions is caused by the dominance of pyrene pi orbitals at the HOMO level of 1F and the relatively low-energy charge transfer from the pyrene ring to the three-coordinate boron center in 1F. The crystal structures of 2, 4, 1F(2), and 5F(2) were determined by X-ray diffraction analyses. The potential use of compound 1 as either a blue emitter or a bifunctional emitter in OLEDs has been demonstrated by the successful fabrication of double- and triple-layer electroluminescent devices.