Ligand-Based Charge-Transfer Luminescence in Ionic Cyclometalated Iridium(III) Complexes Bearing a Pyrene-Functionalized Bipyridine Ligand: A Joint Theoretical and Experimental Study

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Abstract

Two new heteroleptic iridium(III) complexes [Ir(ppy)(2)(pyr(2)bpy)][PF(6)] ([1a][PF(6)]) and [Ir(dfppy)(2)(pyr(2)bpy)][PF(6)] ([2a][PF(6)]), 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][PF(6)]}·2CH(2)Cl(2)·9H(2)O and [2a][PF(6)]·0.25CH(2)Cl(2)·H(2)O 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][PF(6)] and [2][PF(6)]) 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][PF(6)] and [2a][PF(6)] is due to intramolecular charge-transfer (ICT) transitions from the pyrene substituents to the bipyridine ligand. [1a][PF(6)] and [2a][PF(6)] 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^N ancillary ligand.

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