Blue luminescent 2-(2′-pyridyl)benzimidazole derivative ligands and their orange luminescent mononuclear and polynuclear organoplatinum(II) complexes

Five new 2-(2'-pyridyl)benzimidazole derivative ligands, 1,4-bis[2-(2'-pyridyl)benzimidazolyl]benzene (1,4-bmb), 4,4'bis[2-(2'-pyridyl)benzimidazolyl]biphenyl (bmbp), 1-bromo-4-[2-(2'-pyridyl)benzimidazolyl]benzene (Brmb), 1,3-bis[2-(2'-pyridyl)benzimidazolyl]benzene (1,3-bmb), and 1,3,5-tris[2-(2'-pyridyl)benzimidazolyl]benzene (tmb), have been synthesized by Ullmann condensation methods. The corresponding mononuclear and polynuclear Pt-11 complexes, Pt-2(114-bmb)Ph-4 (1), Pt-2(bmbp)Ph-4 (2), Pt(Brmb)Ph-2 (3), Pt-2(1,3-bmb)Ph-4 (4), and Pt-3(trnb)Ph-6 (5), have been obtained by the reaction of the appropriate ligand with [PtPh2(SMe2)](n). The structures of the free ligands 1,4-bmb, bmbp, and tmb, as well as the complexes 1-3, were determined by single-crystal X-ray diffraction. All ligands display fluorescent emissions in the purple/blue region of the spectrum at ambient temperature and phosphorescent emissions in the blue/green region at 77 K, which are attributable to ligand-centered pi-->pi* transition. No ligand-based emission was observed for the PT11 complexes 1-5. All Pt-11 complexes display orange/red emissions at 77 K in a frozen solution or in the solid state, attributable to metal-to-ligand charge transfers (MLCT). Variable-temperature H-1 NMR experiments establish that complexes 1, 4, and 5 exist in isomeric forms in solution at ambient temperature due to the hindered rotation of the square PtC2N2 planes in the complexes.