Understanding the role of cyclometalating ligand regiochemistry on the photophysics of charged heteroleptic iridium(III) complexes

The ability to tune the emission energy of cyclometalated iridium(III) complexes through ligand modification is well understood. Changing the electronic profile of functional groups affords complexes that exhibit emission spanning the visible spectrum, but the regiochemistry of substitution, and its effect on photophysical properties beyond emission energy, remains less explored. In this paper we investigate the substitution pattern of an electron-donating (-OCH3) and electron-withdrawing (-CF3) functional group around the phenyl-ring of 2-phenylpyridine cyclometalating ligands. A family of heteroleptic [Ir(C∧N)2(phen)]PF6 complexes, containing C∧N as the cyclometalating and 1,10-phenanthroline (phen) as the ancillary ligand, were synthesized and characterized using a range of experimental and theoretical techniques. Optical spectroscopy revealed that iridium(III) complex photophysics were most significantly affected when substitution occurred at the 5 phenyl-ring position, para relative to the orthometalated carbon atom. Subtle differences were also observed when substitution occurred at either the 4 or the 6 phenyl-ring position, despite substitution at either site being meta relative to the organometallic bond.