Green-emitting iridium(iii) complexes containing sulfanyl- or sulfone-functionalized cyclometallating 2-phenylpyridine ligands

Abstract

A series of [Ir(C^N)2(bpy)][PF6] complexes in which the cyclometallating ligands contain fluoro, sulfane or sulfone groups is reported. The conjugate acids of the C^N ligands in the complexes are 2-(4-fluorophenyl)pyridine (H1), 2-(4-methylsulfonylphenyl)pyridine (H3), 2-(4-(t)butylsulfanylphenyl)pyridine (H4), 2-(4-(t)butylsulfonylphenyl)pyridine (H5), 2-(4-(n)dodecylsulfanylphenyl)pyridine (H6), 2-(4-(n)dodecylsulfonylphenyl)pyridine (H7). The single crystal structures of H3 and H5 are described. [Ir(C^N)2(bpy)][PF6] with C^N = 1, 3, 4, 5 and 7 were prepared from the appropriate [Ir2(C^N)4Cl2] dimer and bpy; the structure of [Ir2(3)4Cl2]·2CH2Cl2 was determined. [Ir(6)2(bpy)][PF6] was prepared by nucleophilic substitution starting from [Ir(1)2(bpy)][PF6]. The [Ir(C^N)2(bpy)][PF6] complexes have been characterized by NMR, IR, absorption and emission spectroscopic and mass spectrometric methods. The single crystal structures of enantiomerically pure Δ-[Ir(1)2(bpy)][PF6] and of rac-4{[Ir(1)2(bpy)][PF6]}·Et2O·2CH2Cl2 are described, and the differences in inter-cation packing in the structures compared. [Ir(1)2(bpy)][PF6], [Ir(4)2(bpy)][PF6] and [Ir(6)2(bpy)][PF6] (fluoro and sulfane substituents) are yellow emitters (λ(em)(max) between 557 and 577 nm), and the room temperature solution emission spectra are broad. The sulfone derivatives [Ir(3)2(bpy)][PF6], [Ir(5)2(bpy)][PF6] and [Ir(7)2(bpy)][PF6] are green emitters and the emission spectra are structured (λ(em)(max) = 493 and 523 to 525 nm). High photoluminescence quantum yields (PLQYs) of 64-74% are observed for the sulfone complexes in degassed solutions. The emission lifetimes for the three complexes containing sulfone substituents are an order of magnitude longer (2.33 to 3.36 μs) than the remaining complexes (0.224 to 0.528 μs). Emission spectra of powdered solid samples have also been recorded; the broad emission bands have values of λ(em)(max) in the range 532 to 558 nm, and PLQYs for the powdered compounds are substantially lower (≤23%) than in solution. Trends in the redox potentials for the [Ir(C^N)2(bpy)][PF6] complexes are in accord with the observed emission behaviour.

Scrutinizing Design Principles toward Efficient, Long‐Term Stable Green Light‐Emitting Electrochemical Cells

“…In consequence, the use of F and/or F‐containing groups as electron withdrawing substituents has been dropped as a design concept for green emitting LECs and the design and synthesis of fluorine‐free Ir‐iTMCs for LEC application has been pursued. Other electron withdrawing (EW) groups such as sulfur containing groups (e.g., sulfanyl and sulfone) as well as other EW C ^ N ligands like methoxy/methyl substituted 2,3′‐bipyridines have been explored in the attempt to get fluorine‐free green emitting Ir‐iTMCs. Unfortunately, these fluorine‐free Ir‐iTMCs did not lead to an improvement in LEC device lifetime, thereby raising the question whether fluorine indeed is the primary cause of the short lifetime of green LEC devices based on fluorinated iTMCs.…”

Synthesis, Properties, and Light-Emitting Electrochemical Cell (LEEC) Device Fabrication of Cationic Ir(III) Complexes Bearing Electron-Withdrawing Groups on the Cyclometallating Ligands

“…17,18 Thus, while the presence of fluorine atoms on the C^N ligands blue-shifts the emission, its deleterious effect on device lifetime mitigates against its inclusion in the emitter design. Apart from fluorination, the other EWGs that have been used to blue-shift the phosphorescence of Ir(III) complexes include the following: sulfonyl (−SO 2 R), 19,20 trifluoromethyl (−CF 3 ), 21−25 pentafluoro-λ 6 -sulfanyl (SF 5 ), 26 and cyclometalated heterocycles such as 2,3′-bipyridinato. 27−33 Sky-blue and deep-blue emitting cationic Ir(III) complexes bearing biimidazole, 34−36 bis(NHC), 37 substituted triazole or tetrazole, 38−40 or pyrazolyl-pyridine 10 as ancillary ligands have also been explored, but challenges still remain regarding efficiencies and stabilities of these emitters in devices, primarily as a consequence of thermal population of metal-centered states ( 3 MC).…”

Colour tuning by the ring roundabout: [Ir(C^N)2(N^N)]+ emitters with sulfonyl-substituted cyclometallating ligands

“…In our series of complexes, however, no such large drop in PLQY is observed. 31 Excited-state lifetimes (determined using second-order ts) in the solid state are much lower than in MeCN solution. At 77 K the excitation wavelength was fixed at 320 nm.…”

“…31 The dimers were isolated in high yields as yellow solids. The cyclometallating ligands are inequivalent due to the asymmetric pzpy ligand; the most characteristic signals are those for H A6 31 In the [Ir(2) 2 (dmpzpy)] + cation, the sulfone groups are twisted with respect to the phenyl rings to which they are bonded with torsion angles of O1-S1-C18-C17 ¼ 27.4(3), O2-S1-C18-C19 ¼ À22.4(3), O3-S2-C30-C31 ¼ 15.0(3) and O4-S2-C30-C29 ¼ À35.1(2) giving rise to short O sulfone /HC phenyl contacts in the range 2.6-2.7Å. In the LC-ESI mass spectrum of a MeOH solution of the dimer, the base peak at m/z 657.1 was assigned to the [Ir (2) 6 ] is shown in Fig.…”

“…In [Ir(C^N) 2 (N^N)] + complexes, the HOMO contains contributions from the Ir(III) dp orbitals and phenyl p-orbitals of the cyclometallated C^N ligands, and the LUMO is usually localized on the N^N ligand. 30,31 However, the complexes in Scheme 1 are green emitters and we consider here further modication to achieve a shi in emission to higher energies. In 2006, Nazeeruddin et al showed that [Ir(ppy) 2 (4,4 0 -(Me 2 N) 2 bpy)] + (Hppy ¼ 2-phenylpyridine, 4,4 0 -(Me 2 N) 2 bpy ¼ 4,4 0 -bis(dimethylamino)-2,2 0 -bipyridine) was an efficient blue-green emitter, 6 and pointed to the potential use of 1-phenylpyrazole or uorinated 2-phenylpyridine ligands to stabilize the HOMO of [Ir(C^N) 2 (N^N)] + .…”