Derivatives of Imidazole and Carbazole as Bifunctional Materials for Organic Light-Emitting Diodes

New derivatives of carbazole and diphenyl imidazole for potential multiple applications were synthesized and investigated. Their properties were studied by thermal, optical, photophysical, electrochemical, and photoelectrical measurements. The compounds exhibited relatively narrow blue light-emission bands, which is favorable for deep-blue electroluminescent devices. The synthesized derivatives of imidazole and carbazole were tested as fluorescent emitters for OLEDs. The device showed deep-blue emissions with CIE color coordinates of (0.16, 0.08) and maximum quantum efficiency of 1.1%. The compounds demonstrated high triplet energy values above 3.0 eV and hole drift mobility exceeding 10−4 cm2/V·s at high electric fields. One of the compounds having two diphenyl imidazole moieties and tert-butyl-substituted carbazolyl groups showed bipolar charge transport with electron drift mobility reaching 10−4 cm2/V·s at electric field of 8 × 105 V/cm. The synthesized compounds were investigated as hosts for green, red and sky-blue phosphorescent OLEDs. The green-, red- and sky-blue-emitting devices demonstrated maximum quantum efficiencies of 8.3%, 6.4% and 7.6%, respectively.


Introduction
Organic light-emitting diodes (OLEDs) are used in displays and lighting devices due to their low turn-on voltage, high power efficiency, mechanical flexibility and wide range of colors [1]. Generally, electrons and holes injected from the respective electrodes move through charge injection/transport layers into the emitting layer of a device. There, electrons and holes recombine and form singlet and triplet excitons [2]. Fluorescent OLEDs exploit only singlet excitons for light emission [3]. Organic fluorescent materials with high emission quantum yield, restricted intermolecular interactions, and appropriate charge injection/transporting properties are required for stable and efficient OLEDs [4,5]. Because of the high stability of blue singlet emission-based OLEDs, they are still used in commercial displays [6]. Therefore, development of stable, efficient and cost-effective fluorescent blue emitters remains an urgent task. Phosphorescent OLEDs (PHOLEDs) can utilize both singlet and triplet excitons for light emission. This property allows us to increase external quantum efficiency of PHOLEDs [7,8]. Phosphorescent emitters normally have to be doped in a suitable charge transporting host matrix due to their typically poor charge carrier mobility [9]. Host materials with appropriate energy levels and triplet energies, balanced charge carrier transport, as well as high thermal stability and morphological stability of their layers, can enable excellent PHOLED performance [10][11][12].

Synthesis
The synthetic routes for preparation of derivatives of carbazole and diphenyl imidazole are presented in Scheme 1. The target compound 1 was synthesized using the commercial reagents. During the condensation reaction, a tetra-substituted imidazole ring was obtained. Tert-butyl groups were attached to carbazole moieties to obtain compound 2. Compounds 3 and 4 were prepared by the same methods as 1 and 2, respectively.

Synthesis
The synthetic routes for preparation of derivatives of carbazole and diphenyl imidazole are presented in Scheme 1. The target compound 1 was synthesized using the commercial reagents. During the condensation reaction, a tetra-substituted imidazole ring was obtained. Tert-butyl groups were attached to carbazole moieties to obtain compound 2. Compounds 3 and 4 were prepared by the same methods as 1 and 2, respectively. The yields of derivatives 1 and 3 were of 82% and 74%, respectively, whereas the Friedel-Crafts alkylations gave compounds 2 and 4 in 69% and 57% yields, respectively. All the compounds were found to be soluble in common organic solvents. Their structures were confirmed by 1 H NMR, ATR-IR, and mass spectrometry. The yields of derivatives 1 and 3 were of 82% and 74%, respectively, whereas the Friedel-Crafts alkylations gave compounds 2 and 4 in 69% and 57% yields, respectively. All the compounds were found to be soluble in common organic solvents. Their structures were confirmed by 1 H NMR, ATR-IR, and mass spectrometry.

Thermal Properties
The behavior of compounds 1-4 under heating was studied by TGA and DSC. The obtained results are summarized in Table 1. T D -5% weight-loss temperature; T m -melting point; T g -glass transition temperature; T cr -crystallization temperature, λ ab -lowest energy band of absorption; λ fl -wavelength of maximum intensity of fluorescence of the solutions in THF; E T -triplet energy determined from phosphorescence spectrum.
The synthesized compounds showed high thermal stability confirmed by TGA. The 5% weight-loss temperatures (T D ) of the compounds with one imidazole ring (1 and 2) were found to be ca. 400 • C, while compounds with two imidazole units (3 and 4) showed TD higher than 477 • C (Figures 1a and S1a). Compound 1 was obtained as a crystalline substance. In the first DSC heating scan of 1 (Figure 1b), an endothermic melting signal was observed at 270 • C. The second heating scan revealed glass transition temperature (T g ) at 134 • C and further heating showed crystallization and melting signals. Compounds 2, 3 and 4 were isolated after the synthesis and purification as amorphous substances with T g of 146 • C, 151 • C and 172 • C, respectively, observed in the heating and cooling scans (Figure S1b-d). DSC results demonstrate that increased molecular weight by attachment additional carbazole-imidazole moiety or tert-butyl groups results in an increase of T g .
The behavior of compounds 1-4 under heating was studied by TGA and DSC. The obtained results are summarized in Table 1. TD-5% weight-loss temperature; Tm-melting point; Tg-glass transition temperature; Tcr-crystallization temperature, λab-lowest energy band of absorption; λfl-wavelength of maximum intensity of fluorescence of the solutions in THF; ET-triplet energy determined from phosphorescence spectrum.
The synthesized compounds showed high thermal stability confirmed by TGA. The 5% weight-loss temperatures (TD) of the compounds with one imidazole ring (1 and 2) were found to be ca. 400 °C, while compounds with two imidazole units (3 and 4) showed TD higher than 477 °C (Figures 1a and S1a). Compound 1 was obtained as a crystalline substance. In the first DSC heating scan of 1 (Figure 1b), an endothermic melting signal was observed at 270 °C. The second heating scan revealed glass transition temperature (Tg) at 134 °C and further heating showed crystallization and melting signals. Compounds 2, 3 and 4 were isolated after the synthesis and purification as amorphous substances with Tg of 146 °C, 151 °C and 172 °C, respectively, observed in the heating and cooling scans (Figure S1b-d). DSC results demonstrate that increased molecular weight by attachment additional carbazole-imidazole moiety or tert-butyl groups results in an increase of Tg.

Photophysical Properties
Absorption   Table 1). S2a-S5a). THF solutions of 1-4 emitted blue light with the photoluminescence (PL) intensity maxima in the range of 398-403 nm ( Figure 2a, Table 1). The aspecific electrostatic interactions between molecules of the solvent and the materials in the context of Onsager interpretation [47] is in the core of the Lippert-Mataga approach [48][49][50]   The aspecific electrostatic interactions between molecules of the solvent and the materials in the context of Onsager interpretation [47] is in the core of the Lippert-Mataga approach [48][49][50] [51]. The interaction of the solvent and the solute is ongoing in an individual manner for the particular solvent, resulting in the different distribution of LE and CT components. Apparently, the emission of 1-4 in solutions is affected by HLCT, which is characterized as combined transition of the LE and CT states. The observed LE, CT and mixed LE and CT states of the solutions of compounds 1-4 in different solvents may originate from the twisting of the angles between the carbazole and imidazole moieties (Figure 2c) [45].

Photophysical Properties
The wavelengths of intensity maxima of phosphorescence spectra of THF solutions of the compounds recorded at 77 K ranged from 533 nm to 537 nm (Figures 2d and S7a-c). The triplet energies (E T ) of the compounds were estimated from the onset of the phosphorescence spectra (Figure 2d). Compound 1 exhibited the highest E T of 3.18 eV, while compounds 2, 3 and 4 exhibited slightly smaller triplet energy levels 3.16 eV, 3.08 eV and 3.10 eV, respectively ( Table 1).
The emission spectra of the layers of compounds 1-4 had two peaks at ca. 400 nm and a wide red-shifted emission peak at~550 nm (Figure 3a). It is known that an imidazole ring in the presence of oxygen under UV-irradiation is subjected to the ring-opening reaction, which results in the formation of new materials [52,53]. The PL spectra of 1 wt.% solid solution of compound 1 in ZEONEX 480 were recorded after different periods of UV irradiation. They showed consistent increase in intensity of emission band, peaking at 550 nm (Figure 3b). The small concentration of compound 1 in ZEONEX 480 ensures absence of intermolecular interactions as well as formation of intermolecular excimers. PL decay curves of the layers of 1-4 recorded at the different wavelengths of ca. 410 nm and ca. 550 nm exhibited prompt fluorescence ( Figure S8). The observed difference between PL lifetimes demonstrates the different origin of emission. Thus, PL decay curves recorded at 550 nm have a biexponential character. The slightly longer-lived component of the prompt fluorescence is attributed to the oxidized product. Thus, these experiments prove the photochemical process occurring in the layers of compounds 1-4 [54]. The photochemical process was also detected for the THF solutions of the compounds ( Figure S9a-c). It was manifested by substantial decrease of intensity and the redshift of the peak after continuous UV irradiation.
originate from the twisting of the angles between the carbazole and imidazole moieties (Figure 2c) [45].
The wavelengths of intensity maxima of phosphorescence spectra of THF solutions of the compounds recorded at 77 K ranged from 533 nm to 537 nm (Figure 2d and S7a-c). The triplet energies (ET) of the compounds were estimated from the onset of the phosphorescence spectra (Figure 2d). Compound 1 exhibited the highest ET of 3.18 eV, while compounds 2, 3 and 4 exhibited slightly smaller triplet energy levels 3.16 eV, 3.08 eV and 3.10 eV, respectively ( Table 1).
The emission spectra of the layers of compounds 1-4 had two peaks at ca. 400 nm and a wide red-shifted emission peak at ~550 nm (Figure 3a). It is known that an imidazole ring in the presence of oxygen under UV-irradiation is subjected to the ring-opening reaction, which results in the formation of new materials [52,53]. The PL spectra of 1 wt.% solid solution of compound 1 in ZEONEX 480 were recorded after different periods of UV irradiation. They showed consistent increase in intensity of emission band, peaking at 550 nm (Figure 3b). The small concentration of compound 1 in ZEONEX 480 ensures absence of intermolecular interactions as well as formation of intermolecular excimers. PL decay curves of the layers of 1-4 recorded at the different wavelengths of ca. 410 nm and ca. 550 nm exhibited prompt fluorescence ( Figure S8). The observed difference between PL lifetimes demonstrates the different origin of emission. Thus, PL decay curves recorded at 550 nm have a biexponential character. The slightly longer-lived component of the prompt fluorescence is attributed to the oxidized product. Thus, these experiments prove the photochemical process occurring in the layers of compounds 1-4 [54]. The photochemical process was also detected for the THF solutions of the compounds ( Figure S9a-c). It was manifested by substantial decrease of intensity and the redshift of the peak after continuous UV irradiation.

Electrochemical and Photoelectrical Properties
The values of ionization potential (IP) estimated by cyclic voltammetry (CV) and UV photoelectron (PE) spectroscopy are presented in Table 2. The IPCV values of the compounds were estimated from the half-wave potential of the first oxidation relative to ferrocene ( Figure S10a

Electrochemical and Photoelectrical Properties
The values of ionization potential (IP) estimated by cyclic voltammetry (CV) and UV photoelectron (PE) spectroscopy are presented in Table 2. The IP CV values of the compounds were estimated from the half-wave potential of the first oxidation relative to ferrocene ( Figure S10a-d). The electron affinity (EA CV ) values were obtained from the IP CV values and the optical bandgaps, which were deduced from the edges of the absorption spectra of the dilute THF solutions of 1-4. The IP CV and EA CV values of 1-4 are in the small ranges of 5.21-5.26 eV and 1.81-1.94 eV, respectively. The comparable values of the IP CV and EA CV observed for all the studied compounds demonstrate the same oxidation and reduction sites. The photoelectron emission spectra of the solid samples of 1-4 are presented in Figure 4a. The IP PE values of the compounds are in the slightly larger range of 5.18-5.35 eV comparing with that of IP CV . This observation can be explained by the different environments in solutions and solid layers of the compounds. IP CV and IP PE values of compounds 2 and 4, bearing tert-butyl groups, are slightly lower than those of their counterparts 1 and 3, respectively. This trend can be explained by the σ donor effect of tert-butyl groups [55] which reduce the ionization potential values. 1 Determined by cyclic voltammetry; 2 measured by UV photoelectron spectrometry in air; 3 charge mobility at 6.4 × 10 5 V/cm.
The photoelectron emission spectra of the solid samples of 1-4 are presented in Figure 4a. The IPPE values of the compounds are in the slightly larger range of 5.18-5.35 eV comparing with that of IPCV. This observation can be explained by the different environments in solutions and solid layers of the compounds. IPCV and IPPE values of compounds 2 and 4, bearing tert-butyl groups, are slightly lower than those of their counterparts 1 and 3, respectively. This trend can be explained by the σ donor effect of tert-butyl groups [55] which reduce the ionization potential values.  The TOF technique was used to study charge transporting properties of the compounds. TOF current transients for holes and electrons for vacuum deposited films of the compounds 1-4 were recorded at different electric fields ( Figure S11). When the transit times were well recognized from the TOF current transients in log-log scales, charge mobilities were calculated. The electric field dependence values of hole and electron mobilities of the layers of compounds 1-4 are shown in Figure 4b. The layers of compounds demonstrated hole drift mobilities ranging from 10 −5 cm 2 /V·s to 10 −4 cm 2 /V·s at electric field of 6.4 × 10 5 V/cm ( Table 2). Electron mobility was detected only in the layer of compound 4, which reached 10 −4 cm 2 /V·s at high electric fields (>8.1 × 10 5 V/cm). Thus, the difference between hole and electron mobilities in the layer of compound 4 is only one order of magnitude. To achieve high efficiency of PHOLED, host materials with a balanced charge transport are required [56]. This allows generation of a broad charge recombination zone in the emissive layer.

Performance in Fluorescent OLEDs
To investigate electroluminescent properties of compounds 1-4 as materials for doping-free light-emitting layers (EML), OLEDs A-D were fabricated using device structure ITO/MoO3 (1 nm)/NPB (55 nm)/EML (40 nm)/TSPO1 (5 nm)/TPBi (65 nm)/LiF The TOF technique was used to study charge transporting properties of the compounds. TOF current transients for holes and electrons for vacuum deposited films of the compounds 1-4 were recorded at different electric fields ( Figure S11). When the transit times were well recognized from the TOF current transients in log-log scales, charge mobilities were calculated. The electric field dependence values of hole and electron mobilities of the layers of compounds 1-4 are shown in Figure 4b. The layers of compounds demonstrated hole drift mobilities ranging from 10 −5 cm 2 /V·s to 10 −4 cm 2 /V·s at electric field of 6.4 × 10 5 V/cm ( Table 2). Electron mobility was detected only in the layer of compound 4, which reached 10 −4 cm 2 /V·s at high electric fields (>8.1 × 10 5 V/cm). Thus, the difference between hole and electron mobilities in the layer of compound 4 is only one order of magnitude. To achieve high efficiency of PHOLED, host materials with a balanced charge transport are required [56]. This allows generation of a broad charge recombination zone in the emissive layer.
light, respectively (Figure 5b inset, Table 3). The shapes and positions of th troluminescence spectra of the devices were very similar to those of the flu tra of the solutions of compounds 1-4 in THF (Figure 2a and 5b). The shape were practically the same at different applied voltages ( Figure S12). The e of devices C and D were wider and slightly red-shifted compared to thos and B. It was also observed that during operation of the devices at various ages, no additional emission peak appeared in the longer wavelength regi  (Figures 5b-c and S13). Maximum c ciency (0.8 cd/A) and maximum energy efficiency (0.42 lm/W) of device B wer those of devices A, C and D. However, the maximum external quantum effici of device B was the highest one (1.1%) (Figure 5d). This observation can be a the narrowest EL spectrum of device B (Figure 5b).
The obtained results show that the synthesized derivatives of carbazole an imidazole (1)(2)(3)(4) are suitable for the formation of functional layers of OLED. oxidation process of the imidazole ring can be successfully suppressed by fa the devices in an inert atmosphere. This is confirmed by the absence of the add energy peaks in the electroluminescence spectra.
Electroluminescence (EL) spectra of OLEDS A-D are shown in Figures 5b and S12. Electroluminescence intensity maxima of devices A-D were observed in the range of 418-437 nm. According to the CIE color coordinates, A, B and C, D emitted deep-blue and blue light, respectively (Figure 5b inset, Table 3). The shapes and positions of the peaks of electroluminescence spectra of the devices were very similar to those of the fluorescence spectra of the solutions of compounds 1-4 in THF (Figures 2a and 5b). The shapes of EL spectra were practically the same at different applied voltages ( Figure S12). The emission peaks of devices C and D were wider and slightly red-shifted compared to those of devices A and B. It was also observed that during operation of the devices at various electrical voltages, no additional emission peak appeared in the longer wavelength region.
All OLEDs showed relatively low turn-on voltages (V on ) of 4.0-4.8 V (Figure 5c). This observation confirms that injection and transport of holes and electrons towards the emission layer was efficient. Device D showed maximum brightness of 1430 cd/m 2 and minimum turn-on voltage of 4.0 V. Device B, with emissive layer of emitter 2, demonstrated the best electroluminescence properties (Figures 5b,c and S13). Maximum current efficiency (0.8 cd/A) and maximum energy efficiency (0.42 lm/W) of device B were similar to those of devices A, C and D. However, the maximum external quantum efficiency (EQE) of device B was the highest one (1.1%) (Figure 5d). This observation can be attributed to the narrowest EL spectrum of device B (Figure 5b).
The obtained results show that the synthesized derivatives of carbazole and diphenyl imidazole (1)(2)(3)(4) are suitable for the formation of functional layers of OLED. The photooxidation process of the imidazole ring can be successfully suppressed by fabrication of the devices in an inert atmosphere. This is confirmed by the absence of the additional low-energy peaks in the electroluminescence spectra.

Performance in Phosphorescent OLEDs
To study the performance of compounds 1-4 as hosts, the simple PHOLEDs E-H and I-L were fabricated using green or red phosphorescent emitters, i.e., tris [2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy) 3 ) or bis [2-(1-isoquinolinyl-N)phenyl-C](2,4-pentanedionato-O2,O4)iridium(III) ((piq) 2 Ir(acac)), respectively. The concentration of emitters in the hosts was of 10 wt.%. The device structures and energy levels of the materials used in the devices are schematically shown in Figures 6a and S14. The electroluminescence and efficiency characteristics of the devices E-H are given in Table 3 and Figures 6 and S15.
Despite the exploitation of the green phosphorescent emitter Ir(ppy) 3 , most of the PHOLEDs of the series E-H were not characterized by green emission ( Figure S16). Lowintensity violet emission bands were observed in EL spectra of PHOLEDs E-H. The wavelengths of these emission bands were close to those of fluorescence bands of pure 1-4. Thus, the recombination of excitons occurred not only in the emitter but also in hosts. This observation can be attributed to inefficient host-guest energy transfer. Device G containing compound 3 as a host was characterized by green emission with the intensity maximum at ca. 510 nm, confirming the radiative recombination of excitons mainly on Ir(ppy) 3 . Thus, device G was characterized by the most efficient host-guest energy transfer. PHOLEDs E-H were characterized by the relatively low values of V on (3.1-5.2 V), confirming the very efficient injection from the electrodes and transport of holes and electrons to the emitting layer. The highest brightness of 24,600 cd/m 2 and the lowest V on of 3.1 V observed for device G compared to those recorded for the other devices shows more effective exciton recombination and radiative transition in the emitting layer with host 3. Thus, compound 3 can be regarded as the effective host for PHOLEDs. Device G with host 3 exhibited the maximum current, power, and external quantum efficiencies of 31 cd/A, 13.7 lm/W, 8.3%, respectively, in the absence of light outcoupling enhancement. It has to be noted that the simple and unoptimized PHOLED based on host 3 was fabricated. Therefore, the maximum efficiency of device was lower compared to those reported for other PHOLEDs containing Ir(ppy) 3 [57]. Despite the exploitation of the green phosphorescent emitter Ir(ppy)3, most of the PHOLEDs of the series E-H were not characterized by green emission ( Figure S16). Lowintensity violet emission bands were observed in EL spectra of PHOLEDs E-H. The wavelengths of these emission bands were close to those of fluorescence bands of pure 1-4. Thus, the recombination of excitons occurred not only in the emitter but also in hosts. This observation can be attributed to inefficient host-guest energy transfer. Device G containing compound 3 as a host was characterized by green emission with the intensity maximum at ca. 510 nm, confirming the radiative recombination of excitons mainly on Ir(ppy)3. Thus, device G was characterized by the most efficient host-guest energy transfer. PHOLEDs E-H were characterized by the relatively low values of Von (3.1-5.2 V), confirming the very efficient injection from the electrodes and transport of holes and electrons to the emitting layer. The highest brightness of 24,600 cd/m 2 and the lowest Von of 3.1 V observed for device G compared to those recorded for the other devices shows more effective exciton recombination and radiative transition in the emitting layer with host 3. Thus, compound 3 can be regarded as the effective host for PHOLEDs. Device G with host 3 exhibited the maximum current, power, and external quantum efficiencies of 31 cd/A, 13.7 lm/W, 8.3%, respectively, in the absence of light outcoupling enhancement. It has to be noted that the simple and unoptimized PHOLED based on host 3 was fabricated. Therefore, the maximum efficiency of device was lower compared to those reported for other PHOLEDs containing Ir(ppy)3 [57]. Similarly, the highest maximum EQE of 6.4% was obtained for the red PhOLED (device K) based on host 3. In contrast to other red PhOLEDs fabricated in this work, emission from the host was not observed in EL spectra of device K recorded at the different voltages. This observation demonstrates perfect host-guest energy transfer from 3 to (piq) 2 Ir(acac) (Figures 6a and S17). Red PHOLEDs I-K also showed low turn-on voltages of 3.2-4.4 V (Figure 6c, Table 3). Maximum brightness of 8400 cd/m 2 was observed for device K. This device exhibited maximum quantum efficiency of 6.4 % (Figure 6d).
Taking into account that 3 showed the best performance among the compounds of the series, PHOLED M was fabricated with the same structure as that of the red PHOLEDs except the layer of hexaazatriphenylenehexacarbonitrile (HAT-CN) and the layer of FIrpic doped in 3 as an emitting layer (Figure 7). The V on of 3.2 V (Figure 7c) and appropriate spectral properties show complete electronic excitation energy transfer from 3 to FIrpic. Device M with a sky-blue EL reached of maximum EQE of 7.6% (Figure 7d). device K. This device exhibited maximum quantum efficiency of 6.4 % (Figure 6d).
Taking into account that 3 showed the best performance among the compounds of the series, PHOLED M was fabricated with the same structure as that of the red PHOLEDs except the layer of hexaazatriphenylenehexacarbonitrile (HAT-CN) and the layer of FIrpic doped in 3 as an emitting layer (Figure 7). The Von of 3.2 V (Figure 7c) and appropriate spectral properties show complete electronic excitation energy transfer from 3 to FIrpic. Device M with a sky-blue EL reached of maximum EQE of 7.6% (Figure 7d).

Conclusions
Four new derivatives of carbazole and diphenyl imidazole were synthesized and characterized. Optical, photophysical, electrochemical, and photoelectric properties of the compounds were found to be similar, indicating that attachment of the additional carbazole-diphenyl imidazole fragment has no significant impact on these properties. Compounds with two imidazole rings demonstrated higher thermal stability and glass transition temperatures compared with those of the derivatives having one imidazole ring. OLEDs fabricated using the synthesized compounds as emitters showed deep-blue and blue emissions with maximum quantum efficiency ranging from 0.6% to 1.1%. The suitable values of energy levels, triplet energies and charge carrier mobilities allowed the use of the synthesized compounds as hosts for green and red phosphorescent OLEDs. The devices showed sky-blue, green or red emissions, confirming the radiative recombination of excitons in phosphorescent emitters. Maximum quantum efficiencies of 7.6%, 8.3%, and 6.4%, respectively, were observed without optimization of the structures.

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Informed Consent Statement: Not applicable.

Data Availability Statement:
All experimental data to support the findings of this study are available upon request by contacting the corresponding authors.