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Modulating the peripheral large steric hindrance of iridium complexes for achieving narrowband emission and pure red OLEDs with an EQE of 32.0%.

Two pairs of Pt(II) enantiomers ((RR)/(SS)-PyPt, ((RR)/(SS)-Py: N,N'-(1,2-diphenylethane-1,2-diyl)dipicolinamide; (RR)-P/M-QPt, ((RR)/(SS)-Q: N,N'-((1R,2R)-1,2-diphenylethane-1,2-diyl)bis(quinoline-2-carboxamide)) were synthesized, respectively, with good circularly polarized luminescence (CPL) and tunable dissymmetry factors (g) by molecular self-induction with (RR)/(SS)-1,2-diphenylethane-1,2-diamine as carbon chiral sources. In the (RR)-P-QPt and (SS)-M-QPt, specific P- and M-configurations were effectively induced from intrinsic chiral carbon centres (R or S), ingeniously avoiding the racemic mixture formation and chiral separation. Furthermore, the chirality originating from both chiral carbon centres and helicene-like structure improves the g factor significantly, which provides a new molecular design strategy for chiral Pt(II) enantiomers with good CPL properties.

Chiral boron/nitrogen doped multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising for highly efficient and color-pure circularly polarized organic light-emitting diodes (CP-OLEDs). Herein, we report two pairs of MR-TADF materials (Czp-tBuCzB, Czp-POAB) based on planar chiral paracyclophane with photoluminescence quantum yields of up to 98%. The enantiomers showed symmetric circularly polarized photoluminescence spectra with dissymmetry factors |gPL| of up to 1.6 × 10-3 in doped films. Meanwhile, the sky-blue CP-OLEDs with (R/S)-Czp-tBuCzB showed an external quantum efficiency of 32.1% with the narrowest full-width at half-maximum of 24 nm among the reported CP-OLEDs, while the devices with (R/S)-Czp-POAB displayed the first nearly pure green CP electroluminescence with |gEL| factors at the 10-3 level. These results demonstrate the incorporation of planar chirality into MR-TADF emitter is a reliable strategy for constructing of efficient CP-OLEDs.

Direct emission of circularly polarized light from organic light-emitting diodes (OLEDs) is a research hotspot as it could increase the efficiency and significantly simplify device architecture of OLED-based 3D displays. In this study, R/S-OBS-Cz and R/S-OBS-TCz with axial chirality were efficiently prepared by using a stable chiral octahydro-binaphthol unit, carbazole/3,6-ditert-butylcarbazole donors, and a 5,5,10,10-tetraoxide acceptor. The chiral unit-acceptor-donor structure provides them not only thermally activated delayed fluorescence (TADF) characteristics with minor singlet-triplet energy gaps of 0.04 and 0.05 eV but also obvious circularly polarized photoluminescence (CPPL) phenomenon with dissymmetry factors of 8.7 × 10-4 and 6.4 × 10-4 in codoped films. Meanwhile, the CP-OLEDs prepared by enantiomers exhibit good device performances with the maximum external quantum efficiency reaching 20.3% and ideal efficiency roll-off as well as obvious CPEL properties with a |gEL| factor up to 1.0 × 10-3.

Circularly polarized organic light-emitting diodes (CP-OLEDs) have received considerable attention due to their promising applications in 3D displays. However, up to now, CP-OLEDs using iridium(III) phosphors as emissive dopants exhibited low brightness, insufficient efficiency, serious efficiency roll-off, and especially low electroluminescence dissymmetry factors (gEL). In this study, a blue Ir(III) complex Ir(dfppy)2(sdpp) (dfppy is 2-(2,4-difluorophenyl)pyridine, sdpp is pyridinylphosphorothioate) peaking at 466 nm was synthesized, and two pairs of enantiomers containing the λ or δ isomer together with a chiral phosphine center (R or S) were separated into λ-Ir(dfppy)2(S-sdpp), δ-Ir(dfppy)2(R-sdpp), λ-Ir(dfppy)2(R-sdpp) and δ-Ir(dfppy)2(S-sdpp), respectively. It is the first example to investigate phosphine chirality in an Ir(III) complex, and the electron cloud distribution in the chiral conjugated ancillary ligand and λ/δ chirality of the Ir(III) center induced Ir(dfppy)2(sdpp) to exhibit decent circularly polarized luminescence (CPL) activities with a dissymmetry factor (|gPL|) around 1.4 × 10−3. The OLEDs based on Ir(dfppy)2(sdpp) exhibited a maximum external quantum efficiency of 13.8% with mild efficiency roll-off. Notably, the CP-OLEDs based on λ-Ir(dfppy)2(S-sdpp), δ-Ir(dfppy)2(R-sdpp), λ-Ir(dfppy)2(R-sdpp) and δ-Ir(dfppy)2(S-sdpp) showed obvious circularly polarized electroluminescence (CPEL) signals with a gEL of −2.1 × 10−3, 2.4 × 10−3, −2.1 × 10−3 and 1.9 × 10−3, respectively, thus affording an efficient strategy for the design of chiral blue Ir(III) emitters and their CP-OLEDs.

In order to realize efficient organic light-emitting diodes (OLEDs) with good color-purity, iridium(III) (Ir(III)) emitters with high luminous efficiency and narrow full width at half maxima (FWHM) are required. In this work, two main ligands incorporating a pyrido[3′,2′:4,5]pyrrolo[3,2,1-jk]carbazole unit with a rigid structure to realize carrier recombination balance and restrict intramolecular relaxation were developed. Four Ir(III) complexes with isomers peaking at 500 and 551 nm exhibit short phosphorescence lifetimes lower than 1 μs, high photoluminescence quantum yields of up to 89% and narrow FWHMs of 37 nm. Moreover, the devices based on two of the complexes with better thermal stability (>400 °C) show high external quantum efficiencies (EQEmax) of 23.8% and 24.5%, respectively, with efficiency roll-offs of less than 5%. Particularly, the electroluminescence FWHM bandwidth of one device is only 30 nm, which was rarely reported for Ir(III) complex-based OLEDs. Therefore, the strategy for main ligands with rigid structures provides an innovative method for realizing efficient Ir(III) complex-based OLEDs with narrow emissions.

Three green iridium(III) cyclometalated complexes, namely, (4tfmppy)2Ir(dipdtc), (24btfmppy)2Ir(dipdtc), and (TN4T)2Ir(dipdtc), with the sulfur-containing ancillary ligand N,N-diisopropyldithiocarbamate (dipdtc) were synthesized within 15 min at room temperature. The emission colors (λpeak = 497–534 nm) and photoluminescence quantum efficiencies (ΦP = 71.6–94.2%) can be effectively regulated by introducing different main ligands, namely, 2-(4-(trifluoromethyl)phenyl)pyridine (4tfmppy), 2-(2,4-bis(trifluoromethyl)phenyl)pyridine (24btfmppy), and 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine (TN4T). Employing these complexes as emitters, organic light-emitting diodes (OLEDs) with double-emitting layers were fabricated. In particular, the device based on the (TN4T)2Ir(dipdtc) complex showed superior performances with maximum current efficiency of 86.59 cd A−1 and maximum external quantum efficiency of 31.24% with low efficiency roll-off. These results suggest that the Ir(III) complexes with a four-membered ring Ir–S–C–S backbone containing dipdtc have potential applications in OLEDs.

To obtain dopants owing excellent electron mobility, four novel iridium (III) complexes (Ir(tfmppm)2(acac), Ir(tfmppm)2(tpip), Ir(f-4-tfmppm)2(tpip) and Ir(f-3-tfmppm)2(tpip)) with green emission using 2-(2,6-bis(trifluoromethyl)pyridin-4-yl)pyrimidine (tfmppm), 2-(2,6-bis(trifluoromethyl)pyridin-4-yl)-5-fluoropyrimidine (f-4-tfmppm) and 2-(2,6-bis(trifluoromethyl)pyridin-3-yl)-5-fluoropyrimidine (f-3-tfmppm) as main ligands, acetylacetonate (acac) and tetraphenylimidodiphosphinate (tpip) as ancillary ligands, were investigated, respectively. Due to the variation of the structures among the complexes, the photoluminescence quantum yield (up to 94%) and electron mobility can be regulated gradually, which would broaden the recombination zone, balance the distribution of holes and electrons, leading to excellent electroluminescence (EL) performances. Among four emitters, the organic light-emitting diodes (OLEDs) using Ir(f-3-tfmppm)2(tpip) display the best EL performances with a maximum current efficiency of 92.83 cd A−1 and a maximum external quantum efficiency of 29.5%, suggesting these materials have potential application in OLEDs.

A novel ancillary ligand of pyridine sulfonic acid (PySO3) was developed for two green-emitting iridium(III) compounds, Ir1 (λmax = 496 nm) and Ir2 (λmax = 504 nm), with trifluoromethyl-substituted 2-phenylpyridine derivatives as the main ligands. Due to the strong electron-withdrawing ability of PySO3, both complexes have relatively low LUMO energy levels and good electron mobility, which benefit the charge balance in the organic light-emitting diodes (OLEDs) during the electroluminescence process. Therefore, all devices with double light-emitting layers exhibit good performances. In particular, the device using Ir2 as an emitter obtains a maximum luminance above 92 000 cd m−2, a maximum external quantum efficiency (EQEmax) of 25.5% with an extremely low efficiency roll-off, and the EQE still remains at 22.9% at the high luminance of 20 000 cd m−2. These results demonstrate that pyridine sulfonic acid is a potential and charming ligand for Ir(III) complexes and high-performance OLEDs.

Two new cyclometalated platinum(II) complexes ((TN3T)Pt(dptp), (4tfmppy)Pt(dptp)) with 2′,6′-bis(trifluoromethyl)-2,3′-bipyridine (TN3T) and 4-trifluoromethylphenylpyridine (4tfmppy) as the cyclometalated ligands and the nitrogen heterocycle 2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenol (dptp) as the ancillary ligand were developed. Both complexes are green phosphors with high photoluminescence quantum efficiency yields (λmax = 532 nm, Φp = 65% for (TN3T)Pt(dptp) and λmax = 502 nm, Φp = 71% for (4tfmppy)Pt(dptp)) in CH2Cl2 solutions at room temperature, respectively. Organic light-emitting diodes (OLEDs) with single-emitting layer and double-emitting layer structures were fabricated with good performances. Particularly, the double-emitting layer device D2 based on the complex (4tfmppy)Pt(dptp) with 6 wt% doped concentration shows superior performances with a maximum EQE of 26.90% with mild efficiency roll-off. This study demonstrates that the ancillary ligand attached with a 4-phenyl-4H-1,2,4-triazole group could facilitate charge trapping across the bulk of the device for efficient OLEDs.

Two new iridium(III) cyclometalated complexes (Ir-TN3T and Ir-TN4T) with 2′,6′-bis(trifluoromethyl)-2,3′-bipyridine (TN3T) and 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine (TN4T) as the cyclometalated ligands and 2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenol (dptp) as the ancillary ligand were developed. Both complexes are green phosphors with high photoluminescence quantum efficiency yields (λmax = 540 nm, Φ = 71.2% for Ir-TN3T and λmax = 513 nm, Φ = 91.0% for Ir-TN4T, respectively) in CH2Cl2 solution at room temperature. Organic light emitting diodes (OLEDs) with single-emitting-layer and double-emitting-layer structures exhibit good performances. Particularly, device D2 with a double-emitting-layer structure based on a Ir-TN4T complex of 8 wt% doped concentration exhibits superior performances with a peak current efficiency of 94.29 cd A−1 and a peak external quantum efficiency (EQEmax) of 31.43%, and the efficiency roll-off is mild. When the luminance reaches 10 000 cd m−2, the EQE value can still be maintained at 30.26%. This study demonstrates that the Ir(III) complexes containing the ancillary ligand attached with the electron-transporting group of 4-phenyl-4H-1,2,4-triazole could facilitate charge trapping across the bulk of the device and they have promising applications in OLEDs.

To boost intrinsic circularly polarized luminescence (CPL) properties of chiral emitters, an axially chiral biphenyl unit is inlaid in thermally activated delayed fluorescent (TADF) skeleton, urging the participation of chiral source in frontier molecular orbital distributions. A pair of enantiomers, (R)‐BPPOACZ and (S)‐BPPOACZ, containing the cyano as electron‐withdrawing moieties and carbazole and phenoxazine as electron‐donating units are synthesized and separated. The circularly polarized TADF enantiomers exhibit both high photoluminescence quantum yield of 86.10% and excellent CPL activities with maximum dissymmetry factor |g PL| values of almost 10−2 in solution and 1.8 × 10−2 in doped film, which are among the best values of previously reported small chiral organic materials. Moreover, the circularly polarized organic light‐emitting diodes based on the TADF enantiomers achieve the maximum external quantum efficiency of 16.6% with extremely low efficiency roll‐off. Obvious circularly polarized electroluminescence signals with |g EL| values of 4 × 10−3 are also recorded., Taking a new design strategy, a pair of axially chiral thermally activated delayed fluorescent enantiomers are developed with high photoluminescence quantum yield and excellent circularly polarized luminescence signals. Moreover, the resulting circularly polarized organic light‐emitting diodes achieve both high external quantum efficiency with low efficiency roll‐off and obvious circularly polarized electroluminescence signals.

Thermally activated delayed fluorescence (TADF) is one of the most intriguing and promising discoveries towards realization of highly-efficient organic light emitting diodes (OLED) utilizing small molecules as emitters. It has the capability of manifesting all excitons generated during the electroluminescent processes, consequently achieving 100 % of internal quantum efficiency. Since the report of the first efficient OLED based on a TADF small molecule in 2012 by Adachi et al., the quest for optimal TADF materials for OLED application has never stopped. Various TADF molecules bearing different design concepts and strategies have been designed and produced, with the aim to boost the overall performances of corresponding OLEDs. In this minireview, the general principles of TADF molecular design based on three basic categories of TADF species: twisted intramolecular charge transfer (TICT), through-space charge transfer (TSCT) and multi-resonance induced TADF (MR-TADF) are discussed in detail. Several key aspects with respect to each category, as well as some effective methods to enhance the efficiency of TADF materials and corresponding OLEDs from the molecular engineering perspectives, are summarized and discussed to exhibit a general landscape of TADF molecular design to a wide variety of scientific researchers within this particular disciplinary area.

Correction for ‘Fast synthesis of iridium(iii) complexes with sulfur-containing ancillary ligand for high-performance green OLEDs with EQE exceeding 31%’ by Guang-Zhao Lu et al., J. Mater. Chem. C, 2019, 7, 7273–7278.