Impact of the linker on the electronic and luminescent properties of diboryl compounds: Molecules with two BMes2 Groups and the peculiar behavior of l,6-(BMes2)2pyrene

Abstract

To investigate the impact of the linker on the electronic and photophysical properties of diboryl compounds, three new diboryl compounds that contain two BMes2 groups (Mes = mesityl) have been synthesized, including a planar, 1,6-(BMes2)2pyrene (1), a V-shaped bis(p-BMes 2phenyl)diphenylsilane (4), and a U-shaped l,8-bis(p-BMes 2phenyl)naphthalene (5). For comparison, two previously known compounds, p-(BMes2)2benzene (3) and l,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 (E1/2redl = -1.81 V, relative to FeCp20/+), while 5 has the most negative reduction potential (E1/2redl = -2.34 V). All compounds are blue emitters with considerable variation of emission energy and efficiencies (e.g., λem = 446, 402, 395 nm, ℙ = ∼1.0, 0.17, ∼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 l-BMes2pyrene (2) and 3, and with TD-DFT computations on 1 and its fluoride adducts 1F and 1F2, it has been found that the peculiar response of 1 toward fluoride ions is caused by the dominance of pyrene π 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, 1F2, and 5F2 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. © 2008 American Chemical Society.