Tailoring the photoluminescence properties of ionic iridium complexes

Density functional theory/time-dependent density functional theory (DFT/TD-DFT) calculations were performed to investigate the structural, electronic, and optical properties of ionic Ir complexes with several different substituents on the cyclometalated ligand. Geometric parameters, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap, and Mulliken charge on different parts of the molecule were obtained and correlated to the calculated emission and absorption energies. We also discuss the influence of the position of fluoro-substituent on the spectroscopic properties of Ir complexes. As a major trend, the investigated complexes exhibit band shifts that correlate with the electron-withdrawing nature of the ligand substituent. Our results also show that the lowest emission wavelength is observed at ortho position with respect to the coordinating carbon. The different variations observed are discussed in terms of emissive states and, more especially, in terms of the mixture of ligand-ligand charge-transfer (LLCT) and metal-ligand charge-transfer (MLCT) states.