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.