Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III)
complexes
Ir­(2,4-di-X-phenyl-pyridine)2(picolinate) (X = H, F, Cl,
Br) HIrPic, FIrPic, ClIrPic, and BrIrPic has been synthesized and characterized
by NMR, X-ray crystallography, UV–vis absorption and emission
spectroscopy, and electrochemical methods. The addition of halogen
substituents results in the emission being localized on the main cyclometalated
ligand. In addition, halogen substitution induces a blue shift of
the emission maxima, especially in the case of the fluoro-based analogue
but less pronounced for chlorine and bromine substituents. Supported
by ground and excited state theoretical calculations, we rationalized
this effect in a simple manner by taking into account the σp
and σm Hammett constants on both the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
energy levels. Furthermore, in comparison with FIrPic and ClIrPic, the impact of the large bromine atom remarkably
decreases the photoluminescence quantum yield of BrIrPic and switches the corresponding lifetime from mono to biexponential
decay. We performed theoretical calculations based on linear-response
time-dependent density functional theory (LR-TDDFT) including spin–orbit
coupling (SOC), and unrestricted DFT (U-DFT) to obtain information
about the absorption and emission processes and to gain insight into
the reasons behind this remarkable change in photophysical properties
along the homologous series of complexes. According to theoretical
geometries for the lowest triplet state, the large halogen substituents
contribute to sizable distortions of specific phenylpyridine ligands
for ClIrPic and BrIrPic, which are likely
to play a role in the emissive and nonradiative properties when coupled
with the heavy-atom effect.