Efficient Near-UV Emitters Based on Cationic Bis-Pincer Iridium(III) Carbene Complexes

Abstract

We report on the photophysical studies of two cationic near-UV emitters based on bis-pincer Ir(III) carbene complexes: [Ir(nBu)(C(NHC)(Me)CC(NHC))2]X, where Ir(nBu)(C(NHC)(Me)CC(NHC)) is (4,6-dimethyl-1,3-phenylene-κC(2))bis(1-butylimidazol-2-ylidene) and X = I(-) or PF6(-)). The compounds are highly emitting in deaerated CH3CN solution with emission maxima at 384 and 406 nm, and photoluminescence quantum yields of 0.41 and 0.38, for [Ir(nBu)(C(NHC)(Me)CC(NHC))2]I and Ir(nBu)(C(NHC)(Me)CC(NHC))2]PF6, respectively. In order to gain deeper understandings into their structural and electronic features, as well as to ascertain the nature of the excited states involved into the electronic absorption processes, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed on the ground and excited states of the closely related complex [Ir(Me)(C(NHC)(Me)CC(NHC))2](+). In the solid state, an emission at low energy is observed (λ(max) = 500 nm) for both complexes. However, the intensity of the emission at high energy versus the intensity of the new emission at low energy is dependent on the nature of counterions. The origin of this emission is not completely clear, but the experimental data point to the formation of trapping sites induced by aggregation processes involving the interaction between the cationic emitter and the counterion.

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

“…For the ancillary ligand, this complex uses N -heterocylic carbenes (NHCs), which are very strongly σ-donating heterocycles that can strongly destabilize the LUMO, invoking a significant blue shift in the emission. The potency of these heterocycles is well known, with near-UV emission having been reported for iridium complexes containing multiple NHCs within the ligand frameworks [43, 58]. Furthermore, the cyclometalating ligand incorporates a nitrogen ring into the 5-position of the cyclometalating ring.…”

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

“…6,7 With EWGs positioned on the C^N ligands, the HOMO is stabilized more than the LUMO, which translates to an increased band gap and, generally, bluer emission. Currently, it is a near-universal strategy to employ C^N ligands like 2-(4,6-difluorophenyl)-pyridine, dFppy, to obtain blue/blue-green-emitting Ir complexes in solution, a subset of which have been used in LEECs.…”

Formation of 5-membered metallacycles at iPrPCPIr by C–H, O–H, and C–CO bond cleavage

“…Pincer iridium species have been shown to form metallacycles by C-H activation of phosphine alkyl substituents in POCOP [23] and PCP [24] systems, by metalation of a second pincer ligand in less sterically demanding systems [25], as well as during the catalytic cleavage of C-O bonds in substituted alkyl aryl ethers [26]. However, while the use of pincer iridium species has been extensively investigated in alkane dehydrogenation and similar reactions [19,22,27], examples of processes or complexes employing metallacycles are limited.…”