Article pubs.acs.org/cm
Novel Luminescent Benzimidazole-Substituent Tris(2,4,6trichlorophenyl)methyl Radicals: Photophysics, Stability, and Highly Efficient Red-Orange Electroluminescence Yingchang Gao,† Wei Xu,‡ Hongwei Ma,† Ablikim Obolda,† Wenfu Yan,‡ Shengzhi Dong,† Ming Zhang,† and Feng Li*,† †
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Qianjin Avenue 2699, Changchun 130012, P. R. China ‡ State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Qianjin Avenue 2699, Changchun 130012, P. R. China S Supporting Information *
ABSTRACT: Luminescent radicals have various applications because they simultaneously possess optoelectronic, electronic, and magnetic properties. Despite the development of some luminescent tris(2,4,6trichlorophenyl)methyl (TTM)-based radicals, all the substituents directly attached to the TTM skeleton are electron-donating groups. Herein, the electron-withdrawing group is first attached to a p carbon of the parent TTM radical, and two novel stable open-shell adducts based on the benzimidazole unit with red-orange emission are obtained. Their photophysical properties, photochemical stabilities, and electroluminescent performances are fully investigated. Because of the introduction of the benzimidazole unit, the intramolecular charge transfer property of D− A type molecules is suppressed to a large extent, and the delocalization of the sole electron is strengthened. Both radicals exhibit largely improved photostability compared to that of the TTM core. High PL quantum yields (ΦF) of 0.39 and 0.36 in doped films are achieved, which are among the highest values for luminescent radicals. Extremely high-voltage-durable characteristic is demonstrated in the organic light-emitting diodes utilizing them as emitters. One device has a maximal external quantum efficiency that even exceeds the classical theoretical upper limit of 5%.
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series of strong red-emissive carbazole-pendant radicals;12−15 they also studied the charge-transport properties of some TTM derivatives, which showed high mobility values for both holes and electrons.15,16 In addition, Lambert et al. reported a number of perchlorotriphenylmethyl (PTM)-cored luminescent radicals;17,18 Nishihara and co-workers have synthesized several TTM-similar luminescent radicals by incorporating a pyridine ring into the TTM skeleton.19,20 OLEDs are thought to have promising application as flatpanel displays and solid state illumination because they are easy to process, lightweight, high-contrast, and flexible.21−24 However, triplet harvesting has always been a challenge for OLEDs using closed-shell molecules as emitters because the transition of a triplet exciton to the ground state is spinforbidden.25 We confirmed that using open-shell molecules, in particular the electrically neutral radicals, as emitters of OLEDs is a way to circumvent the transition problem of the triplet exciton because the transition of the doublet exciton to the
INTRODUCTION Organic radicals have abundant potential applications in fields such as spintronics,1 polarizing agents,2 organic magnetism,3 and accelerating chemical reactions4 because of their open-shell electronic structures. Recently, the applications of organic radicals in optoelectronic materials and devices have seen some progress and attracted much interest. For example, Nishide and co-workers successively reported nitroxide radicals with high hole-drift mobility values that are comparable to typical holetransporting materials.5,6 Liu et al. utilized an imino nitroxide radical as the semiconductor layer to fabricate an organic fieldeffect transistor (OFET) device, which showed excellent p type FET characteristics with a low operating voltage.7 Bao et al. significantly improved the conductivity and air stability of nchannel organic thin-film transistors (OTFTs) by using an organic radical as the n type dopant.8,9 Duan et al. employed a derivative of this radical as a dopant in Bphen or directly as an electron injection layer (EIL) to improve the electron injection and conductivity10,11 and thus obtained highly efficient and stable organic light-emitting diodes (OLEDs). Juliá and coworkers introduced carbazole and its derivatives into a tris(2,4,6-trichlorophenyl)methyl (TTM) core and obtained a © XXXX American Chemical Society
Received: April 12, 2017 Revised: July 6, 2017 Published: July 9, 2017 A
DOI: 10.1021/acs.chemmater.7b01521 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials ground state is spin-allowed.26,27 Nevertheless, the reported luminescent radicals are rare and almost deep-red/near-infrared molecules. Until now, the luminescent radicals are confined to the derivatives of TTM, PTM, and TTM-similar radicals.12−20,28 All the substituent groups directly attached to the center core are electron-donating groups (such as carbazole and triphenylamine units18), which makes the emission of these molecules have the characteristic of intramolecular charge transfer (ICT) due to the electron-withdrawing feature of the radical core. We are curious about what would happen if the electron-accepting groups take the place of the electron-donating units. Benzimidazole has good electron mobility and a lower LUMO level,29,30 so we incorporated it into the TTM core and obtained two novel and stable luminescent radicals, bis[4(1H-benzimidazolyl)-2,6-dichlorophenyl](2,4,6trichlorophenyl)methyl radical (TTM-2Bi) and tris[4-(1Hbenzimidazolyl)-2,6-dichlorophenyl]methyl radical (TTM-3Bi) in this work. To the best of our knowledge, they are the first examples of directly coupling electron-withdrawing groups to a p carbon of the TTM center. Because of the electronwithdrawing nature of both TTM core and benzimidazole units, the ICT in TTM-2Bi and TTM-3Bi is suppressed to a large extent. In such a case, the absorption and emission spectra of them are expected to hypsochromically shift compared to those of TTM−carbazole compounds. Because benzimidazole is also a nitrogen-containing heterocyclic compound, the conjugated effect also needs to be taken into account. Thus, we studied their photophysical properties and provided a strategy for tuning the spectra of luminescent radicals toward the blue direction. As two novel radicals, their photochemical stabilities were also investigated. At last, we fabricated OLEDs using them as the emitters. Both devices exhibited excellent high-voltage-durable characteristic and red-orange emission. Meanwhile, the device based on TTM-2Bi showed better performance with a maximal external quantum efficiency (EQE) of 5.4% and a current efficiency (CE) of 9.8 cd A−1.
Scheme 1. Synthesis Route of Radicals Based on a TTM Moleculea
Reagents and conditions: (a) Cs2CO3, DMF, 160 °C, reflux for 10− 12 h; (b) Bu4NOH, anhydrous THF, room temperature, stirring for 5 h, adding tetrachloro-p-benzoquinone, stirring for an additional 1.5 h. a
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RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 shows the chemical structures and synthesis routes of the target molecules: TTM-2Bi and TTM-3Bi. The synthesis and dehydrogenation of αHTTM were performed according to the literature. 31 Then we combined TTM core with benzimidazole units under analogous reaction conditions between TTM and carbazole that have been reported.12 Unexpectedly, in a manner different from the synthesis of TTM−carbazole adducts, we simultaneously obtained αHTTM-2Bi and αHTTM-3Bi in a relatively high yield, while the product with one group substituted is seldom obtained. This might be relevant to the distinction of reactivity between benzimidazole and carbazole. Then pure TTM-2Bi and TTM-3Bi radicals were obtained by treatment of their precursors with a suitable volume of tetrabutylammonium hydroxide and 2,3,5,6-tetrachloro-p-benzoquinone successively in tetrahydrofuran. We can also prepare the αHTTM-3Bi and its radical starting from TTM-2Bi. The synthesized compounds were characterized by nuclear magnetic resonance (NMR), mass spectrometry (MS), element analysis, and Fourier transform infrared spectroscopy (FTIR). The FTIR spectra of radicals were compared with those of their precursors, which are shown in Figure S4. The detailed synthetic procedure and
Figure 1. (a) Normalized UV−vis and PL spectra of TTM, TTM-2Bi, and TTM-3Bi in a chloroform solution. The inset shows an enlarged view of absorptions around 515−640 nm. (b) Solvation effect, shown in the solvatochromic absorption and PL spectra of TTM-2Bi and TTM-3Bi in solvents of different polarities: (1) cyclohexane, (2) toluene, (3) chloroform, (4) acetone, and (5) N,N-dimethylformamide.
analysis can be seen in the experimental section in the Supporting Information. Thermal and Electrochemical Properties. The thermal properties of the two open-shell molecules were investigated by means of thermogravimetric analysis (TGA) under a nitrogen B
DOI: 10.1021/acs.chemmater.7b01521 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials Table 1. Physical Properties of TTM-2Bi and TTM-3Bi λem (nm)
ΦF
SOMO/SUMO (eV)
molecule
λabsa (nm)
solutiona
filmb
solutionc
filmd
calculatede
experimentalf
Egg (eV)
TTM-2Bi TTM-3Bi
384/512/558 405/508/564
588 593
603 606
0.16 0.30
0.39 0.36
−5.66/−3.51 −5.64/−3.54
−5.37/−3.90 −5.35/−3.97
2.10 2.07
Measured in chloroform at room temperature. bEmitters doped into TPBi films (5 wt %). cMeasured in chloroform using rhodamine 6G (R6G) as the standard. dPL quantum yield of the doped film (emitter, 5 wt % TPBi) measured in an integrating sphere. eCalculated from Gaussian 09 at the B3LYP/6-31G(d) level. fObtained from cyclic voltammetry (CV) measurement. gDetermined with the onset absorption wavelength of the solution. a
Figure 2. Transient PL spectra of TTM-2Bi and TTM-3Bi on a time scale of 500 ns in a dilute CHCl3 solution.
Figure 4. (a) Contrast images of (1) TTM, (2) TTM-2Bi, and (3) TTM-3Bi in dilute CHCl3 (∼1 × 10−5 mol L−1) under irradiation (365 nm) with an UV lamp at room temperature under ambient conditions. (b) Photochemical stability of the radicals characterized by the degeneration of their emission in chloroform under the irradiation of a pulsed laser at 355 nm with an energy density of 98.1 kW cm−3 (pulse width of 8 ns, frequency of 10 Hz) with N2 protection.
weight loss) for TTM-2Bi and TTM-3Bi are approximately 210 and 195 °C, respectively. These are moderate values among those of the reported TTM-based radicals. The Td of the threebenzimidazole-substituted molecule is slightly lower. This might be attributed to the different molecular weight. Cyclic voltammetry (CV) measurements were performed using the conventional three-electrode system to study the energy levels of the highest singly occupied molecular orbital (SOMO) and the lowest singly unoccupied molecular orbital (SUMO) whose definitions are shown in Figure 3. The results of CV measurements are displayed in Figure S11. From the oxidation potential of CV curves, the SOMO levels were calculated to be −5.37 eV for TTM-2Bi and −5.35 eV for TTM-3Bi, with ferrocenium/ferrocene as the reference. Accordingly, we also obtained their SUMO levels, −3.90 and −3.97 eV, respectively, from the onset of reduction curves. The reduction potentials from TTM to TTM-3Bi shift from −0.68 to −0.60 V, which indicates the electron-withdrawing nature of the benzimidazole substituent. In addition, the electrochemical band gaps of TTM-2Bi and TTM-3Bi were determined to be 1.47 and 1.38 eV, respectively.
Figure 3. Spatial distribution of the frontier molecular orbitals and corresponding energy levels of (a) TTM, (b) TTM-2Bi, and (c) TTM-3Bi calculated at the B3LYP/6-31G(d) level.
atmosphere. The TGA curves for them are shown in Figure S10; the decomposition temperatures (Td, corresponding to 5% C
DOI: 10.1021/acs.chemmater.7b01521 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 5. (a) Schematic structure of the device used in this work, where the emitter represents TTM-2Bi or TTM-3Bi. (b) Normalized EL spectra of TTM-2Bi and TTM-3Bi in doped devices recorded at 10 V with CIE coordinates. The inset is the image of device I at 12 V. (c) Current density− voltage−luminance (J−V−L) characteristics of the OLEDs based on TTM-2Bi and TTM-3Bi. (d) External quantum efficiency (ηEQE) and current efficiency (CE) as a function of current density.
Table 2. Electroluminescent Performance of Devices I and II Vona (V)
CEb (cd A−1)
EQEc (%)
λELd (nm)
device I
7.2
9.8
5.4
600
device II
8.0
6.9
4.1
605
CIEe (x, y) (0.605, 0.384) (0.622, 0.367)
χDf (%) 69 57
Turn-on voltage at a brightness of 1 cd m−2. bMaximal current efficiency. cMaximal external quantum efficiency. dMaximal EL wavelength. eMeasured at 10 V. fFormation ratio of doublet excitons. a
Photophysical Properties. The ultraviolet−visible (UV− vis) absorption and photoluminescence (PL) spectra of TTM2Bi and TTM-3Bi in chloroform are shown in Figure 1a. For comparison, the photophysical properties of TTM were also tested. The key photophysical data are summarized in Table 1. As one can see, both TTM-2Bi and TTM-3Bi show an intense absorption band around 380−400 nm that can be ascribed to the characteristic π−π* transition of TTM-based radicals, and a much weaker broad band at longer wavelengths of 500−600 nm. The featured absorption bands of the radicals can be further identified by comparing their absorption with that of their precursors [TTM-2Bi compared to αHTTM-2Bi, and TTM-3Bi compared to αHTTM-3Bi (Figure S12)]. The absorption onset of TTM-2Bi is at ∼590 nm, and that of TTM-3Bi has a slight red-shift to 600 nm. The corresponding optical gaps are thus determined to be 2.10 and 2.07 eV, respectively. The results coincide with the trend of electrochemical band gaps obtained from the CV method. For PL spectra, both TTM-2Bi and TTM-3Bi exhibit similar profile
Figure 6. EPR spectra of TTM-2Bi (left) and TTM-3Bi (right) powder measured at room temperature before (up) and after (down) evaporation.
peaking around 585−595 nm, which are distinctly blue-shifted compared to the D−A type radical molecules, such as TTM− carbazole series (peaking at ∼680 nm in chloroform).12,14 Benzimidazole lacks one benzene ring compared to carbazole, which would decrease the degree of conjugation of the molecule. Decreased levels of both conjugation and ICT can induce the blue-shifted spectra. To verify which is the main factor, we connected indole that has a structure similar to that of benzimidazole to TTM to synthesize one D−A type TTMD
DOI: 10.1021/acs.chemmater.7b01521 Chem. Mater. XXXX, XXX, XXX−XXX
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calculated spin density maps of frontier orbitals (HOMO, SOMO, SUMO, and LUMO) and corresponding energy levels are displayed in Figure 3. Both the SOMO and SUMO of TTM are localized on the whole molecule. Compared to TTM, the electron-cloud densities of SOMO and SUMO of TTM-2Bi and TTM-3Bi have a small extension into the peripheral benzimidazoles, which means enhanced π-conjugation that is consistent with the photophysical data. All the calculated orbital energy levels of TTM-2Bi and TTM-3Bi are almost identical. The much lower-lying energy levels of SUMO with respect to TTM again prove the electron-withdrawing property of the benzimidazole unit. The data of SOMO and SUMO energy levels according to theoretical calculation are listed in Table 1. Both TTM-2Bi and TTM-3Bi have very similar electron densities and sterically hindered structures that can be seen from their spatial distribution of SOMO and SUMO orbitals and the values of dihedral angles (shown in Figure S15). Photochemical Stabilities. As novel TTM-based luminescent radicals, it is necessary to explore the photochemical stability of TTM-2Bi and TTM-3Bi. We measured the degeneration of their fluorescence intensity in a diluted chloroform solution irradiated by the UV pulsed laser with the protection of N2 bubbling and compared the results with those of TTM. As shown in Figure 4, the luminescence of TTM degenerates rapidly and thoroughly disappears after irradiation for 3 min while the emission intensities of TTM-2Bi and TTM-3Bi degenerate much more slowly. The introduction of the peripheral groups greatly improves the photochemical stability of these radicals. The degenerate lifetime (t1/2) of TTM-2Bi is 41 times as long as that of TTM, which is comparable to that of a new luminescent radical reported in 2014 (47 times as long as that of TTM in chloroform).19 TTM3Bi exhibits even higher photochemical stability with a longer degenerate lifetime (62 times as long as that of TTM). We ascribed the enhanced photochemical stability to the steric hindrance and the electron-withdrawing property of benzimidazole that induces the strengthened delocalization of the sole electron.33 Electroluminescent Properties. To evaluate the performance of TTM-2Bi and TTM-3Bi in their potential application in OLEDs, we also fabricated devices with a doped architecture. Initially, we used 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) as the host of the luminescent radicals. Unfortunately, we did not obtain the desired results (EL spectra of the devices with different electron-transporting materials and doped ratios are shown in Figure S20). After numerous attempts, we finally found that TPBi is appropriate as the host of the two compounds. It is noteworthy that TPBi is usually used as an electron-transporting material, and utilizing it as the host in OLEDs has rarely been reported.34,35 We then fabricated multilayer OLEDs using TTM-2Bi and TTM-3Bi as the emitters with the following structure: indium tin oxide (ITO)/MoO3 (3 nm)/4,4′-cyclohexylidenebis[N,N-bis(ptolyl)aniline] (TAPC) (70 nm)/emitters:TPBi (5 wt %, 40 nm)/2,4,6-tris[3-(diphenylphosphoryl)phenyl]-1,3,5-triazine (PO-T2T) (70 nm)/LiF (0.8 nm)/Al (100 nm). As shown in Figure 5a, MoO3 and LiF act as the hole and electron-injecting layer and TAPC and PO-T2T serve as the hole-transporting (HTL) and electron-transporting layer (ETL), respectively. TTM-2Bi or TTM-3Bi doped into TPBi film is employed as the emitting layer (EML), and the corresponding devices are defined as device I and device II, respectively. Both devices display red-orange emission with EL peaks at 600 and 605 nm,
based radical, [4-(N-indolyl)-2,6-dichlorophenyl]bis(2,4,6trichlorophenyl)methyl radical (TTM-1ID). The synthesis and characterization are shown in the Supporting Information. We measured the PL spectra of TTM-2Bi, TTM-3Bi, TTM1ID, and two other D−A TTM-based radicals (TTM-2ID and TTM-1Cz reported in refs 28 and 26, respectively) in a dilute chloroform solution (Figure S13). We can see that TTM-2Bi and TTM-3Bi still have an intense blue-shift compared to TTM-1ID and TTM-2ID, though one more N atom of benzimidazole may enhance the molecular conjugation effect. This suggests that the weakened ICT of TTM-2Bi and TTM3Bi plays a leading role in their blue-shifted PL spectra compared to those of the D−A type radical molecules. Meanwhile, the tiny red-shift either in absorption or in PL spectra with an increase in the number of benzimidazoles indicates that the introduction of benzimidazole may enhance the π-conjugation of TTM. This can be further supported by comparing the absorption and PL spectra of TTM-2Bi and TTM-3Bi to those of TTM. The red-shifts are 15 and 20 nm for absorption spectra and 22 and 28 nm for PL spectra of TTM-2Bi and TTM-3Bi, respectively, as shown in Figure 1a. To investigate the ground and excited state properties of TTM-2Bi and TTM-3Bi, their solvent-polarity effect on UV− vis absorption and PL spectra was also measured in solvents with different polarities. As shown in Figure 1b, because of the structural similarity of the two molecules, they show similar characteristics. With the increase in solvent polarity ranging from cyclohexane to N,N-dimethylformamide, the solvatochromic shift of PL spectra is merely 12 and 8 nm for TTM-2Bi and TTM-3Bi, respectively, and the absorption spectra have almost no change. The photophysical data of TTM-2Bi and TTM-3Bi in different solvents are listed in Table S1. For comparison, we also measured the solvation effect of TTM-1ID, TTM-2ID, and TTM-1Cz, and the PL spectra and photographs of all the related radicals in solutions of different polarities are shown in Figure S14. The red-shifts of TTM-1ID, TTM-2ID, and TTM1Cz are 30, 23, and 62 nm, respectively, ranging from cyclohexane to chloroform. It is noteworthy that all the D−A TTM-based radicals have a common character: their emissive intensity decreases sharply with an increase in polarity and even disappears thoroughly in acetone and DMF. TTM-2Bi and TTM-3Bi show luminescence in solvents of all polarities, which is similar to the behavior of TTM, a typical LE molecule. The results described above suggest that the ICT property is suppressed to a large extent in TTM-2Bi and TTM-3Bi. The transient PL decay spectra in a diluted chloroform solution were recorded using a time-correlated single-photon counting method. As illustrated in Figure 2, both TTM-2Bi and TTM-3Bi present a single-exponential decay with transient PL lifetimes of 21.3 and 27.3 ns, respectively. Fluorescent quantum yields in a chloroform solution are estimated to be 0.16 for TTM-2Bi and 0.30 for TTM-3Bi using rhodamine 6G (R6G) as the standard.32 The PL quantum yields (ΦF) of TTM-2Bi and TTM-3Bi doped in 1,3,5-tri(phenyl-2-benzimidazolyl)benzene (TPBi) films with a 5% dopant concentration are measured as 0.39 and 0.36 in an integrating sphere, respectively. These are among the highest PL yield values of luminescent radical-doped films. Density Functional Theory Simulation. To gain better insight into the influence of appending electron-deficient groups onto the TTM skeleton, unrestricted density functional theory (DFT) calculations were performed using the Gaussian 09 series of programs at the B3LYP/6-31G(d) level. The E
DOI: 10.1021/acs.chemmater.7b01521 Chem. Mater. XXXX, XXX, XXX−XXX
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respectively, as shown in Figure 5b. The EL spectra remain almost unchanged with increasing driving voltage even up to 21 V (Figures S16 and S17). The current density−voltage− luminance (J−V−L) characteristics of the OLEDs are shown in Figure 5c. The results described above suggest that both devices exhibit an excellent high-voltage-durable characteristic; that is, they can work without breakdown at very high working voltages. Especially for device II, the brightness reaches its highest value at 19.5 V and hardly decreases even at 21 V. However, they also show a relatively high turn-on voltage (Von, recorded at a luminance of 1 cd m−2) around 7−8 V, which may be caused by the thick EML and large energy barrier between the HOMO levels of TAPC and TPBi. Additionally, as presented in Figure 5d, device I exhibits better performance with a maximal EQE of 5.4% and a maximal CE of 9.8 cd A−1. All electroluminescence data are summarized in Table 2. As shown in Table 2, the maximal EQE of device I is higher than the theoretical upper limit of 5% in conventional orangered devices. Therefore, it is necessary to know about the formation ratio of doublet excitons in the two devices. Analogous to the traditional closed-shell molecules, the formation ratio of doublet excitons can be determined by the following equation:
Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01521. Experimental section, including general information, OLED fabrication and testing, and the details of the synthesis; 1H NMR and FTIR spectra; GC−MS mass spectra; thermal properties and electrochemical properties; photophysical and electroluminescent properties; and the solvation effect (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Feng Li: 0000-0001-5236-3709 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support from the National Key R&D Program of China (Grant 2016YFB0401001), the National Key Basic Research and Development Program of China (973 program, Grant 2015CB655003) Founded by MOST, and the National Natural Science Foundation of China (Grant 51673080).
Φ = χD ϕPLηr ηout
where Φ is the maximal EQE, χD is the ratio of doublet excitons, ϕPL is the PL quantum yield of the emissive layer (here, the values are 0.39 for TTM-2Bi and 0.36 for TTM-3Bi), ηr is the fraction of injected charge carriers that form excitons and can be regarded as a unit if the leaky current can be neglected, and ηout is the light out-coupling efficiency, usually considered to be 20%.36 Therefore, the values of χD of device I and device II were determined to be 69 and 57%, respectively, which are nearly 2−3 times as high as the formation ratio of the singlet exciton (25%) in traditional fluorescent OLEDs. This result shows the advantage of using neutral π radical as the emitter in OLEDs. Figure 6 shows the electron paramagnetic resonance (EPR) spectra of TTM-2Bi and TTM-3Bi. The typical signals of one unpaired electron before and after evaporation indicate that TTM-2Bi and TTM-3Bi can endure the process of vacuum thermal evaporation and maintain the radical characteristic. Furthermore, the almost identity between PL and EL spectra confirms that the EL emissions come from the neutral radicals (Figures S18 and S19).
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REFERENCES
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CONCLUSION Two novel air-stable and luminescent radicals with red-orange emission were synthesized and studied. It is the first time electron-withdrawing groups have been directly coupled to a p carbon of the parent TTM center. A greatly improved photostability compared to that of the TTM core is achieved, which is ascribed to the protection of the peripheral groups and the enhanced delocalization of the sole electron due to the electron-withdrawing property of benzimidazole. The doped OLEDs based on TTM-2Bi and TTM-3Bi exhibited an excellent high-voltage-durable characteristic. In particular, the device using TTM-2Bi as an emitter showed a maximal EQE of 5.4%, a maximal CE of 9.8 cd A−1, and a doublet exciton formation ratio of 69%. Our work suggests directly coupling electron withdrawal to the center radical may induce the blueshifted emission, improved photostability, and enhanced PL quantum yields. F
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Chemistry of Materials
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DOI: 10.1021/acs.chemmater.7b01521 Chem. Mater. XXXX, XXX, XXX−XXX
