Preparation and Photophysical Properties of Mixed-Ligand Cyclometallated Complexes of Ir(III) with a Dendritic Bipyridine Ligand

Abstract

Complexes [Ir(C^N) 2 (G1-bpy)]PF 6 , where C^N is a cyclometallating ligand derived from 2-(2`-thienyl)pyridine and 2-phenylpyridine, and G1-bpy is a dendritic bipyridine ligand of the first generation, 4,4`-bis[3",5"-bis(benzyloxy)phenylethyl]-2,2`-bipyridine, were prepared and characterized by 1 H NMR, electronic absorption, and emission spectroscopy. The polyether dendritic substituents exert a [soft] effect on the spectral and luminescence properties of the complexes, manifested as slight destabilization of the electronically excited charge-transfer state involving the bipyridine ligand, as compared to the model complexes [Ir(C^N) 2 (bpy)]PF 6 .Bis-and triscyclometallated Ir(III) complexes attract increased researchers' attention owing to the possibility of using them as phosphorescing materials in organic light-emitting devices (OLEDs) [133]. In these devices, singlet and triplet excited states arising from charge recombination are [trapped] with a chromophore in which fast intercombination conversion results in room-temperature electrophosphorescence. One of the factors governing the performance of phosphorescing OLEDs is the spin3orbital coupling causing the singlet3triplet mixing of the excited states of the chromophore. As compared to Ru(II) and Os(II) complexes, Ir(III) complexes (d 6 ) are characterized by higher quantum yields of the luminescence in liquid solutions at room temperature {for example [4], j phos of ( fac-[Ir(ppy) 3 ] is 0.4, where ppy is 2-phenylpyridinate). This is due to efficient intercombination conversion between the singlet and triplet excited states, caused by spin3orbital coupling in the Ir(III) ion.In mixed-ligand biscyclometallated complexes of the type [Ir(C^N) 2 (bpy)]PF 6 [here C^N is 2-(2`-thienyl)pyridinate (tpy) or 2-phenylpyridinate (ppy); bpy is 2,2`-bipyridine], studied in [5,6], the orbital nature of the luminescing state strongly depends on the solvent polarity and matrix hardness. Colombo et al. [5,6] believe that this effect, especially in the case of the phenylpyridinate complex, is due to the fact that the intraligand excited state and the charge-transfer state involving, respectively, the cyclometallating and bipyridine ligands have close energies. Therefore, it is interesting to study the spectral and luminescence properties of the related mixed-ligand biscyclometallated complexes of Ir(III) containing a 4,4`-substituted dendritic bipyridine ligand. The choice of polyether dendritic substituents is governed, on the one hand, by their donor power and, on the other hand, by the possibility of specific solvation of the bipyridine moiety. Both these factors can affect the energy of the excited charge-transfer state involving the bipyridine ligand.

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“…The current is higher in the OLED using the neat G1 level system than those using the dendrimer consisting of the Ir(ppy) 3 core and phenylene-based dendrons 204 indicating that the dendrimer's hole drift mobility was enhanced by the hole-transporting dendrons. Balashev and co-workers 205 synthesized Ir-based complexes [Ir(C^N) 2 (G1-bpy)(PF 6 )], where C^N is a cyclometallating ligand derived from 2-(2 0 -thienyl)pyridine and 2-phenylpyridine, and G1-bpy is 4,4 0 -bis[3 00 ,5 00 -bis(benzyloxy)phenylethyl]-2,2 0 -bipyridine. Their studies demonstrated that the luminescence behavior of the mixed-ligand Ir(III) complexes with a dendritic bipyridine ligand strongly depended on the cyclometalating ligand, temperature, and matrix (effect of the hardness and polarity).…”