Highly Efficient Blue Organic Light-Emitting Diode Based on a Pyrene[4,5- <i>d</i> ]Imidazole-Pyrene Molecule

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Highly Efficient Blue Organic Light-Emitting Diode Based on a Pyrene[4,5-d]Imidazole-Pyrene Molecule Yulong Liu†, Xiaxia Man†, Qing Bai, Hui Liu, Pengyuan Liu, Ying Fu, Dehua Hu, Ping Lu and Yuguang Ma Yulong Liu† State Key Laboratory of Supramolecular Structure and Materials, Department of Chemistry, Jilin University, Changchun 130012 Department of Chemistry, College of Arts and Sciences, Northeast Agricultural University, Harbin 150030 , Xiaxia Man† State Key Laboratory of Supramolecular Structure and Materials, Department of Chemistry, Jilin University, Changchun 130012 The First Hospital of Jilin University, Changchun 130021 , Qing Bai State Key Laboratory of Supramolecular Structure and Materials, Department of Chemistry, Jilin University, Changchun 130012 , Hui Liu State Key Laboratory of Supramolecular Structure and Materials, Department of Chemistry, Jilin University, Changchun 130012 , Pengyuan Liu State Key Laboratory of Supramolecular Structure and Materials, Department of Chemistry, Jilin University, Changchun 130012 , Ying Fu Department of Chemistry, College of Arts and Sciences, Northeast Agricultural University, Harbin 150030 , Dehua Hu Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 , Ping Lu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, Department of Chemistry, Jilin University, Changchun 130012 and Yuguang Ma Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 https://doi.org/10.31635/ccschem.021.202000627 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Organic light-emitting diodes (OLEDs), which have been recently utilized in some flat-panel display screens such as mobile phones and televisions, show many merits, including light weight, high flexibility, energy preservation, and so forth, and are considered the next-generation displays and solid-state lightings. Blue-emitting materials that can be applied in nondoped OLEDs with little efficiency roll-offs at high brightness are of great importance. Here, a highly efficient, blue-emitting material, 9-phenyl-10-(4-(pyren-1-yl)phenyl)-9H-pyreno[4,5-d]imidazole (PyPI-Py), is achieved using pyrene[4,5-d]imidazole and pyrene as the weak electron donor and electron acceptor, respectively. The nondoped blue OLED exhibits excellent performance with a maximum brightness of 75,687 cd m−2, a maximum current efficiency of 13.38 cd A−1, and a maximum external quantum efficiency (ηext) of 8.52%. Moreover, the ηext is maintained at 8.35% and 8.05% at a brightness of 10,000 and 50,000 cd m−2, respectively, displaying extremely small efficiency roll-offs of 2.0% and 5.5%. The device characteristics are among the highest values for nondoped blue OLEDs and correspond to the best performance obtained for nondoped pyrene-based blue OLEDs. The superior performance is attributed to the proper donor–acceptor design strategy which results in a quasi-equivalent hybrid local and charge-transfer excited state with the maximum generation of an 82% fraction of singlet excitons. Download figure Download PowerPoint Introduction Pyrene, consisting of four fused benzene rings, is one of the most widely investigated polycyclic aromatic hydrocarbons in recent decades.1,2 The rigid planar structure and flat π-system of the pyrene-based component results in favorable π-electron flow, flexible self-assembly, and excellent photoelectric properties, such as bright emission, efficient excimer formation, and special-stacking topological configuration.3–7 Pyrene-based materials have been widely applied as biological probes and fluorescent sensors because of their tunable fluorescence in response to environmental alteration.8–10 Pyrene[4,5-d]imidazole-based organic dyes are also suitable in the applications of dye-sensitized solar cells owing to their rigid structure.11,12 In recent years, there has been increasing interest for developing pyrene as a semiconductor in optoelectronics.1,2,13–17 Organic light-emitting diodes (OLEDs) have greatly progressed in the past decades and shown potential as the next-generation flat-panel displays and solid-state lightings.18–20 Because of excellent thermal stability, efficient blue light emission, and high mobility of charge carrier, pyrene has emerged as a very appealing chromophore for blue OLEDs, which are urgently needed for full-color displays.21–25 External quantum efficiency (ηext) is a crucial parameter for the characterization of OLED performance. The theoretical value of ηext can be calculated by the following equation: η ext = η r × η out × η PL × γ where ηr is the exciton utilization efficiency (EUE), ηout represents the light extraction efficiency (20–30%),26 ηPL stands for the intrinsic photoluminescence (PL) quantum efficiency in film, and γ is the holes-electrons recombination efficiency (ideally 100%).27 Although the rigid architecture of pyrene allows for the formation of face-to-face π–π stacking and enhanced charge migration, the strong tendency toward excimers generation will substantially quench the fluorescence in the solid state to a certain extent, yielding a relatively low ηPL and further limiting the ηext of the resulting device.28,29 Thus, in prior studies, OLEDs of pyrene derivatives are mostly fabricated by doping technology aiming to achieve high efficiency in OLEDs to avoid the fluorescence-quenching effect in the aggregated state.30–32 To improve the performance in nondoped OLEDs, rational modifications of the pyrene structure at various positions were extensively investigated as well to adjust the π–π stacking of pyrene-based molecules, thereby reducing their aggregation tendency in condensed media and fine-tuning the optical properties. For example, Cheng et al.33 prepared a dipyrenylbenzene-containing compound with a twisted molecular structure, which led to a low extent of π–π stacking and a high ηPL of 75% in the film state. The nondoped blue-emitting OLED displayed a high ηext of 5.2% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.11) and a very high brightness of 40,400 cd m−2. Huang et al.34 prepared a dumbbell-shaped spirocyclic aromatic hydrocarbon, which realized a large pyrene–pyrene distance of up to 10.4 Å. Its nondoped OLED achieved a maximum current efficiency (CE) of 7.4 cd A−1 and a maximum ηext value of 4.6% with CIE coordinates of (0.16, 0.15). Chan et al.35 demonstrated an efficient blue OLED based on the solution-processable pyrene-1,3-alt-calix arene, which exhibited a record CE of 10.5 cd A−1 in the nondoped OLED, corresponding to a ηext value of 6.4% and CIE coordinates of (0.15, 0.24). Such results represent typical chromophores that are not inherently prone to aggregate. Recently, we have also reported a nondoped OLED based on pyrene[4,5-d]imidazole derivative N,N-diphenyl-4'-(9-phenyl-9H-pyreno(4,5-d)imidazol-10-yl)-(1,1'-biphenyl)-4-amine (PyPPA). with high ηPL of 45% in the solid state which obtained a maximum ηext of 8.47% with CIE coordinates of (0.14, 0.13).36 These previous studies have reported significant achievements in increasing the ηPL of solid-state pyrene derivatives. Furthermore, ηext values exceeding 5%, which is the theoretical upper limit for traditional fluorescent OLEDs have been successfully attained using pyrene-based materials. Exploiting a material's potential is a never-ending mission for a scientist. According to the expression for ηext, another important factor that must be considered in addition to the ηPL to achieve higher ηext is the ηr, which also refers to the EUE. The phosphorescent and thermally activated delayed fluorescent (TADF) materials represent the most investigated materials in recent years to realize high efficiency and brightness through efficacious triplet harvesting.37–40 However, they usually need to be doped into appropriate hosts, and the problem of severe efficiency roll-offs at high brightness in OLEDs remains unresolved. In recent years, materials with hybrid local and CT (HLCT) excited state were also well developed, which successfully achieved the merits of high ηPL and a high output of singlet excitons in a series of donor (D)–acceptor (A) materials.36,41–52 HLCT materials also show the potential to maintain stable ηext at high brightness. Thus, there is still room for further enhancement of the ηext of blue OLEDs and low efficiency roll-offs in pyrene-based D–A system. The electron-donating and -withdrawing strengths of D and A should be carefully considered when adopting a D–A design strategy for a blue light-emitting material because neither a strong D nor a strong A should be employed to avoid the variation of the emission region. In our previous works, imidazole was an ideal unit for blue-emitting materials owing to its high ηPL and bipolar transport characteristics.36,42–44,53–55 It tends to be a weak electron-withdrawing substituent when linked to a stronger D but acts as a weak electron donator otherwise.42,51 Pyrene[4,5-d]imidazole is thus chosen as the weak D and obtained by creating a melt of pyrene and imidazole. In contrast, pyrene is expected to play the role of a relatively weak A when connected to the pyrene[4,5-d]imidazole group. The corresponding D–π–A molecule, 9-phenyl-10-(4-(pyren-1-yl)phenyl)-9H-pyreno[4,5-d]imidazole (PyPI-Py), was constructed by linking the D (pyrene-imidazole) and A (pyrene) moieties via a phenyl bridge, which may improve the ηPL of the resulting molecule on the premise of guaranteeing the charge-transfer (CT) character. In this case, it is expected that the CT state between pyrene[4,5-d]imidazole and pyrene will increase the ratio of the singlet exciton, and the phenylene π bridge between the D of pyrene-imidazole and the A of pyrene will enhance the generation of the local excited (LE) state and increase the ηPL. As a result, PyPI-Py exhibits high quantum efficiencies in both solution (82% in hexane at 10−5 M) and the solid state (52% in neat film with thickness of 30 nm). Moreover, its single-crystal structure displays a twisted stacking mode, suggesting the efficient inhibition of an intense aggregation effect. The theoretical calculations indicate a typical HLCT character with an intensive intercrossed coupling of CT and LE states, and therefore leads to the predominant HLCT feature in the S1 → S0 transition, which is consistent with the recently proposed HLCT principle.36,48–51 The nondoped OLED based on PyPI-Py displays stable blue emission peaks at 475 nm with a CIE coordinate of (0.15, 0.22). A maximum CE of 13.38 cd A−1 and a maximum brightness of 75,687 cd m−2 are achieved. The highest ηext can reach 8.52%, corresponding to a maximum EUE as high as 82%, which is almost 3.2-fold higher than that of the model compound 9,10-diphenyl-9H-pyreno[4,5-d]imidazole (PyPI) without a pyrene unit. More importantly, for the nondoped blue device, ηext of 8.35% and 8.05% can still be reached even when the brightness is as high as 10,000 and 50,000 cd m−2, respectively, displaying extremely low efficiency roll-offs of 2.0% and 5.5%. The device characteristics are among the highest values reported to date for nondoped blue OLEDs based on the pyrene unit. Consequently, through the introduction of a pyrene unit onto the PyPI segment, the nature of the molecular excited state of the resulting PyPI-Py is changed ideally from a traditional LE state to a preponderant HLCT state, resulting in excellent device performance. Experimental Methods Synthesis of materials Synthesis of 4-pyren-1-yl-benzaldehyde 1-Bromopyrene (4.90 g, 17.4 mmol), (4-formylphenyl)boronic acid (2.25 g, 15.0 mmol), dry toluene (24.0 mL), and K2CO3 aqueous solution (16.0 mL, 2.0 mol L−1) were added to a reaction flask followed by Pd(PPh3)4 (1.05 g, 0.9 mmol). The mixture was refluxed at 90 °C under N2 atmosphere for 48 h. The resulting crude product was extracted with trichloromethane and further purified by column chromatography. The product was isolated as white powder (3.70 g, Yield: 80%). Proton nuclear magnetic resonance (1H NMR) [500 MHz, dimethyl sulfoxide (DMSO)-d6, 25 °C, δ]: 10.19 (s, 1H), 8.43 (d, J = 7.9 Hz, 1H), 8.38 (d, J = 7.6 Hz, 1H), 8.34 (d, J = 7.3 Hz, 1H), 8.28 (s, 2H), 8.23 (d, J = 9.3 Hz, 1H), 8.15 (dd, J = 10.0, 7.9 Hz, 3H), 8.13–8.08 (m, 2H), 7.91 (d, J = 8.1 Hz, 2H). Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) (mass m/z): [M+] calcd for C23H14O, 306.36; found, 307.20. Synthesis of PyPI A mixture of pyrene-4,5-dione (0.50 g, 2.1 mmol), aniline (5.0 mL, 55.1 mmol), benzaldehyde (5.0 mL, 49.0 mmol), acetic acid (15.0 mL), and ammonium acetate (0.80 g, 10.5 mmol) was stirred for 2 h. When cooled down from 120 °C to room temperature, the mixture was subsequently filtered and flushed with a water/acetic acid mixture (3:1, 40.0 mL). Then, the crude product was further purified by column chromatography and sublimation to give white product (0.65 g, Yield: 78%). 1H NMR (500 MHz, DMSO-d6, δ) 8.95 (d, J = 7.0 Hz, 1H), 8.32 (d, J = 7.0 Hz, 1H), 8.24 (t, J = 8.6, 8.6 Hz, 1H), 8.21–8.17 (m, 3H), 7.83–7.79 (m, 2H), 7.79–7.71 (m, 4H), 7.69–7.65 (m, 2H), 7.43–7.36 (m, 3H), 7.32 (d, J = 7.4 Hz, 1H). Carbon nuclear magnetic resonance (13C NMR) (126 MHz, CDCl3, δ): 132.20, 131.69, 130.17, 129.87, 129.49, 129.22, 128.91, 128.28, 127.97, 127.58, 126.40, 125.29, 124.54, 124.29, 122.85, 122.46, 119.82, 117.98. MALDI-TOF (mass m/z): [M+] calcd for C29H18N2, 394.47; found, 395.32. Anal. Calcd (%) for C29H18N2: C, 88.30; H, 4.60; N, 7.10. Found: C, 88.37; H, 4.58; N, 7.04. Synthesis of PyPI-Py A mixture of pyrene-4,5-dione (0.50 g, 2.1 mmol), aniline (5.0 mL, 55.1 mmol), 4-pyren-1-yl-benzaldehyde (PyP-CHO) (0.80 g, 2.6 mmol), acetic acid (15.0 mL), and ammonium acetate (0.80 g, 10.5 mmol) was stirred for 2 h. When cooled down from 120 °C to room temperature, the mixture was subsequently filtered and flushed with a water/acetic acid mixture (3:1, 40.0 mL). Then, the crude product was further purified by column chromatography and sublimation to give white product (0.77 g, Yield: 62%). 1H NMR (500 MHz, DMSO-d6, δ) 9.02 (d, J = 6.7 Hz, 1H), 8.40 (d, J = 7.9 Hz, 1H), 8.38–8.31 (m, 3H), 8.28–8.20 (m, 7H), 8.13 (dd, J = 8.5, 6.1 Hz, 2H), 8.06 (d, J = 7.8 Hz, 1H), 7.99–7.90 (m, 4H), 7.87–7.82 (m, 3H), 7.80 (t, J = 7.8, 7.8 Hz, 1H), 7.70 (d, J = 8.3 Hz, 2H), 7.35 (d, J = 8.2 Hz, 1H). 13C NMR (126 MHz, CDCl3, δ): 132.24, 131.72, 131.47, 130.96, 130.79, 130.61, 130.39, 130.17, 130.13, 129.41, 129.30, 128.44, 128.00, 127.68, 127.61, 127.45, 127.40, 126.52, 126.09, 125.37, 125.24, 125.07, 124.98, 124.96, 124.87, 124.66, 124.43, 123.64, 122.93, 118.06. MALDI-TOF (mass m/z): [M+] calcd for C45H26N2, 594.72; found, 595.35. Anal. Calcd (%) for C45H26N2: C, 90.88; H, 4.41; N, 4.71. Found: C, 90.75; H, 4.64; N, 4.61. Theoretical calculation Gaussian 09 (version D.01) package on a PowerLeader cluster was used to carry out all the density functional theory (DFT) calculations. Molecule structures of PyPI and PyPI-Py were selected from optimized ground-state geometries according to time-dependent DFT (TD-DFT) at the level of CAMB3LYP/6-31G(d,p). For examining the character of electronic transitions, the dominant "particle"–"hole" pair contributions based on the TD-M062X/6-31G(d,p) method was used to evaluate the natural transition orbitals (NTOs). Results and Discussion Synthesis and characterization As shown in Scheme 1, the model compound PyPI was conveniently synthesized via a click-like one-pot reaction in which benzaldehyde, ammonium acetate, aniline, and pyrene-4,5-dione were refluxed under N2 atmosphere.24 PyPI-Py was also prepared in good yield using the same procedure with PyP-CHO instead of benzaldehyde as the starting material. The structures of PyPI and PyPI-Py were fully characterized by 1H and 13C NMR spectroscopies, mass spectrometry, and elemental analysis and in good accordance with their expected structures. Supporting Information Figures S1–S5 show the detailed characterization data. Scheme 1 | Molecular structures and synthesis of PyPI and PyPI-Py. Download figure Download PowerPoint Thermal properties These two polycyclic aromatic hydrocarbon skeletons featured high thermal stability. As displayed in Figure 1a, in the differential scanning calorimetry (DSC) heating cycles, the glass transition temperature (Tg) and melting point (Tm) of PyPI-Py appeared at 150 and 317 °C, respectively. The Tg and Tm values of PyPI-Py were much higher compared with the PyPI values of 74 °C and 215 °C, respectively. In addition, upon further heating of PyPI beyond Tg, an exothermal peak was detected at 139 °C, which was assigned as the crystallization temperature (Tc). From thermogravimetric analysis (TGA), PyPI exhibited a decomposition temperature (5% weight loss) of 355 °C, whereas that of PyPI-Py was 122 °C higher reaching 477 °C (Figure 1b), demonstrating its superior thermal stability because of the introduction of an additional pyrene unit. The favorable thermal properties of PyPI-Py are sufficient to meet the requirements for evaporative manufacture. Figure 1 | DSC (a) and TGA (b) curves of PyPI and PyPI-Py under nitrogen atmosphere. Download figure Download PowerPoint Theoretical calculations To estimate their molecular configuration and distribution of frontier molecular orbitals, DFT calculations were carried out. As shown in Figure 2a, PyPI presented nearly coplanar geometry in the ground state. The highest occupied molecular orbitals (HOMOs) as well as the lowest unoccupied molecular orbitals (LUMOs) were both delocalized on the entire molecule, indicating a typical pure π → π* transition. In contrast, the HOMO of PyPI-Py was mostly distributed on the PyPI segment with a residual on the substituted pyrene, whereas the LUMO was mostly located on the pyrene group with sizable extension on the central linking phenylene. This finding suggested a significant change in the frontier molecular orbital distribution. In addition, PyPI-Py showed a highly twisted molecular geometry with a calculated torsion angle of 55° between the PyPI and Py units in the ground state (Figure 2a). Unlike the fully separated LUMO and HOMO distributions in the molecules containing strong D–A units, the relatively weak electron-donating and -withdrawing strengths of the D and A moieties allowed for partially delocalized HOMOs and LUMOs in PyPI-Py. Therefore, pyrene serves as a weak A, which accommodates our design strategy. Figure 2 | (a) in the ground state and distributions of PyPI-Py and (b) S1 geometries and of S0 → S1 for PyPI-Py and PyPI represents the Download figure Download PowerPoint To and the nature of the excited state of analysis of singlet as well as triplet excited were The angle between the D and A units was greatly from 55° to a coplanar configuration in the excited state (Figure The was the PyPI and phenylene which its HOMO whereas the was mostly delocalized on the pyrene and phenylene but to the PyPI segment to some because of the angle the phenyl and pyrene (Figure Therefore, distributions of well in the pyrene[4,5-d]imidazole and phenylene but separated in the pyrene The large of the orbital a strong transition for S1 → S0 resulting in a for the ηPL. The of the and distributions to a certain extent upon pyrene a CT transition that singlet exciton In the and of PyPI were distributed the molecule because of nearly molecular configuration (Figure The analysis of the higher singlet and triplet excited and the energy for PyPI-Py are in Supporting Information Figures and HLCT and and small energy between S1 and = and were which the and a high fluorescence and EUE ratio in Furthermore, it has been that a phenyl unit between the D and A serves as an appropriate method for the of HLCT excited properties The LUMO and HOMO energy by were to by theoretical calculations Supporting Information Figure The and of PyPI-Py were to be and respectively, corresponding to a LUMO level of and a HOMO level of The of PyPI-Py and PyPI (0.80 that their HOMO were both by the PyPI In contrast, the LUMO level of PyPI-Py was than that of PyPI This be attributed to the of an additional pyrene group that as a weak A and substantially the molecular orbital distribution as in the theoretical The be thus which the device performance. The energy calculated from the of PyPI and PyPI-Py were and respectively, in good with their corresponding optical energy obtained from the results 1 | and of PyPI and PyPI-Py PyPI PyPI-Py and PL peaks in solution with a of 1 × 10−5 mol and PL peaks in a neat film 30 the values in are the quantum efficiency of a neat film at room optical energy is from the in is from by properties The and PL of PyPI-Py and PyPI in are displayed in Figure and the corresponding are in PyPI displayed because of the structure from the pyrene-imidazole The of PyPI-Py was to that of PyPI in However, the in two was the of PyPI-Py. The of PyPI-Py may partially from a of CT and LE transitions, demonstrating the of the lowest CT and LE The CT character was further by the PL PyPI in the at nm with of structure in the of whereas PyPI-Py a large of at nm and a Unlike the in emission of PyPI as the was enhanced Supporting Information Figure the emission of PyPI-Py were as well as (Figure For example, the emission peak at nm in the but to nm in highly This effect further the of a CT in the excited state as in increasing , the of of both PyPI and PyPI-Py including peak and in the ground state upon variation (Figure and Supporting Information Figure As for the PL of PyPI in there was little its that they all demonstrated structure the typical LE of the excited state. Figure | PL and of PyPI-Py in M) with PyPI for Download figure Download PowerPoint Figure | PL and of PyPI-Py in Download figure Download PowerPoint To further the of singlet excited state for the the of the was constructed based on the As in Figure PyPI-Py showed a with a of that one excited state in both low and high This which is than that of the typical CT molecule a quasi-equivalent of CT and LE because of the strong coupling between the CT and LE states, which is in good with the theoretical and high PL efficiency in highly as in the previous Therefore, a quasi-equivalent consisting of an HLCT state in the S1 state of PyPI-Py. Because of the of CT and LE even in the highly PyPI-Py a high ηPL of which was with the values in both and Supporting Information The high ηPL in to the good device performance. Furthermore, a displayed a and of in with (Figure that the CT and LE of PyPI-Py into a HLCT state. The ηPL of PyPI and PyPI-Py in was by using as a Supporting Information S1 and PyPI which was than the values of PyPI-Py Supporting Information Figure | of PyPI-Py with the fluorescence the of the Download figure Download PowerPoint In a neat film, PyPI showed a maximum emission peak at nm with a of nm in to that in Supporting Information Figure This peak a relatively large at maximum of which was to the strong aggregation effect of PyPI with planar In the were to some extent because of the pyrene to a of nm under the same and a of nm The ηPL in was for PyPI and for indicating that pyrene a role in molecular aggregation its properties, such as high ηPL and good thermal stability, were successfully the PL of PyPI-Py in film was also without (Figure The PL of PyPI neat film also with a of as shown in Supporting Information Figure Furthermore, the and and were calculated according to the in the Supporting In the the of PyPI-Py was calculated to be × which is higher than that of PyPI × and the of PyPI-Py was calculated to be × which was

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