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We report the development of light-emitting dendrimers with heteroleptic or homoleptic iridium(III) complexes at the core, with one, two, or three first- or second-generation biphenyl-based dendrons attached to the 2-phenylpyridyl ligand that is involved in the emission of light. The 5-phenyl-1-methyl3-n-propyl-1H-1,2,4-triazole co-ligand was not dendronised and was not involved in the emission process. The dendrimers with two emissive ligands were found to have the highest solution photoluminescence quantum yields (PLQYs) which were greater than 90% independent of the dendrimer generation. The second-generation dendrimers were all found to have higher neat film PLQYs than their first-generation counterparts due to the increased shielding of the emissive core by the dendrons and surface groups. The maximum external quantum efficiencies of devices made from the neat film and a guest:host blend of the second-generation dendrimer with two emissive ligands were found to be the highest (7.5% and 15.8%, respectively), which was attributed to a balance between the interactions required for charge transport and minimising those that lead to non-radiative processes. Thus, we have demonstrated that combining dendrimer generation and number of emissive dendronised ligands is a viable strategy for optimising light-emitting dendrimer OLED performance. [ABSTRACT FROM AUTHOR]

A series of solution-processable green emitting dendronised bis-tridentate iridium(III) complexes composed of [2,6-diimidazolylidene]benzene and the dianionic ligand, 2-[3-(trifluoromethyl)-1H-pyrazol-5-yl]-6-[4-(trifluoromethyl)phenyl]pyridine, have been synthesised to investigate the role of surface groups (t-butyl versus 2-ethylhexyloxy) and dendron generation (first versus second) on the photoluminescent (PL) and electroluminescent (EL) properties of the dendrimers. The dendrimers exhibited similar solution photoluminescent quantum yields (PLQYs) in toluene (60–70%) independent of the surface groups and dendrimer generation. In contrast, the PLQY values of the second-generation dendrimers were significantly higher than those with the first-generation dendrons in the neat film due to the higher generation dendrons providing more effective encapsulation of the emissive cores. When the dendrimers were blended at a concentration of 2 mol% in tris[4-carbazoyl-9-ylphenyl]amine (TCTA) the PLQYs increased but were still below the values measured in solution suggesting that the dendrimers were not evenly distributed throughout the host. Emission from the guest and host in the 2 mol% blends provided evidence for uneven distribution of the guest and inefficient energy transfer from the host to the dendrimers. The difference in encapsulation of the cores by the dendrons also affected charge transport in the neat films. In the case of the second-generation dendrimer with 2-ethylhexyloxy groups, the hole mobility was <10−8 cm2 V−1 s−1, which was an order of magnitude lower than for the less sterically encumbered dendrimers. Simple two-layer organic light-emitting diodes (OLEDs) with a solution-processed light-emitting layer were prepared. The best of the host-free devices exhibited a maximum external quantum efficiency (EQE) of 13.4%. The devices with the dendrimers blended with TCTA had more uniform EQEs of 12.2% to 13.5%, which are amongst the highest reported for solution-processed OLEDs containing a bis-tridentate iridium(III) complex emitter. Finally, it was found that devices containing the dendrimer with the t-butyl surface groups had greater stability. [ABSTRACT FROM AUTHOR]

Dual emission light‐emitting dendrimers composed of phosphorescent fac‐tris[2‐phenylpyridyl]iridium(III) [Ir(ppy)3] cores and thermally activated delayed fluorescence‐based (TADF‐based) dendrons have been prepared. The TADF‐based dendrons are designed to have green or blue emission. The dendrimers show solvatochromism, with the emission switching between phosphorescence and TADF depending on the medium in which the measurement is undertaken. Time‐dependent photoluminescence (PL) spectra measurements show that the TADF dendrons can act as an energy pool for emission from the phosphorescent core. The PL quantum yields (PLQYs) are found to be strongly dependent on the dielectric constant of the solvent, ranging from as high as 76% to as low as 0.3%. Neat films of the dendrimers are found to have relatively balanced hole and electron mobilities of order 10–6 cm2 V–1 s–1, with bilayer organic light‐emitting diodes (OLEDs) containing neat emissive layers having a maximum external quantum efficiency (EQE) of 4.7% for a film having a PLQY of 12%. Finally, the solution processed OLEDs fabricated using 0.4 mol% of the dendrimers blended with 9‐[3‐(9H‐carbazol‐9‐yl)phenyl]‐9H‐carbazole‐3‐carbonitrile result in green emission with maximum EQEs of 9.8% and 15.1% for the dendrimers with green and blue emissive TADF dendrons, respectively. [ABSTRACT FROM AUTHOR]

Five Pt(II) complexes are described in which the metal ion is bound to anionic N∧N∧N ‐coordinating ligands. The central, deprotonated N atom is derived from an imide Ar−C(=O)−NH−C(=O)−Ar {PtL1–2Cl; Ar=pyridine or pyrimidine}, an amide py−C(=O)−NH−CH2−py {PtL3Cl}, or a hydrazide py−C(=O)−NH−N=CH−py {PtL4Cl}. The imide complexes PtL1–2Cl show no significant emission in solution but are modestly bright green/yellow phosphors in the solid state. PtL3Cl is weakly phosphorescent. PtL4Cl is formed as a mixture of isomers, bound through either the amido or imino nitrogen, the latter converting to the former upon absorption of light. Remarkably, the imino form displays fluorescence in solution, λ0,0=535 nm, whereas the amido shows phosphorescence, λ0,0=624 nm, τ=440 ns. It is highly unusual for two isomeric compounds to display emission from states of different spin multiplicity. The amido‐bound PtL4Cl can act as a bidentate O∧N ‐coordinating ligand, demonstrated by the formation of bimetallic complexes with iridium(III) or ruthenium(II). [ABSTRACT FROM AUTHOR]

The synthesis and characterization of a series of cobalt compounds, coordinated by the redox-active macrocyclic biquinazoline ligand, Mabiq [2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2'-biquinazolino)-[15]-1,3,5,8,10,14-hexaene-1,3,7,9,11,14-N6], is presented. The series includes the monometallic Co(Mabiq)Cl2 (1), Co(Mabiq)Cl (2), and Co(Mabiq) (4), with formal metal oxidation states of 3+ → 1+. A binuclear cobaltous compound, Co2(Mabiq)Cl3 (3), also was obtained, providing the first evidence for the ability of the Mabiq ligand to coordinate two metal ions. The electronic structures of the paramagnetic 2 and 3 were examined by electron paramagnetic resonance spectroscopy and magnetic susceptibility studies. The CoII ion that resides in the N4-macrocylic cavity of 2 and 3 adopts a low-spin S = ¹/2 configuration. The bypirimidine functionality in 3 additionally coordinates a high-spin S = ³/2 cobaltous ion in a tetrahedral environment. The two metal ions in 3 are weakly coupled by magnetometry. The square-planar, low-valent 4 offers one of a limited number of examples of structurally characterized N4-macrocyclic CoI compounds. Spectroscopic and density functional theory computational data suggest that a CoII(Mabiq·) description may be a reasonable alternative to the CoI formalism for this compound. [ABSTRACT FROM AUTHOR]