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(pyr2bpy)][PF6] ([1a][PF6]) and [Ir(dfppy)2(pyr2bpy)][PF6] ([2a][PF6]), where Hppy = 2-phenylpyridine, Hdfppy = 2-(3,5-difluorophenyl)pyridine, and pyr2bpy = 5,5′-bis(pyren-1-yl)-2,2′-bipyridine, have been synthesized and fully characterized. The single-crystal structures of pyr2bpy and the complexes 4{[1a][PF6]}·2CH2Cl2·9H2O and [2a][PF6]·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][PF6]) 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.