Positional Fluorination of Phenylpyridine: Unexpected Electronic Tuning in Bis-Cyclometalated Iridium(III) Acetylacetonate Complexes

We report a systematic study on the effect of positional fluorination of 2-phenylpyridine (Hppy) cyclometalating ligands in neutral iridium(III) complexes of general formula [Ir(Fppy)₂(acac)], where Fppy = 2-(fluorophenyl)pyridine and acac = acetylacetonate. Five complexes (C1-C5) were synthesized, including a nontrivial asymmetric derivative. The archetypal complexes [Ir(ppy)₂(acac)] (C6) and [Ir(dFppy)₂(acac)] (C7) were also prepared, to serve as reference. We demonstrate that fluorination does not simply induce a general blue shift, but that the specific substitution site on the phenyl ring of the cyclometalating ligands tunes HOMO and LUMO levels in a nontrivial fashion. Indeed, para fluorination (as in C3) affords the most red-shifted emission of the series, even compared to the fluorine free C6 complex. On the other hand, meta and ortho substitutions (as in C4 and C2, and C1, respectively) result in progressively bluer emission, in line with electrochemical and computational data. The asymmetric complex (C5) exhibits intermediate properties, reflecting the averaged contribution of the two parent symmetric complexes (C1 and C3). This work establishes positional fluorination as a powerful design tool for tuning the electronic structure and emission color of cyclometalated iridium(III) complexes, with relevance for photocatalytic applications, OLED development, and other photonic technologies.