New phosphorescent polynuclear Cu(I) compounds based on linear and star-shaped 2-(2′-pyridyl)benzimidazolyl derivatives:: Syntheses, structures, luminescence, and electroluminescence

Four dinuclear and trinuclear Cu(1) complexes that contain 2-(2'-pyridyl)benzimidazolyl derivative ligands including 1,4-bis[2-(2'-pyridyl)benzimidazolyl]benzene (1,4-bmb), 1,3-bis[2-(2'-pyridyl)benzimidazolyl]benzene (1,3-bmb), 1,3,5-tris[2-(2'-pyridyl)benzimidazolyl]benzene (tmb), and 4,4'-bis[2-(2'-pyridyl)benzimidazolyl]biphenyl (bmbp) have been 4-bmb)(PPh3)(4)][BF4](2) (1), [Cu-2(1,3-bmb)(PPh3)(4)][BF4](2) synthesized. The formulas of these complexes are [Cu-2( (2), [Cu-3(tmb)(PPh3)(6)][BF4](3) (3), and [Cu-2(bmbp)(PPh3)(4)][BF4](2) (4), respectively. The crystal structures of 2-4 have been determined by single-crystal X-ray diffraction analyses. The Cu(1) ions in the complexes have a distorted tetrahedral geometry. For 3, two structural isomers (syn and anti) resulted from two different orientations of the three 2-(2'-pyridyl) benzimidazolyl chelating units were observed in the crystal lattice, Variable-temperature H-1 NMR experiments established the presence of syn and anti isomers for 1-3 in solution which interconvert at ambient temperature. Complexes 1-4 have a weak MLCT absorption band in the 350-450 rim region and display a yellow-orange emission when irradiated by UV light. One unexpected finding is that the yellow-orange emission of complexes 1-4 has a very long decay lifetime (similar to 200 mu s) at 77 K. An electroluminescent (EL) device using 4 as the emitter and PVK as the host was fabricated. However, the long decay lifetime of the copper complexes may limit their applications as phosphorescent emitters in EL devices.