Sensitized Near-Infrared Emission from Ir<sup>III</sup>-Ln<sup>III</sup> (Ln = Nd, Yb, Er) Bimetallic Complexes with a (N<sup>∧</sup>O)(N<sup>∧</sup>O) Bridging Ligand

A (N<sup>∧</sup>O)­(N<sup>∧</sup>O) bridging ligand, 5-bromopyrimidine-2-carboxylic acid (bpmc), was used for connecting Ir<sup>III</sup> and Ln<sup>III</sup> centers to construct the series of d–f bimetallic complexes Ir­(pdt)<sub>2</sub>(μ-bpmc)­Ln­(TTA)<sub>3</sub>, where pdt = 1,3-dimethyl-5-phenyl-1<i>H</i>-[1,2,4]­triazole, TTA = 4,4,4-trifluoro-1-(thiophen-2-yl)-butane-1,3-dionate, and Ln = Nd, Yb, Er, Gd. Crystallographic analyses reveal that there are very short spatial distances between the d–f centers (about 6 Å), and photophysical studies demonstrate the appropriate energy level of the Ir<sup>III</sup> chromophore for sensitization of near-infrared (NIR) lanthanide ions, both of which are two important factors for efficient Ir<sup>III</sup> → Ln<sup>III</sup> energy transfer. The energy transfer rates for Ir<sup>III</sup>-Nd<sup>III</sup>, Ir<sup>III</sup>-Yb<sup>III</sup>, and Ir<sup>III</sup>-Er<sup>III</sup> are calculated to be 3.6 × 10<sup>9</sup>, 2.8 × 10<sup>8</sup>, and 4.0 × 10<sup>8</sup> s<sup>–1</sup>, respectively.