The potential energy surfaces associated with the triplet electronic state of the Ir(ppz)3 and Ir(ppy)3 cyclometalated complexes, respectively, are calculated using density functional theory [where ppz = 1-phenylpyrazolyl-N,C2′, ppy = 2-phenylpyridyl]. Both surfaces present a metal-to-ligand charge-transfer energy minimum and a ligand-field energy minimum, connected by a chemical path involving only the rotation of one pyrazolyl or pyridyl group. Unlike Ir(ppy)3, the lowest energy minimum of Ir(ppz)3 is the ligand-field state.

Substitution effects on the photoluminescence and electrochemical properties and the photochemical stability of tris-(phenylbenzimidazolinato)Ir III complexes were investigated. Facial and meridional isomers of a series of complexes having the general structure of Ir(C C) 3 , where (C C) is (4-Rphenyl)benzimidazolinato (R = H, CF 3 , CN, OCH 3 ), were prepared. They are abbreviated to Ir(pmb) 3 (1), Ir-(CF 3 pmb) 3 (2), Ir(CNpmb) 3 (3), and Ir(Opmb) 3 (4), respectively. Electron-donating or -withdrawing groups on the phenyl ring lead to both higher emission quantum yields and longer emission lifetimes compared to those of 1 in each mer and fac series. Particularly, the emission quantum yields were high in the cases of 2a, 3a, 3b, and 4a. No photochemical isomerization was observed in both fac-(a) and mer (b)

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MethodCalculations were carried out using unrestricted density functional theory (DFT). Unrestricted DFT was used to investigate several cyclometalated complexes and the validity of the results obtained by the calculations were confirmed experimentally [2,3]. The unrestricted DFT permits the analysis of the triplet state properties and the calculation of energy variation as a function of the geometric changes, i.e., the potential energy surface (PES) [4]. During the searches for minima structures, the geometries were fully optimized without geometric constraints. We performed two sets of calculations to control the accuracy of the model with details of the methods. The first set of calculations used a B3LYP functional, 6-31G basis sets and a LANL2DZ core potential. In the second set of calculations, a polarization function was added to the atom of the ligands and a SDD core potential was used. Both sets gave similar results. The data obtained with polarization functions were used for the analysis. For each system, the ground state facial isomer is taken as the energy reference, i.e., E o = zero. The calculations were performed using the Gaussian package [5]. Fig. 3 shows the frontier orbitals of the ground state of Ir(N-methyl-pim) 3 facial isomer. Performing an excitation from the HOMO to the LUMO would correspond to a MLCT transition. On the triplet PES, the precise assignation of the MLCT character is complicated by the presence of a strong metal-ligand mixing in the orbitals, nonetheless, the calculations confirm that the lowest excited state corresponds to a triplet MLCT. The results obtained for Ir(N-phenyl-pim) 3 , are similar. On the triplet PES associated with Ir(N-methyl-pim) 3 , the isomer of type AA, has an energy E AA of +69.2 kcal/mol. and the isomer of type A has an energy E A of + 64.6 kcal/mol. The calculations predict therefore that the departure of the methyl group stabilizes the complex. On the triplet PES associated with Ir(N-phenyl-pim) 3 , the isomer of type AA, has an energy E AA of +59.3 kcal/mol. and the isomer of type A has an energy E A of + 72.3 kcal/mol. In contrast with the methyl case, the departure of the phenyl group destabilizes largely the complex. The calculations show therefore that the relative stability of the isomers of type A and AA depends critically on the nature of the group R bonded to the nitrogen atom. The aromatic nature of the group R is essential to stabilize the isomer of type AA It provides a delocalization of the π electrons. Results and discussionIn conclusion, the DFT calculations predict that the lowest excited state of Ir(N-R-pim) 3 complexes is a triplet MLCT. In this configuration, the aromatic character of the complex is not preserved. The departure of the radical R together with the rotation of the nitrogen atom orbitals permits the recovery of an aromatic pattern of six  π electrons on the imidazole moiety. It explains that the departure of one methyl group stabilizes the Ir(N-methyl-pim) 3 complex. Selecting an aromatic R group should prevent the ...