2,7(3,6)-Diaryl(arylamino)-substituted Carbazoles as Components of OLEDs: A Review of the Last Decade

Organic light emitting diode (OLED) is a new, promising technology in the field of lighting and display applications due to the advantages offered by its organic electroactive derivatives over inorganic materials. OLEDs have prompted a great deal of investigations within academia as well as in industry because of their potential applications. The electroactive layers of OLEDs can be fabricated from low molecular weight derivatives by vapor deposition or from polymers by spin coating from their solution. Among the low-molar-mass compounds under investigation in this field, carbazole-based materials have been studied at length for their useful chemical and electronic characteristics. The carbazole is an electron-rich heterocyclic compound, whose structure can be easily modified by rather simple reactions in order to obtain 2,7(3,6)-diaryl(arylamino)-substituted carbazoles. The substituted derivatives are widely used for the formation of OLEDs due to their good charge carrier injection and transfer characteristics, electroluminescence, thermally activated delayed fluorescence, improved thermal and morphological stability as well as their thin film forming characteristics. On the other hand, relatively high triplet energies of some substituted carbazole-based compounds make them useful components as host materials even for wide bandgap triplet emitters. The present review focuses on 2,7(3,6)-diaryl(arylamino)-substituted carbazoles, which were described in the last decade and were applied as charge-transporting layers, fluorescent and phosphorescent emitters as well as host materials for OLED devices.


Introduction
Since the first discovery of organic electroluminescent compounds, huge attention has been devoted to the creation of new materials and optimized multilayer device architectures for viable and practical organic light emitting diodes (OLEDs), which would demonstrate low-driving voltage, high brightness, full-color emission, a long lifetime, and the easy formation of flexible thin-film devices. The commercial benefit of OLED devices is seen in the emergence of digital cameras, mobile cell phones as well as display technologies; however, there is still a strong demand to considerably improve the performance and lifetime of the devices for lighting technologies [1][2][3][4][5].
Formerly, structurally and chemically well-defined, carbazole-based compounds and their characteristics were investigated due to their properties of biological activity [6][7][8]. However, in the last three decades, the carbazole rings or substituted carbazole-containing compounds have been widely investigated in the field of organic optoelectronics, particularly in OLED technologies [9][10][11][12]. The substituted carbazole-based derivatives are widely used as components of the devices due to their good charge injection and transport characteristics in the layers of the materials [13], electroluminescence [14], thermally activated delayed fluorescence [15], improved thermal and morphological stability as well as their homogenous thin film formation properties [16]. In addition, the rather high triplet energies of some aryl(arylamino)-substituted derivatives make them useful components, acting as host materials for the phosphorescent triplet emitters in electrophosphorescent OLED technologies [17].
The 9H-carbazole core as electron donor (D) can be easily substituted at the 9,2,7 (3,6)-positions by using different aromatic fragments. Donor-acceptor (D-A) derivatives (9-arylcarbazoles), asymmetric 9,2(3)-diarylcarbazoles as well as symmetric 2,7(3,6)diarylcarbazoles could be synthesized by using various arylation or amination reactions. D-D fragments containing derivatives are usually used as hole-transporting layer materials or fluorescent emitters [18][19][20]. The D-A and A-D-A structures that have carbazoles are more suitable as host materials for phosphorescent triplet emitters, thermally activated delayed fluorescence (TADF)-based emitters as well as in other less investigated fields such as exciplex components, intramolecular hosts, dopants, etc. [21]. D-A structure derivatives and asymmetric 9,2(3)-diaryl(diarylamino)carbazoles are rather widely described in earlier review articles [22,23]. In the present review, we analyze and present only symmetric 2,7(3,6)-diaryl(arylamino)-substituted carbazoles, which were developed and described in last decade and were applied as charge-transporting layers, fluorescent and phosphorescent emitters as well as host materials for the OLED devices.
The Ullmann reaction is a coupling reaction between aryl halides and aromatic amines or aromatic heterocyclic compounds in the presence of copper-based catalysts. The mechanism of the Ullmann reaction was extensively studied for many years. Complications arise because the reactions are often heterogeneous, especially those catalyzed with metallic copper. [33,34]. Suzuki cross-coupling procedures, developed by Nobel laureate Akira Suzuki, are among the most widely investigated reactions in the formation of carbon-carbon bonds in aromatic compounds. These reactions were generally catalyzed by inorganic catalysts, such as soluble palladium (Pd) complexes having various ligands, and more recently, also in aqueous media [35,36]. The Buchwald-Hartwig amination procedure is a chemical reaction used in organic chemistry for the formation of carbon-nitrogen bonds via palladium-catalyzed coupling reactions of aromatic amines with aryl halides [37]. Other reactions as the Stille coupling [38,39], Diels Alder [40][41][42], and Friedel-Craft [43] have also been used to obtain the target carbazole compounds, but in rarer cases. In the general case, diarylamino carbazoles (27DNCr, 27DNArCr, 36DNCr, and 36DNArCr) are usually obtained by Ullmann or Buchwald-Hartwig reactions. On the other hand, diaryl carbazoles (27DCr, 27DArCr, 36DCr, and 36DArCr) are usually prepared by Suzuki or Stille reactions, as presented in Scheme 2.

Diaryl(diarylamino)-substituted Carbazoles as Charge-Transporting Layer Materials for OLEDs
Structures of the diaryl-substituted compounds (Cx), which were used to form the hole-transporting layers (HTLs) in OLED devices, are shown in Scheme 3. The target compounds C1-C14 [44][45][46][47][48] and C17-C26 [49][50][51][52][53][54] were obtained under the conditions of the Suzuki reaction. Compounds C15-C16 were generated by the Diels-Alder reaction between conjugated diene and substituted alkene, forming the substituted cyclohexene fragments [55]. Authors of the research studied properties of the derivatives and used these materials for the formation of HTLs in OLED devices. Thermal properties were examined for compounds C1-C13, C17-C19, C21-C22, and C24-C26. The reported material C22 demonstrated the highest thermal stability in this group, with a very high thermal decomposition temperature (T d ) of 575 • C, as well as the highest glass transition temperature (T g ) of 260 • C. The values of ionization potentials (I p ) for compounds C9-C11, C17-C20, and C23 were 5.65 eV, 5.55 eV, 5.8 eV, 5.38 eV, 5.42 eV, 5.19 eV, and 5.50 eV, respectively, and confirmed suitable hole-injecting properties for thin layers of many of the materials. HOMO and LUMO levels of the materials C1-C8, C13-C17, C19-C22, and C24-C26 are different and depend on the nature of the substituents. The HOMO level of the diarylcarbazoles varied between −4.93 and −6.02 eV, and the LUMO level was in a broad range between −0.87 and −2.93 eV due to different electron-withdrawing or donating substituents at the carbazole core. Positive charge drift mobility (µ h ) in the thin layers of the derivatives C12, C17-C19, C21-C22, and C25-C26 were reported. The compounds demonstrated rather high holedrift mobility in their amorphous films ranging from 5 × 10 −5 cm 2 ·V −1 ·s −1 to 1.5 × 10 −4 cm 2 ·V −1 ·s −1 at high electrical fields. The charge-injecting/transporting properties of these materials confirmed that they are suitable hole-transporting layer materials in OLEDs.
Kumar and co-workers investigated the hole-transporting properties of C25 and C26 for fluorescent OLEDs ITO/PEDOT:PSS/NPB/C25 or C26/Alq 3 /LiF/Al, as well as for phosphorescent devices ITO/PEDOT:PSS/NPB/C25 or C26 /Ir(ppy) 3 /CBP/TPBi/LiF/Al. At 1000 cd/m 2 , the fluorescent green device with C26 showed a current efficiency of 4.0 cd/A, which was about 135% higher than that of the typical HTM NPB-based device. Green phosphorescent devices with C25 showed a high current efficiency of 58.4 cd/A (power efficiency 54.8 lm/W and EQE 16.1%), while 45.1 cd/A (power efficiency 40.8 lm/W and EQE 12.5%) was measured in the C26-containing device.
Shen and co-authors investigated two types of devices. The structure of device I was ITO/C27-C30/TPBi/Mg:Ag, and that of device II was ITO/C27-C30/(Alq 3 )/Mg:Ag. In type I, the derivatives functioned as both hole-transporting and emitting materials. In the type II devices, light was emitted from either the disubstituted carbazole layers or from Alq 3 . Device II with C30 reached a maximum external quantum efficiency of 1.3%.
Kim et al. investigated the phosphorescent devices ITO/NPD/H1 or H2: Ir(ppy) 3 / Alq 3 /LiF/Al. A maximum brightness of 4165 cd/m 2 , low current efficiency of 1.58 cd/A, and also a low external quantum efficiency of 0.59% were achieved in the group of the most efficient device with the host H1 [74]. The PhOLEDs ITO/PEDOT/H3:Ir(ppy) 3 /TPBi/LiF/ Al containing the green emitter Ir(ppy) 3 in hosts H3 using wet and dry processes as well as dry process, with the incorporation of an additional TAPC hole-transporting layer, were fabricated by Jou and co-workers. The efficiency of the best device was reported as 11.6 lm/W (21 cd/A). The host material H4 was used in a solution processable multilayer device ITO/PEDOT/ H4 Ir(ppy) 3  The structures of the diarylamino-substituted carbazoles, which were used as host materials for PhOLED devices, are shown in Scheme 6. The objective compounds H16-H17 [77,78], H22-H23 [79], and H24 [80] were obtained by Ullmann reaction. The compounds H18-H21 [81] and H25 [57] were prepared by the Buchwald-Hartwig amination procedure. TGA confirmed that the compounds are highly and thermally stable. The T d values of the derivatives H16-H23 were 419 • C, 384 • C, 364 • C, 389 • C , 399 • C, 417 • C, 335 • C, and 404 • C, respectively. The T g values of the materials H16-H23 ranged from 81 • C to 210 • C. I p was reported for some of the derivatives. For example, the phenothiazine-substituted compound H16 had a lower I p of 5.25 eV as compared with the carbazole-based compound H17 (5.57 eV). The described HOMO and LUMO energies of the compounds H17 and H19-H25 had different values and depended considerably on arylamino fragments. The energy levels of these derivatives ranged from −4.86 to −5.62 eV for HOMO and from −1.78 to −2.74 eV for LUMO.  3 ] demonstrated improved efficiency (7.4% and 16 lm/W). Most importantly, the superior stability of the device using H22 enabled a lifetime well above 10,000 h at a practical luminance of 1000 cd/m 2 [84]. PhOLED devices ITO/PEDOT/PSS/TAPC/H24 or H25: Ir(ppy) 3 /BmPyPb/LiF/Al were fabricated by Park and Lee. The green device with H24 showed the best performance, with a maximal EQE of 21.3%.

Diaryl(arylamino)-substituted Carbazoles as Fluorescent Emitters of OLEDs
Structures of the 2,7-diaryl-or 3,6-diaryl-substituted carbazoles, which were used as emitters (E) in the fluorescent OLED devices, are shown in Scheme 7. All the target diarylsubstituted compounds E1-E13 [85][86][87][88][89][90][91][92][93] and E15-E20 [94][95][96][97], except for E14, were obtained during the Suzuki reaction of the di-halogenated carbazoles with the corresponding boronic acids or their esters. The objective material E14 was synthesized by fusing triphenylethylene and one dimesitylboron group moiety into a system, with one attached to the carbazole fragment [98]. The thermal properties of some of the emitters were investigated [99]. It was described that the compounds E1-E7, E9-E10, E14, and E17-E18 exhibited rather high thermal stability, with a T d value ranging from 283 to 603 • C. Meanwhile, the T d values of the compounds E15 and E16 were lower and reached 219 • C and 188 • C, respectively. It was mentioned that the combination of fragments of carbazole with various conjugated aromatic fragments is favorable for the formation of homogeneous thin films with high T g temperature values. The T g of E1, E3-E6, E9-E10, E14, E17, and E18 were 105,138,65,138,77,70,211,88,170, and 172 • C, respectively. The values of I p for the derivatives E1-E2 and E19-E20 were 5.70 eV, 5.56 eV, 5.17 eV, and 5.30 eV, respectively. It was evident that the attached fragments at the 2,7-and 3,6-positions of the carbazole core provided an effective tool for the HOMO and LUMO of the compounds. The HOMO and LUMO of E2-E7 and E9-E20 ranged from −4.43 to −5.66 and from −2.61 to −3.01 eV, respectively. Hole drift mobilities in the amorphous layer of the compounds E17-E18 were reported, and the values varied from 1 × 10 −5 to 3 × 10 −8 cm 2 ·V −1 ·s −1 at a high electric field.
The device ITO/PEDOT:PSS/CBP: E2/TPBi/LiF/Al was fabricated by Konidena and co-workers. This OLED reached a power efficiency of only 0.5 lm/W and a current efficiency of 1.1 cd/A. The same structure device just using the emitter E5 reached a power efficiency of 3.9 lm/W and a current efficiency of 4.9 cd/A, while the same device with E7 reached a power efficiency of 4.12 lm/W and a current efficiency of 9.52 cd/A. Dang and co-workers experimented with the device ITO/HATCN (dipyrazino[2,3-f:2 ,3h]quinoxaline-2,3,6,7,10,11hexacarbonitrile)/NPB/TCTA/E3 or E4/TPBi/LiF/Al. The material E4, with a much balanced molecular conjugation and a twisted molecular conformation, exhibited much better performance in non-doped OLEDs with maximum photometric efficiency, power efficiency, and EQE values of 7.38 cd/A, 6.81 lm/W, and 3.0%, respectively. The 2,7-di(arylamino)-or 3,6-di(arylamino) carbazoles, which were used as fluorescent emitters (Ex) in the OLED devices, are shown in Scheme 8. The compounds 27DBrCr, 36DBr9ArCr, or 36DI9ArCr were firstly obtained as starting materials for the synthesis, as shown in Scheme 1. The target compounds E21 [100,101] and E23 [102] were obtained during the Buchwald-Hartwig amination reaction, and the materials E22 [79], E24 [103], and E25 [104] were obtained during the Ullmann reaction of the starting derivatives.
It was reported that the compounds E21-E25 have high thermal stability, with temperatures of decomposition at 436, 440, 387, 285, and 413 • C, respectively. The derivatives E21, E24, and E25 are also suitable for glass formation, with T g values at 88, 110, and 119 • C. HOMO and LUMO energy levels were described for all the molecules of

Diary(arylamino)-substituted Carbazoles as TADF Emitters for OLEDs
It should be mentioned that only 3,6-diaryl-substituted carbazoles are still reported as the TADF compounds suitable for emitting layers (Tx in Scheme 9). The starting materials (36DBrCr and 36DBr9ArCr) for the synthesis of the emitters as objective derivatives were prepared, as shown in Scheme 1. The target compound T1 was obtained during the Suzuki reaction [106]. The compounds T2-T3 were obtained by nucleophilic substitution [107,108]. Other compounds, T4-T5, were obtained during the Buchwald-Hartwig reaction [109].
The T d of compounds T1-T3 were very high and reached 178 • C, 242 • C, and 287 • C, respectively. These destruction temperatures were lower than those of compounds T4-T5, where they exceeded 500 • C. The Ip of the studied derivative T2 in solid-state was reported as 5.89 eV. The values of HOMO/LUMO for compounds T1, T4, and T5 were −5.77/−2.40 eV, −5.63/−2.64 eV and −5.62/−2.67 eV. Only the charge transporting properties for layers of compounds T2 were described. They exhibited a bipolar charge transport with balanced hole and electron mobility exceeding 10 −4 cm 2 ·V −1 ·s −1 at higher than 3 × 10 5 V/cm electric fields. The 3,6-Di(arylamino)carbazoles, which were used as the TADF emitters for OLEDs, are shown in Scheme 10. The target compounds T6-T12 were obtained during the Ullmann amination reaction of the corresponding diiodo or dibromo derivatives shown in Scheme 1 [110][111][112]. The compounds T13-T14 were synthesized during the Buchwald-Hartwig reactions of the starting materials [113]. Thermal stability was presented for the conjugates T6-T9 and T11-T14. These derivatives have rather high stability, with T d values from 291 • C to 530 • C. The highest T d value, which exceeded 530 • C, was mentioned for derivative T12. DSC measurements showed that the conjugates T11-T14 formed amorphous films, with the T g ranging from 86 • C to 155 • C. The derivatives T13-T14 containing carbazolyl or diphenylamino fragments had the highest T g values, reaching 155 • C. The HOMO and LUMO energies of T11-T14 were presented to be −5.60/−2.63 eV, −5.53/−2.58, −5.06/−2.31 eV, and −5.17/−2.24 eV, respectively.
In order to evaluate the potential of the mentioned TADF emitters for OLEDs, authors fabricated devices by using the synthesized TADF materials. Compounds T6-T9 were tested in TADF-based devices of the following structures:

Concluding Remarks
Recent developments on 2,7(3,6)-diaryl(arylamino)-substituted carbazole-based electroactive derivatives are presented here with a short description of their preparation, physical properties, and the characteristics of organic light emitting diodes using the materials. The reviewed derivatives had different functions in the OLED devices, including hole transport in the thin layers of the materials, electroluminescence or thermally activated delayed fluorescence in emitting layers as well as host functions for phosphorescent dopants. Some of the derivatives function very effectively as positive charge-transporting layer compounds, enhancing quantum efficiencies, lowering driving voltages, and increasing the life time of the OLEDs. Positive charge drift mobility in the thin films of the most effective derivatives can exceed 2 × 10 −3 cm 2 ·V −1 ·s −1 at high electrical fields.
Among various carbazole-based host derivatives, the diaryl-substituted conjugates are very effective as hosts for the blue (EQE > 27%), green (EQE > 210%), and red (EQE > 20%) phosphorescent organic light emitting diodes. As derivatives for the light emitting layer, the diaryl(arylamino)-substituted carbazoles cover the broad range of emitted lights, from the color blue to green, through the substitution and introduction of different aromatic fragments into the carbazole core at the 2,7(3,6) positions. For example, by using the thermally activated delayed fluorescence (TADF) function of the carbazole-based emitters, blue devices showed a maximum EQE efficiency that exceeded 30%. Therefore, the low molar mass 2,7(3,6)-diaryl(arylamino)-substituted carbazoles are promising as charge-transporting layer derivatives, emitters, and hosts for various configurations of OLED devices, and further research in the field of new carbazole-based electroactive materials is actively ongoing to improve the characteristics of future OLED devices.
Author Contributions: Writing-review and editing, G.K.; writing-review and editing, S.G. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Kaunas University of Technology, Vytautas Magnus University and Lithuanian Academy of Sciences.

Conflicts of Interest:
The authors declare no conflict of interest.