Differences of Structures and Electronic Properties in the Triplet States between Dibromo and Dichloro Mononuclear Polypyridine Iridium(III) Complexes

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Emission and electronic properties of eight iridium(III)-centered polypyridine complexes were studied systematically. A crystal structure of [IrBr2(phen)2]PF6 was obtained by X-ray diffraction study, where phen is 1,10-phenanthroline. Those in the triplet states were also determined by DFT calculations to investigate the source of emission spectra. Two triplet structures of [IrX2(bpy)2]+ and [IrX2(phen)2]+ and the transition state of interconversion were obtained with X− = Cl− and Br−. Here, bpy is 2,2′-bipyridine. For the chloro complexes, two triplet species have nonequivalent bpy and phen ligands through pseudo-Jahn–Teller geometric distortion. The dichloro complexes with asymmetric spin density distributions were found to correspond to stronger emission spectra than those for the dibromo compounds. Energies (Eem’s) corresponding to the observed emission wavelengths (λem’s) are in good agreement with the theoretical S0–T1 energy differences (ΔE’s). For [IrBr2(bpy)2]+, Eem = 2.29 eV and ΔE = 2.26 eV. For [IrCl2(bpy)2]+, Eem = 2.29 eV and ΔE = 2.33 eV. For [IrBr2(phen)2]+, Eem = 2.26 eV and ΔE = 2.28 eV. For [IrCl2(phen)2]+, Eem = 2.28 eV and ΔE = 2.34 eV. For all the four complexes, 1A, 1B, 3A, and 3B, DFT calculations indicate Ir–N bond rupture. Tc with a small S0–T1 energy-gap would undergo nonradiative relaxation. Tc indicates a triplet state which is a five-coordinate species with triplet metal-centered character. Coexistence of the triplet state species are discussed in terms of their electronic structures.

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