Ligand-based charge-transfer luminescence in ionic cyclometalated iridium(III) complexes bearing a pyrene-functionalized bipyridine ligand: A joint theoretical and experimental study

Two new heteroleptic iridium(III) complexes [Ir(ppy)₂(pyr ₂bpy)][PF₆] ([1a][PF₆]) and [Ir(dfppy) ₂(pyr₂bpy)][PF₆] ([2a][PF₆]), where Hppy = 2-phenylpyridine, Hdfppy = 2-(3,5-difluorophenyl)pyridine, and pyr ₂bpy = 5,5′-bis(pyren-1-yl)-2,2′-bipyridine, have been synthesized and fully characterized. The single-crystal structures of pyr ₂bpy and the complexes 4{[1a][PF₆]}·2CH ₂Cl₂·9H₂O and [2a][PF ₆]·0.25CH₂Cl₂·H₂O have been determined. The effect of the pyrene substituents on the electronic properties is investigated through a comprehensive photophysical and theoretical study on the two complexes in comparison to reference complexes without substituents on the ancillary ligand ([1][PF₆] and [2][PF ₆]) and by making absorption and luminescence titrations of ligand pyr₂bpy. Both theory and experiment show that the intense and broad band appearing in the 400-500 nm region of the absorption spectra of [1a][PF₆] and [2a][PF₆] is due to intramolecular charge-transfer (ICT) transitions from the pyrene substituents to the bipyridine ligand. [1a][PF₆] and [2a][PF₆] exhibit luminescence bands centered above 650 nm, attributed to a charge-transfer triplet state located on the pyr₂bpy ligand, lying at lower energy than the strongly emitting Ir-ppy→bpy triplet states of the complexes lacking the pyrene fragments. Such luminescence, detected both at room temperature and 77 K, shows that the appendage of luminophoric moieties to luminescent Ir-based centers may further widen the emission tuneability of this exploited class of luminescent materials through purely electrostatic effects exerted on a properly designed N^N ancillary ligand.