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

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Abstract
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 these rhodium(III), iridium(III), and ruthenium(II) complexes reveals the influence of the carboxylic groups that affect in different ways the ILCT, MLCT, and LC states.