Two are not always better than one: ligand optimisation for long-living light-emitting electrochemical cells

Abstract

The complex [Ir(ppy) 2 (dpbpy)] [PF 6 ] (Hppy = 2-phenylpyridine, dpbpy = 6,6 0 -diphenyl-2,2 0 -bipyridine) has been prepared and evaluated as an electroluminescent component for light-emitting electrochemical cells (LECs); the complex exhibits two intramolecular face-to-face p-stacking interactions and long-lived LECs have been constructed; the device characteristics are not significantly improved in comparison to analogous LECs with 6-phenyl-2,2 0 -bipyridine.Light-emitting electrochemical cells (LECs) are a minimalist derivative of organic light-emitting devices (OLEDs) and in their simplest form consist of a film of an ionic transition metal complex placed between two electrodes. 1,2 LECs offer considerable technological advantages over OLEDs as they require a less reactive cathode material (Al instead of Ca or Mg) because the device is no longer dependent upon the work function of the electrode and hence do not require stringent protection from environmental oxygen or water. The disadvantage of LECs is the short operating lifetime, in the order of hours to days, compared to OLEDs. [3][4][5] We have recently reported the use of intra-and intermolecular face-to-face p-stacking for the stabilisation of the ground and excited state of electroluminescent iridium complexes and shown that this leads to exceptionally long-living LEC devices. 6,7 The long lifetimes of these devices establish LECs as a viable alternative to OLED technology. In [Ir(ppy)(pbpy)] + (Hppy = 2-phenylpyridine, pbpy = 6-phenyl-2,2 0 -bipyridine) the pendant phenyl group of the pbpy ligand forms a face-to-face p-stack with the metallated ring of a ppy ligand (3.2-3.5 Å ). This interaction minimises the expansion of the metal-ligand bonds in the excited state and precludes the attack by water and other nucleophiles resulting in the long observed lifetimes. We concluded that analogous complexes with 6,6 0 -diphenyl-2,2 0 -bipyridine would have an even greater stabilisation of the excited state as the two pendant phenyl groups would stack with different ppy ligands giving a very ''tight'' complex.The ligand 6,60 -diphenyl-2,2 0 -bipyridine, dpbpy, was obtained from the reaction of four equivalents of phenyllithium with 2,2 0 -bipyridine in THF followed by oxidation of the intermediate tetrahydro-species with MnO 2 according to the general procedure of Sauvage et al. ) and the complex is luminescent exhibiting an emission in MeCN solution with a maximum at 595 nm with a lifetime t = 0.6 ms and a quantum yield (PLQE) of 3%.We have determined the structure of [Ir(ppy) 2 (dpbpy)][PF 6 ]z and the [Ir(ppy) 2 (dpbpy)] + cation present in the lattice is shown in Fig. 1a. The Ir-N(ppy) (2.0504(17), 2.0341(17) Å ) and Ir-C(ppy) distances (2.0120 (18) . We stress here that the intramolecular p-stacking is a direct and inevitable consequence of the ligand structure and will be present in the solid state, solution and thin film phases. To summarise, as observed from the crystal structure, the use of the dpbpy ligand for optimising the

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

“…LECs based on 31 with a small amount of IL [BMIM] + [PF 6 ] À showed t 1/2 = 1290 h and E tot = 13.6 J at a luminance value of 100 cd m À2 (Table 2). [99] Notably, this stability can be enhanced using a pre-biasing at high voltages, which leads to efficient (10 lm W À1 ) and stable (t 1/2 = 3000 h, E tot = 73 J) LECs with luminance levels of 200 cd m À2 . [168] LECs made with the reference complex [Ir(ppy) 2 À exhibit much poorer performance (Table 2), highlighting the relevance of the "locked" structure for device stability.…”

“…[99] Because HOMO and LUMO orbitals are usually located on different ligands (except in the case of high-field-strength ancillary ligands, for which the HOMO and the LUMO are located on different moieties of the same cyclometalated ligand), the HOMO-LUMO energy gap can be fine-tuned by playing independently on the cyclometalated (HOMO) or on the ancillary ligands (LUMO), or even on both of them. As a rule of thumb, it can be said that 1) electron-withdrawing substituents on the C^N ligands reduce the s donation to the metal, decrease the electron density on the iridium ion, and lead to a stabilization of the HOMO; and 2) electrondonating substituents on the ancillary ligand or use of intrinsically electron-rich ligands bring about a destabilization of the LUMO.…”

“…The same group also showed that LECs with lifetimes of thousands of hours are obtained when using intramolecularly caged iridium(III) complexes ( Figure 25 and Table 2). [99,102,168,176] The cage formation effect occurs through an intramolecular p-p interaction between a pendant phenyl group attached to the N^N bpy ligand and the phenyl of the C^N cyclometalating ppy moiety. Complex 31 ( Figure 25) keeps its rigid cage conformation in the ground and excited states acquiring high stability against reactions with water.…”

“…[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.…”

Luminescent Iridium Complexes Used in Light-Emitting Electrochemical Cells (LEECs)

“…Although the intramolecular p-stack is an effective strategy for increasing device stability, it does have limits: complex 26, with two incorporated p-stacking phenyl rings on the ancillary ligand results in poorer device performance compared to that with 25 [68]. Devices based on 25 and 26 both show long lifetimes (t 1/2 = 1300 h for 25 and 26).…”