Iridium Metal Complexes Containing N-Heterocyclic Carbene Ligands for Blue-Light-Emitting Electrochemical Cells

Abstract

A new series of cationic blue-emitting, heteroleptic iridium(III)-based metal complexes were systematically synthesized using two 4,6-difluorophenylpyridine ligands as well as one methyl- or n-butyl-substituted bisimidazolium salt carbene-type ligand. In degassed CH(2)Cl(2), all complexes display highly efficient, blue phosphorescence (λ(max) ∼ 452 nm; emission quantum yield ∼ 0.30) at room temperature and also show blue emission in a thin film. The measured photophysical properties of the complexes have been rationalized with the help of quantum-chemical calculations. Because of the high solubility of the complexes, solution-processed devices, light-emitting electrochemical cells (LEECs), were made. The results showed that true blue emission and short turn-on time is achieved when an ionic conductor, tetrabutylammonium trifluoromethanesulfonate, was used as the matrix for the film containing the emitters. These iridium complexes and the described devices are the bluest materials ever reported and the first case of LEECs based on carbene ligands.

Luminescent Ionic Transition‐Metal Complexes for Light‐Emitting Electrochemical Cells

“…[84] It is even possible to avoid this two-step procedure by directly treating the [{(C^N) 2 Ir(m-Cl)} 2 ] dimer with a stoichiometric amount of neutral ligands in the presence of Ag 2 O, to promote the removal of chloride during the reaction. [85] …”

“…If this is the case, as expected from the larger electronegativity of the N atom, the new LUMO is mainly located on the heterocyclic moiety of the (C^N) ligand (e.g., the pyridine ring for ppy-based ligands). In this way, the use of ancillary ligands with relatively inaccessible p* orbitals (e.g., carbenes [85] or isocyanides [92,93] ) leads to low-lying emitting excited states centered on the cyclometalated ligands and the emitting T 1 state exhibits a more ligand-centered character. In such cases, highly structured emissions and strongly increased radiative lifetimes (i.e., small values of k r ) are found.…”

“…[93] In 2007, Chin et al [120] synthesized a series of cationic iridium complexes equipped with even stronger-field ligands, such as CO. Complexes [Ir(ppy) 2 Figure 19) emit at 451-482 nm and 441-443 nm, respectively. Following an analogous approach, Yang et al [85] have recently reported the first LECs based on N-heterocyclic carbene ligands displaying the bluest emission ever reported for a LEC device (456-488 nm). This class of complexes displays true-blue emission both in solution and in neat film, although exhibit a strong PLQY decrease in the solid state because of self-quenching.…”

“…[76] As discussed in Section 3, there are several reasons that justify their selection over Ru II complexes: 1) high PLQY; [19,86,98,156] 2) easily tunable HOMO-LUMO energy gap by independent chemical modifications of the ligands; [29,37,75,81,82,85,91,99,103,105,111,113,116,118,[157][158][159][160] 3) high stability against degradation processes owing to higher ligand-field splitting. [19,86,97,98,156,[161][162][163][164] Over the last eight years, the use of Ir-iTMCs has brought very significant advances in LEC performances, that is, color, efficiency, stability, and turn-on times.…”

“…They also claimed that the addition of cyano groups at the end of the alkyl substituents is responsible of the enhancement of the efficiency. [117] A new approach, based on the use of carbene-type ligands, was recently introduced by Yang et al [85] They demonstrated that the combination of methyl-or n-butylsubstituted bisimidazolium carbene-type ligands together with the dfppyH cyclometalating ligand yields blue LECs Figure 19) to cover the whole visible spectrum. [121] For instance, devices with complex 22 and its fluorinated version (i.e., complex with fluoro groups in the cyclometalating ligand) show blue-green emissions at 544 and 512 nm, respectively.…”