Emission energy of azole-based ionic iridium(iii) complexes: a theoretical study

Abstract

A theoretical density functional theory study has been performed on different families of cationic cyclometallated Ir(III) complexes with the general formula [Ir(C^N)2(N^N)](+) and azole-based ligands. The goal was to investigate the effect that the number and position of the nitrogen atoms of the azole ring have on the electronic structure and emission wavelength of the complex. The increase in the number of nitrogen atoms changes the relative energy of the HOMO and LUMO levels and leads to a gradual shift in the emission wavelength that can be larger than 100 nm. The direction of the shift however depends on the ligand in which the azole ring is introduced. The emission shifts to bluer wavelengths when the azole forms part of the cyclometallating C^N ligands, whereas it shifts to the red when the 5-membered ring is incorporated into the ancillary N^N ligand. The position of the nitrogen atoms in the azole ring also plays an important role in determining the emission energy. Complexes with phenyl-azole C^N ligands bearing a nitrogen in the azole position to which the phenyl is linked show a markedly blue-shifted emission compared to complexes with the same number of nitrogen atoms in the azole ring and bearing a carbon atom in that position. Therefore, when comparing the emission properties of azole-based [Ir(C^N)2(N^N)](+) complexes, not only the number of nitrogen atoms of the azole but also their position in the ring and the ligand where the azole ring is incorporated should be taken into account.

Highly Stable and Efficient Light-Emitting Electrochemical Cells Based on Cationic Iridium Complexes Bearing Arylazole Ancillary Ligands

“…Both the cyclometalating and the ancillary ligands display coplanarity for their constituting rings, as it is usually found for iTMC complexes possessing five-atoms chelate rings and without bulky substituents in their molecular structure. 32 The largest deviation from coplanarity is indeed found for the pyMebim N^N ligand (7.3°) in [3][PF 6 ] due to the methyl group introduced in the interannular region. The values predicted for the bite angles C−Ir−N (~80.1°) and N−Ir−N (~74.5° for [1] + , [2] + , and [4] + , and ~74.0° for [3] + and [5] + ) are very close to those obtained from the X-Ray structures of [4] [1] + to [5] + .…”

“…Arylazole ligands can therefore be used to fine tuning the photophysical and chemical properties of Ir-iTMCs. 32 In particular, the imidazole ring can furthermore be functionalized through the reactive N−H bond. The electroluminescence properties of Ir-iTMCs bearing arylazole units as cyclometalating ligands have been widely investigated in LECs.…”

Blue-green emitting cationic iridium complexes with 1,3,4-oxadiazole cyclometallating ligands: synthesis, photophysical and electrochemical properties, theoretical investigation and electroluminescent devices

“…6,9,10 To widen the energy gap and thus increase the emission energy of the complex, two strategies have been proposed, i.e., i) to stabilize the highest occupied molecular orbital (HOMO) residing on the phenyl ring of ppy and the iridium ion, and ii) to destabilize the lowest unoccupied molecular orbitals (LUMO) delocalized over bpy. 30 In addition to these techniques, an alternative approach is to replace the pyridine rings of ppy-type CΛN ligands with 5-membered nitrogen-rich heterocycles, such as triazole or tetrazole, [32][33][34][35] which has been shown to effectively enlarge the energy gap of the complex via stabilizing the HOMO. 30 In addition to these techniques, an alternative approach is to replace the pyridine rings of ppy-type CΛN ligands with 5-membered nitrogen-rich heterocycles, such as triazole or tetrazole, [32][33][34][35] which has been shown to effectively enlarge the energy gap of the complex via stabilizing the HOMO.…”

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

“…Note that we use the 3 MLCT term to characterize the T 1 state, although the reader must interpret that a ligand-to-metal charge transfer occurs in the deactivation process; the latter is used to be in agree with the literature discussions 44 . Moreover, the increased multiconfigurational character of the T 1 state in the complexes 8-9 is associated to its proximity to high excited states as was observed for related Ir III complexes 33 . The 3 LC state of the ppy ligand was computed to 3.14 eV above its ground state; taking into account that 7-9 shows their T 1 states at least 2.90 eV above their ground states, a mixing with the nearing 3 LC state of the ppl ligand can be expected in these cases as noted below.…”