TD-DFT study of the absorption and emission spectra of blue to red phosphorescent Ir(III) complexes

The electronic structures and spectroscopic properties of a series of blue to red emitted tris-chelated Ir(III) complexes containing two cyclometalating ligands (2-(4,6-difluorophenyl)-4-methoxypyridine (LB), 4,4'-difluoro-5-methyl-2,3-diphenylpyrazine (LG), 5-methyl-2,3-diphenylpyrazine (LY), 2,3-bis(4-fluorophenyl)-quinoxaline (LO), 2,3-diphenylquinoxaline (LR)) and one ancillary ligand (acetylacetonate (acac)) were investigated at the B3LYP/6-31G(d)+LANL2DZ, B3LYP/TZVP, PBE0/6-31G(d)+LANL2DZ and PBEO/TZVP levels. Time-dependent density functional theory (TD-DFT) method with PCM model in CH2Cl2 solution was employed to explore the absorption and emission properties based on the optimized geometries in the ground state. The lowest lying absorptions of complexes were calculated to be of triplet character, essentially corresponding to HOMO -> LUMO and HOMO -> LUMO+1 transitions with metal-to-ligand charge transfer ((MLCT)-M-3) transitions. The phosphorescence of each complex originates from the lowest triplet excited state, which is assigned to the mixing of (MLCT)-M-3 and intraligand charge transfer characters. Finally we found extremely good agreement between theory and experiment, thus we suggest that the computational analysis applied here can open the way to a theoretical and computational strategy for the design and preparation of new red to blue colored phosphorescent iridium compounds.