Inter-Ligand Energy Transfer and Related Emission Change in the Cyclometalated Heteroleptic Iridium Complex:  Facile and Efficient Color Tuning over the Whole Visible Range by the Ancillary Ligand Structure

We report a novel color tuning methodology in the electrophosphorescent iridium complex by substituting one cyclometalating ligand to an ancillary ligand as an emitting center. Highly efficient exothermic inter-ligand energy transfer (ILET) from the MLCT3 state produced between iridium and cyclometalating 2-(2,4-difluorophenyl)pyridine to LX3 state of the ancillary ligand offers a chance to access a wider range of color from sky blue (478 nm) to red (666 nm). Characteristic shapes of photoluminescence spectra, strong solvatochromic phenomena as well as DFT calculations gave evidences for this exothermic ILET.