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.

About the electronic and photophysical properties of iridium(iii)-pyrazino[2,3-f][1,10]-phenanthroline based complexes for use in electroluminescent devices

“…Indeed, the hole and electron injection ability of the iTMCs play an important role in the luminance and lifetime, limiting the current density in the device; in this case, a low charge transport is responsible for the poor LEC performance 57 . This description is coherent with a good performance of the device due to the low turn-on voltage 20 Table 3. Theoretical studies have related the easier hole injection from the anode into the emissive compound to a small IP, while a larger EA is related to the easy electron injection from the cathode 30,32,58 .…”

“…We tailored the electronic properties of the Ir III -ppl complexes using three different cyclometalating ligands ( Fig. Complexes 1, 2 and 4 have been reported in literature and they serve as a comparison for the obtained results 20,23,24 . These three ligands show different acceptor characters, and they would serve to change the HOMO energy.…”

Facile Synthesis of Luminescent Ir–Pt–Ir Trimetallic Complexes

“…The C^N nitrogen atoms are in a trans orientation relative to each other. The bond length between the Ir atom and the nitrogen atom of the pyridine, Ir(1)−N(3), is 2.184(4) Å, which is slightly longer than those of complexes [Ir(C^N) 2 (2‐picolinate)] (FIrpic, 2.135(3) Å) and [Ir(C^N) 2 (N^N)] + (N^N=neutral diimine, 2.126Å–2.148(4) Å), which also contain pyridine in the ancillary ligand.…”

A Family of IrIII Complexes with High Nonlinear Optical Response and Their Potential Use in Light‐Emitting Devices

“…Ground state geometrical parameters computed in the gas phase were calculated for all four complexes. The calculated structure corresponding to 3 was compared with the experimental crystal structure of this compound that was reported elsewhere 7. The crystal structure and the calculated structure are depicted in Figure 2; selected bond lengths and angles for these structures are compared in Table 1.…”

“…The synthesis of 3 was carried out similar to that described for 1 , except that [Ir(F 2 ‐ppy) 2 (μCl)] 2 was used instead of [Ir(ppy) 2 (μCl)] 2 and the reactants were heated to reflux in a mixture of MeOH/CH 2 Cl 2 (3:1). The complex formed was then purified by silica column chromatography eluting with acetone and then with ethanol, yield 89 mg (78 %) 7…”