Triplet Emitters Research Articles

Spontaneous Fluctuations of Transition Dipole Moment Orientation in OLED Triplet Emitters.

The efficiency of an organic light-emitting diode (OLED) depends on the microscopic orientation of transition dipole moments of the molecular emitters. The most effective materials used for light generation have 3-fold symmetry, which prohibits a priori determination of dipole orientation due to the degeneracy of the fundamental transition. Single-molecule spectroscopy reveals that the model triplet emitter tris(1-phenylisoquinoline)iridium(III) (Ir(piq)3) does not behave as a linear dipole, radiating with lower polarization anisotropy than expected. Spontaneous symmetry breaking occurs in the excited state, leading to a random selection of one of the three ligands to form a charge-transfer state with the metal. This nondeterministic localization is revealed in switching of the degree of linear polarization of phosphorescence. Polarization scrambling likely raises out-coupling efficiency and should be taken into account when deriving molecular orientation of the guest emitter within the OLED host from ensemble angular emission profiles.

Controlled 2D-Confinement of Phosphorescent Pt(II) Complexes on Quartz and 6H-SiC(0001) Surfaces

The photophysical properties of Pt(II)-based triplet emitters can be controlled by tuning the Pt(II)...Pt(II) distance. Herein we show that the functionalization and characterization of insulating and semiconducting substrates with triplet emitters can be achieved by a convenient wet-chemical grafting of a pyridine-terminated silane linker on amorphous quartz and on silicon carbide single crystal surfaces. Coordination of the monodentate pyridine unit with phosphorescent Pt(II) complexes bearing tridentate luminophores forces the triplet emitters to aggregate by surface constraints, even if bulky substituents such as adamantyl moieties are present. We found that X-ray photoemission spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR FT-IR) constitute adequate tools to ascertain the degree of functionalization; in particular, high resolution XPS spectra revealed the electronic states at the surfaces, which were correlated with the frontier orbitals of the tridentate luminopho...

A luminescent poly(amidoamine)-iridium complex as a new singlet-oxygen sensitizer for photodynamic therapy.

A polymer complex (1P) was synthesized by binding bis(cyclometalated) Ir(ppy)2(+) fragments (ppy = 2-phenylpyridyl) to phenanthroline (phen) pendants of a poly(amidoamine) copolymer (PhenISA, in which the phen pendants involved ∼6% of the repeating units). The corresponding molecular complex [Ir(ppy)2(bap)](+) (1M, bap = 4-(butyl-4-amino)-1,10-phenanthroline) was also prepared for comparison. In water solution 1P gives nanoaggregates with a hydrodynamic diameter of 30 nm in which the lipophilic metal centers are presumed to be segregated within polymer tasks to reduce their interaction with water. Such confinement, combined with the dilution of triplet emitters along the polymer chains, led to 1P having a photoluminescence quantum yield greater than that of 1M (0.061 vs 0.034, respectively, in an aerated water solution) with a longer lifetime of the (3)MLCT excited states and a blue-shifted emission (595 nm vs 604 nm, respectively). NMR data supported segregation of the metal centers. Photoreaction of O2 with 1,5-dihydroxynaphthalene showed that 1P is able to sensitize (1)O2 generation but with half the quantum yield of 1M. Cellular uptake experiments showed that both 1M and 1P are efficient cell staining agents endowed with two-photon excitation (TPE) imaging capability. TPE microscopy at 840 nm indicated that both complexes penetrate the cellular membrane of HeLa cells, localizing in the perinuclear region. Cellular photodynamic therapy tests showed that both 1M and 1P are able to induce cell apoptosis upon exposure to Xe lamp irradiation. The fraction of apoptotic cells for 1M was higher than that for 1P (74 and 38%, respectively) 6 h after being irradiated for 5 min, but cells incubated with 1P showed much lower levels of necrosis as well as lower toxicity in the absence of irradiation. More generally, the results indicate that cell damage induced by 1M was avoided by binding the iridium sensitizers to the poly(amidoamine).

Poly(2-(N-carbazolyl)ethyl acrylate) as a host for high efficiency polymer light-emitting devices

Poly(2-(N-carbazolyl)ethyl acrylate), a poly(acrylate) comprised of carbazole-side groups attached via a flexible chain to the polymer backbone (PVAK) has been tested as host for solution-processed polymer light-emitting devices (PLEDs). This non-conjugated polymer proved to be an excellent candidate to host wide-bandgap phosphors. Notably, this polymer exhibited a high thermal stability (Td=322°C), a glass transition temperature (Tg) of 91°C and a wide bandgap corresponding to the pendent carbazole units and the disrupted π-conjugation of the polymer main chain, making this polymeric host a suitable candidate for wide bandgap triplet emitters. When tested as a host for FIrpic and Ir(ppy)3, the resulting blue and green light-emitting devices showed a maximum luminous efficiency of 18.25 and 17.74cd/A, respectively, which are comparable to recent reports of devices made using other carbazole-based oxygen-rich polymeric hosts. The polymer was also characterized by UV–visible absorption, photoluminescence spectroscopy as well as cyclic voltammetry.

Green phosphorescent organic light-emitting devices based on wide bandgap host materials

Two homoleptic iridium(III) complexes Ir3F and Ir6F were synthesized and tested as wide bandgap organometallic hosts for green phosphorescent OLEDs. While using Ir(ppy)3 as the green triplet emitter, Ir3F-based devices clearly outperformed similar devices fabricated with the traditional CBP host. On the opposite, Ir6F-based devices showed lower performances than the reference device. As appealing feature, Ir6F-based devices showed almost no efficiency roll-off i.e., no reduction of the luminous efficiency with the current density contrarily to that commonly observed for phosphorescent OLEDs.

Scanning-tunneling-spectroscopy-directed design of tailored deep-blue emitters.

Frontier molecular orbitals can be visualized and selectively set to achieve blue phosphorescent metal complexes. For this purpose, the HOMOs and LUMOs of tridentate Pt(II) complexes were measured using scanning tunneling microscopy and spectroscopy. The introduction of electron-accepting or -donating moieties enables independent tuning of the frontier orbital energies, and the measured HOMO-LUMO gaps are reproduced by DFT calculations. The energy gaps correlate with the measured and the calculated energies of the emissive triplet states and the experimental luminescence wavelengths. This synergetic interplay between synthesis, microscopy, and spectroscopy enabled the design and realization of a deep-blue triplet emitter. Finding and tuning the electronic "set screws" at molecular level constitutes a useful experimental method towards an in-depth understanding and rational design of optoelectronic materials with tailored excited state energies and defined frontier-orbital properties.

Structure-property relationships in an iridium(III) bis(terpyridine) complex with extended conjugated side chains.

Iridium(III) bis(terpyridine) complexes are known as excellent triplet emitters with emission lifetimes in the order of microseconds. We report the homoleptic complex [IrL2](3+) (L = 4'-(4-2,5-bis(octyloxy)-4-styrylphenyl)ethinyl)phenyl)-2,2':6',2″-terpyridine) that shows no detectable phosphorescence at room temperature but shows fluorescence. Emission spectra of [IrL2](3+) depend on the excitation wavelength. The origin of this behavior is studied with the help of results from (TD-)DFT calculations and is attributed to the selective excitation of different rotamers and isomers. Femtosecond-transient absorption experiments give further support for this interpretation as the specific excited-state absorption features of Z- and E-stilbene motives can be identified.

Spectroscopic mapping and selective electronic tuning of molecular orbitals in phosphorescent organometallic complexes - a new strategy for OLED materials.

The improvement of molecular electronic devices such as organic light-emitting diodes requires fundamental knowledge about the structural and electronic properties of the employed molecules as well as their interactions with neighboring molecules or interfaces. We show that highly resolved scanning tunneling microscopy (STM) and spectroscopy (STS) are powerful tools to correlate the electronic properties of phosphorescent complexes (i.e., triplet emitters) with their molecular structure as well as the local environment around a single molecule. We used spectroscopic mapping to visualize several occupied and unoccupied molecular frontier orbitals of Pt(II) complexes adsorbed on Au(111). The analysis showed that the molecules exhibit a peculiar localized strong hybridization that leads to partial depopulation of a dz² orbital, while the ligand orbitals are almost unchanged. We further found that substitution of functional groups at well-defined positions can alter specific molecular orbitals without influencing the others. The results open a path toward the tailored design of electronic and optical properties of triplet emitters by smart ligand substitution, which may improve the performance of future OLED devices.

Thermally stable indoloacridine type host material for high efficiency blue phosphorescent organic light-emitting diodes

High triplet energy materials derived from carbazole or α-carboline modified indoloacridine were synthesized and device characteristics of blue triplet emitter doped devices were investigated. The indoloacridine derived host materials showed a high triplet energy above 2.80eV and a high glass transition temperature over 170°C due to rigid nature of the molecular structure. The indoloacridine based host materials could approach high external quantum efficiency above 20% in blue phosphorescent organic light-emitting diodes.

Carboline modified dibenzofuran as a high triplet host material for blue phosphorescent organic light-emitting diodes

A carboline modified dibenzofuran compound, 2,8-bis(pyrido[2,3-b]indol-9-yl)dibenzofuran (DBFCb), was synthesized and evaluated as the host material for blue phosphorescent organic light-emitting diodes by doping iridium (III) bis((3,5-difluorophenyl)pyridine) picolinate (FIrpic) as a blue triplet emitter. The DBFCb host showed balanced charge density in the emitting layer and a triplet energy of 2.88 eV by carboline moiety. The FIrpic doped DBFCb devices realized high external quantum efficiency of 21.3% at a low doping concentration of 3%.

All-phosphorescent three-color two-stack tandem white organic light emitting diodes with high-color-rendering index values

Reported herein are high-color-rendering index two-stack phosphorescent tandem white organic light-emitting diodes using three-color phosphorescent dopants: iridium(III) bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) (472 nm) as a blue emitter for the first stack layer, and iridium(III) bis(4-methyl-2-(3,5-dimethylphenyl)quinolinato-N,C2′) tetramethylheptadionate (612 nm) and iridium(III) bis(4-(4-tbutylphenyl)thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (562 nm) as red and yellow triplet emitters for the second stack layer, respectively. By locating the blue and red emissive layers slightly far from their antinode position in terms of photonic mode density, the electroluminescent spectra could be changed, and the CRI values could be increased from 74 to over 85. There is some efficiency loss, however, from 38.6 to 29.4 lm/W due to the inevitable electrical and optical losses.

Synthesis, Characterization, and Electroluminescent Properties of Iridium(III) 2‐Phenylpyridine‐type Complexes Containing Trifluoromethyl Substituents and Various Main‐Group Moieties

AbstractNew heteroleptic cyclometalated iridium(III) 2‐phenylpyridine‐type complexes with trifluoromethyl substituents and various main‐group moieties were synthesized and their photophysical, electrochemical, and electroluminescent (EL) properties studied. The emission color can be tuned by a facile derivatization of the phenyl moiety of 2‐phenylpyridine with various main‐group moieties, and we have prepared new yellowishgreen to orange triplet emitters with enhanced charge injection/charge transporting features, which can furnish attractive EL performance in phosphorescent organic light‐emitting devices (OLEDs). Attempts were also made to fabricate two‐color white‐light OLEDs based on a combination of fluorescent blue and phosphorescent orange emitters.

Phosphorescent iridium(III) complexes based on 2-phenylimidazo[1,2-a]pyridine-type ligands: Synthesis, photophysical, electrochemical, and electrophosphorescent properties

New iridium(III) cyclometalated complexes based on 2-phenylimidazo[1,2-a]pyridine-type ligands were synthesized and their photophysical, electrochemical and electroluminescent (EL) properties were investigated. The detailed insight into the characters of the emissive excited states was obtained by frontier molecular orbital analysis. These metal complexes exhibit very balanced charge transporting ability for both kinds of charge carriers, which can furnish decent EL performance in the phosphorescent organic light-emitting devices (PHOLEDs) with peak luminance of 15491 cd m−2 at ca. 13.5 V, external quantum efficiency of 6.81%, luminance efficiency of 34.74 cd A−1, and power efficiency of 17.54 lm W−1. These results not only provide a better understanding of the inherent characters of IrIII phosphors with phenylimidazo[1,2-a]pyridine units, but also valuable information on future molecular design of triplet emitters with unique electronic features for high-performance PHOLEDs.

Molecular asterisks with a persulfurated benzene core are among the strongest organic phosphorescent emitters in the solid state

A series of functionalized persulfurated benzene molecules were synthesized. Their photophysical properties and crystal structures were analyzed. All compounds are non-emitting in solution at room temperature, but in a sharp contrast, quantum yields can be very high (up to 100%) in the solid state at 298 K or in a rigid matrix at low temperatures. This is a consequence of a decrease of intramolecular rotations and motions, but conformational and rotamer issues along with substituent effects might also play a role. These compounds are among the rare examples of highly phosphorescent organic materials, due to a Crystallization Induced Phosphorescence or to an Aggregation Induced Phosphorescence. Compound 1 is among the most phosphorescent solid known to date. They thus represent an alternative to heavy metal ion-based triplet emitters in solid state.

High triplet energy Al complex as a host material for blue phosphorescent organic light-emitting diodes

An Al complex, tris((2-(pyrazol-1-yl)pyridin-3-yl)oxy)aluminum (Al(pypy)3), was synthesized as a high triplet energy host material for blue phosphorescent organic light-emitting diodes. A high triplet energy ligand, 2-(1H-pyrazol-1-yl)pyridin-3-ol, was coordinated to the Al to develop the high triplet energy host material derived from Al. The Al(pypy)3 host showed a high triplet energy of 2.86eV for efficient energy transfer to blue triplet emitter. A maximum quantum efficiency of 20.5% was achieved in blue device using the Al(pypy)3 host material.

Solution‐Processible 2,2′‐Dimethyl‐biphenyl Cored Carbazole Dendrimers as Universal Hosts for Efficient Blue, Green, and Red Phosphorescent OLEDs

A series of solution‐processible 2,2′‐dimethyl‐biphenyl cored dendrimers, namely G1MP, G2MP, and G3MP, is designed and synthesized by tuning the generation of periphery carbazole dendron. The resulting dendrimers all show excellent solubility in common organic solvents, and their high‐quality thin films can be formed via spin‐coating with a root‐mean‐square roughness in the range of 0.38–0.54 nm. G3MP, which contains the third‐generation carbazole dendron, has the greatest potential among those made here as an ideal universal host for multicolored triplet emitters. G3MP exhibits good thermal stability, with a glass transition temperature of 368 °C, a triplet energy as high as 2.85 eV enough to prevent the loss of triplet excitons, and suitable HOMO/LUMO levels of –5.30/–2.11 eV to facilitate both hole and electron injection and transport. When using G3MP as the host, highly efficient deep‐blue, blue, green, and red phosphorescent organic light‐emitting diodes (PhOLEDs) are successfully demonstrated, revealing a maximum luminous efficiency up to 18.2, 28.2, 54.0, and 12.7 cd A–1 with the corresponding Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.23), (0.15, 0.35), (0.38, 0.59), and (0.64, 0.34), respectively. The state‐of‐art performance indicates that dendritic hosts have a favorable prospect of applications in solution‐processed white PhOLEDs and full‐color displays.

Highly efficient blue and deep-blue emitting zwitterionic iridium(iii) complexes: synthesis, photophysics and electroluminescence

We report on a series of blue and deep-blue emitting zwitterionic iridium(III) complexes, consisting formally of a cationic Ir centre and a N,N′-heteroaromatic (N^N) ligand bearing negatively charged side groups, i.e. sulfonate and borate. The synthesis, photophysical and electrochemical properties of this series are described in detail together with their X-ray crystal structure determination. The reported complexes exhibit intense blue (λmax at 450 nm) and deep blue (λmax at 435 nm) emission in deaerated solution, similar to the related cationic complexes. The strategy employed, namely the internal salt formation, allows high solubility in many organic solvents as well as for some of the complexes a low sublimation temperature. For this reason, one of the complexes was further tested as an emitter in phosphorescent organic light emitting diodes (PhOLEDs). Despite the zwitterionic nature of the triplet emitter employed, the devices were fabricated by means of sublimation process. The devices showed a peak external quantum efficiency (EQE) as high as 11.0% and Commission Internationale d'Enclairage (CIE) coordinates x = 0.21 and y = 0.33.

High quantum efficiency and color stability in white phosphorescent organic light emitting diodes using a pyridine modified carbazole derivative

A high triplet energy material derived from carbazole and pyridine, 9,9′-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H-carbazole), was synthesized as the host material for blue and white phosphorescent organic light-emitting diodes. The 9,9′-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H-carbazole) host showed a high triplet energy of 2.99 eV for efficient energy transfer to blue triplet emitter. High quantum efficiencies of 19.8% and 17.1% were achieved in blue and white phosphorescent organic light-emitting diodes using the 9,9′-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H-carbazole) host material. In addition, the 9,9′-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H-carbazole) based white phosphorescent organic light-emitting diodes showed good color stability.

Metal‐Free OLED Triplet Emitters by Side‐Stepping Kasha’s Rule

Keeping it light: Most organic fluorophores do not show useful emission from the triplet excited state. A new class of materials is presented for organic light-emitting diodes that exhibit phosphorescence under electrical excitation, even at room temperature, without employing the heavy-atom effect. Blocking of internal conversion is so effective that emission from higher-lying triplet states, above the singlet, can be observed at low temperature. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Metal‐Free OLED Triplet Emitters by Side‐Stepping Kasha’s Rule

Leicht und leuchtend: Die meisten organischen Fluorophore zeigen keine nützliche Emission aus dem angeregten Triplettzustand. Eine neue Materialklasse für organische Leuchtdioden, die auf elektrische Anregung hin auch bei Raumtemperatur phosphoreszieren, kommt ohne Schweratomeffekt aus, denn die interne Konversion wird so effektiv blockiert, dass Emissionen aus höherliegenden Triplettzuständen (oberhalb des Singuletts) bei tiefen Temperaturen zu beobachten sind.