Rigid biimidazole ancillary ligands as an avenue to bright deep blue cationic iridium(iii) complexes

Abstract

Herein we report the synthesis and optoelectronic characterisation of three deep blue-emitting cationic iridium complexes, of the form [Ir(dFppy)(2)(N^N)]PF(6), bearing biimidazole-type N^N ancillary ligands (dFppyH = 2-(2,4-difluorophenyl)pyridine). Complex 1 contains the parent biimidazole, biim, while 2 contains a dimethylated analog, dMebiim, and 3 contains an ortho-xylyl-tethered biimidzole, o-xylbiim. We explore a strategy of tethering the biimidazole in order to rigidify the complex and increase the photoluminescent quantum yield, culminating in deep blue (λ(max): 457 nm in MeOH at 298 K) ligand-centered emission with a very high photoluminescent quantum yield of 68% and microsecond emission lifetime. Density functional theory calculations elucidate the origin of such disparate excited state kinetics across this series, especially in light of virtually identical optoelectronic properties observed for these compounds.

Lessons learned in tuning the optoelectronic properties of phosphorescent iridium(iii) complexes

“…49 We hypothesized that the reason for the low Φ PL was due to the presence of an undesired twisting of the biimidazole in the excited triplet state, which would subsequently twist back into planarity upon relaxation to the ground state via a non-radiative pathway. 51 DFT modelling supported this proposal whereby in particular the steric bulk of the methyl groups in 50 enforced a twisting out of planarity of the ancillary ligand (Figure 16). Indeed, both 49 and 50 show blue, ligand-centred emission (λ PL = 464, 490 nm for 49 and 457, 486 nm for 50, in MeOH) but with expectedly low photoluminescence quantum yields (Φ PL = 20% for 49…”

Protonation‐Dependent Luminescence of an Iridium(III) Bibenzimidazole Chromophore

“…In general, the emission range of this type of iridium complexes strongly depends on the type and the functionalization of the ligands used. [49] A pronounced difference in the photophysical behavior of ruthenium and iridium complexes can be seen in the emission properties of the mononuclear complex and the dinuclear compound Ir(tmBBI)Ir. Both the position and the shape of the emission and absorption spectra of the dinuclear complex exhibit features observed in the twofold-deprotonated Na-Ir(tmBBI) and the partially deprotonated Ir(tmBBI)-H. The absorption spectra of the dinuclear and mononuclear complexes are similar in terms of their shape in the visible region, but the emission of Ir(tmBBI)Ir is twice as intense as that of Ir(tmBBI)-H 2 ; therefor, the intensity corresponds with the number of independent iridium metal cores (see Figure 7).…”

Cyclometalated Ir(iii) complexes of deprotonated N-methylbipyridinium ligands: effects of quaternised N centre position on luminescence

“…2.2-2.5 V, which might be formally assigned to an Ir IV/III couple. The relatively high potentials when compared with related complexes of ppy 6,[11][12][13][33][34][35][36][37][38][39]42 are attributable to the presence of the electron-deficient pyridinium units. DFT calculations (see below) show that the C^N ligands contribute to the HOMO significantly.…”

“…[14][15][16][17][18][19][20] Most of the reported complexes fall into two classes; neutral homoleptic Ir III (C^N) 3 and heteroleptic Ir III (C^N) 2 (L-X) (C^N = cyclometalating ligand; L-X = monoanionic ancillary ligand), 8,9,[21][22][23][24][25][26][27][28][29][30][31][32] and monocationic heteroleptic species [Ir III (C^N) 2 (L-L)] + (L-L = neutral bidentate ligand). 7,[11][12][13][33][34][35][36][37][38][39][40] The emission properties of the latter are tuned readily, and various strategies have been adopted. These include changing the degree of π-conjugation in the ligands, 34 incorporating electron-rich S-heterocycles, [41][42][43][44] or most commonly, functionalising C^N or L-L to adjust the energies of the metal and ligand orbitals.…”