Luminescent Cyclometalated RhIII, IrIII, and (DIP)2RuII Complexes with Carboxylated Bipyridyl Ligands: Synthesis, X-ray Molecular Structure, and Photophysical Properties

Luminescent cyclometalated rhodium(III) and iridium(III) complexes of the general formula [M(ppy)2(N∧N)][PF6], with N∧N = Hcmbpy = 4-carboxy-4′-methyl-2,2′-bipyridine and M = Rh (1), Ir (2) and N∧N = H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine and M = Rh (3), Ir (4), were prepared in high yields and fully characterized. The X-ray molecular structure of the monocarboxylic iridium complex [Ir(ppy)2(Hcmbpy)][PF6] (2) was also determined. The photophysical properties of these compounds were studied and showed that the photoluminescence of rhodium complexes 1 and 3 and iridium complexes 2 and 4 originates from intraligand charge-transfer (ILCT) and metal-to-ligand charge-transfer/ligand-centered MLCT/LC excited states, respectively. For comparison purposes, the mono- and dicarboxylic acid ruthenium complexes [Ru(DIP)2(Hcmbpy)][Cl]2 (5) and [Ru(DIP)2(H2dcbpy)][Cl]2 (6), where DIP = 4,7-diphenyl-1,10-phenanthroline, were also prepared, whose emission is MLCT in nature. Comparison of the photophysical behavior of th...