Phosphorescent Cu(I) Complexes of 2-(2‘-pyridylbenzimidazolyl)benzene:  Impact of Phosphine Ancillary Ligands on Electronic and Photophysical Properties of the Cu(I) Complexes

Four mononuclear Cu(I) complexes of 2-(2‘-pyridyl)benzimidazolylbenzene (pbb) with four different ancillary phosphine ligands PPh₃, bis[2-(diphenylphosphino)phenyl]ether (DPEphos), bis(diphenylphosphino)ethane (dppe), and bis(diphenylphosphinomethyl)diphenylborate (DPPMB) have been synthesized. The crystal structures of [Cu(pbb)(PPh₃)₂][BF₄] (1), [Cu(pbb)(dppe)][BF₄] (2), [Cu(pbb)(DPEphos)][BF₄] (3), and the neutral complex [Cu(pbb)(DPPMB)] (4) were determined by single-crystal X-ray diffraction analyses. The impact of the phosphine ligands on the structures of the copper(I) complexes was examined, revealing that the most significant impact of the phosphine ligands is on the P−Cu−P bond angle. The electronic and photophysical properties of the new complexes were examined by using UV−vis, fluorescence, and phosphorescence spectroscopies and electrochemical analysis. All four complexes display a weak MLCT absorption band that varies considerably with the phosphine ligand. At ambient temperature, no emission was observed for any of the complexes in solution. However, when doped into PMMA polymer (20 wt %), at ambient temperature, all four complexes emit light with a color ranging from green to red-orange, depending on the phosphine ligand. The emission of the new copper complexes has an exceptionally long decay lifetime (>200 μs). Ab initio MO calculations established that the lowest electronic transition in the copper(I) complexes is MLCT in nature. The electronic and photophysical properties of the new mononuclear Cu(I) complexes were compared with those of the corresponding polynuclear Cu(I) complexes based on the 2-(2‘-dipyridyl)benzimidazolyl derivative ligands and the previously extensively studied phenanthroline-based Cu(I) complexes.