Abstract: A convergent strategy for the synthesis of three generations of dendrons comprised of carbazole moieties is described. The procedure to build the dendrons involves an iterative palladium catalysed amination–debenzylation sequence using N-benzyl-3,6-dibromocarbazole. The three carbazolyl focussed dendrons are then attached to a reactive fac-tris[2-phenylpyridyl]iridium(III) core by a palladium catalysed amination to give the dendrimers. The three generations of dendrons have one, three, and seven carbazole units leading to dendrimers with fac-tris[2-phenylpyridyl]iridium(III) cores and three, nine and twenty one carbazole units. The use of 9,9-dialkylfluorenyl surface groups gave the dendrimers excellent solubility. The attachment of the carbazolyl-based dendrons did not change the emission colour significantly with the dendrimers emitting green phosphorescence. The dendrimers were highly luminescent with solution photoluminescence quantum yields of the order of 70%. Ground state molecular orbital calculations showed that while the LUMO was concentrated on the core iridium(III) complex the HOMO was delocalised across the core and each of the dendrons. This was reflected in the oxidation properties of the dendrimers whereby the increased carbazolyl character of the HOMO resulted in the first oxidation being moved to more positive potentials.
Abstract: By using phosphorescent emitters, organic light emitting displays can be up to four times more efficient than those of fluorescent materials. Full colour displays based on phosphorescent materials have not been achieved thus far due to the poor efficiency of blue phosphorescent emitters. We show that there is a correlation between non-radiative decay of phosphorescence and vibrational coupling for related blue emissive materials containing similar iridium(III) complex chromophores. The materials had solution photoluminescence quantum yields (PLQYs) of up to 55% at room temperature with Commission Internationale de l’Eclairage co-ordinates of (0.155, 0.16). Stronger vibrational coupling was found to lead to an increased non-radiative decay rate and decreased PLQY. The activation energy for non-radiative decay was found to depend on the environment with the non-radiative decay rate being decreased when the emissive materials were placed in a solid host.
Abstract: Dendrimers are now an important class of light-emitting material for use in organic light-emitting diodes (OLEDs). Dendrimers are branched macromolecules that consist of a core, one or more dendrons, and surface groups. The different parts of the macromolecule can be selected to give the desired optoelectronic and processing properties. The first light-emitting dendrimers were fluorescent but more recently highly efficient phosphorescent dendrimers have been developed. OLEDs containing light-emitting dendrimers have been reported to have external quantum efficiencies of up to 16 %. The solubility of the dendrimers opens the way for simple processing and a new class of flat-panel displays. In this Review we show how the structure of the light-emitting dendrimers controls key features such as intermolecular interactions and charge transport, which are important for all OLED materials. The advantages of the dendrimer architecture for phosphorescent emitters and the way the structure can be varied to enhance materials performance and device design are illustrated.
Abstract: A convergent procedure has been developed for the preparation of fac-tris(2-phenylpyridyl)iridium(III) cored dendrimers with first- and second-generation dendrons that each contain one and two carbazole units, respectively. The carbazole moieties are both an electroactive moiety and a branching unit in the dendron. The photoluminescence quantum yields of neat films of the first- and second-generation dendrimers were 48 ± 5% and 39 ± 4%, respectively. These values are substantially higher than for equivalent first- and second-generation dendrimers with phenyl moieties at the branching points of the dendrons instead of the carbazole units. The improved solid state luminescent properties can be attributed to the increased steric demand of the carbazole unit relative to the phenyl ring, which reduces more effectively the intermolecular interactions that cause the cores to be less emissive. Electrochemical experiments showed that both the core of the dendrimers and the dendrons were electroactive. Thin film hole mobilities of the first-generation dendrimer with the carbazolyl branching units in the dendrons were found to be higher than the equivalent dendrimer with biphenyl branching units across a range of fields.
Abstract: We report a new family of homoleptic iridium(III) complexes that emit blue phosphorescence at room temperature. The iridium(III) complexes are comprised of phenyltriazole ligands and were easily prepared via short synthetic routes. The parent fac-tris(1-methyl-5-phenyl-3-propyl-[1,2,4]triazolyl)iridium(III) complex exhibits blue photoluminescence (PL) with emission peaks at 449 and 479 nm and has a solution PL quantum yield of 66%. The emission was sequentially blue-shifted by the attachment of one and two fluorine atoms to the ligand phenyl ring with the fac-tris{1-methyl-5-(4,6-difluorophenyl)-3-propyl-[1,2,4]triazolyl}iridium(III) complex having the 1931 Commission Internationale de l'Eclairage coordinates of (0.16, 0.12) at room temperature. In contrast, when the phenyl ring of the ligands was substituted by trifluoromethyl, the PL spectrum was red-shifted when compared to the parent compound whereas if the trifluoromethyl group was attached to the triazole ring, the emission was blue-shifted. The radiative rates of these new blue iridium(III) complexes were found to be in the range of 2-6 × 105 s-1, indicating that the emission had varying amounts of metal-to-ligand charge-transfer character. Molecular orbital calculations showed that for the fluorinated complexes the contribution of the ligand triplet character to the emissive energy state increased with the hypsochromic shift in emission. This was confirmed by time-resolved PL measurements, which showed that the complex with the deepest blue emission had the slowest radiative decay rate.
Abstract: We have studied triplet-triplet annihilation in neat films of electrophosphorescent fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3]-cored dendrimers containing phenylene- and carbazole-based dendrons with 2-ethylhexyloxy surface groups using time-resolved photoluminescence. From measured annihilation rates, the limiting current densities above which annihilation would dominate in dendrimer light-emitting devices are found to be > 1 A/cm2. The triplet exciton diffusion length varies in the range of 2–10 nm depending on the dendron size. The distance dependence of the nearest-neighbor hopping rate shows that energy transfer is dominated by the exchange mechanism.
Abstract: We describe the preparation of a dendrimer that is solution-processible and contains 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a fac-tris[2-(2,4-difluorophenyl)pyridyl]iridium(III) core. The homoleptic complex is highly luminescent and the color of emission is similar to the heteroleptic iridium(III) complex, bis[2-(2,4-difluorophenyl)pyridyl]picolinate iridium(III) (FIrpic). To avoid the change in emission color that would arise from attaching a conjugated dendron to the ligand, the conjugation between the dendron and the ligand is decoupled by separating them with an ethane linkage. Bilayer devices containing a light-emitting layer comprised of a 30 wt.-% blend of the dendrimer in 1,3-bis(N-carbazolyl)benzene (mCP) and a 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene electron-transport layer have external quantum and power efficiencies, respectively, of 10.4 % and 11 lm W-1 at 100 cd m-2 and 6.4 V. These efficiencies are higher than those reported for more complex device structures prepared via evaporation that contain FIrpic blended with mCP as the emitting layer, showing the advantage of using a dendritic structure to control processing and intermolecular interactions. The external quantum efficiency of 10.4 % corresponds to the maximum achievable efficiency based on the photoluminescence quantum yield of the emissive film and the standard out-coupling of light from the device.
Abstract: A simple way of tuning the emission color in solution processed phosphorescent organic light emitting diodes is demonstrated. For each color a single emissive spin-coated layer consisting of a blend of three materials, a fac-tris(2-phenylpyridyl)iridium (III) cored dendrimer (Ir–G1) as the green emitter, a heteroleptic [bis(2-phenylpyridyl)-2-(2[prime]-benzo[4,5-alpha]thienyl)pyridyl]iridium (III) cored dendrimer [Ir(ppy)2btp] as the red emitter, and 4,4[prime]-bis(N-carbazolyl) biphenyl (CBP) as the host was employed. By adjusting the relative amount of green and red dendrimers in the blends, the color of the light emission was tuned from green to red. High efficiency two layer devices were achieved by evaporating a layer of electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI) on top of the spin-coated emissive layer. A brightness of 100 cd/m2 was achieved at drive voltages in the range 5.3–7.3 V. The peak external efficiencies at this brightness ranged from 31 cd/A (18 lm/W) to 7 cd/A (4 lm/W).
Abstract: Electrophosphorescent dendrimers are promising materials for highly efficient light-emitting diodes. They consist of a phosphorescent core onto which dendritic groups are attached. Here, we present an investigation into the optical and electronic properties of highly efficient phosphorescent dendrimers. The effect of dendrimer structure on charge transport and optical properties is studied using temperature-dependent charge-generation-layer time-of-flight measurements and current voltage (I–V) analysis. A model is used to explain trends seen in the I–V characteristics. We demonstrate that fine tuning the mobility by chemical structure is possible in these dendrimers and show that this can lead to highly efficient bilayer dendrimer light-emitting diodes with neat emissive layers. Power efficiencies of 20 lm/W were measured for devices containing a second-generation (G2) Ir(ppy)3 dendrimer with a 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene electron transport layer.
Abstract: We report methodology for the preparation of symmetric and asymmetric solution processable phosphorescent dendrimers that are comprised of 2-ethylhexyloxy surface groups, biphenyl based dendrons, and iridium(III) complex cores. The symmetric dendrimer has three dendritic 2-benzo[b]thiophene-2-ylpyridyl (BTP) ligands with the dendritic ligands responsible for red emission. The asymmetric dendrimer has two dendritic 2-phenylpyridyl ligands and one unsubstituted BTP ligand. Iridium(III) complexes comprised of 2-phenylpyridyl ligands are normally associated with green emission whereas those containing BTP ligands emit red light. Red emission is observed from the asymmetric dendrimer demonstrating that emission occurs primarily from the metal-to-ligand charge transfer state associated with the ligand with the lowest HOMO–LUMO energy gap. The photoluminescence quantum yields (PLQYs) of the symmetric and asymmetric dendrimers were strongly dependent on the dendrimer structure. In solution the PLQYs of the asymmetric and symmetric dendrimers were 47 ± 5% and 29 ± 3% respectively. The photoluminescence lifetime of the emissive state of both dendrimers in solution was 7.3 ± 0.1 µs. In the solid state the comparative PLQYs were reversed with the symmetric dendrimer having a PLQY of 10 ± 1% and the asymmetric dendrimer a PLQY of 7 ± 1%. The comparatively larger decrease in PLQY for the asymmetric dendrimer in the solid state is attributed to increased core–core interactions. The intermolecular interactions are greater in the asymmetric dendrimer because there is no dendron on the BTP ligand. Electrochemical analysis shows that charge is injected directly into the cores of the dendrimers.
Abstract: A simple convergent procedure has been developed for the preparation of solution processable phosphorescent dendrimers with biphenyl-based dendrons and fac-tris(2-phenylpyridyl)iridium(III) cores. We found that the attachment point and branching of the dendrons are important for controlling the color of the light emission. Photoluminescence excitation measurements showed that energy could be transferred efficiently from the dendrons to the core. Solution photoluminescence quantum yield (PLQY) measurements of the dendrimers were of order 70%, showing that the attachment of the dendron did not decrease the luminescence efficiency of the core iridium complex. The PLQYs of the neat dendrimer films increased with generation with the second-generation dendrimer having a neat film PLQY of 31%, 11/2 times higher than the first-generation dendrimers and almost 3 times that of the nondendritic iridium complex, demonstrating the power of the dendrimer architecture to control intermolecular interactions. Electrochemical experiments showed that charge was injected directly into the core of the dendrimers.
Abstract: High-efficiency single-layer-solution-processed green light-emitting diodes based on a phosphorescent dendrimer are demonstrated. A peak external quantum efficiency of 10.4% (35 cd/A) was measured for a first generation fac-tris(2-phenylpyridine) iridium cored dendrimer when blended with 4,4[prime]-bis(N-carbazolyl)biphenyl and electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene at 8.1 V. A maximum power efficiency of 12.8 lm/W was measured also at 8.1 V and 550 cd/m2. These results indicate that, by simple blending of bipolar and electron-transporting molecules, highly efficient light-emitting diodes can be made employing a very simple device structure.
Abstract: Iridium-based phosphorescent dendrimers have shown much promise as highly efficient light emitting materials for organic light emitting diodes (OLEDs). Here we report the effects of modifying the chemical structure on the emissive and charge transport properties of Ir(ppy)3 based electrophosphorescent dendrimers. We investigate a novel para linked first generation (G1) iridium dendrimer. This material is compared to G1 and G2 meta linked dendrimers. We show that by blending these dendrimers into a CBP host, high external quantum efficiencies of over 10% and luminous efficiencies of 27 lm/W can be achieved.
Abstract: Photoluminescence and electroluminescence (EL) from a conjugated dendrimer consisting of three distyrylbenzene units linked by a central nitrogen atom as core and meta-linked biphenyl units as dendrons were investigated. The conjugated dendrimer emits green light and shows photoluminescence quantum efficiency of 9%. Bright electroluminescence was realized by using bilayer devices with blurred interface, which were fabricated by sequentially spin coating a neat dendrimer and a dendrimer doped with 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). The devices have the following structure: indium tin oxide/3,4-polyethylenedioxythiothene-polystyrenesulfonate/dendrimer/ dendrimer:PBD/Al. By optimizing the concentration of PBD, the maximum brightness and EL quantum efficiency reach 4100 cd/m2 and 0.17%, respectively. This is the best result reported so far on organic light-emitting diodes using dendrimer as an active material with an Al cathode.
Abstract: High-efficiency single-layer-solution-processed green light-emitting diodes based on a phosphorescent dendrimer are demonstrated. A peak external quantum efficiency of 10.4% (35 cd/A) was measured for a first generation fac-tris(2-phenylpyridine) iridium cored dendrimer when blended with 4,4[prime]-bis(N-carbazolyl)biphenyl and electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene at 8.1 V. A maximum power efficiency of 12.8 lm/W was measured also at 8.1 V and 550 cd/m2. These results indicate that, by simple blending of bipolar and electron-transporting molecules, highly efficient light-emitting diodes can be made employing a very simple device structure
