Phosphorescent Cationic Iridium Complexes Research Articles

Highly-efficient solution-processed organic light-emitting diodes using cationic iridium complexes with a pyridine-imidazole type ancillary ligand as the dopants

Phosphorescent cationic iridium complexes have emerged as promising dopants for solution-processed OLEDs, but further enhancing their device performances to the level achieved by neutral iridium complexes has remained challenging. Herein, we report a series of cationic iridium complexes, namely [Ir(ppz)2(Mepyim)]PF6 (1), [Ir(Mephtz)2(Mepyim)]PF6 (2), [Ir(tPhTAZ)2(Mepyim)]PF6 (3) and [Ir(CF3-dPhTAZ)2(Mepyim)]PF6 (4) for highly-efficient solution-processed OLEDs, which use 1-phenyl-1H-pyrazole (ppz), 1,3-dimethyl-5-phenyl-1H-1,2,4-triazole (Mephtz), 3,4,5-triphenyl-4H-1,2,4-triazole (tPhTAZ), and 3,4-diphenyl-5-(trifluoromethyl)-4H-1,2,4-triazole (CF3-dPhTAZ) as the cyclometalating (C^N) ligands, respectively, and 2-(1-methyl-1H-imidazole-2-yl)pyridine (Mepyim) as the ancillary (N^N) ligand. Their chemical structures have been verified by X-ray crystallography and their photophysical and electrochemical properties have been comprehensively characterized. In lightly doped poly(methylmethacrylate) films, complexes 1–4 afford green-blue to sky-blue emission, with high phosphorescent efficiencies at 0.75–0.98; in particular, complex 2 exhibits not only a near-unity phosphorescent efficiency (0.98) but also a short triplet lifetime (1.19 μs), because of its rather high radiative decay rate (8.2 × 105 s−1). Theoretical calculations reveal that the emission originates from either charge-transfer states (Ir/C^N → N^N) or C^N/N^N-centered 3π−π* states. By using complexes 1–4 as the dopants, solution-processed OLEDs with a simple double-layer structure afford efficient green-blue to sky-blue electroluminescence, with external quantum efficiencies (EQEs) at 6.8%–18.9%. In particular, the green-blue OLED based on complex 2 shows not only a high peak EQE of 18.9% but also a small efficiency roll-off (EQE = 16.5% at 1000 cd m−2), which is the best-performing device based on cationic iridium complex reported so far.

Elucidating the Nonradiative Deactivation Pathways in a Cationic Iridium Complex with 2,4-di(1H-pyrazol-1-yl)Pyridine as the Ancillary Ligand

Deep insight into the nonradiative deactivation pathways in phosphorescent cationic iridium complexes is critically important for developing efficient blue-emitting complexes toward advanced applications. Here, we report the synthesis and photophysical and electrochemical characterizations of a blue-green-emitting cationic iridium complex [Ir(ppy)2(bipzpy)]PF6 (Hppy is 2-phenylpyridine, and bipzpy is 2,4-di(1H-pyrazol-1-yl)pyridine). The nonradiative deactivation pathways in [Ir(ppy)2(bipzpy)]PF6 have been elucidated through extensive density functional theory calculations. The calculations reveal that the higher-lying charge-transfer (CT) state in [Ir(ppy)2(bipzpy)]PF6, which arises from Ir/ppy → bipzpy transitions, favors nonradiative deactivation because of its large structural distortion compared to the ground state. Both the CT state and the dark metal-centered (3MC) state can be thermally accessed by the lowest-lying emitting triplet state at room temperature, with the former being much more easily ...

Anchoring a Sulfonate Group to an Electron‐Transporting Molecule by an Alkyl Chain and Its Use as the Counter Anion in a Phosphorescent Cationic Iridium Complex

An electron-transporting anion prepared by anchoring a sulfonate group onto an electron-transporting molecule through a flexible alkyl chain and its use as the counter anion in a cationic iridium complex is reported. The flexible alkyl chain improves the solubility of the bulky anion in water or in polar organic solvents. The anion exhibits similar photophysical and electrochemical properties to the parent electron-transporting molecule. Within the complex, the anion does not disturb the phosphorescence of the cation in the solid film. Solution-processed small-molecule organic light-emitting diodes (OLEDs) using the complex as the dopant show superior performances over the reference device using the conventional complex with a PF6 - counter anion. It is revealed that anchoring anionic groups to optoelectronically active molecules with flexible alkyl chains is a feasible approach to develop optoelectronically active anions for assembling ion pairs with advanced optoelectronic properties.

High-color-rendering-index white polymer light-emitting electrochemical cells based on ionic host-guest systems: Utilization of blend films of blue-fluorescent cationic polyfluorenes and red-phosphorescent cationic iridium complexes

Abstract The authors present high-color-rendering-index (CRI) white polymer light-emitting electrochemical cells (PLECs) based on ionic host-guest systems. PLECs were fabricated with blend films composed of a blue fluorescent cationic π-conjugated polymer, poly(9,9-bis[6′-(N,N,N,-trimethylammonium)hexyl]fluorine-co-alt-1,4-phenylene)bromide (PFN+Br−), used as a host, and a red phosphorescent cationic iridium complex, [Ir(ppy)2(biq)]+(PF6)− (where (ppy)− = 2-phenylpyridinate and biq = 2,2′-biquinoline), used as a guest. By optimizing the composition of PFN+Br− and [Ir(ppy)2(biq)]+(PF6)− in the active layers, white light emission with Commission Internationale de l'Eclairage (CIE) coordinates of (x = 0.28, y = 0.31) and a very high CRI of 95.8 was achieved through mixing of blue and red light at an applied voltage of 4.0 V. The white light emissions were obtained at a low [Ir(ppy)2(biq)]+(PF6)− concentration (PFN+Br−:[Ir(ppy)2(biq)]+(PF6)− = 1:0.01 (mass ratio)), showing that [Ir(ppy)2(biq)]+(PF6)− acts as a suitable energy acceptor. The emission mechanism of the ionic host-guest PLECs can be explained by Forster resonance energy transfer (FRET) and Dexter energy transfer (DET) from the excited PFN+Br− to [Ir(ppy)2(biq)]+(PF6)−. Utilization of ionic host-guest PLECs is a very promising method for realizing white light-emitting devices with very high CRI.

Phosphorescent cationic iridium complexes with phenyl-imidazole type cyclometalating ligands: A combined experimental and theoretical study on photophysical, electrochemical and electroluminescent properties

Phosphorescent cationic iridium complexes with phenyl-imidazole type cyclometalating ligands have been synthesized for the first time and their photophysical, electrochemical properties have been comprehensively investigated. By changing the ancillary ligands, the complexes give orange-red or green-blue light. Compared to 2-phenylpyridine (ppy), the phenyl-imidazole ligands destabilize simultaneously the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of the complexes. Their emitting triplet states show dominant charge-transfer (iridium/cyclometalating ligands→ancillary ligands) character. The complexes have been used to fabricate solid-state light-emitting electrochemical cells (LECs). The orange-red LEC gives a high peak current efficiency of 14.3 cd A−1, which is among the highest reported for orange-red LECs; the green-blue LEC gives a peak current efficiency of 6.3 cd A−1. It is shown that the phenyl-imidazole cyclometalating ligands hold promise for the invention of iridium-based cationic phosphorescent dyes with tunable energy levels and emission properties.

Ratiometric Molecular Probes Based on Dual Emission of a Blue Fluorescent Coumarin and a Red Phosphorescent Cationic Iridium(III) Complex for Intracellular Oxygen Sensing.

Ratiometric molecular probes RP1 and RP2 consisting of a blue fluorescent coumarin and a red phosphorescent cationic iridium complex connected by a tetra- or octaproline linker, respectively, were designed and synthesized for sensing oxygen levels in living cells. These probes exhibited dual emission with good spectral separation in acetonitrile. The photorelaxation processes, including intramolecular energy transfer, were revealed by emission quantum yield and lifetime measurements. The ratios () between the phosphorescence () and fluorescence () intensities showed excellent oxygen responses; the ratio of under degassed and aerated conditions ( was 20.3 and 19.6 for RP1 and RP2. The introduction of the cationic Ir (III) complex improved the cellular uptake efficiency compared to that of a neutral analogue with a tetraproline linker. The emission spectra of the ratiometric probes internalized into living HeLa or MCF-7 cells could be obtained using a conventional microplate reader. The complex RP2 with an octaproline linker provided ratios comparable to the ratiometric measurements obtained using a microplate reader: the ratio of the value of RP2 under hypoxia (2.5% O2) to that under normoxia (21% O2) was 1.5 and 1.7 for HeLa and MCF-7 cells, respectively. Thus, the intracellular oxygen levels of MCF-7 cells could be imaged by ratiometric emission measurements using the complex RP2.

Two in One: Luminescence Imaging and 730 nm Continuous Wave Laser Driven Photodynamic Therapy of Iridium Complexes

A phosphorescent cationic iridium complex (1) with a donor−π-donor type unit as a N∧N ligand and 2-phenylpyridine as a C∧N ligand was synthesized and characterized. 1 exhibits an intense absorption peak centered at ∼450 nm with a large molar extinction coefficient (ε) of ∼2.4 × 104 M–1 cm–1 and displays highly efficient orange-red phosphorescence located at 620 nm with a quantum efficiency of ∼3% in PBS buffer solution at room temperature, which is beneficial for a luminescence probe in living cells excited by visible light. Importantly, the generation of singlet oxygen (1O2) by 1 was observed by 730 nm continuous wave (CW) laser irradiation owing to its reverse saturable absorption property, which was confirmed by the 1O2 scavenger of 1,3-diphenylbenzo[c]furan (DPBF) and ESR spectra. The photodynamic therapy effect of 1 in vivo by a 730 nm CW laser was further assessed by histological examination and immunohistochemistry.

Control of Intramolecular π–π Stacking Interaction in Cationic Iridium Complexes via Fluorination of Pendant Phenyl Rings

Intramolecular π-π stacking interaction in one kind of phosphorescent cationic iridium complexes has been controlled through fluorination of the pendant phenyl rings on the ancillary ligands. Two blue-green-emitting cationic iridium complexes, [Ir(ppy)(2)(F2phpzpy)]PF(6) (2) and [Ir(ppy)(2)(F5phpzpy)]PF(6) (3), with the pendant phenyl rings on the ancillary ligands substituted with two and five fluorine atoms, respectively, have been synthesized and compared to the parent complex, [Ir(ppy)(2)(phpzpy)]PF(6) (1). Here Hppy is 2-phenylpyridine, F2phpzpy is 2-(1-(3,5-difluorophenyl)-1H-pyrazol-3-yl)pyridine, F5phpzpy is 2-(1-pentafluorophenyl-1H-pyrazol-3-yl)-pyridine, and phpzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine. Single crystal structures reveal that the pendant phenyl rings on the ancillary ligands stack to the phenyl rings of the ppy ligands, with dihedral angles of 21°, 18°, and 5.0° between least-squares planes for complexes 1, 2, and 3, respectively, and centroid-centroid distances of 3.75, 3.65, and 3.52 Å for complexes 1, 2, and 3, respectively, indicating progressively reinforced intramolecular π-π stacking interactions from complexes 1 to 2 and 3. Compared to complex 1, complex 3 with a significantly reinforced intramolecular face-to-face π-π stacking interaction exhibits a significantly enhanced (by 1 order of magnitude) photoluminescent efficiency in solution. Theoretical calculations reveal that in complex 3 it is unfavorable in energy for the pentafluorophenyl ring to swing by a large degree and the intramolecular π-π stacking interaction remains on the lowest triplet state.

Solid-state light-emitting electrochemical cells employing phosphor-sensitized fluorescence

We report highly efficient phosphor-sensitized solid-state light-emitting electrochemical cells (LECs) utilizing a phosphorescent cationic iridium complex [Ir(dFppy)2(SB)]+(PF6−) as the host and a fluorescent cationic dye (R6G) as the guest. Photophysical studies show that R6G retains a high photoluminescence quantum yield (PLQY) in highly polar media, revealing its suitable use as an emitting guest in an ionic host matrix. Such solid-state LECs achieve quantum efficiency (cd A−1) efficiency, and power efficiency up to 5.5% photon/electron, 19 cd A−1 and 21.3 lm W−1, respectively. The device quantum efficiency achieved is among the highest reported for fluorescent LECs and is higher than one would expect from the PLQY of the R6G fluorescent dye in the host film, thus indicating that phosphor-sensitization is useful for achieving highly efficient fluorescent LECs. Moreover, using narrow-band fluorescent emitters, such as R6G (FWHM, ∼50 nm), is effective in improving the color saturation of solid-state LECs based on cationic complexes.