Ionic Iridium Complexes Research Articles

Phosphorescence Color Tuning of Ionic Iridium Complexes by Manipulating Excited State Properties

The new ionic iridium complexes, [Ir(bpy)2(CN)2]− and [Ir(dtbbpy)2(CN)2]+, were prepared and their luminescence properties were investigated, where bpy, dtbbpy and CN represent 2,2′-dipyridyl, 4,4′-di-tert-butyl-2,2′-dipyridyl and cyanide, respectively. We expected that introduction of the electron-withdrawing nonchromophoric ligand, -CN, to an Ir complex might effectively lower the HOMO level and lead to the blue-shifted emission compared to [Ir(bpy)3]3+. The main ligand with a sterically bulky group was also considered to orient the binding between the main ligand and iridium center and to improve the solubility. Thus, the new ionic complex, [Ir(dtbbpy)2(CN)2]+, was synthesized and its properties was compared with the other complex, [Ir(bpy)2(CN)2]−. The maximum PL peak of [Ir(dtbbpy)2(CN)2]+ appeared at 491 nm, which was hypsochromically shifted, compared with 528 and 566 nm of [Ir(bpy)3]3+ and [Ir(bpy)2(CN)2]−, respectively. Investigation of their electrochemical properties led us to inferring that the cyanide ligand could effectively modulate the HOMO level of its iridium complex for the blue emission. Also, the introduction of the bulky alkyl group in the main ligand might effectively cause direct bindings between both Ns of biphenyl and the iridium center, which also might result in the relative blue-shifted emission in comparison with PL peak of [Ir(bpy)2(CN)2]− in which biphenyl was C- and N-bound to the Ir center.

Cationic Heteroleptic Cyclometalated IridiumIII Complexes Containing Phenyl-Triazole and Triazole-Pyridine Clicked Ligands

Novel heteroleptic iridium complexes containing the 1-substituted-4-phenyl-1H-1,2,3-triazole (phtl) cyclometalating ligand have been synthesized. The 3+2 Huisgen dipolar cycloaddition method (‘click’ chemistry) was utilized to prepare a class of bidentate ligands (phtl) bearing different substituents on the triazole moiety. By using various ligands (phtl-R1 and pytl-R2) (R1=adamantane, methyl and R2=adamantane, methyl, β-cyclodextrin, ursodeoxycholic acid), we prepared a small library of new luminescent ionic iridium complexes [Ir(phtr-R1)2(pytl-R2)]Cl and report on their photophysical properties. The flexibility of the clicking approach allows a straightforward control on the chemical-physical properties of the complexes by varying the nature of the substituent on the ligand.

Ionic Iridium(III) Complexes with Bulky Side Groups for Use in Light Emitting Cells: Reduction of Concentration Quenching

AbstractHere, the photophysics and performance of single‐layer light emitting cells (LECs) based on a series of ionic cyclometalated Ir(III) complexes of formulae $\left[ {{\rm Ir}\left( {{\rm ppy}} \right)_{\rm 2} \left( {{\rm bpy}} \right)} \right]^ + {\rm PF}_{\rm 6}^ -$ and $\left[ {{\rm Ir}\left( {{\rm ppy}} \right)_{\rm 2} \left( {{\rm phen}} \right)} \right]^ + {\rm PF}_{\rm 6}^ -$ where ppy, bpy, and phen are 2‐phenylpyridine, substituted bipyridine and substituted phenanthroline ligands, respectively, are reported. Substitution at the NˆN ligand has little effect on the emitting metal‐ligand to ligand charge‐transfer (MLLCT) states and functionalization at this site of the complex leads to only modest changes in emission color. For the more bulky complexes the increase in intermolecular separation leads to reduced exciton migration, which in turn, by suppressing concentration quenching, significantly increases the lifetime of the excited state. On the other hand, the larger intermolecular separation induced by bulky ligands reduces the charge carrier mobility of the materials, which means that higher bias fields are needed to drive the diodes. A brightness of ca. 1000 cd m−2 at 3 V is obtained for complex 5, which demonstrates a beneficial effect of bulky substituents.

Red-light-emitting electrochemical cell using a polypyridyl iridium(iii) polymer

A deep-red phosphorescent ionic iridium(III) complex is prepared and incorporated into a polymer. Both the complex (1) and the polymer (2) were used as the single active material in solid-state light-emitting electrochemical cells (LECs). The devices built up using 1 and 2 emit in the deep-red region of the visible spectrum with CIE coordinates x = 0.710; y = 0.283 and x = 0.691; y = 0.289, respectively, making them one of the deepest-red emitting LECs reported. It is the first example of a polymeric LEC incorporating an ionic iridium complex, which exhibits increased stabilities compared with the device based on the small molecular weight complex.

Decreased Turn‐On Times of Single‐Component Light‐Emitting Electrochemical Cells by Tethering an Ionic Iridium Complex with Imidazolium Moieties

We report a significant decrease in turn-on times of light-emitting electrochemical cells (LECs) by tethering imidazolium moieties onto a cationic Ir complex. The introduction of two imidazolium groups at the ends of the two alkyl side chains of [Ir(ppy)(2)(dC6-daf)](+)(PF(6))(-) (ppy = 2-phenylpyridine, dC6-daf = 9,9'-dihexyl-4,5-diazafluorene) gave the complex [Ir(ppy)(2)(dC6MIM-daf)](3+)[(PF(6))(-)](3) (dC6MIM-daf = 9,9-bis[6-(3-methylimidazolium)hexyl]-1-yl-4,5-diazafluorene). Both complexes exhibited similar photoluminescent/electrochemical properties and comparable electroluminescent efficiencies. The turn-on times of the LECs based on the latter complex, however, were much lower than those of devices based on the former. The improvement is ascribed to increased concentrations of mobile counterions ((PF(6))(-)) in the neat films and a consequent increase in neat-film ionic conductivity. These results demonstrate that the technique is useful for molecular modifications of ionic transition-metal complexes (ITMCs) to improve the turn-on times of LECs and to realize single-component ITMC LECs compatible with simple driving schemes.

Efficient solution-processed electrophosphorescent devices using ionic iridium complexes as the dopants

Efficient solution-processed electrophosphorescent devices using two blue-emitting ionic iridium complexes (complex 1 and complex 2) were fabricated, with poly( N-vinylcarbazole) (PVK):1,3-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7) as the host and Cs 2CO 3/Al as the cathode. Using complex 1 as the dopant, we obtained efficient blue-green electrophosphorescence from single-layer devices with a maximum efficiency of 12.2 cd A −1, a maximum brightness of 12,600 cd m −2 and CIE (Commission Internationale de l’Éclairage) coordinates of (0.19, 0.45). And the maximum efficiency of the device based on complex 1 can be further improved to 20.2 cd A −1, when a thin 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI) layer was inserted between the light-emitting layer and the cathode. Using complex 2 as the dopant, we obtained deep-blue electrophosphorescence with the emission peak at 458 nm and CIE coordinates of (0.16, 0.22). Our work suggests that ionic iridium complexes are promising phosphors for obtaining efficient electrophosphorescence in the blue region.

A Supramolecularly-Caged Ionic Iridium(III) Complex Yielding Bright and Very Stable Solid-State Light-Emitting Electrochemical Cells

A new iridium(III) complex showing intramolecular interligand pi-stacking has been synthesized and used to improve the stability of single-component, solid-state light-emitting electrochemical cell (LEC) devices. The pi-stacking results in the formation of a very stable supramolecularly caged complex. LECs using this complex show extraordinary stabilities (estimated lifetime of 600 h) and luminance values (average luminance of 230 cd m-2) indicating the path toward stable ionic complexes for use in LECs reaching stabilities required for practical applications.

A Series of Red‐Light‐Emitting Ionic Iridium Complexes: Structures, Excited State Properties, and Application in Electroluminescent Devices

AbstractA series of ionic diiminoiridium complexes [Ir(piq‐C∧N)2(L‐N∧N)](PF6) were prepared, where piq‐C∧N is 1‐phenylisoquinolinato and L‐N∧N are bidentate N‐coordinating ligands: 2,2′‐bipyridine (bpy), 4,4′‐dimethyl‐2,2′‐bipyridine (mbpym), 5,5′‐bis(thiopen‐2‐yl)‐2,2′‐bipyridine (tbpyt), and 5,5′‐bis(9,9‐dioctylfluoren‐2‐yl)‐2,2′‐bipyridine (FbpyF). X‐ray diffraction studies of [Ir(piq)2(mbpym)](PF6) revealed that the iridium center adopts a distorted octahedral geometry. All complexes exhibited intense and long‐lived emission at room temperature. The substituents on the 2,2′‐bipyridine moieties influence the photophysical and electrochemical properties. The excited states were investigated through theoretical calculations together with photophysical and electrochemical properties. It was found that the excited state of the [Ir(piq)2(FbpyF)](PF6) complex can be assigned to a mixed character of 3LC (πN∧N→π*N∧N), 3MLCT, 3LLCT (πC∧N→π*N∧N), and 3LC (πC∧N→π*C∧N). In addition, the alkylfluorene‐substituted complex, [Ir(piq)2(FbpyF)](PF6), hadrelatively high quantum efficiency and good film‐forming ability, and it was expected to be a good candidate for lighting and display applications. A nondoped, single‐layer device that incorporates this complex as a light‐emitting layer was fabricated and red phosphorescence was obtained.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Single-Layer Electroluminescent Devices and Photoinduced Hydrogen Production from an Ionic Iridium(III) Complex

We report the electronic structure and diverse applications of a highly luminescent ionic transition metal complex, [Ir(dF(CF3)ppy)2(dtbbpy)](PF6) (where dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine and dtbbpy = 4,4‘-di-tert-butyl-2,2‘-dipyridyl). The large HOMO−LUMO gap (ΔE = 3.06 V) enabled high-energy electroluminescence from the complex. Single-layer devices were fabricated and found to emit blue-green light (500 nm). The strong reducing strength of the excited state (E*ox = 1.21 V) enabled effective catalysis of the photoinduced reduction of H2O to H2. It was found that the relative quantum yield of hydrogen was over an order of magnitude improved from the standard photosensitizer Ru(dmphen)32+ (dmphen = 4,7-dimethyl-1,10-phenanthroline).

Synthesis and reactions of ene–hydrazone diphosphine iridium complexes and related species †

Treatment of the azine diphosphine Z,Z-PPh2CH2C(But)N–NC(But)CH2PPh2 I with [IrCl(CO)2(H2NC6H4Me-4)] in benzene gave the ene–hydrazone diphosphine iridium(III) hydride [IrH(Cl)(CO){PPh2CHC(But)N–NC(But)CH2PPh2}], 1, which isomerised reversibly to the ionic square planar iridium(I) complex [Ir(CO){PPh2CH2C(But)N–NC(But)CH2PPh2}]Cl 2a, containing an azine diphosphine. Treatment of 1 with NEt3 gave the neutral ene–hydrazone diphosphine iridium(I) complex [Ir(CO){PPh2CHC(But)N–NC(But)CH2PPh2}] 3 which is reactive and undergoes oxidative addition of H2 to give the iridium(III) dihydride mer,cis-[IrH2(CO){PPh2CH C(But)N–NC(But)CH2PPh2}] 4 and oxidative addition of MeI to give the methyliridium(III) complex [IrMe(I)(CO){PPh2CHC(But)N–NC(But)CH2PPh2}] 5. It reacted rapidly with olefins or acetylenes (L), i.e.N-methylmaleimide, ethene or dimethyl acetylenedicarboxylate, to give the five-co-ordinate adducts [Ir(CO)L{PPh2CH C(But)N–NC(But)CH2PPh2}], 6a, 6b or 6c, respectively, also with O2 to give the η2-dioxygen adduct [Ir(CO)(η2-O2){PPh2CHC(But)N–NC(But)CH2PPh2}] 7. Treatment of 3 with 1 mol of picric acid protonated the ene–hydrazone diphosphine backbone to give the azine diphosphine iridium(I) salt [Ir(CO){PPh2CH2C(But)N–N C(But)CH2PPh2}][OC6H2(NO2)3] 2b. The N-methylmaleimide adduct 6a was similarly protonated to give the corresponding azine diphosphine iridium(I) salt [Ir(CO)(η2-COCHCHCONMe){PPh2CH2C(But)N–NC(But)CH2PPh2}][OC6H2(NO2)3] 8. Complex 1 was protonated by HCl to give the corresponding azine diphosphine iridium(III) salt [IrH(Cl)(CO){PPh2CH2C(But)N–NC(But)CH2PPh2}]Cl 9a, which is converted into 3 when treated with NEt3. The 1H, 13C and 31P NMR and some IR data are given.