Structures of polypyridine mononuclear Ir complexes in the ground state and the lowest triplet state

The structures of eight Ir^III centered polypyridine complexes were determined by density-functional-theory calculations. The differences in the optimized geometries between the ground state and the lowest excited triplet state were mainly considered. A crystal structure of IrCl(bpy)(terpy)](PF₆)₂ was also obtained by the X-ray diffraction study, where bpy is 2,2′-bipyridine and terpy is 2,2′:6′,2″-terpyridine. The computed geometries are in good agreement with the experimental ones. Those in the triplet biradical states were determined to evaluate the energy difference between the triplet and the ground states. The resulted values correlate well with the observed emission energies. To investigate the nature of the electronic transition involving the ground and the first excited triplet states, a Mulliken population analysis of the spin densities on the eight complexes was performed. The geometric changes from free tterpy ligand {tterpy = 4′-(4-tolyl)-2,2′:6′,2″-terpyridine} to the Ir^III complexed ligand, and then to triplet biradical were examined. The planarity enhanced the π-π^∗ excitation in the ligand and consequently gave the stable triplet biradical of the complex. It was found that efficient phosphorescence should be impacted by the presence of one coplanar polypyridine ligand.