Crucial role of palladium(0/II) catalysts in the synthesis of multi-resonance thermally activated delayed fluorescence emitters

Abstract

Boron and nitrogen/oxygen-based multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters represent cutting-edge OLED technology in both academic and industrial research, demonstrating high color purity and power efficiency. However, the advancement of these emitters is somewhat constrained due to limited synthetic methodologies and low yields of the emitters and their respective precursors. Therefore, comprehensive knowledge of synthetic approaches is necessary, particularly concerning the precursors for improved molecular development. Most precursors for the final emitter are synthesized by forming C–C/C–X bonds, involving palladium-based catalysts and additives as crucial reagents. In this review, we thoroughly discuss the synthetic approaches used to prepare the intermediates with the palladium catalyst alongside various ligands, along with most of the recent reports. We also outline the general criteria for selecting the Pd catalyst and ligand, their respective molar equivalents, and the corresponding yields.

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Ajeet Chandra

Dr Ajeet Chandra is highly motivated for science and works as a research professor in the Department of Information Display at Kyung Hee University, Seoul, South Korea. At present, he is involved in the synthesis of display materials based on BODIPYs and MR emitters and also in their applications. Additionally, he gained an institute post-doctoral experience in synthetic methodology from the Department of Chemistry at the Indian Institute of Technology (IIT) Bombay. Previously, he completed his B. Sc. and M. Sc. (Organic Chemistry) from VBS Purvanchal University and the University of Allahabad, UP, India, respectively. Thereafter, he joined a research program at IIT Delhi. Due to his orientation towards organic synthesis, he moved to IIT Kanpur to complete his doctoral program. During that time, he was involved in the oxidative transformations of organic compounds using hypervalent iodine(V) reagents. Dr Chandra is always thankful to the present and past colleagues, teaching mentors, research advisors, and educational and research institutions for funding and infrastructure.

Paramasivam Palanisamy

Mr Paramasivam Palanisamy completed his bachelor's degree in 2016 from Sri Ramakrishna Mission Vidyalaya College of Arts and Science in Coimbatore, Tamil Nadu, India, and received a master's degree in chemical sciences from the National Institute of Technology in Tiruchirappalli, Tamil Nadu, India, in 2018. Currently, he is pursuing his doctoral research program under the guidance of Prof. Jang Hyuk Kwon at the Organic Optoelectronic Device Laboratory in the Department of Information Display at Kyung Hee University in Seoul, South Korea. His research focuses on the design and synthesis of MR-TADF materials for optoelectronic devices.

Aradhya Rajput

Aradhya Rajput obtained her bachelor's degree from the University of Delhi, India. She is currently pursuing an integrated master's and Ph.D. program in the Department of Information Display at Kyung Hee University in Seoul, South Korea, under the supervision of Prof. Jang Hyuk Kwon. Her research focuses on designing and developing boron-based multi-resonance thermally activated delayed fluorescence (MR-TADF) materials and their applications in optoelectronic devices.

Jang Hyuk Kwon

Prof. Dr Jang Hyuk Kwon is an honorable professor in the Department of Information Display at Kyung Hee University in Seoul, South Korea. He earned his PhD in chemistry from the Korea Advanced Institute of Science and Technology (KAIST), Seoul, in 1993. From 1993 to 2005, he served as a chief researcher at Samsung SDI in South Korea. His research interests include developing organic materials for OLEDs, QLEDs, OPVs, organic nanodots, and electrochromic devices.

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Introduction

Over the past few decades, the prevalence of displays across different applications has grown considerably. Life without information displays, like mobile phones, watches, TVs, or computers, seems bland nowadays. These devices play a crucial role in keeping us connected to our environment and the broader world. Several display technologies have been used on these devices for better visual performance.^1–3 Among these, organic light-emitting diodes (OLEDs) have secured a significant position in the commercial display industry due to their benefits over other display technologies, including large grayscale, ultra-thin flexible screens, high color purity, and low power consumption.^1,2

A basic OLED comprises several organic layers sandwiched between two metal electrodes, allowing carriers to flow and recombine at the emissive layer to emit light. Among all the layers, the emissive layer (EML) is crucial for determining the emission color and stability of OLED devices.^3,4 It is a co-evaporated layer made up of a host material and an emitter molecule, with the emitter molecule being essential for optimizing the device performance.^5–7 According to the spin-statistics rule, the hole and electron recombine in the EML to produce 25% singlet excitons and 75% triplet excitons.^5–7 Based on exciton utilization efficiency, OLEDs are categorized into several generations.⁸ The first-generation OLEDs utilize conventional fluorescence emitters, where only singlet excitons decay to the ground state to emit light.⁷ Consequently, the internal quantum efficiency (IQE) is limited to 25%.^9,10 To enhance the IQE, heavy metal-based emitters were employed in second-generation phosphorescence OLEDs (Ph-OLEDs). Metal complexes utilize both singlet and triplet excitons, theoretically enhancing the IQE to 100%. Although Ph-OLEDs offer high efficiency and stability, incorporating noble metals in these emitters makes them less economical for mass production.^11,12 To address these issues, thermally activated delayed fluorescence (TADF) emitters were developed by Adachi et al. as third-generation OLED emitters. This is a pure organic molecule capable of harvesting dark triplet excitons through an efficient reverse intersystem crossing (RISC) process and achieves 100% IQE, resulting in high efficiency and stability across a broad spectrum of colors.^13–18 However, TADF emitters have a donor–acceptor structure connected by a single covalent bond, leading to significant structural deformation in the excited state.^17,18 Consequently, this results in a broader emission spectrum, which causes poor color purity in the OLED devices.

Considering these shortcomings, Hatakeyama et al. developed a subclass of TADF emitters known as multi-resonance TADF (MR-TADF) emitters in 2016.¹⁹ MR-TADF emitters consist of boron (B) and nitrogen (N) embedded in polycyclic aromatic hydrocarbons (PAHs), featuring atomically separated highest-occupied molecular orbitals (HOMOs) and lowest-occupied molecular orbitals (LUMOs). This unique molecular design yields an MR effect that produces a narrow emission spectrum, while high molecular rigidity boosts photoluminescence quantum efficiency (PLQY).²⁰ As a result, it provides high color purity and improved external quantum efficiency (EQE) in OLED devices.^21–35 However, tuning the emission color is somewhat limited due to the synthetic challenges associated with the intermediate and final borylated compounds. Herein, wide range of intermediate precursors are produced by C–C and C–X (X = N, O, etc.) bond formation reactions. The most frequently utilized synthetic methods for these reactions are Buchwald–Hartwig cross-coupling,^32,36–38 Heck coupling,^39,40 Suzuki reaction,^41–44 and Hiyama coupling,^45,46 all of which crucially involve palladium (Pd) in its 0/II oxidation state as a catalyst.^47,48 These reactions serve as the building blocks for generating the desired intermediates for the respective emitters, helping to achieve the desired λ_max while also simplifying the reaction protocol with high yields. Therefore, it is essential to understand the reaction protocols and clear synthetic approaches for effective emitter synthesis.^49,50

In this review, we focus on C–C and C–X (where X = N, O, etc.) bond formation reactions that utilize a Pd(0/II) catalyst and provide a comprehensive discussion of the reaction conditions necessary for preparing the intermediates of MR emitters. Additionally, we address the challenges in selecting suitable catalysts and their co-reagents. The primary aim of this review is to summarize the major synthetic methods for the preparation of MR-TADF precursors, facilitating further comparison and understanding of each palladium catalyst in relation to the various types of reacting groups.^15,21–23

During the 20^th century, many scientists concentrated on addressing the difficulties associated with one-step C–C bond formation. Although there were already numerous established reactions for creating C–C and C–X bonds, these often exhibited sensitivity and yielded low results due to issues with self- and cross-coupling products. Consequently, in the early 21^st century, Richard F. Heck,^51–53 Eiichi Negishi,^54–56 and Akira Suzuki^57–59 advanced self- and cross-coupling reactions of carbon–carbon bonds using Pd(0) catalysts alongside alkoxide or hydroxide bases under elevated temperature conditions, and they were awarded the Nobel Prize in Chemistry in 2010 for this work.^53,60,61 In many cases, these coupling reactions were facilitated using trialkyl/aryl phosphine complexes of Pd(0) combined with a base for coupling aryl halides with boronic acids or boronate complexes (Scheme 1a).^62,63 Later, the protocol was enhanced by utilizing Pd(II) salts in conjunction with phosphine ligands to generate the active Pd(0) complex in situ. As shown in Scheme 1b, a general mechanistic investigation revealed that the Pd(0) complex undergoes oxidative insertion with aryl halides via oxidative addition (OA), which then reacts with a base to form intermediate 3.⁶¹ At the same time, the base-activated boronic acid/ester forms an activated complex of boronate derivative 5, which subsequently reacts with intermediate product 3 to yield intermediate complex 6 through transmetalation (TM). Ultimately, this intermediate leads to the C–C bond product Ar–Ar′ 7, along with the regeneration of the active Pd(0) complex via reductive elimination in the presence of phosphine ligands. The advantages of the Suzuki reaction include its atom economy regarding cost and yield, as well as its reduced sensitivity compared to traditional organometallic reactions.

Scheme 1 The carbon–carbon bond(s) formation reaction via Suzuki coupling.

In addition to the previously mentioned reactions, other Pd(0)-catalyzed processes are also recognized for facilitating the formation of C–C and C–X bonds (X = heteroatoms). Such reactions include Chan–Lam coupling,⁶⁴ the Heck reaction, Hiyama coupling, Kumada coupling,⁶⁵ Negishi coupling, the Petasis reaction,⁶⁶ Sonogashira coupling,⁶⁷ and the Stille reaction,⁶⁸ among others.^62,69 This review highlights the Buchwald–Hartwig cross-coupling reaction,^48,70,71 which plays a crucial role in constructing OLED materials, particularly in designing core skeletons. This reaction follows a similar mechanistic pathway for C–C bond formation, where amine analogues substitute the boronate during core construction. A general representation of the reaction is shown in Scheme 2.

Scheme 2 The carbon–carbon bond(s) formation reaction via Buchwald–Hartwig cross-coupling.

In the continuation of the synthesis of the final emitter, the borylation is one of the key steps for B–N-core-containing materials. Borylation occurs in two distinct modes. First, a single-pot one-shot reaction takes place, i.e., direct borylation using boron trihalide (BX₃, X = Br, I) (Scheme 3). Second, a cascade of three-step reactions begins with an aryl halide exchange reaction with Li-amide, followed by borylation with boron trihalide, and concludes with the arylation reaction mediated by nitrogenous base deprotonation, as illustrated in Scheme 3.⁷² In all cases of borylation, the boron atom integrates into the PAH core in such a way as to form a stable six-membered ring, achieving both stability and a higher yield of the product. Interestingly, one-pot borylation occurs with low selectivity, and the position of boron substitution is directed by electronically rich functional groups, such as secondary amines and methyl groups, which regioselectively favour the para-position, while steric dominance restricts substitution at the ortho-position. Therefore, the tandem approach is typically employed for selective borylation without the need for any directing motifs. Recently, Chuluo Yang et al. reported possible transitional paths and intermediates for the cascade of borylation/boron-annulation by calculating their energy states using density functional theory (DFT) calculations.⁷³

Scheme 3 Single-pot and cascade approaches for the borylation reactions.

Although the incorporation of B–N into PAHs was reported by Feng et al. in 2013,^74–76 the broader emission spectrum and absence of TADF properties limited their effectiveness in OLEDs.⁷⁶ Following the introduction of the DABNA molecule by Hatakeyama et al.,¹⁹ B–N-based MR-TADF materials have been extensively studied,^74,75 resulting in numerous reported derivatives at the current stage.⁷⁷ Some of the variations of B–N-based emitters are shown in Fig. 1.^77–84

Fig. 1 Different types of BN-based emitters.

Note: Typically, all reaction steps are performed in dry solvents and under inert atmospheric conditions, leading to a higher yield. During the addition of Pd(0/II) catalysts, the reaction mixture must be deoxygenated by degassing or bubbling nitrogen or argon gas through the solvents to prevent the oxidation of the Pd catalyst by oxygen dissolved in the solvent. The completion of reactions can be monitored by thin-layer chromatography (TLC), and the metals can be removed from the reaction mixture using silica-supported ciliate filtration. Finally, the compound can be highly purified using recrystallization or column chromatography. Additionally, it should be noted that the reactive species of the palladium complex has a charge of zero. In contrast, the palladium(II) precursor (Pd(II) catalyst) is an inactive species that is activated in situ during the reaction with the aid of additional phosphine-derived ligands, generating the short-lived active palladium(0) complex.^85,86 The participation of the C–C or C–N reactions is discussed in Scheme 1.

Synthetic development of the palladium-catalyzed display materials

Starting in 2016, Hatakeyama synthesized new B–N embedded polycyclic heteroaromatic compounds, DABNA-1 and DABNA-2, which garnered huge attention due to their unique properties and synthesis.¹⁹ While the first purely discovered B–N-derived molecule, borazine—also known as borazole or inorganic benzene—exhibits similar aromatic properties to organic compounds like benzene and was synthesized through the reaction of diborane with ammonia under rigorous conditions, such as a high temperature of 250 to 300 °C. After several reports, Feng's group developed mono-boron and nitrogen-doped organic compounds for OLED applications under minimal synthetic conditions. Additionally, the synthesis of DABNA-1/2, achieved using the commercially available compounds dichlorobromobenzene and diphenylamine, remained simple and straightforward. As shown in Scheme 4, the synthetic procedure was elaborated during the establishment of paths A and B by modifying the secondary amine constituents diphenylamine and bis([1,1′-biphenyl]-3-yl)amine. In constructing DABNA-1/2, the palladium(II) catalyst, specifically bis(di-tert-butyl(4-dimethylaminophenyl)phosphine)dichloropalladium(II) ((A-^taPhos)₂PdCl₂, i.e., Pd(amphos)Cl₂), played a significant role in the Buchwald–Hartwig (B–H) cross-coupling in the development of precursor compounds 1 and 2, respectively. The developed protocols using these Pd(II) catalysts are promising candidates for constructing C–N bonds. Furthermore, the cascade of borylation of these compounds 1 and 2 produces TADF materials, DABNA-1/2, with narrow emission spectra of PL/FWHM at 462/33 and 470/34 nm, respectively.

Scheme 4 Synthetic route of DABNA-1 and DABNA-2.

As shown in Scheme 5, in 2019, our group developed two blue TADF materials, TDBADI and TDBA-AC.⁸⁷ The intermediate, 7-bromo-2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (Br-TDBA), was generated from the commercially available compounds dibromo-difluorobenzene and t-butylphenol via an aromatic substitution reaction, followed by a borylation reaction using potassium carbonate as the base and BBr₃ as the main reagent. Furthermore, Br-TDBA was utilized to generate C–N bonds between 5-phenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (3) and 9,9-dimethyl-9,10-dihydroacridine (4) to produce the emitters TDBA-DI and TDBA-AC, respectively. Both TADF emitters exhibited PL/FWHM (nm/nm) values of 456/55 and 458/50, respectively. Notably, the dihydroacridine derivative provided improved yield compared to 5-phenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole due to the absence of steric bulk in the former compared to the other polyaromatic amine.

Scheme 5 Synthesis of TDBA-DI and TDBA-AC from dibromodifluorobenzene and p-tert-butyl phenol.

In the same year, our group designed two modified novel compounds through the cross-coupling reaction between 3-bromo-10-(2,4,6-triisopropylphenyl)-10H-dibenzo[b,e][1,4]oxaborinine (5) and 5-phenyl-5,12-dihydroindolo[3,2-a]carbazole (6), as well as 5,10-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (3), leading to the formation of PXB-mIC and PXB-DI, respectively.⁸⁸ Among these reactions, the less-substituted indolocarbazole afforded a superior yield within a shorter time duration (path A) compared to the bisindolocarbazole (path B), shown in Scheme 6.

Scheme 6 Synthesis of PXB-mIC and PXB-DI.

At this time, our group introduced a novel procedure for synthesizing DBA-DI. Notably, the borylation method for this molecule is entirely different from that of TDBA-DI, as depicted in Scheme 7.⁸⁹ Here, the well-known BBr₃ was replaced by i-Pr-Bpin in a tandem bora-Friedel–Crafts-type reaction.⁹⁰ However, this synthetic procedure for generating the borylated compound is superior to the previous approach, as the overall yield is nearly three times higher than that of the TDBA-DI synthesis. In the first step, a chemoselective lithium–bromine exchange reaction occurs due to the ortho-lithiation effect via oxygens, and then it undergoes isopropyl displacement to afford the (4-bromo-2,6-dimethoxyphenyl)-Bpin derivative. Finally, the AlCl₃-catalysed bora-Friedel–Crafts-type reaction⁹⁰ affords a superior yield due to the chelation effect by ortho-arylethers.⁹¹ Furthermore, this protocol was optimized in toluene over a longer duration by adjusting the loading concentration of the phosphine ligand.

Scheme 7 Synthesis of DBA-DI.

In 2022, Yang et al. also developed similar types of TADF emitters by changing the amine partner in the reaction involving Br-TDBA (Scheme 8).⁹² The reaction is highly selective, yielding mono- and di-substituted SBA analogues known as pre-TDBA-SBA and 2-TDBA-SBA. The formation of both products depends on the stoichiometry of Br-TDBA and the conditions of the employed reagents, as outlined in paths A and B. In the case of TDBA-SBA synthesis, the diverse N-arylation of both nitrogens in SBA was effectively controlled by the stoichiometry of Br-TDBA, followed by the subsequent addition of t-butyl-4-bromobenzene. Both reactions were carried out in a single-pot operation by incorporating a one-time palladium catalyst combined with the ligand tri-tert-butylphosphonium tetrafluoroborate. The overall yield for both emitters remarkably ranged from 36% to 42%.

Scheme 8 Synthesis of TDBA-SBA and 2TDBA-SDA from 10H,10′H-9,9′-spirobis[acridine].

In 2020, our group was involved in synthesizing 6,12-diphenyl-5,11-dihydroindolo[3,2-b]carbazole (7) using indoles and benzaldehyde as starting components, in the presence of a catalytic amount of the catalyst DCDBTSD, and oxidizing it with a catalytic mol% of DDQ. Next, we subjected this to monophenylation of nitrogen, which led to the successful inclusion of palladium-catalyzed TDBA-Br, resulting in the product TDBA-TPDICz. At the same time, we synthesized Trz-TPDICz by substituting the acceptors with 2-bromo-4,6-diphenyl-1,3,5-triazine (Scheme 9).⁹³

Scheme 9 Synthesis of TDBA-TPDICz and TRZ-TPDICz.

Furthermore, our group reported the M3CzB emitter developed from the commercial source 3-methyl-9-phenyl-9H-carbazole (9). This compound underwent synthesis to M3Cz through bromination, followed by the Suzuki cross-coupling reaction (Scheme 10).^94a The reaction utilized well-established conditions with only a 6 mol% load of the Pd(0) complex. The obtained intermediate product was further treated with DBA-Br, as applied under the previous reaction conditions, resulting in an optimized reaction protocol that provides an improved yield of 77% due to the low load of phosphine ligands over a longer duration.

Scheme 10 Synthesis of M3CzB from methyl phenyl carbazole.

Choi et al. developed three diverse emitters: TB-3Cz, TB-P3Cz, and TB-DACz, each molecule undergoing three steps of tandem reactions, shown in Scheme 11.^94b Herein, the synthesis of TB-P3Cz involved two steps of palladium-catalyzed C–C and C–N bond formation via the Suzuki coupling and Buchwald–Hartwig cross-coupling reactions, respectively. Firstly, the synthesis of P3Cz was achieved through the reaction between 3,6-diiodo-9H-carbazole and 9-phenyl-3-(Bpin-2-yl)-9H-carbazole using 3 mol% of Pd(0) complex in a highly polar solvent. Finally, this intermediate was further used to synthesize TB-P3Cz by reaction of TDBA-Br and P3Cz with 5 mol% tris(dibenzylideneacetone)dipalladium(0) and 20 mol% tri-tert-butylphosphine in a basic medium. Each step of the reaction was independently supported by palladium catalysts, yielding higher results of 72% and 56%, respectively. However, it was reported that the more electronically rich carbazoles (3Cz and DACz) produced better coupling results with a similar reaction protocol, yielding C–N coupling results of 59–60% at the final step.

Scheme 11 Synthesis of TB-3Cz, TB-P3Cz and TB-DACz.

In 2021, our group developed novel synthetic highly rigid emitters, PzDBA and PzTDBA. Among these, the peripheral t-butyl group was introduced to reduce the aggregation in PzTDBA instead of the hydrogens in PzBA. These emitters were effectively utilized for C–N bond formation between DBA/TDBA-Br and 5,10-dihydrophenazine by using 6 mol% of Pd(II) catalyst and 22 mol% of tri-tert-butylphosphine in a basic medium at a very high temperature (Scheme 12). The reactions were well tolerated, yielding excellent results of 75–78%.¹⁶

Scheme 12 Synthetic procedures for the synthesis of PzDBA and PzTDBA.

Our team has successfully modified the previously reported molecule DBA-DI (Scheme 13) by substituting one of the (N-phenyl)indolyl groups with benzofuranyl and thiobenzofuranyl moieties to develop the highly rigid emitters DBA-BFICz and DBA-BTICz.⁹⁵ Notably, the results demonstrated superior performance regarding PLQY, low roll-off efficiency, and high k_RISC. The synthesis of DBA-Br and the methods for creating the highly rigid structures BFICz and BTICz have also been documented. Subsequently, fusing BFICz and BTICz with DBA-Br yields DBA-BFICz and DBA-BTICz, respectively. In this context, the adopted methodology provides similar reaction yields and requires a more potent palladium reagent than tris(dibenzylideneacetone)dipalladium(0) along with tri-tert-butylphosphine ligands in a basic medium.

Scheme 13 The synthetic route of DBA-BFICz and DBA-BTICz.

Hatakeyama and his coworkers modified the parent DABNA-core by peripheral decoration and the extension of boron and nitrogen atoms in polyheterocyclic aromatic compounds. His group remains active and constantly generates new ideas by introducing a new MR-core and a novel nitrogenous ligand donor. Recently, they introduced the construction of the new cores ν-DABNA and BOBNA-OAr.⁹⁶ Interestingly, both materials exhibit good performance in display technology, with narrow FWHM, excellent OLED properties, and color purity.⁹⁶ For the synthesis of v-DABNA, dibromochlorobenzene and diphenylamine were treated using the well-established B-HCC reaction. Chemoselectively, the more reactive bromines were substituted by the secondary amine to form intermediate compound 13. This intermediate then reacts with N¹,N³-diphenyl-1,3-benzenediamine according to the previous protocol, yielding compound 14, which is subsequently subjected to borylation to obtain the desired product (Scheme 14a). Simultaneously, the synthesis of DOBNA-OAr was performed, which does not involve the Pd(0) catalyst, and the synthetic route of the reaction is shown in Scheme 14b.

Scheme 14 The synthetic route of ν-DABNA and DOBNA-OAr.

In 2022, our group significantly modified the core of ν-DABNA by peripherally decorating it with diverse substituents and successfully developed the synthesis of m-v-DABNA, 4F-v-DABNA, and 4F-m-v-DABNA.⁹⁷ The newly synthesized molecules exhibited narrow FWHM, high PLQY, and small ΔE_ST. In Scheme 15a, the synthetic methodology for synthesizing m-v-DABNA from the commercially available source dibromochlorobenzene is presented. This methodology parallels that of v-DABNA, where diphenylamine is replaced by bis(m-tolyl)amine, and a higher mol% of palladium catalyst is used in combination with different ligands compared to Scheme 14. Consequently, the current synthetic approach yields excellent results at each step. Finally, it is applied to the direct borylation reaction using BBr₃ at a very high reflux temperature (Scheme 15a). Furthermore, the synthesis of 4F-v-DABNA and 4F-m-v-DABNA was also developed by replacing the reactive secondary amine partner in both steps. During the development of these materials, m-dibromobenzene with difluoroaniline was subjected to C–N coupling to achieve compound 18, which serves as an intermediate compound for 19 and 20. Another implementation of the amine coupling partner in paths A and B employs a similar catalyst, akin to the previous pattern of the B-HCC reactions. Our methodology, utilizing a combination of palladium catalysts and controlled amounts of phosphine additives, yields excellent products, which, after the one-shot borylation, also furnish remarkably moderate yields (Scheme 15b).

Scheme 15 The synthetic route of m-v-DABNA, 4F-v-DABNA, and 4F-m-v-DABNA.

In the same year, our group developed not only m-v-DABNA, 4F-v-DABNA, and 4F-m-v-DABNA but also the synthesis of pMDBA-DI, mMDBA-DI, and t-Bu-ν-DABN.⁹⁸ The synthetic route for pMDBA-DI and mMDBA-DI is similar to that of TDBA-DI synthesis, with only changes made by using meta- and para-cresols, as shown in Scheme 16a. Next, we compared the synthesized molecule t-Bu-ν-DABNA. The synthetic approach can be applied similarly to the synthesis of diphenyl-1,3-diamine, as shown in Scheme 16b. Furthermore, the next B-HCC methodology was initiated to develop the polyarylated amine component of a precursor for the borylation product, achieving a yield of 85%. In the subsequent context, this polyarylated amine was transferred to the borylation step to achieve the desired product (Scheme 16b).

Scheme 16 Facile synthesis of pMDBA-DI, mMDBA-DI and t-Bu-ν-DABNA.

In 2024, Adachi and his coworkers reported another approach for developing modified v-DABNA as ν-DABNA-O₂-TB and ν-DABNA-O₂-M through the intermixing of nitrogen and oxygen.⁹⁹ As shown in Scheme 17, the compound was synthesized from dibromobutylbenzene with the addition of diarylamine. Compounds 23 and 25 differed in their aryl substituents, and the reaction was carried out using the B-HCC reaction. Meanwhile, intermediates 23 and 25 were subjected to the synthesis of the precursors of the borylated compounds 24 and 26 with the aid of the Ullmann coupling reaction. At this time, the coupling agent was 2-methylresorcinol in the presence of a catalytic mol% of Cu(I) salt and an organic proton donor. These newly formed intermediates were directly utilized for the borylation reaction due to their high electronic abundance at the reactive sites. However, the excessive use of BBr₃ altered the product yields by forming unwanted byproducts, such as the triborylated compounds Δ-DABNA-O2-TB and Δ-DABNA-O2-M.

Scheme 17 The synthetic route of ν-DABNA-O2-TB and ν-DABNA-O2-M.

Adachi et al. modified the core of CzBN by para-hydrogen displacement using naphthyl and perylene groups.¹⁰⁰ Typically, incorporating naphthyl and perylene enhances the PLQY but increases the FWHM. At this time, their successful incorporation maintained the reserved FWHM of CzBN, while the PLQY dropped. The synthesis was highly straightforward; the first step of the reaction involved the Suzuki reaction between 2-bromo-1,3-difluoro-5-iodobenzene and naphthyl boronic acid and perylene boronic acid, maintaining a polar protic medium using THF and water as a mixed source. Next, the direct displacement of both fluorine groups occurred via carbazole, followed by stepwise single-pot borylation, leading to the formation of the products CzBNNa and CzBNPy (Scheme 18).

Scheme 18 Synthetic procedures for the synthesis of CzBNNa and CzBNPy.

Recently, Yoo, Ha, and Lee reported the modified t-BADNA by introducing the π-extended cores as triphenyl (TN), DN, and DOB to developed the emitters as tDABNA-TP and tDABNA-DN and tDABNA-DOB.¹⁰¹ The synthesis of this series of materials is similar, and the reaction proceeds through a cascade involving the B-HCC reaction, one-pot borylation, and Suzuki reactions. In the subsequent B-HCC reaction, bis(di-tert-butyl(4-dimethylaminophenyl)phosphine)dichloropalladium(II) was used as the active catalyst for C–N bond formation for compound 31 and was applied in the synthesis of the rigid skeleton for compound 32. Finally, compound 32 was subjected to the Suzuki coupling using various donors with arylpinacoboranate. The products were developed at this stage by altering the boronate derivative in a mild basic medium, yielding 44–66% of the desired product, as shown in Scheme 19.¹⁰¹

Scheme 19 Synthetic procedures for the synthesis of tDABNA-TP and tDABNA-DN and tDABNA-DOB.

In 2022, Kwon and coworkers modified the skeletons of Cz-DABNA by considering it as a reference core in developing the two novel blue materials, namely mICz-DABNA and BFCz-DABNA, by expanding the carbazole unit to 5-phenyl-5,12-dihydroindolo[3,2-a]carbazole (6) and 12H-benzofuro[3,2-a]carbazole (33), respectively (Scheme 20).¹⁰² Here, the synthetic route for both materials are similar, and they successfully performed these reactions in just two steps. The reaction methodology was developed using a Pd₂(dab)₃ catalyst and diversely reactive phosphines, supported by the construction of X = NPh for XPhos and X = O for t-Bu₃P·HBF₄ via B-HCC reactions.

Scheme 20 Synthetic procedures for the synthesis of mICz-DABNA and BFCz-DABNA.

In 2024, our group reported a couple of pure green emitters, namely BpIC-DPA and BpIC-Cz.¹⁰³ In this work, the curvilinear indolocarbazole units were fused onto a DABNA core, with N-donors attached para to the boron atom. As shown in Scheme 21, N-donors such as diphenylamine and carbazole were fused onto the core by replacing the iodine using palladium(II) acetate. Such specific catalyst selection resulted in a high synthetic yield of over 50%. Furthermore, the tandem borylation yielded the respective green emitters.

Scheme 21 Synthetic procedures for the synthesis of BpIC-DPA and BpIC-Cz.

Hatakeyama developed a novel V-shape MR emitters, named as V-DABNA, and V-DABNA-F. These emitters differ in their core due to the change of R groups, as of phenyl and difluorophenyl-substituents.¹⁰⁴ In the following methodology, all the initial four steps were performed with the support of the B-HCC reaction. As shown in Scheme 22, the products formed at each reaction step achieved moderate to good yields. At the final step of borylation, the excessive use of BBr₃, affording three times borylation in a single-shot reaction, significantly increased yields as the ranging of 11–35%, with more than 26% EQE in both cases. Remarkably, these emitters exhibited both V-DABNA and V-DABNA-F photoluminescence, with FWHM values of 481/17 and 464/16 nm, respectively.

Scheme 22 The synthetic route of V-DABNA and V-DABNA-F.

In 2024, Adachi et al. reported the synthesis of f-DOBNA and achieved a high PLQY of 90% with a low gas ΔE_ST and k_RISC.¹⁰⁵ The designed molecule reached a photoluminescence wavelength of 445 nm with an FWHM of 24 nm by touching the CIEx,y coordinates at (0.150, 0.041) with a maximum EQE of 20%. The molecular synthesis began with dichlorobromobenzene. Interestingly, the initial three steps involved in the synthesis of compound 48 occurred via H-BCC reactions. These reactions afforded excellent yields, although the final step had a lower yield of 60%. Furthermore, the precursor of this borylated product was transferred for the borylation reaction, leading to the formation of f-DOBNA with a yield of 40% (Scheme 23).

Scheme 23 The synthetic route of f-DABNA.

Yang et al. developed two rigid orange-yellow TADF molecules, namely, tBuInPz-BO and InPz-BO, with the PL peaks in toluene at 588 nm and 575 nm, respectively.¹⁰⁶ These molecules are constructed by joining novel rigid polycyclic phenazine-derived donors with a tDAB–acceptor unit, as shown in Scheme 24. The newly synthesized emitters have a good rate of RISC and high PLQY. Among these, tBuInPz-BO was explored for OLED applications, achieving a high EQE of 22% with an electroluminescence peak at 585 nm. The synthesis of tBuInPz-BO began with di-t-Bu-Cz. Controlled selective monobromination was performed using a stoichiometric amount of NBS at low temperature to room temperature, followed by the ArSN reaction with 1-fluoro-2-nitrobenzene in a strong basic medium to produce the intermediate product tBuCzN-Br. Furthermore, the nitro group was selectively reduced using Fe and then subjected to the B-HCC reaction for the annulation process, forming the C–N bond product tBuInPz. Finally, another C–N bond formation occurred between tDAB-Br and tBuInPz to obtain the desired product, tBuInPz-BO. A similar reaction can also be applied for the synthesis of InPz-BO, where di-t-Bu-Cz can be replaced by carbazole. This methodology can also open a novel path for developing red and green TADF molecules by varying the donating strength of the donors.

Scheme 24 The synthetic route of tBuInPz-BO and InPz-BO.

In 2022, Xu and colleagues studied a bipolar host of a blue tCBNDADPO MR-TADF emitter after its synthesis and reported that it performed with 100% internal quantum efficiency and 466 nm of PL with 26 nm of FWHM in DCM.¹⁰⁷ However, this molecule has a very high value of ΔE_ST and exhibits a red shift in the film state with an EL/FWHM of 470/47 nm. tCBNDA can be synthesized by following various reports. In this approach, they performed bromination followed by palladium-catalyzed C–P bond formation, as shown in Scheme 25. The ensuing two-step reactions provided an overall yield of 30.8%.

Scheme 25 The synthetic route of tCBNDADPO.

Hatakeyama and his coworkers developed a novel synthetic procedure for synthesizing tribenzo[b,d,f]azepine (TBA) from commercially available sources as a ligand donor partner.¹⁰⁸ This ligand was terminally incorporated into the fundamental core of ν-DABNA, leading to the synthesis of a series of blue emitters: ν-DABNA-Az-1, ν-DABNA-Az-2, and ν-DABNA-Az-3. In this work, we highlight one of the easiest emitters, ν-DABNA-Az-1, which was developed through the two independent steps of the B-HCC reactions, transforming compounds 49 to 50 with the assistance of the Pd₂(dba)₃ catalyst. The reaction differed based on the use of phosphine ligands, ranging from SPhos to tBu₃P·HBF₄, and employed high-boiling solvents, from toluene to mesitylene. The precursor of the borylated product was obtained by a coupling reaction involving diverse amines in a cascade manner, followed by a one-shot borylation protocol, as illustrated in Scheme 26.

Scheme 26 Synthetic route of ν-DABNA-Az.

Later, Zhang's group was influenced by Prof. Hatakeyama's previous study concerning deep blue emitters. Considering the high FWHM and deep blue color requirements, this group developed novel molecules, BN-TCzTBA and BN-TPCzTBA, which exhibited reduced FWHM, good color purity, and high k_RISC and PLQY.¹⁰⁹ They synthesized these new molecules using 1-bromo-2-chloro-3-fluorobenzene and employed the coupling partner TBA during the B-HCC reaction. In subsequent steps, a variety of carbazoles were utilized to generate intermediates 52 and 53, and following independent borylation, the products BN-TCzTBA and BN-TPCzTBA were produced (Scheme 27).

Scheme 27 Synthetic route of BN-TCzTBA and BN-TPCzTBA.

In 2023, our group reported the two novel symmetrical and asymmetrical meta-B-π-B based diborylated MR-emitters, NO-DBMR and Cz-DBMR, which exhibit small ΔE_ST values with FWHM/PL values of 458/14 nm (pure blue emission) and 480/14 nm (bluish-green emission), respectively.¹¹⁰ These materials are promising for developing pure blue and green MR-emitters based on π-extension by considering ligand strength or other alternatives. During the synthesis of unsymmetrical NO-DBMR, 5-chloro-tetraphenylbenzene-1,3-diamine was utilized, and the initial step involved C–N bond formation between the amines and haloarene using a low molar concentration of the Pd₂(dba)₃ catalyst in combination with diverse phosphine ligands for synthesizing the intermediate compound 55. To develop the unsymmetrical core, 3,5-bis(diphenylamino)phenol was employed for the ArSN reaction, which was further used in the double borylation process to achieve the desired emitter. Simultaneously, the carbazole-derived symmetrical emitter was developed, starting with 3-chloro-5-(N-carbazolyl)-1-bromobenzene. The first two steps of the B-HCC reaction, followed by borylation, produce the desired product Cz-DBMR (Scheme 28).

Scheme 28 The synthetic route of NO-DBMR and Cz-DBMR.

In 2021, Hatakeyama reported the ultrapure blue emission of an asymmetrical meta-B-π-B based diborylated polyheteroaromatic MR-emitter known as v-DABNA-O-Me.¹¹¹ This initial concept provides sufficient knowledge to develop the novel manifest of meta-B-π-B cores. The developed core demonstrates high values of both PLQY and RISC, achieving a CIE coordinate of (0.13, 0.10) along with a high EQE and minimal roll-off. As depicted in Scheme 29, the initial compound can be produced using the previous methodology. Borylation was carried out with boron tribromide, which was then utilized for new C–N bond formation between the diaryl amine and the intermediate product, using bis(di-tert-butyl(3-methyl-2-butenyl)phosphine)dichloropalladium (NECO-296) in the essential medium.

Scheme 29 The synthetic route of v-DABNA-O-Me.

In 2024, Tang and coworkers reported four MR-emitting compounds: SAC2MN1B, DPA2MN2B, Cz2MN2B, and SAC2MN2B, all exhibiting ultrapure blue emissions.¹¹² Among these, SAC2MN1B and DPA2MN2B share the same isostructural core as the mono- and diborylated products, where the diarylamine is replaced by 10H-spiro[acridine-9,9′-fluorene], resulting in a PL/FWHM of 468/30 and 449/29 nm in toluene at 10⁻⁶ M, respectively. SAC2MN2B and Cz2MN2B molecules display C2 symmetry in a 2D plane, where the meta–meta-B-π-B structure of diphenylamine and carbazole bisects through the middle of the nitrogen and benzene rings, reflecting the mirror image of all tolyl and phenyl substituents of the amine/carbazole. The open structure of DPA2MN2B arises from the diaryl amine ligand being replaced by the carbazole, which creates a locked skeleton in Cz2MN2B. The transition from unlocked to locked structures influences the donating strength. Thus, it shifts to the red region of PL/FWHM to 446/21 and 481/23 nm in toluene at 10⁻⁶ M, respectively. The following schematic representation illustrates the synthesis process, which proceeds from the dibromofluorobenzene carbazole unit via an ArSN reaction, followed by bis(tolylamine) through a palladium-catalyzed B-HCC reaction. The final step involves a borylation reaction, yielding 32% of the desired product, Cz2MN2B. Conversely, a tellurium-derived precursor was employed for the borylation pathway, resulting in the decarbametallated and borylated product DPA2MN2B, with a lower yield of 15% (Scheme 30).

Scheme 30 The synthetic route of DPA2MN2B and Cz2MN2B.

However, we have discussed the non-sensitized MR emitters of SAC2MN1B and DPA2MN2B and the switching effects of monoborylation to diborylation reactions. The effects of diborylation cover the blue region due to the enhancement of the acceptor unit as boron and also improve the FWHM. During the total synthesis of the S2MN intermediate, the process was the same for both emitters. The final step of the controlled borylation afforded superior yields for SAC2MN1B and DPA2MN2B at 39% and 51%, respectively, which is schematically represented in Scheme 31.

Scheme 31 The synthetic route of SAC2MN1B and SAC2MN2B.

At this time, the Hatakeyama group is involved in synthesizing ω-DABNA, where three boron atoms lie in the meta-B-π-B based triborylated system, forming the polyheteroaromatic borylated core.¹¹³ The photophysical properties of ω-DABNA were examined in a 1 wt% poly(methyl methacrylate) (PMMA) dispersion film, which showed a photoluminescence (PL) and full width at half maximum (FWHM) of 509/22 nm in the green region. The reaction intermediate product was prepared using a previously known method, followed by borylation and tetra-diphenylamine coupling with a mol% of palladium catalyst in the B-HCC reaction for compound 65. Finally, the last step involved the borylation of the achieved intermediate using a highly reactive borylating reagent. All steps of the reaction followed a well-established procedure, resulting in an overall yield of 18.5% for intermediate product 63 to ω-DABNA (Scheme 32).

Scheme 32 The synthetic route of ω-DABNA.

The compound 3,6-di-tert-butyl-9-(3,5-dichlorophenyl)-9H-carbazole (66) has been synthesized from the commercially available inexpensive compound 1,3-dichloro-5-fluorobenzene with 3,6-di-tert-butyl-9H-carbazole (1.2 equiv) using cesium carbonate as the base (1.5 equiv) at 120 °C for 24 hours in NMP solvent,¹¹⁴ which further undergoes B-HCC with bis(4-tolyl)amine in the presence of a catalytic amount of both Pd₂(dba)₃ and SPhos in a basic medium at high temperature to yield compound 67. Next, the reaction involves the controlled addition of boron tribromide to perform regioselective mono- and di-borylation reactions, resulting in products CzDABNA-NP-M/TB and CzB2-M/TB, respectively (Scheme 33).

Scheme 33 The synthetic route of CzDABNA-NP-M/TB and CzB2-M/TB.

Suh and coworkers developed sterically crowded blue-MR-TADF emitters, specifically mono-mx-CzDABNA and tri-mx-CzDABNA, with PL/FWHM values of 474/34 nm and 462/26 nm, respectively.¹¹⁵ Among these, the tri-mx-CzDABNA exhibited narrower spectral broadening due to the inhibition of intermolecular π–π stacking in the film state. The compound 69 has been synthesized from dichloroiodobenzene via a sequence of two B-HCC reactions, followed by annulation, using a Pd(0) catalyzed C–C bond formation in a basic medium to yield compound 70. Furthermore, treatment of this intermediate with various amines produced the intermediate skeletons of compounds 71 and 72. Independently, both products were subjected to borylation using BI₃ as the reagent, resulting in equal maximum yields of 22% (Scheme 34).

Scheme 34 The synthetic route of mono-mx-CzDABNA and tri-mx-CzDABNA.

In 2023, Li et al. reported the synthesis of sky-blue compounds BNCz-SAF, BNCz-DMAC, and BNCz-PXZ from the commercial source 5-bromo-1,3-difluoro-2-iodobenzene with di-t-BuCz via the ArSN reaction and produced BNCz-Br as the borylation reaction product (Scheme 35).¹¹⁶ It is known that the DtBuCz core exhibits photoluminescence at 483 nm with a full width at half maximum (FWHM) of 23 nm. Herein, the newly synthesized three emitters depend on the corresponding secondary amines, which are essential for building the desired rigid emitters.

Scheme 35 The synthetic route of BNCz-SAF, BNCz-DMAC, and BNCz-PXZ.

This year, Wang et al. developed the sky-blue emitters DtCzB-SFN and DtCzB-DFN, and this design can be correlated with the previous emitters BNCz-SAF, BNCz-DMAC, and BNCz-PXZ.¹¹⁷ In this approach, to prevent the free rotation of the MR core and the secondary sterically crowded amine, bis(dibenzo[b,d]furan-4-yl)amine (74) and N-(dibenzo[b,d]thiophen-4-yl)dibenzo[b,d]furan-4-amine (75) were utilized. Furthermore, this derivative was delivered for the next C–N coupling reaction to obtain the target emitters (Scheme 36). At this time, both emitters exhibited a constant PL/FWHM of 475/21 nm. However, the subsequent molecules exhibit a wider FWHM compared to the previous scheme. We assert that these emitters can be beneficial for synthesizing blue and green emitters after the peripheral decoration.

Scheme 36 The synthetic route of DtCzB-SFN and DtCzB-DFN.

In the upcoming compound development, the highly sterically crowded MR-TADF emitters, specifically Me-PABO (PL/FWHM 453/21 nm) and Me-PABS (463/21 nm), have been reported.¹¹⁸ Both emitters vary in their constituents, featuring dibenzofuran and dibenzothiophene, with a consistent substituent of 2,6-dimethylphenyl groups. The introduction of the 2,6-dimethylphenyl group not only addresses solubility issues but also prevents borylation sites during the one-shot reaction compared to other ortho-free functional groups with aryl groups. Both emitters can be readily constructed using a palladium-supported B-HCC reaction in two steps as precursors for the borylation intermediate isolated products, 78 and 79 (Scheme 37). Due to the controlled borylation site, a significant amount of BBr₃ was employed to enhance the desired emitters.

Scheme 37 The synthetic route of Me-PABO and Me-PABS.

Yang et al. elaborated on the synthesis of two MR-TADF emitters, PSeZBN₁ (PL/FWHM 475/23 nm) and PSeZBN₂ (PL/FWHM 519/51 nm), by incorporating the heavy atom selenium as the 10H-phenoselenazine donor.¹¹⁹ As we know, increasing the donating strength in the MR core at the para- and meta-positions of boron causes blue-shift and red-shift effects with respect to DtCzBN (PL 483 nm), respectively. Notably, pre-BNCzBr (X = Br, Y = H) for path A was prepared from 2,5-dibromo-1,3-difluorobenzene through a reaction with Di-t-ButCz in the presence of cesium carbonate via the conventional ArSN reaction. Furthermore, its borylation reaction produces the intermediate product BN-Cz-Br. Finally, its reaction with 10H-phenoselenazine via the B-HCC reaction produces the emitter PSeZBN1. Simultaneously, 80 (X = H, Y = Br) was derived from 1,3-dibromo-2,4-difluorobenzene for path B. Herein, a reverse methodology was employed for the synthesis of PSeZBN2, i.e., the B-HCC reaction followed by the borylation reactions (Scheme 38).

Scheme 38 The synthetic route of PSeZBN1 and PSeZBN2.

Miao and coworkers studied the additional effects in the DtCzBN core by incorporating the negative inductive electron-withdrawing substituent, the pyridyl group, and synthesized three near-green MR emitters: 2PyBN (PL/FWHM 499/21 nm), 3PyBN (PL/FWHM 490/23 nm), and 4PyBN (PL/FWHM 495/24 nm).¹²⁰ All these molecules can be independently synthesized using methods A and B (Scheme 39). The yields of the reactions can fluctuate, from borylation followed by Suzuki coupling to vice versa. Significantly, a pattern of red shift was observed when moving from 3PyBN to 4PyBN to 2PyBN due to differences in inductive effect strength, ranging from low to high. Among these, the strong intramolecular hydrogen bonding in 2PyBN reduces the FWHM, which also helps improve the high PLQY.

Scheme 39 The synthetic route of 2PyBN, 3PyBN, and 4PyBN.

In 2024, Zheng et al. reported the synthesis of MR-TADF emitters, namely p-ICz-BNCz, m-ICz-BNCz, and dm-ICz-BNCz, exhibiting PL/FWHM values of 484/24 nm, 508/28 nm, and 526/41 nm, respectively.² These emitters were synthesized from the recombination of ICz and DtBuCzB units. During reaction development, ICz-Bpin was synthesized from indolocarbazole (ICz) via bromination using NBS, resulting in the intermediate product ICz-Br. This was then treated with B₂pin₂ and a catalytic amount of Pd(dppf)Cl₂ in the presence of potassium acetate. To elaborate on the synthesis of p-ICz-BNCz, tBuCzB-Bpin was regioselectively synthesized with stoichiometric control of B₂pin₂ and catalytic mol% of both [Ir(COD)(OMe)]₂ and 4,4′-di-tert-butyl-2,2′-bipyridine. Furthermore, the intermediate was coupled with ICzBr via Suzuki coupling, yielding p-ICz-BNCz. Meanwhile, the stoichiometrically controlled bromination of DtBuCzB with NBS produced DtBuCzB-Br and DtBuCzB-DBr, which were coupled with ICz-Bpin in the presence of the Pd(PPh₃)₄ complex, resulting in m-ICz-BNCz and dm-ICz-BNCz, respectively, as shown in Scheme 40. However, the exploration of the photophysical properties of p-ICz-BNCz showed no significant change in PL and FWHM values. In contrast, moving the ICz group to the meta-position of the boron enhanced both PL and FWHM due to the greater donating ability and charge-transfer character of the molecule, alongside a reduction in MR nature. Thus, the presence of two ICz units significantly increases PL and FWHM values. These emitters exhibit good EQE, but the core requires more significant changes by adjusting the donating strength of the donor and widening the FWHM.

Scheme 40 The synthetic route of p-ICz-BNCz, m-ICz-BNCz, and dm-ICz-BNCz.

In 2023, Yang and a coworker explored the synthesis of three MR-TADF emitters: BNIP-tBuDPAC (PL/FWHM 544/43 nm), BNIP-CzDPA (PL/FWHM 583/49 nm), and BNIP-tBuCz (PL/FWHM 564/60 nm), conducting a comparative study with DNDIP (PL/FWHM 558/38 nm).¹⁰ From a molecular perspective, replacing one of the di-tert-butylindolophenazine (IPZ) groups with 9,9-diphenyl-9,10-dihydroacridine, di-tert-butylcarbazole, and tetraphenyl-9H-carbazole-3,6-diamine in a symmetrical emitter DNDIP generates three unsymmetrical emitters: BNIP-tBuDPAC, BNIP-CzDPA, and BNIP-tBuCz. The shifting of blue to red region of the emission spectra were common with the enhancement of donor strength from acridine to di-tert-butylcarbazole and then to tetraphenyl-9H-carbazole-3,6-diamine. The synthesis is straightforward, proceeding through three steps: ArSN reaction, B-HCC reaction, followed by borylation. The employed palladium-catalyzed reactions yield between 42% and 50%, depending on the bulkier amines (Scheme 41).

Scheme 41 The synthetic route of BNIP-tBuDPAC, BNIP-CzDPA, BNIP-tBuCz and BNDIP.

Zhang and coworkers re-joined di-tert-butylcarbazole (D-t-Bu-Cz) with the main core of DABNA to synthesize TCZ-F-DABNA. The synthesis proceeded in two steps: firstly, the coupling of 1,3-dibromo-2-chlorobenzene with 2.4 mol of 7,10-di-tert-butyl-5H-indolo[3,2,1-de]phenazine via the B-HCC reaction, followed by a single-pot cascade borylation reaction (Scheme 42).¹²¹ The resulting emitter belongs to the orange-yellow region, with a wavelength of 588 nm and a full width at half maximum (FWHM) of 38 nm, showing a high PLQY and an EQE of 39.2%. Thus, the core can be used to inherit the good EQE, but a crucial change in its structure is required to achieve pure blue or green emitters.

Scheme 42 The synthetic route of TCZ-F-DABNA.

In the following approach, the Yang group reported the conventional method for constructing Na-sBN and Na-dBN. In this procedure, the developed molecules exhibited EQE values of 28.8% and 25.2%, with high PLQY and both small ΔE_ST and FWHM (31 nm).¹²² During the synthetic development, they used 1-bromo-2-chloro-3-fluorobenzene with di-t-Bu-Cz in a strong basic medium to develop 83. Simultaneously, dibromonaphthalene (84) was transformed into 85 with the aid of a palladium catalyst and B₂Pin₂ in a mild basic medium. Next, the Suzuki coupling between 83 and 85 produced 86, which was subjected to a borylation reaction (Scheme 43). However, the reaction primarily produced the monoborylated green emitter Na-sBN (PL 516 nm) and the diborylated red emitter Na-dBN (PL 612 nm). Importantly, it can be assumed that this study will aid in the development of new molecules while maintaining FWHM for both solution and OLED applications.

Scheme 43 The synthetic route of Na-sBN and Na-dBN.

In 2024, Zhang et al. reported the novel emitters tCzBN-PQ and tCzBN-PQCz, incorporating a secondary acceptor or donor–acceptor unit. The designed tCzBN-PQ and tCzBN-PQCz are classified as green emitters, exhibiting a high EQE of 30.2% to 35.1%, respectively.¹²³ Conversely, the DtBuCzB core was modified into TCzBN-DPF, TCzBN-TMPh, and TCzBN-oPh by substituting the para-hydrogen of the boron with 9,9-diphenyl-9H-fluoren-2-yl, mesityl, and ortho-diphenyl groups. Although several modified cores were also developed, this discussion will concentrate on the reports of tCzBN-PQ and tCzBN-PQCz, where the desired compounds were easily synthesized from 2,4-dichloroquinazoline and the respective arylboronate, with the intermediates further utilized with DtCzBN-Bpin via the Suzuki reaction, as shown in Scheme 44. All the reaction steps proceeded with good to excellent yields.

Scheme 44 The synthetic route of tCzBN-PQ and tCzBN-PQCz.

As the previously developed molecule, CzBN (PL/FWHM of 474/25 nm, cf. Scheme 18), was additionally substituted with naphthyl and pyranyl units at the para-position of the benzene ring, the Adachi group developed CzBNNa and CzBNPyr emitters.¹²⁴ Consequently, these emitters exhibit a significant bathochromic shift with PL values of 483 and 480 nm due to extended π-conjugation while maintaining a constant FWHM like CzBN. Additionally, both the Zhuang and Li groups reported a new molecule, BN-R, which features peripheral decoration with a donor at the carbazole unit, enhancing the donor strength of the carbazoles and resulting in a red shift from CzBN (PL 474 nm, sky-blue) to BN-Y (PL 567 nm, a mix of orange and yellow). Furthermore, they incorporated an acceptor triazine unit at the para-boron of BN-R, allowing for a further redshift into the pure red region of PL 624 nm. The newly synthesized molecules, BN-Y and BN-R, derived from CzBN, show significant increments in FWHM (24 to 36 nm). The synthetic approaches for the target molecules are convenient for BN-Y due to limited steps in synthetic development, yielding high efficiency at each step, such as B-HCC, ArSN, and borylation reactions. Moreover, the direct regioselective introduction of the triazine unit at the para-boron is also straightforward within two additional steps. This process involves using an iridium complex and 4,4-di-tert-butyl-2,2-dipyridyl (dtbpy) to support the introduction of Bpin with the B₂pin₂ reagent, followed by the Suzuki reaction to introduce the diphenyl triazine unit using diphenyl triazine chloride with the aid of a catalytic mol% of Pd(0) complex in a basic medium (Scheme 45).

Scheme 45 The synthetic route of BN-R.

In 2023, the Zhuang and Bi groups collectively explored the synthesis of a series of emitters, namely, BN-R1, BN-R2, and BN-R3.¹²⁵ As shown in Scheme 46, the synthetic methodology was adopted from their previous report. Here, the triazine unit of BN-R was replaced with 2,6-diphenylpyrimidinyl, 4,6-diphenylpyrimidinyl, and 2-phenylpyrimidinyl, renaming them as BN-R1, BN-R2, and BN-R3, respectively. The next step involves constructing the skeleton via the Suzuki reaction, yielding 77–82%. Unfortunately, they were unable to achieve a high EQE, matching the previous record of 20%. We believe that the methodology developed by Zhuang can pave the way for the advancement of pure red emitters with improved EQE and PLQY.

Scheme 46 The synthetic route of BN-R1, BN-R2 and BN-R3.

In 2021, our group reported the innovative red emitters, BP-2DPA and BP-4DPA, which were assembled from tetraphenyl-5-(Bpin-2-yl)benzene-1,3-diamine and perylene mono/dibromide via the conventional Suzuki coupling method.¹²⁶ The optimized reaction conditions are sufficient for these reactions, producing excellent yields ranging from 83% to 90%. Furthermore, the controlled stoichiometric amounts of BBr₃ are adequate for the subsequent reaction conditions, yielding remarkably good results of 37% to 39% (Scheme 47). Fascinatingly, both rationally designed molecules, BP-2DPA and BP-4DPA, exhibit photoluminescence peaks at 599 nm and 605 nm, respectively, achieving high photoluminescence quantum yields (PLQY) of 91.3% and 96.4%.

Scheme 47 The synthetic route of BP-2DPA and BP-4DPA.

In 2024, Zhang's group developed a highly rigid and strained orange-red emission MR emitter known as B4N6-Me.¹²⁷ The name of the molecule is derived from the number of boron and nitrogen atoms in the polycyclic aromatic hydrocarbon and underscores its proof-of-concept as a multiple resonance emitter. Consequently, this molecule shows photoluminescence at 580 nm with a very narrow FWHM of 19 nm in toluene, achieving an EQE of 28.4% at 1000 cd m⁻². The synthesis of these emitters is highly convenient, yielding good to excellent results at each step of the reaction. During molecular development, they selected indolocarbazole (90) as the central core to achieve a narrow FWHM by developing a rigid core and a centrosymmetric molecule. In the first step, the excessive amount of dibromofluorobenzene undergoes nucleophilic aromatic substitution (S_NAr or ArSN) reaction with the NH of compound 90, that produces precursor 91, which is then utilized to assemble mesityl tolylamine into precursor 92 with the help of a palladium catalyst. Finally, as shown in Scheme 48, it undergoes borylation using 40 mol of BBr₃ in a one-shot reaction to obtain the desired product with a high yield of 71%.

Scheme 48 The synthetic route of B4N6-Me.

Zysman-Colman et al. reported that AZB-Ph-Trz and AZB-Trz form 93.¹²⁸ The synthesis related to AZB was not found to be more effective in reducing the yield in step 2, which was at 20%. Next, the B-HCC reactions were applied independently using Pd(OAc)₂ in a mild basic medium for the synthesis of the materials AZB-Ph-Trz and AZB-Trz, aided by 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine (95) and 2-bromo-4,6-diphenyl-1,3,5-triazine (96), respectively (Scheme 49). However, these materials are less important than sky blue TADF molecules in OLED applications, even though they exhibit nearly high k_RISC values. Naturally, this strategy can be helpful in designing novel blue TADF molecules through new designs and peripheral π-electron decorations.

Scheme 49 The synthetic route of AZB-Ph-Trz and AZB-Trz.

In 2020, Duan and his coworkers reported the neatly locked BCzBN (484 nm) core via the simplest method for synthesizing fused isoquinoline motifs that generate the 6-butylphenanthridine-derived pure green AZA-BN (527 nm) OLED materials.¹²⁹ The developed molecules possess significant advantages due to their nearly standard CIEy of 0.69 (BT2020), low ΔE_ST, high PLQY of 99.7%, and EQE of 28.2%. The synthesis was conducted in three steps, with excellent yields in the first two steps, namely, aromatic nucleophilic substitution and Suzuki reactions. The synthesis was developed from 5-bromo-2-chloro-1,3-difluorobenzene, where the first step involved di-tert-butylcarbazole followed by palladium-catalyzed couplings with (2-cyanophenyl)boronic acid (Scheme 50). We assume their goal was to develop a novel pure green molecule in the final step, where the cyanide should remain intact throughout the reaction, which may also fall within the green region. Fortunately, they found that an excessive amount of butyl lithium was beneficial for the borylation reaction to achieve the pure green AZA-BN product. During the establishment of this synthesis, they conducted controlled experiments and found that one and two moles of butyl lithium were unsuitable for synthesizing their desired product.

Scheme 50 The synthetic route of AZA-BN.

Zheng and his coworkers successfully synthesized LTCz-BN. The method is closely related to developing a rigid polyaromatic conjugated B–N embedded core.¹⁷ During the synthetic development of this novel material, they transformed the benzene core of BCzBN (484 nm) into the indolo[3,2,1-jk]carbazole core of LTCz-BN (497 nm). The material was synthesized from commercially available trifluorobenzene. The reaction took place in a strong basic medium with an excess stoichiometric amount of di-tert-butyl carbazole, which was then subjected to palladium-catalyzed annulation to develop the indolo[3,2,1-jk]carbazole core, yielding the precursor of the borylated compound LTCz-NL. The formed intermediate product was treated with liquid Br₂ and subsequently underwent a cascade of single-pot borylation reactions (Scheme 51). However, each step of the synthetic approach yielded excellent results except for the borylation reaction. This method may also open new avenues for designing other analogues of indolo[3,2,1-jk]carbazole, which could contribute to developing pure green compounds.

Scheme 51 The synthetic route of LTCz-BN.

Zhang et al. directly performed a cascade transformation to convert the benzene group into anthracene in the established MR-TADF emitters by incorporating naphtha fusion, designating the compound as BCzBN (484 nm) to AN-BN (601 nm).¹³⁰ Fascinatingly, this structure not only achieves a high PLQY and roll-off efficiency with a wide FWHM but also exhibits a significant increase in its wavelength. The authors aimed to achieve a green color rather than the observed red. To justify the high wavelength of the emitting molecule, they based their explanation on the fundamental principles of Clar's aromatic π-sextet rule. The synthesis was carried out using (2-(bromomethyl)phenyl)methanol, oxidized with pyridinium chlorochromate (PCC), followed by Suzuki coupling with the well-known core DtCz-Bpin. Moreover, Lewis acid-catalyzed annulation was noted after condensing the aldehyde with tosylamine at room temperature (Scheme 52). The yields from each reaction step were substantial and can be employed for further peripheral decoration to achieve excellent color purity and narrow FWHM.

Scheme 52 The synthetic route of AN-BN.

Collectively, Shi, Wang, Zhang, and their colleagues successfully developed two novel blue MR-TADF emitters: BNCz-aDMAC and BNCz-PaDMAC.¹³¹ During the construction of these emitters, the BNCz core was combined with two types of units: 9-adamantyl-dihydroacridine and 10-phenyl-extended 9-adamantyl-dihydroacridine. They were found to be effective at low concentrations, maintaining high color purity due to the weak aggregation quenching effects of the adamantyl group. BNCz-Br can be synthesized using a well-established conventional method, which is then treated with adamantyl dihydroacridine (aDMAC) to yield BNCz-aDMAC. In contrast, the synthesis of BNCz-PaDMAC involved an additional step of the Suzuki reaction, utilizing 4-chlorophenyl pinacoborate and a Pd(PPh₃)₄ catalyst under mild basic conditions. The synthetic yields for both BNCz-aDMAC and BNCz-PaDMAC emitters were 71% and 56%, respectively, outperforming that of BNCz-Br, as shown in Scheme 53.

Scheme 53 The synthetic route of BNCz-aDMAC and BNCz-PaDMAC.

While many emitters are well known, we concentrated on some recently developed circularly polarized electroluminescent chiral blue display materials, S-AX-BN and SO₂-AX-BN, which exhibit narrowband emission and were introduced by Zheng and his colleagues.¹³² Interestingly, the synthesis was performed using commercially available dibromoiodobenzene. The starting compound was introduced similarly to the Ullmann coupling reaction, employing the strong base n-BuLi and a Cu(II) salt, followed by a crucial halogen exchange step from Br to I. Next, S-AX-BN was synthesized by adding the Grignard reagent and diphenyl disulfide using the halogen exchange product, followed by Buchwald–Hartwig cross coupling (B-HCC) with DtCzB-Bpin and the Pd₂(dba)₃ catalyst in a strongly basic medium. Unfortunately, the product synthesis was inefficient at this stage, limiting the yield to 18%. In contrast, synthesizing SO2-AX-BN requires an extra step for sulfide oxidation using m-CPBA, which is then subjected to B-HCC, resulting in a better yield of 45% (Scheme 54).

Scheme 54 The synthetic route of S-AX-BN and SO2-AX-BN.

Bin et al. developed the gram-scale synthesis of the chiral MR-emitter, SFDBN-CN, for both enantiomeric products. This study presents the schematic diagram representing the synthesis of (R)-SFDBN-CN and (R)-SFDBN-H.¹³³ Although the synthesis can begin from the readily available enantiopure (R)-109, their convenient synthesis was successfully developed from the commercially available, inexpensive compound spirobifluorene. As shown in Scheme 55, the reactions involved the simplest steps: Friedel–Crafts acylation followed by Baeyer–Villiger oxidation and basic hydrolysis for the production of racemic product (±)-109. In the subsequent step, to achieve a high yield of the desired enantiopure product, the chiral reagent (2R,3R)-N¹,N¹,N⁴,N⁴-tetracyclohexyl-2,3-dimethoxysuccinamide ((R,R)-110), was employed for the kinetic resolution of one product as the diastereomer, which was then subjected to basic hydrolysis, yielding the enantiopure product (R)-109 at 99% purity. This enantiopure product was subsequently utilized for the synthesis of (R)-SFDBN-CN and (R)-SFDBN-H in four steps. The triflation and B-HCC reactions were employed to generate the intermediate isolated product 113, followed by the diborylation reaction. Notably, the presence of the chlorine atom enhances the effectiveness of para-borylation due to the influence of the chlorine atom, minimizing side products via steric crowding. In the final borylation step, ipso-halogen substitution occurs, producing (R)-SFDBN-Br due to the presence of an excess amount of bromide counter ions. Independently, (R)-SFDBN-Br was converted into the hydrogenated product via the B-HCC protocol, yielding (R)-SFDBN-H, and then underwent ipso-substitution of Br to CN via CuCN, resulting in (R)-SFDBN-CN.

Scheme 55 The synthetic route of (R)-SFDBN-CN and (R)-SFDBN-H.

In 2023, Prof. L. Duan's group elaborated on the synthesis of highly rigid chiral, active, circularly polarized hetero[n]helicenes.¹³⁴ However, it was reported that modifications in the parent core of RTBN (PL/FWHM 692/36 nm) occurred by substituting two antiparallel units of carbazoles with two phenols and thiophenols, resulting in BNO1 and DBNS-tBu as the red emitters, with PL/FWHM values of 605/32 nm and 632 nm, respectively. This approach demonstrated low and high shifts, as expected, with a consideration of PL at 620 nm. Consequently, this method achieved a near-pure red region at 617 nm with a PLQY of nearly 100% and EQE values of 36.6%/34.4% for RBNN within three steps. Additionally, these molecules satisfied BT2020 with CIEx,y coordinates of 0.67 and 0.33, with LT95 recorded at 400 hours. Trifluorobromochlorobenzene was treated with an excess of di-t-Bu-Cz and then with Cz-BPin via Suzuki coupling. Ultimately, the borylation produced the desired product RBNN, and the molecules were separated into isomers P,P-RBNN and M,M-RBNN (Scheme 56).

Scheme 56 The synthetic route of RBNN.

At the same time, the Zheng group reported a heterohelicene, namely, o[B–N]₂N₂, whose properties were not significantly more satisfactory than the previous record.¹³⁵ The material synthesis shows high yields in the first two steps to prepare intermediate 118 from carbazole. The final borylation yielded a poor result of only 20% (Scheme 57). Nonetheless, this molecule exhibited PL at 556 nm, a narrow FWHM of 21 nm in toluene, and a large value of ΔE_ST. We believe that this approach can pave the way for developing novel helicene-based green MR emitters. Virtually, N-borylation with boron tribromide affords improved yields due to the strong reactivity of N–H with B. Recently, Meng, Ding, and collaborators developed the novel emitter tCz[B–N]N and this newly developed emitter as a pure-green emitter with a PL of 526 nm and a wide FWHM of 41 nm (Scheme 58).¹³⁶ The core skeleton has efficient planarity for monoborylation, and they reported a very high yield of the borylation product of 94%. Herein, the o-[B–N]₂N₂ emitter caused the drastic reduction of the borylation step. Insightfully, this molecule exhibited PL at 556 nm and a narrow FWHM of 21 nm in toluene, along with a large value of ΔE_ST. We believe that this approach can pave the way for developing novel helicene-based green MR emitters. Practically, the tandem (one-pot) protocol is a less effective method than the one-shot borylation. Basically, one-shot borylation occurs through the position with the lowest activation energy and a lack of selectivity. In the case of tandem borylation, the decrease in yield may also be due to the longer duration of the three-step protocol for borylation reactions, which is sensitive to solubility and moisture. Enhancing the temperature and diversely optimizing the solvent produces a random yield. The para–para double borylation is minimized due to the diminished electron density on the para-position of the one boron-substituted compound, which fairly affects the electrophilic substitutions. Hence, the second borylation may not be effective.

Scheme 57 The synthetic route of o-[B-N]₂N.

Scheme 58 The synthetic route of tCz[B–N]N.

Conclusions

In this review, we explore various palladium-derived reactions that contribute to developing different precursors leading to MR-TADF emitters used in the wide color gamut of OLEDs. Advancements are continually being made to achieve highly effective, less challenging, and commercialized MR-TADF-OLED materials with superior characteristics. Considering several competitive synthetic approaches, the Pd(0/II) method is the most straightforward and commercially viable for forming C–C and C–X (X = O, N, P) bonds, requiring a comprehensive understanding of the reagents used. Considering this, we have thoroughly discussed the synthetic methodologies reported for MR-TADF preparation. Additionally, different variants of palladium catalysts, along with their respective additives and solvent conditions, are discussed in detail. We anticipate that this review will assist researchers in collectively acquiring knowledge regarding the variants of the palladium (Pd) catalyst that are appropriate for diverse reactive sites, thereby promoting further advancements in the synthesis of MR-TADF emitters.

Reaction analysis for future vision

This review can provide sufficient knowledge about the synthesis and design of novel B–N and B–O emitters and their corresponding intermediates by interchanging the strength of ligands and catalysts in symmetric and asymmetric variations, incorporating specific reactive functionalities, facile steric effects, etc., to achieve narrow FWHM, high EQE, PLQY, small ΔE_ST, and a good CIE color gamut, among others.

1. The aromatic substitution nucleophilic (ArSN) reaction is employed for the direct displacement of the labile group, fluorine (F), with carbazole and its analogues. This bond formation uses strong bases like K₂CO₃, Na₂CO₃, and Cs₂CO₃ in high-boiling aprotic polar solvents at elevated temperatures in DMF, DMA, NMP, etc. However, the diaryl ether bond(s) have also been developed under similar conditions with the help of DMSO or NMP in the presence of excess base.

2. The halogen derivatives (except for F) can be selectively utilized for the Pd-catalysed coupling reactions. The reactivity order of the halogens is I > Br > Cl. Bromine is a borderline soft base that acts as a powerful coupling motif and offers high yields. Iodine is also the best choice, but sometimes yields lower results as a coupling product due to its more labile nature.

3. Furthermore, boronates and iodoarenes are utilized as reactive starting materials for constructing the precursors via the Suzuki reaction to develop new C–C bonds on the PAH core.

4. In the Suzuki reaction, tetrakis(triphenylphosphine)palladium(0) [palladium-tetrakis(triphenylphosphine), i.e., Pd(PPh₃)₄] is utilized as an effective catalyst, using 1 to 5 mol% for each C–C bond forming reaction in a mixed solvent system that satisfies mild basic conditions (K₂CO₃ or Na₂CO₃ or Cs₂CO₃). Typically, the most effective mixed solvent systems are Tol : EtOH : H₂O (v/v/v) and THF : H₂O (v/v); sometimes, methanol is replaced with higher alcohols to enhance solubility and meet the high-temperature reaction conditions.

5. Notably, boronic acid and aryl halides are also used for cross-coupling by developing modified reaction conditions for the conventional Suzuki reaction, instead of using the Pd(dppf)Cl₂ conditions, with or without a CuCl combination in a basic medium and the aprotic polar solvent DMF/DME (Schemes 39 and 58).

6. The Buchwald–Hartwig cross-coupling (B-HCC) is one of the best choices for aryl–aryl rigid C–N bond construction between the primary/secondary amines and iodoarenes with the help of catalytic control of Pd₂(dba)₃ and nitrogenous ligands such as t-Bu₃P·HBF₄, t-Bu₃P, X/SPhos, etc., using high-boiling aromatic solvents such as toluene, xylene, mesitylene, etc. (Table S1†).

7. Chemoselectively, the B-HCC is highly selective for the bromine group over the chlorine group at a lower temperature of reflux of toluene. The cascade of multistep reactions was also reported using the B-HCC reaction.

8. The B-HCC is also effective for the intramolecular annulation reaction by constructing an indolophenazine derivative via C–N bond generation (Scheme 24). On the other hand, a Pd(II) based five-membered annulated product was also generated via C–C bond formation (Scheme 51).

9. The C–P bond is also constructed using Pd(OAc)₂ in the presence of a mild basic medium.

Abbreviations

PhMe	Toluene
t-BuPh	tert-Butylbenzene
MesH	Mesitylene or 1,3,5-trimethylbenzene
ODCB	ortho-Dichlorobenzene
1,2,4-TCB	1,2,4-Trichlorobenzene
DCM	Dichloromethane
DMF	Dimethylformamide
DMA/DMAc	Dimethyl acetamide
B2pin2	Bis(pinacolato)diboron
Cz	Carbazole
3,6-Di-t-BuCz	3,6-Di-tert-butylcarbazole
ICz	Indolocarbazole
DIPEA	N,N-Diisopropylethylamine or Hünig's base
Pre	Precursor
DBA-Br	7-Bromo-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene
TDBA-Br	7-Bromo-2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene
DCDBTSD	N,2-Dibromo-6-chloro-3,4-dihydro-2H-benzo[e][1,2,4]thiadiazine-7-sulfonamide-1,1-dioxide
DMPU	1,3-Dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone
tBu3P	Tri-tert-butylphosphine
t-Bu3P·HBF4	Tri-tert-butylphosphonium tetrafluoroborate
SPhos	Dicyclohexyl(2′,6′-dimethoxy[1,1′-biphenyl]-2-yl)phosphane
XPhos	Dicyclohexyl[2′,4′,6′-tris(propan-2-yl)[1,1′-biphenyl]-2-yl]phosphane
DPEPhos	Bis[(2-diphenylphosphino)phenyl] ether
BINAP	(2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl) or ([1,1′-binaphthalene]-2,2′-diyl)bis(diphenylphosphane)
Dtbpy	4,4-Di-tert-butyl-2,2-bipyridine
Pd(PPh3)4	Tetrakis(triphenylphosphine)palladium(0) or [palladium-tetrakis(triphenylphosphine)]
Pd(OAc)2	Palladium(II) acetate
(Amphos)PdCl2	Bis(di-tert-butyl(4-dimethylaminophenyl)phosphine)dichloropalladium(II) or (A-^taPhos)2PdCl2
Pd(dba)2	Bis(dibenzylideneacetone)palladium(0) or palladium(0) bis(dibenzylideneacetone)
Pd2(dba)3	Tris(dibenzylideneacetone)dipalladium(0)
Pd(dppf)Cl2	1,1′-Bis(diphenylphosphino)ferrocene palladium(II)dichloride or [1,1′-bis(diphenylphosphino)ferrocene]dichloropalladium(II)
NECO-296	Bis(di-tert-butyl(3-methyl-2-butenyl)phosphine)dichloropalladium
[Ir(COD)(OMe)]2	(1,5-Cyclooctadiene)(methoxy)iridium(I) dimer or bis(1,5-cyclooctadiene)di-μ-methoxydiiridium(I)

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by the Development of Materials and Parts Technology (RS-2024-00439679, the Development of Device and Materials for Blue Devices with CIEy ≦ 0.15 having High Efficiency and Long Device Lifetime based on Phosphorescent Sensitized Fluorescent OLEDs) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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Footnote

† Electronic supplementary information (ESI) available: Pd(0/II) catalysed coupling reaction chart for the construction of C–N/P bond(s); entry of the compounds and photophysical data. See DOI: https://doi.org/10.1039/d5qo00522a

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