A comparative study of Ir(iii) complexes with pyrazino[2,3-f][1,10]phenanthroline and pyrazino[2,3-f][4,7]phenanthroline ligands in light-emitting electrochemical cells (LECs)

Abstract

We report the comparative study of the electrochemical and photoluminescent properties of two Ir(III) complexes described as [Ir(F₂ppy)₂(N^N)][PF₆], where the F₂ppy ligand is 2-(2,4-difluorophenyl)pyridine and the N^N ligands are pyrazino[2,3-f][1,10]phenanthroline (ppl) and pyrazino[2,3-f][4,7]phenanthroline (ppz). The complexes were used for the fabrication of light-emitting electrochemical cells (LECs). The structures of the complexes have been corroborated by X-ray crystallography. Theoretical calculations were performed to understand the photophysical behavior of the complexes. Both in solution and solid state, the photoluminescence spectra shows that emission is significantly red-shifted in the [Ir(F₂ppy)₂(ppz)][PF₆] complex compared with the [Ir(F₂ppy)₂(ppl)][PF₆] complex. Besides, the [Ir(F₂ppy)₂(ppl)][PF₆] complex exhibits a higher quantum yield and a longer excited state lifetime than the [Ir(F₂ppy)₂(ppz)][PF₆] complex; therefore, in the last case non-radiative decay is predominant due to the stabilization of LUMO orbital (energy gap law). In the fabrication of LEC devices with the [Ir(F₂ppy)₂(ppl)][PF₆] complex, light emission was obtained with a maximum value of luminance equal to 177 cd m⁻², while in the case of the [Ir(F₂ppy)₂(ppz)][PF₆] complex, no luminance was observed. According to the photophysical data, the performance in LEC devices could be explained by the different position of the nitrogens in the ppl and ppz structural isomers, electronically affecting the complex, and therefore its properties. In addition, from the crystallographic analysis it is possible to note that the [Ir(F₂ppy)₂(ppz)][PF₆] complex shows enhanced intermolecular and intramolecular interactions compared with [Ir(F₂ppy)₂(ppl)][PF₆], and consequently a higher ordering of the molecules in the complex with ppz ligand can be expected. This higher order could favour the quenching processes, and consequently enhance the non-radiative deactivation.