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(2)bpy)][PF6] ([1a][PF6]) and [Ir(dfppy)(2)(pyr(2)bpy)][PF6] ([2a][PF6]), where Hppy = 2-phenylpyridine, Hdfppy = 2-(3,5-difluorophenyl)pyridine, and pyr(2)bpy = 5,5'-bis(pyren-1-yl)-2,2'-bipyridine, have been synthesized and fully characterized. The single-crystal structures of pyr(2)bpy and the complexes 4{[1a][PF6])center dot 2CH(2)Cl(2)center dot 9H(2)O and [2a][PF6]center dot 0.25CH(2)Cl(2)center dot 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 pyr(2)bpy. 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 pyr(2)bpy 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 boolean AND N ancillary ligand.