Emissive Ir(III) complexes bearing thienylamido groups on a 1,10-phenanthroline scaffold

The synthesis, structures and photophysical properties of a series of bis-cyclometallated Ir(III) complexes bearing phenylpyrazole (ppz) cyclometallating ligands and phenanthroline-based ancillary ligands containing thienyl-and bithienylamido groups are reported. All complexes are emissive in solution, while in PMMA films strong emission is observed from the thienylamido substituted complex with no emission from the bithienylamido complex. The bithienylamido substituted complex has an excited state lifetime which is significantly longer than the emission lifetime, attributed to the population of non-equilibrated (MLCT)-M-3 and (LC)-L-3 states in this complex. This represents a rare example of this unusual excited state behaviour. DFT calculations show that the emitting (MLCT)-M-3 state and the dark (LC)-L-3 state on bithiophene are close in energy and that a large change in the triplet state geometry occurs upon excitation that effectively lowers the energy of the (MLCT)-M-3 state below that of the dark (LC)-L-3 state. The low quantum yield of the bithienylamido complex is attributed to a structural rearrangement upon relaxation back to the ground state, opening a non-radiative decay pathway.