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 FeCp 2 0/+ ), while 5 has the most negative reduction potential ( E 1/2 red1 = −2.34 V). All compounds are blue emitters with considerable variation of emission energy and efficiencies (e.g., λ em = 446, 402, 395 nm, Φ = ∼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 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 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, 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.