Effects of Deuterium Isotopes on Pt(II) Complexes and Their Impact on Organic NIR Emitters.

Insight into effect of deuterium isotopes on organic near-IR (NIR) emitters was explored by the use of self-assembled Pt(II) complexes H-3-f and HPh-3-f, and their deuterated analogues D-3-f and DPh-3-f, respectively (Scheme 2). In vacuum deposited thin film, albeit having nearly identical emission spectral feature maximized at ~810 nm, H-3-f and D-3-f exhibit remarkable difference in photoluminescence quantum yield (PLQY) of 29 % and 50 %, respectively. Distinction in PLQY is also observed for HPh-3-f (800 nm, 50 %) and DPh-3-f (798 nm, 67 %). We then elucidated the theoretical differences in the impact on near-infrared (NIR) luminescence between Pt(II) complexes and organic small molecules upon deuteration. The results establish a general guideline for the deuteration on NIR emission efficiency. From a perspective of practical application, NIR OLEDs based on D-3-f and DPh-3-f emitters attain EQEmax of 15.5 % (radiance 31,287 mW Sr-1 m-2 ) and 16.6 % (radiance of 32,279 mW Sr-1 m-2 ) at 764 nm and 796 nm, respectively, both of which set new records for NIR OLEDs of >750 nm.

Exciplex-forming cohost systems with 2,7-dicyanofluorene acceptors for high efficiency red and deep-red OLEDs.

Two 2,7-dicyaonfluorene-based molecules 27-DCN and 27-tDCN are utilized as acceptors (A) to combine with hexaphenylbenzene-centered donors (D) TATT and DDT-HPB for probing the exciplex formation. The photophysical characteristics reveal that the steric hindered 27-tDCN not only can increase the distance of D and A, resulting in a hypsochromic emission, but also dilute the concentration of triplet excitons to suppress non-radiative process. The 27-tDCN-based exciplex-forming blends exhibit better photoluminescence quantum yield (PLQY) as compared to those of 27-DCN-based pairs. In consequence, among these D:A blends, the device employing DDT-HPB:27-tDCN blend as the emissiom layer (EML) exhibits the best EQE of 3.0% with electroluminescence (EL) λmax of 542 nm. To further utilize the exciton electrically generated in exciplex-forming system, two D-A-D-configurated fluorescence emitter DTPNT and DTPNBT are doped into the DDT-HPB:27-tDCN blend. The nice spectral overlap ensures fast and efficient Förster energy transfer (FRET) process between the exciplex-forming host and the fluorescent quests. The red device adopting DDT-HPB:27-tDCN:10 wt% DTPNT as the EML gives EL λmax of 660 nm and maximum external quantum efficiency (EQEmax) of 5.8%, while EL λmax of 685 nm and EQE of 5.0% for the EML of DDT-HPB:27-tDCN:10 wt% DTPNBT. This work manifests a potential strategy to achieve high efficiency red and deep red OLED devices by incorporating the highly fluorescent emitters to extract the excitons generated by the exciplex-forming blend with bulky acceptor for suppressing non-radiative process.

Entropy-driven charge-transfer complexation yields thermally activated delayed fluorescence and highly efficient OLEDs.

Exciplex-forming systems that display thermally activated delayed fluorescence are widely used for fabricating organic light-emitting diodes. However, their further development can be hindered through a lack of structural and thermodynamic characterization. Here we report the generation of inclusion complexes between a cage-like, macrocyclic, electron-accepting host (A) and various N-methyl-indolocarbazole-based electron-donating guests (D), which exhibit exciplex-like thermally activated delayed fluorescence via a through-space electron-transfer process. The D/A cocrystals are fully resolved by X-ray analyses, and UV-visible titration data show their formation to be an endothermic and entropy-driven process. Moreover, their emission can be fine-tuned through the molecular orbitals of the donor. Organic light-emitting diodes were fabricated using one of the D/A systems, and the maximum external quantum efficiency measured was 15.2%. An external quantum efficiency of 10.3% was maintained under a luminance of 1,000 cd m-2. The results show the potential of adopting inclusion complexation to better understand the relationships between the structure, formation thermodynamics and properties of exciplexes.

Modifications of Pyridine-3,5-dicarbonitrile Acceptor for Highly Efficient Green-to-Red Organic Light-Emitting Diodes.

The strategy of acceptor modification is a powerful technique for tuning the emission color of thermally activated delayed fluorescence (TADF) emitters. In this study, we have successfully designed and synthesized three TADF emitters with donor-acceptor (D-A) structures using a 4-(diphenylamino)-2,6-dimethylphenyl (TPAm) donor and various pyridine-3,5-dicarbonitrile (PC) acceptor units. As a result, three compounds named TPAmbPPC, TPAm2NPC, and TPAmCPPC exhibited greenish-yellow to orange-red emissions with high photoluminescent quantum yields (76-100%) in thin films. Remarkably, a greenish-yellow device based on TPAmbPPC and TPAm2NPC showed a high maximum external quantum efficiency (EQEmax) of 39.1 and 39.0%, respectively. Furthermore, benefiting from the suitable steric hindrance between the acceptor and donor, the nondoped organic light-emitting diodes (OLEDs) based on TPAmbPPC demonstrated an exceptional EQEmax of 21.6%, indicating its promising potential as an efficient emitter for the application of OLED applications. Furthermore, orange-red OLED devices based on TPAmCPPC exhibited a high EQEmax of 26.2%, a CE of 50.1 cd A-1, and a PE of 52.4 lm W-1.

Engineering of Cyano Functionalized Benzo[d]imidazol-2-ylidene Ir(III) Phosphors for Blue Organic Light-Emitting Diodes.

In this study, we designed and synthesized three series of blue emitting homoleptic iridium(III) phosphors bearing 4-cyano-3-methyl-1-phenyl-6-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (mfcp), 5-cyano-1-methyl-3-phenyl-6-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (ofcp), and 1-(3-(tert-butyl)phenyl)-6-cyano-3-methyl-4-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (5-mfcp) cyclometalates, respectively. These iridium complexes exhibit intense phosphorescence in the high energy region of 435-513 nm in the solution state at RT, to which the relatively large T1 → S0 transition dipole moment is beneficial for serving as a pure emitter and an energy donor to the multiresonance thermally activated delayed fluorescence (MR-TADF) terminal emitters via Förster resonance energy transfer (FRET). The resulting OLEDs achieved true blue, narrow bandwidth EL with a max EQE of 16-19% and great suppression of efficiency roll-off with ν-DABNA and t-DABNA. We obtained the FRET efficiency up to 85% using titled Ir(III) phosphors f-Ir(mfcp)3 and f-Ir(5-mfcp)3 to achieve true blue narrow bandwidth emission. Importantly, we also provide analysis on the kinetic parameters involved in the energy transfer processes and, accordingly, propose feasible ways to improve the efficiency roll-off caused by the shortened radiative lifetime of hyperphosphorescence.

Exciplex-Forming Cohost Systems with 2,3-Dicyanopyrazinophenanthrene-based Acceptors to Achieve Efficient Near Infrared OLEDs.

Two new 2,3-dicyanopyrazinophenanthrene-based acceptors (A) p-QCN and m-QCN were synthesized to blend with a donor (D) CPTBF for the exciplex formation. The energy levels of p-QCN and m-QCN are modulated by the peripheral substituents 4- and 3-benzonitrile, respectively. Exciplex-forming blends were identified by the observation of the red-shifted emissions from various D : A blends with higher ratios of donor for suppressing the aggregation of acceptor. The two-component relaxation processes observed by time-resolved photoluminescence support the thermally activated delayed fluorescence (TADF) character of the exciplex-forming blends. The device employing CPTBF : p-QCN and (2 : 1) and CPTBF : m-QCN (2 : 1) blend as the emitting layer (EML) gave EQEmax of 1.76 % and 5.12 %, and electroluminescence (EL) λmax of 629 nm and 618 nm, respectively. The device efficiency can be further improved to 4.32 % and 5.57 % with CPTBF : p-QCN and (4 : 1) and CPTBF : m-QCN (4 : 1) as the EML, which is consistent with their improved photoluminescence quantum yields (PLQYs). A new fluorescent emitter BPBBT with photoluminescence (PL) λmax of 726 nm and a high PLQY of 67 % was synthesized and utilized as the dopant of CPTBF : m-QCN (4 : 1) cohost system. The device employing CPTBF : m-QCN (4 : 1): 5 wt.% BPBBT as the EML gave an EQEmax of 5.02 % and EL λmax centered at 735 nm, however, the weak residual exciplex emission remains. By reducing the donor ratio, the exciplex emission can be completely transferred to BPBBT and the corresponding device with CPTBF : m-QCN (2 : 1): 5 wt.% BPBBT as the EML can achieve EL λmax of 743 nm and EQEmax of 4.79 %. This work manifests the high efficiency near infrared (NIR) OLED can be realized by triplet excitons harvesting of exciplex-forming cohost system, followed by the effective energy transfer to an NIR fluorescent dopant.

Requirements for Additional Strength Biowaivers for Modified Release Solid Oral Dosage Forms in International Pharmaceutical Regulators Programme Participating Regulators and Organisations: Differences and Commonalities.

This article describes an overview of waivers of in vivo bioequivalence studies for additional strengths in the context of the registration of modified release generic products and is a follow-up to the recent publication for the immediate release solid oral dosage forms. The current paper is based on a survey among the participating members of the Bioequivalence Working Group for Generics (BEWGG) of the International Pharmaceutical Regulators Program (IPRP) regarding this topic. Most jurisdictions consider the extrapolation of bioequivalence results obtained with one (most sensitive) strength of a product series as less straightforward for modified release products than for immediate release products. There is consensus that modified release products should demonstrate bioequivalence not only in the fasted state but also in the fed state, but differences exist regarding the necessity of additional multiple dose studies. Fundamental differences between jurisdictions are revealed regarding requirements on the quantitative composition of different strengths and the differentiation of single and multiple unit dosage forms. Differences in terms of in vitro dissolution requirements are obvious, though these are mostly related to possible additional comparative investigations rather than regarding the need for product-specific methods. As with the requirements for immediate release products, harmonization of the various regulations for modified release products is highly desirable to conduct the appropriate studies from a scientific point of view, thus ensuring therapeutic equivalence.

Triarylamine-Pyridine-Carbonitriles for Organic Light-Emitting Devices with EQE Nearly 40.

Highly efficient thermally activated delayed fluorescence (TADF) molecules are in urgent demand for solid-state lighting and full-color displays. Here, the design and synthesis of three triarylamine-pyridine-carbonitrile-based TADF compounds, TPAPPC, TPAmPPC, and tTPAmPPC, are shown. They exhibit excellent photoluminescence quantum yields of 79-100% with small ΔEST values, fast reverse intersystem crossing (RISC), and high horizontal dipole ratios (Θ//  = 86-88%) in the thin films leading to the enhancement of device light outcoupling. Consequently, a green organic light-emitting diode (OLED) based on TPAmPPC shows a high average external quantum efficiency of 38.8 ± 0.6%, a current efficiency of 130.1 ± 2.1 cd A-1 , and a power efficiency of 136.3 ± 2.2 lm W-1 . The highest device efficiency of 39.8% appears to be record-breaking among TADF-based OLEDs to date. In addition, the TPAmPPC-based device shows superior operation lifetime and high-temperature resistance. It is worth noting that the TPA-PPC-based materials have excellent optical properties and the potential for making them strong candidates for TADF practical application.

Highly Efficient Simple-Structure Sky-Blue Organic Light-Emitting Diode Using a Bicarbazole/Cyanopyridine Bipolar Host.

Up to now, the most efficient blue phosphorescent organic light-emitting diode (PhOLED) was achieved with a maximum external quantum efficiency (ηext) of 34.1% by using an exciplex cohost. It still remains a challenge to obtain such high efficiencies using a single-host matrix. In this work, a highly efficient sky-blue PhOLED is successfully fabricated using a newly developed bipolar host material, namely 5-(2-(9H-[3,9'-bicarbazol]-9-yl)phenyl)nicotinonitrile (o-PyCNBCz), which realizes a ηext of 29.4% at a practical luminance of 100 cd m-2 and a maximum ηext of 34.6% (at 23 cd m-2). The present device is characterized by simple configuration with a single host and single emitting layer. o-PyCNBCz also reveals high efficiency of 28.2% (94.8 cd A-1) when used as the host for green PhOLED. Under identical conditions, o-PyCNBCz always outperforms than its isomer 3-PyCNBCz (5-(9-phenyl-9H-[3,9'-bicarbazol]-6-yl)nicotinonitrile) in terms of more balanced charge transportation, higher photoluminescent quantum yields of over 90%, and higher horizontal orientation ratio of the emitting dipole for the host-dopant films, which finally lead to its superior performance in PhOLEDs. It is observed that all these merits of o-PyCNBCz benefit from its ortho-linking style of carbazole (p-type unit) and cyanopyridine (n-type unit) on the phenylene bridge and the resultant molecular conformation.

Rational Tuning of Bis-Tridentate Ir(III) Phosphors to Deep-Blue with High Efficiency and Sub-microsecond Lifetime.

A new class of bis-tridentate Ir(III) complexes (Dap-1-4) was synthesized using carbene pincer pro-chelates PC1·H3(PF6)2 or PC2·H3(PF6)2 with either imidazolylidene or imidazo[4,5-b]pyridin-2-ylidene appendages, together with a second cyclometalating 2,6-diaryoxypyridine chelate, L1H2 and L2H2, differed by a NMe2 donor at the central pyridinyl fragment. The respective emission tuning between the ultraviolet and blue region was rationalized using time-dependent density functional theory (TD-DFT) approaches. Next, a highly efficient blue emitter (Dap-5) was synthesized by concomitant addition of two methyl groups and a single CF3 substituent at the central phenyl and peripheral imidazo[4,5-b]pyridin-2-ylidene entities of the carbene pincer chelate, respectively. The organic light-emitting diode (OLED) device with 15 wt % Dap-5 in DPEPO shows electroluminescence at 468 nm and with CIE (0.14, 0.15) and a max external quantum efficiency (max EQE) of 16.8% with low efficiency roll-off (EQE of 14.4% at 1000 cd m-2); the latter is attributed to the relatively shortened triplet excited-state radiative lifetime. These results highlight the adequateness of bis-tridentate Ir(III) phosphors in fabrication of practical blue-emitting OLEDs.

A Survey of the Regulatory Requirements for the Waiver of In Vivo Bioequivalence Studies of Generic Products in Certain Dosage Forms by Participating Regulators and Organisations of the International Pharmaceutical Regulators Programme.

The requirements to waive in vivo bioequivalence studies for immediate release solid oral dosage forms based on the Biopharmaceutics Classifications System (BCS) are well known, and biowaivers[1] for other types of oral dosage forms based on pre-defined criteria may also be acceptable. Similarly, biowaivers for dosage forms such as injectable products may also be allowed if certain criteria are met. The current paper summarises the biowaiver requirements for oral solutions and suspensions, soft gelatin capsules and injectable products (intravenous injections, subcutaneous and intramuscular injections, emulsions for injection and micellar solutions for injection) among the participants of the Bioequivalence Working Group for Generics (BEWGG) of the International Pharmaceutical Regulators Programme (IPRP). A review of the requirements indicated that there was a trend towards convergence when the dosage form became less complex; however, the most common approach used by each of the jurisdictions was a case-by-case approach given that most jurisdictions do not have well defined guidelines to support all possible scenarios. Even in the simplest case of intravenous solutions, the acceptability of qualitative changes in excipients differ between the IPRP members.  Notwithstanding the differences, the dissemination of the information is a first step towards regulatory convergence regarding biowaivers for certain dosage forms and should be useful for pharmaceutical companies currently developing generic medicinal products for IPRP jurisdictions.

Diboron-Based Delayed Fluorescent Emitters with Orange-to-Red Emission and Superior Organic Light-Emitting Diode Efficiency.

For the application of organic light-emitting diodes (OLEDs) in lighting and panels, the basic requirement is to include a full spectrum color range. Compared with the development of blue and green luminophores in thermally activated delayed fluorescence (TADF) technology, the progress of orange-to-red materials is slow and needs further investigation. In this study, three diboron compound-based materials, dPhADBA, dmAcDBA, and SpAcDBA, were designed and synthesized by nucleophilic arylation of three amine donors on 9,10-diboraanthracene (DBA) in a two-step reaction. With increasing electron-donating ability, they show orange-to-red emission with TADF characteristics. The electroluminescence of these diboron compounds exhibits emissions λmax at 613, 583, and 567 nm for dPhADBA, dmAcDBA, and SpAcDBA, respectively. It is noteworthy that the rod-like D-A-D structures can achieve high horizontal ratios (84-86%) and outstanding device performance for orange-to-red TADF OLEDs: the highest external quantum efficiencies for dPhADBA, dmAcDBA, and SpAcDBA are 11.1 ± 0.5, 24.9 ± 0.5, and 30.0 ± 0.8%, respectively. Therefore, these diboron-based molecules offer a promising avenue for the design of orange-to-red TADF emitters and the development of highly efficient orange-to-red OLEDs.

Versatile Pt(II) Pyrazolate Complexes: Emission Tuning via Interplay of Chelate Designs and Stacking Assemblies.

Three homoleptic Pt(II) metal complexes [Pt(imPz)2] (1), [Pt(imiz)2] (2), and [Pt(imMz)2] (3) were synthesized from the treatment of Pt(DMSO)2Cl2 and functional imidazolyl pyrazole in refluxing tetrahydrofuran (THF). Alternatively, the heteroleptic Pt(II) complexes [Pt(imPz)(fppz)] (4), [Pt(imiz)(fppz)] (5), and [Pt(imMz)(fppz)] (6) were obtained from the treatment of a common intermediate [Pt(fppzH)Cl2] with a corresponding imidazolyl chelate. Pt(II) complexes 1, 2, and 5 were studied by single-crystal X-ray diffraction to reveal the corresponding packing arrangement in their crystal lattices, among which both homoleptic complexes 1 and 2 formed monomeric species, while heteroleptic 5 aligned as a dimer with a nonbonding Pt···Pt contact of 3.574 Å. Subsequent photophysical examinations showed that the homoleptic 1-3 and heteroleptic 4-6 exhibited the structured sky-blue ππ* emission and structureless light-green-emitting metal-metal-to-ligand charge transfer (MMLCT) emission in the solid state, respectively. A shortened Pt···Pt interaction of approximately 0.34-0.35 nm was confirmed in thin films of all heteroleptic Pt(II) complexes 4-6 by grazing-incidence X-ray diffraction (GIXD) analyses. Finally, Pt(II) complex 6 was employed as a dopant in the fabrication of organic light-emitting diode (OLED) devices with varied doping ratios, among which OLEDs with only 1 wt % 6 in the SimCP host exhibited a maximum external quantum efficiency (EQE) of 5.8% and CIEx,y values of 0.20, 0.31. In contrast, OLEDs using a nondoped architecture (i.e., 100% of 6 in the emitting layer (EML)) achieved a maximum EQE of 26.8%, current efficiency (CE) of 91.7 cd A-1, and power efficiency (PE) of 80.1 lm·W-1 and CIEx,y values of 0.41, 0.55, manifesting their versatility in various degrees of stacking assemblies and hence facile color-tuning capability on OLEDs.

Harnessing a New Co-Host System and Low Concentration of New TADF Emitters Equipped with Trifluoromethyl- and Cyano-Substituted Benzene as Core for High-Efficiency Blue OLEDs.

A strategic approach combining a new co-host system and low concentration of new thermally activated delayed fluorescence (TADF) emitters to make efficient blue TADF organic light-emitting diode (OLED) was developed. The benchmark TADF molecule, 4CzIPN, was adopted as a probe to examine the feasibility of a co-host composing of a hole transporter SimCP and an electron transporter oCF3-T2T. As a result, a sky blue device with 1 wt % 4CzIPN doped in SimCP:oCF3-T2T co-host exhibited 100% energy transfer and achieved a high external quantum efficiency (EQE) up to 26.1%. Importantly, this device showed a limited efficiency rolloff with an EQE of 24% at 1000 cd m-2. To further shift the emission toward blue, three new TADF molecules, 4CzIPN-CF3, 3CzIPN-H-CF3, and 3CzIPN-CF3, modified either by lowering the electron-withdrawing ability of the acceptor group or by reducing the number of carbazole donors of 4CzIPN, have been synthesized and characterized. Among them, 4CzIPN-CF3 and 3CzIPN-H-CF3 display hypsochromic shift emissions compared to that of 4CzIPN. These new compounds were then explored for their potential applications as TADF emitters. Blue TADF OLEDs with 1 wt % of 4CzIPN-CF3 and 3CzIPN-H-CF3 dispersed in SimCP:oCF3-T2T co-host achieved EQEs of 23.1 and 16.5% and retained high EQEs of 20.9 and 14.7% at 1000 cd m-2, respectively.

A Survey of the Regulatory Requirements for the Acceptance of Foreign Comparator Products by Participating Regulators and Organizations of the International Generic Drug Regulators Programme.

The acceptance of foreign comparator products is the most limiting factor for the development and regulatory assessment of generic medicines marketed globally. Bioequivalence studies have to be repeated with the local comparator products of each jurisdiction because it is unknown if the comparators of the different countries are the same product, with the consequent duplication of efforts by regulators and industry alike. The regulatory requirements on the acceptability of foreign comparator products of oral dosage forms differ between countries participating in the Bioequivalence Working Group for Generics of the International Pharmaceutical Regulators Programme. Brazil, Colombia, the European Union member States, Japan, Mexico, South Korea and the United States only accept bioequivalence studies with their local comparator. In contrast, Australia, Canada, New Zealand, Singapore, South Africa, Switzerland and Taiwan accept studies with foreign comparators under certain conditions. Canada limits its use to highly soluble drugs with a wide therapeutic range in immediate release products. Australia requires a comparison of the quantitative composition. In contrast, there are fewer restrictions on the acceptance of foreign comparators in New Zealand, Singapore, South Africa, Switzerland and Taiwan. For the WHO Prequalification of Medicines and for developing generics of the essential medicines the WHO lists comparators from different countries. In conclusion, there is currently no consensus amongst regulators on the acceptability of foreign comparator products.

The Requirements for Additional Strength Biowaivers for Immediate Release Solid Oral Dosage Forms in International Pharmaceutical Regulators Programme Participating Regulators and Organisations: Differences and Commonalities.

In relation to the registration of generic products, waivers of in vivo bioequivalence studies (biowaivers) are considered in three main cases: certain dosage forms for which bioequivalence is self-evident (e.g. intravenous solutions), biowaivers based on the Biopharmaceutics Classification System and biowaivers for additional strengths with respect to the strength for which in vivo bioequivalence has been shown. The objective of this article is to describe the differences and commonalities in biowaivers for additional strengths of immediate release solid oral dosage forms between the participating members of the International Pharmaceutical Regulators Program (IPRP). The requirements are based on five main aspects; the pharmacokinetics of the drug substance, the manufacturing process, the qualitative and quantitative composition of the different strengths, and the comparative dissolution profiles. For the pharmacokinetic aspects, many regulators/agencies have the same requirements. All strengths must be manufactured with the same process, although a few regulators/agencies accept small differences. In relation to the formulation aspects, the data required breaks down into three major approaches based initially on one of those of the EU, the USA or Japan, but there are some differences in these three major approaches with some country specific interpretations. Most regulators/agencies also have the same requirements for the dissolution data, though there are some notable exceptions.

Exciplex Cohosts Employing Nonconjugated Linked Dicarbazole Donors for Highly Efficient Thermally Activated Delayed Fluorescence-Based Organic Light-Emitting Diodes.

Two new nonconjugated linked dicarbazole materials, dCzPSi and dCzPSO2, with high triplet energy were synthesized and characterized. dCzPSi and dCzPSO2 were adopted as unipolar host materials for the green thermally activated delayed fluorescence (TADF) emitter (4CzIPN) to achieve high-efficiency organic light-emitting diodes (OLEDs). The electron-transporting acceptor, PO-T2T, was introduced to mix with dCzPSi and dCzPSO2 to give two new exciplex-forming systems that can improve the exciton formation propensity in the emitting layer. The relevant properties of these new exciplexes were characterized, and they were suggested as promising cohosts for the green TADF emitter, 4CzIPN. The characteristics of the devices employing single hosts (dCzPSi and dCzPSO2) and exciplex-forming cohosts (dCzPSi:PO-T2T and dCzPSO2:PO-T2T) were explored. The obtained results indicate that the Si-bridged dicarbazole compound dCzPSi outperforms its counterpart dCzPSO2 in which two carbazole groups are linked by an SO2 group. The device employed with the dCzPSi:PO-T2T cohost with 10 wt % 4CzIPN achieved a low Von of 2.2 V and maximum efficiencies of 21.1% (external quantum efficiency), 56.4 cd A-1 (current efficiency), 59.1 lm W-1 (power efficiency), as compared to those (18.7%, 56.6 cd A-1, and 68.5 lm W-1) of the dCzPSO2:PO-T2T-hosted device. This work verifies the advantages of using a cohost that can form an exciplex for boosting the device efficiency with reduced efficiency roll-off of TADF-based OLEDs.

Probe exciplex structure of highly efficient thermally activated delayed fluorescence organic light emitting diodes.

The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). We report here the pump-probe Step-Scan Fourier transform infrared spectra of exciplex composed of a carbazole-based electron donor (CN-Cz2) and 1,3,5-triazine-based electron acceptor (PO-T2T) codeposited as the solid film that gives intermolecular charge transfer (CT), TADF, and record-high exciplex type cyan organic light emitting diodes (external quantum efficiency: 16%). The transient infrared spectral assignment to the CT state is unambiguous due to its distinction from the local excited state of either the donor or the acceptor chromophore. Importantly, a broad absorption band centered at ~2060 cm-1 was observed and assigned to a polaron-pair absorption. Time-resolved kinetics lead us to conclude that CT excited states relax to a ground-state intermediate with a time constant of ~3 µs, followed by a structural relaxation to the original CN-Cz2:PO-T2T configuration within ~14 µs.

Cyanopyrimidine-Carbazole Hybrid Host Materials for High-Efficiency and Low-Efficiency Roll-Off TADF OLEDs.

Two isomeric host materials (Sy and Asy) comprising carbazole (donor) and CN-substituted pyrimidine (acceptor) were synthesized, characterized, and utilized as host materials for green and blue thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). Both molecules have high triplet energy and small energy difference between singlet and triplet states, leading to feasible TADF. The different linking topologies of carbazole and CN groups on the pyrimidine core provide distinct photophysical properties and molecular packing manners, which further influence the efficiency as they served as hosts in TADF OLEDs. As compared to Asy-based cases, the Sy-hosted TADF OLED device gave higher maximum external quantum efficiencies (EQE) of 24.0% (vs 22.5%) for green (4CzIPN as a dopant) and 20.4% (vs 15.0%) for blue (2CzTPN as a dopant) and low efficiency roll-off. The high horizontal dipole ratio (Θ ≈ 88%) for both emitters dispersed in Sy and Asy hosts accounts for the high device efficiency. A clear molecular structure-physical property-device performance relationship has been established to highlight the importance of symmetrical structure in TADF host material design.