The structure optimization of phenanthroimidazole based isomers with external quantum efficiency approaching 7% in non-doped deep-blue OLEDs

Jie-Ji Zhu a, Yuwen Chen b, Yong-Hong Xiao a, Xin Lian a, Guo-Xi Yang a, Shan-Shun Tang a, Dongge Ma *b, Ying Wang c and Qing-Xiao Tong *a
aDepartment of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Material of Guangdong Province, Shantou University, Guangdong, 515063, P. R. China. E-mail: qxtong@stu.edu.cn
bInstitute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, People's Republic of China. E-mail: msdgma@scut.edu.cn
cKey Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: wangy@mail.ipc.ac.cn

Received 4th December 2019 , Accepted 3rd February 2020

First published on 3rd February 2020


In this work, four phenanthroimidazole (PI) based isomers TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA for high-efficiency deep-blue organic light-emitting diodes (OLEDs) have been designed and synthesized. The structure–property relationship is systematically studied. Devices based on TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA achieved deep-blue emissions with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.07), (0.15, 0.07), (0.15, 0.09) and (0.15, 0.05) and high external quantum efficiencies (EQEmax) of 4.12%, 4.66%, 6.88% and 5.59%, respectively. The PBI-PPI-TPA based device exhibited negligible efficiency roll-off with an EQE of 6.48% at practical 1000 cd m−2. Moreover, the EQE is still above 5% even at a high brightness of 10[thin space (1/6-em)]000 cd m−2. Comparing the four isomers, we found that the substituent at the C2 position of the PI core has a significant influence on the emission wavelength and CIE coordinates. This work provides a rational design strategy where modifying an electron acceptor (A) at the C2 position and an electron donor (D) at the N1 position of the PI core will be an effective way to fabricate high-performance PI-based bipolar emitters.


Introduction

As one of the primary colors, blue is indispensable for full-color displays, and illumination sources through either a primary or complementary color strategy, which has made blue emitters very important in the field of OLEDs.1 Designing and synthesizing new molecular systems with highly efficient deep-blue emission are very important, not only to widen the color gamut and reduce power consumption in full-color displays but also to excite lower energy emissive sources to generate other color coordinates and white lighting via an energy cascade.2 In the recent two decades, many blue emitters have been reported.1,3–7 Rare-metal based phosphorescent materials are of good quality,8 however, they encounter stability and economic benefit problems. Whereas most of the purely organic luminescent materials with thermally activated delayed fluorescence can only give sky blue emission and suffer efficiency roll-off at high brightness.9,10 As a consequence, high-quality blue emitters are highly insufficient, and the exploration of robust blue emitters remains a challenging task.11,12

In general, the electron transport properties of organic semiconductors are inferior compared to hole-transport properties, especially for blue-emitting compounds.13 One of the basic requirements for blue emitters is their enhanced electron affinities, which enable achieving balanced charge injection and transport. Phenanthroimidazole (PI) is an excellent fluorophore for blue materials, by virtue of the unique electronic structure of the imidazole ring, showing decent fluorescence quantum yields and excellent charge transport properties.14 Ma and Yang et al. reported a series of linear and branched blue-emitting PI derivatives.15–17 They provided an effective strategy to design high performance PI-based hybridized local and charge-transfer state (HLCT) blue materials: attaching electron donating (D) groups to the C2 position of the PI core, which expands the electron distribution and provides high quantum yield. Meanwhile, attaching electron accepting (A) groups to the N1 position can enhance electron transportability and improve the current efficiency in devices. To date, the majority of studies regarding PI-based D–A luminophores have been based on C2-substituted systems;2,13,18–21 some focused on the N1-connected D–A paired PI derivatives.22–24 Few works focused on the relationship between isomer's structure and device performance.22,25 The understanding of the structure–property relationship is crucial in developing materials with desired photophysical, electrochemical, and thermal properties for OLEDs.26–28

We aim to investigate the structure–property relationship and then provide a strategy to design high performance D–π–A or A–π–D type bipolar emitters. Inspired by the high electron mobility of electron transport material 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) and derivatives,29–31 we employ 1,2-diphenyl-1H-benzo[d]imidazole (PBI) as an electron transport enhancing group, meanwhile, triphenylamine (TPA) as a hole transport enhancing group, to fabricate PI-based isomers. As both PBI and PI have two connected positions (N1 and C2, shown in Scheme 1), through switching substituent positions, we got four isomers, denoted TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA (Scheme 1), respectively. Four non-doped devices using them as emitters have been fabricated and show good efficiency (maximum EQE of 4.12%, 4.66%, 6.88%, and 5.59%, respectively). In particular, the PBI-PPI-TPA based device shows negligible efficiency roll-off in the 10–1000 cd m−2 region and its performance is comparable to those of other state-of-the-art non-doped blue OLEDs.


image file: c9tc06658f-s1.tif
Scheme 1 Molecular structure of new compounds.

Results and discussion

Theoretical calculation

To better understand the structure–property relationships of the new compounds, geometry optimization in the gas phase and frontier orbitals are performed using the Gaussian 09 program package at the B3LYP/6-31g(d,p) level. As shown in Fig. 1, the substituent positions have little influence on the twisting angles between the PI core and the D/A unit. The dihedral angle at the joint point is between 34.0 and 38.5°. However, the frontier orbital distribution of isomers varies with each other. For TPA-PPI-PBI and TPA-PPI-NPBI, the highest occupied molecular orbital (HOMO) delocalizes on the TPA unit and PI core, while the N1-connected PBI (NPBI) unit is highly twisted to the PI core, the lowest unoccupied molecular orbital (LUMO) of TPA-PPI-NPBI tends to delocalize on the diphenyl rings between two imidazole cores. For both compounds, the HOMO and LUMO are well separated; this indicates that the oxidation occurs on the TPA unit, and reduction occurs on the PBI unit. For PBI-PPI-TPA, the TPA unit has little contribution to frontier orbital distribution; the HOMO delocalizes along the PI core and π-bridge, while the LUMO mainly delocalizes along the PBI and π-bridge. The suitable electron cloud overlap in frontier molecular orbitals results in a large oscillator strength (f = 1.007). This kind of cloud overlap is conducive to obtain a bipolar charge transport feature and realize high PLQY.32,33 Interestingly, as NPBI is a weak electron-withdrawing group, the HOMO of NBPI-PPI-TPA delocalizes on the TPA unit, and the LUMO mainly delocalizes on the π-bridge between NBPI and PI.
image file: c9tc06658f-f1.tif
Fig. 1 Optimized molecular geometries and distributions of the molecular frontier orbitals.

For further understanding the excited-state properties of the four isomers, we calculated the natural transition orbitals (NTOs) of the singlet and triplet states to analyze the electron transition feature and excited-state energy levels (Fig. S1 and Tables S1–S4, ESI). The calculated results were analyzed by software Multifwn.34 For TPA-PPI-PBI and TPA-PPI-NPBI, the S0 → S1 transition reveals the CT transition feature (major molecular orbital contribution is listed in Fig. S2, ESI), in which holes and electrons are well separated. For PBI-PPI-TPA, the S0 → S1 transition exhibits an HLCT transfer feature, which is beneficial for improving the fluorescence yield (Φf).32,35 Interestingly, the HOMO/LUMO distribution of NPBI-PPI-TPA is inverting to a classic HLCT strategy designed compound TPA-PPI-PBI. However, NPBI-PPI-TPA exhibits the HLCT feature of the S0 → S1/2/3 transition, while TPA-PPI-PBI only exhibits the HLCT feature of the S0 → S2 transition. This proves that connecting D at the N1 position and A at the C2 position of the PI core is easier to realize the HLCT feature.

Photophysical properties

The UV/vis absorption and PL spectra of the compounds were measured in diluted DCM solution (Fig. 2) and neat films (Fig. S3, ESI). Key photophysical data of the four isomers are listed in Table 1. As shown in Fig. 2, all compounds show similar absorption profiles in solution from 240–270 nm. The absorption band around 260 nm should have originated from the π–π* transition of the common benzene ring. The longer wavelength absorption bands vary in the range of 300–400 nm, which originated from the π–π* transition;36 the details of major molecular orbital contribution is shown in Fig. S2 (ESI). The emission peaks of the compounds in DCM solution are at 450, 449, 432 and 427 nm, respectively (Table 1); meanwhile, emission peaks in neat film are at 457, 451, 460, and 438 nm, respectively (Fig. S3, ESI). To further study the excited state properties, the solvation effects on the photophysical properties were investigated in solvents with different polarities (Fig. S4, ESI). Upon increasing the solvent polarities gradually from n-hexane to acetonitrile (ACN), the emission of TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA exhibits a red-shift of 57, 55, 45 and 24 nm, respectively. Furthermore, the fine vibrational structure of the fluorescence disappears in acetonitrile, indicating a sign of ICT in the excited state of all compounds. It is worth noting that TPA substituents at the C2 position of PI exhibit a stronger intramolecular charge transfer effect, resulting in red-shifted absorption peak compared with the other two.
image file: c9tc06658f-f2.tif
Fig. 2 Normalized UV/Vis absorption and PL spectra of compounds in DCM (10−5 M).
Table 1 Key physical data of the new compounds
Compound λ abs [nm] λ PL [nm] Φ f [%] τ [ns] E T1/ES1e [eV] HOMO/LUMOf [eV] E g [eV] T g/Tm/Tdh [°C]
a Absorption and photoluminescence peaks measured in 10−5 M DCM. b Photoluminescence peaks measured in 10−5 M DCM and neat films, respectively. c Photoluminescence quantum yield measured in DCM using 9,10-diphenylanthracene as a standard (Φf = 0.90 in cyclohexane) and absolute quantum yields measured in neat films, respectively. d PL lifetimes of prompt decay components in dilute DCM and neat films, respectively. e Singlet/triplet energy levels calculated according to the highest energy peaks of fluorescence and phosphorescence spectra at 77 K. f HOMO estimated from cyclic voltammetry, the LUMO calculated from the equation: LUMO = HOMO + Eg, Eg is the optical energy gap estimated from the absorption onset of the compound in dilute DCM. g Optical energy gap estimated from the absorption onset in DCM and neat films, respectively. h Glass transition temperature, melting point and decomposition temperature (5% weight loss), — stands for not detected.
TPA-PPI-PBI 259, 312, 366 450/457 0.56/0.51 3.72/2.70 2.37/3.05 −5.28/−2.27 3.01/2.92 —/—/524
TPA-PPI-NPBI 261, 278, 366 449/451 0.77/0.52 2.15/1.75 2.37/3.04 −5.35/−2.35 3.00/2.97 —/317/521
PBI-PPI-TPA 260, 309, 344 442/460 0.83/0.90 1.96/3.28 2.48/3.15 −5.33/−2.28 3.05/2.99 —/334/529
NPBI-PPI-TPA 260, 283, 341 427/438 0.61/0.73 2.51/3.26 2.50/3.20 −5.23/−2.08 3.15/3.06 —/—/514


We tested the fluorescence and phosphorescence spectra of four compounds at 77 K (Fig. S5, ESI). For TPA-PPI-PBI and TPA-PPI-NPBI, their low-temperature phosphorescence spectra show two emission bands (around 430 nm and 520 nm). As the measurement was taken in a low polar solvent (toluene) and molecular rotation was inhibited at 77 K, we infer that the first emission band around 420 nm is from the PI core, while the second emission band is from the whole molecule. According to the highest peaks of the low-temperature fluorescence and phosphorescence spectra, the singlet energy states (ES1) of the compounds were estimated to be 3.05, 3.04, 3.15 and 3.20 eV for TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA, and NPBI-PPI-TPA respectively, while their triplet energy states (ET1) were estimated to be 2.37, 2.37, 2.48 and 2.50 eV respectively. The ET1 of PBI-PPI-TPA and NPBI-PPI-TPA is over 2.48 eV; this makes them suitable to act as hosts for yellow or orange phosphors.37,38 Besides, the calculated first-ten ES and ET values are presented in Tables S1–S4 (ESI). Both experimental (Table 1) and calculated results (Tables S1–S4, ESI) of four compounds indicate that the energy gap between ES1 and ET1EST) is over 0.4 eV, with which it is hard to enable an efficient reverse intersystem crossing (RISC) process from T1 to S1.39 We also tested the fluorescence lifetime of all compounds in DCM and neat films. All compounds exhibited fluorescence with short lifetimes less than 4 ns (Table 1 and Fig. S6, ESI), indicating that they are not TADF or TTA materials. Besides, we found that TPA-PPI-PBI and TPA-PPI-NPBI have similar optical properties (emission peaks, solvatochromic shifts, ES1, and ET1). This indicates that substituents at the C2 position of PI control these properties.

Single-crystal structure analysis

Single-crystal X-ray crystallographic analysis is applied to confirm the structure of TPA-PPI-NPBI and investigate the molecular packing arrangement. Single crystals of TPA-PPI-NPBI (CCDC number: 1948950) were obtained by slow vaporization of the DCM solution covered with an n-hexane layer. As shown in Fig. 3, the star shape steric hindrance unit TPA inhibits π–π stacking in the solid-state. The PI core and benzimidazole core are nearly coplanar, which agrees with the optimized structure (Fig. 1). Also, weak π–π interactions between PI and BI cores can be found with a distance of 4.15 Å. However, strong intermolecular C–H⋯N (2.91 Å and 2.72 Å) hydrogen bonds exist. The intermolecular hydrogen bonds are conducive to providing a potential electron transport pathway.40 We also get the crystal structure of intermediate NPBI-PPI-Br (Fig. S7, ESI, CCDC No: 1948948), which clearly shows that the NPBI unit is twisted to the PI core, indicating that the limited conjunction degree of NPBI-PPI-TPA ensures deep blue emission.
image file: c9tc06658f-f3.tif
Fig. 3 Molecular structure (a) and packing arrangement (b) of TPA-PPI-NPBI in crystals.

Thermal properties

The thermal properties of the new compounds were analyzed with DSC and TGA under N2 protection, as shown in Fig. 4 and Table 1. The melting temperatures Tms of TPA-PPI-NPBI and PBI-PPI-TPA were 317 °C and 334 °C, respectively. Benefitted from the rigid skeleton of PPI and PBI units, all of them exhibit high Tds over 510 °C, especially for PBI-PPI-TPA, the Td was up to 529 °C. It is worth noting that no glass transport temperature (Tg) was obtained in the DSC measurement, probably due to the twisting and bulky configuration.41 The high Td demonstrates that they could form morphologically stable amorphous films upon thermal evaporation, which is highly important for application in OLEDs.13
image file: c9tc06658f-f4.tif
Fig. 4 (a) DSC and (b) TGA curves of the new compounds.

Electrochemical properties

The electrochemical properties of compounds were studied in solution through cyclic voltammetry (CV) measurements using ferrocene as the internal standard. The oxidation scans were carried out in DCM to estimate the HOMO levels, which were deduced from the onset oxidation potentials concerning the HOMO level of ferrocene (−4.8 eV). Their cyclic voltammograms are shown in Fig. 5 and the respective electrochemical data are summarized in Table 1. All the compounds displayed two reversible oxidation waves, and no reduction wave was detected within the electrochemical window of DCM. All the compounds display similar onset oxidation potentials, and their HOMO levels are estimated to be −5.28, −5.35, −5.33 and −5.23 eV for TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA, respectively, while their LUMO levels are −2.21, −2.35, −2.28 and −2.08 eV by subtracting Eg (Eg was estimated from the absorption onset in DCM). As deduced from a much easier electrochemical oxidation of TPA (oxidation potential: ca. 0.45 V vs. Fc/Fc+)42 than PI (oxidation potential: ca. 0.84 V vs. Fc/Fc+),43 the first oxidation wave may belong to the TPA unit and the second belongs to the PI core. However, the calculated result of PBI-PPI-TPA (Fig. 1) indicates that TPA has little contribution to the HOMO; thus the first oxidation peak shouldn’t be from the TPA unit. To explain this phenomenon, the calculated HOMOn (n=−3 to 0) distribution of PBI-PPI-TPA is presented in Fig. S8 (ESI). The energy levels of the HOMO (−5.11 eV) and HOMO−1 (−5.18 eV) are very close. The corresponding molecular orbitals may be degenerated. Thus, the first oxidation mainly occurs at the TPA unit of PBI-PPI-TPA.
image file: c9tc06658f-f5.tif
Fig. 5 Cyclic voltammetry of compounds recorded in DCM and Bu4NPF6; the inset shows a cyclic voltammetry curve of potential calibration material ferrocene.

Electroluminescence properties

To evaluate the electroluminescence (EL) performances of the materials, non-doped OLEDs were fabricated with a device structure of ITO/HAT-CN (15 nm)/TAPC (65 nm)/TCTA (5 nm)/EML (20 nm)/TPBi (40 nm)/Liq (1.25 nm)/Al (120 nm) (see Fig. 6a). Here, ITO is the anode, 2,3,6,7,10,11-hexacyanohexaazatriphenylene (HAT-CN) is the hole injection layer, and 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) is the hole transporting layer. According to our previous experience, inserting a layer of 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA) will improve the device performance. Here, TPBi and 8-hydroxyquinolinolato-lithium (Liq) serve as electron transporting and electron injecting layers, respectively. Besides, the deep HOMO level (∼6.30 eV) of TPBi can also block the holes in the devices.21 Taking TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA as emitters, we got devices A, B, C and D, respectively.
image file: c9tc06658f-f6.tif
Fig. 6 (a) The energy level diagram of the devices; (b) current density–voltage–luminance characteristic curves; (c) power efficiency–luminance curves; and (d) EQE–luminance curves.

Fig. 6 shows the device performances, and the key performance data are summarized in Table 2. Devices A and B display deep blue EL emission with the same CIE coordinates of (0.15, 0.07), and their EQEmax is 4.12% and 4.66%, respectively. TPA-PPI-NPBI based device B displays better device performance than TPA-PPI-PBI based device A, especially at high brightness (>100 cd m−2), including EQE, power efficiency (PE), current efficiency (CE) and efficiency roll-off. The smaller roll-off may result from the coplanar structure of TPA-PPI-NPBI providing a potential carrier transport pathway in the solid-state and improving charge balance. PBI-PPI-TPA based device C displays the highest performance, with a low Von of 2.8 V and PEmax of 5.77 lm W−1 (Fig. 6c). The EQEmax of device C reaches 6.88% (at 100 cd m−2) and slightly rolls off to 6.46% at a luminance of 1000 cd m−2 (Fig. 6d). It is worth noting that the EQE of device C is up to 5.34% at 10[thin space (1/6-em)]000 cd m−2, revealing one of the best performances in non-doped blue OLEDs.44–48 The excellent performance of device C may result from the balanced charge transfer ability of PBI-PPI-TPA. To verify this, hole- and electron-only devices were fabricated with the configuration of ITO/NPB (10 nm)/PBI-PPI-TPA (70 nm)/NPB (10 nm)/Al (120 nm) and ITO/TPBi (10 nm)/PBI-PPI-TPA (70 nm)/TPBi (10 nm)/LiF (1 nm)/Al (120 nm), respectively. As shown in Fig. S9 (ESI), PBI-PPI-TPA has both hole and electron transporting ability. NPBI-PPI-TPA based device D displays the deepest blue EL emission with emission peak at 429 nm and CIE coordinates of (0.15, 0.05). Its EQEmax is 5.59% and rolls off to 4.18% at a luminance of 1000 cd m−2. The roll-off is pretty small compared with those of other PI emitters with CIEy ≤ 0.05. Moreover, the almost unchanged EL spectra of devices A–D with increasing voltages from 3 to 7 V (Fig. S10, ESI) indicate that the devices have good stability during the electrical charge injection process.

Table 2 EL performances of the fabricated devices
Device Emitter V on [V] λ EL [nm] L max [cd m−2] CEb [cd A−1] PEb [lm W−1] EQEb [%] CIEc [x, y]
a Turn on voltage at a luminance of 1 cd m−2. b Current efficiency, power efficiency, and external quantum efficiency at the maximum value/100/1000 cd m−2. c Color coordinates at 7 V.
A TPA-PPI-PBI 3.2 437 2563 2.98/2.67/2.07 2.93/1.73/0.80 4.12/3.83/2.83 (0.15, 0.07)
B TPA-PPI-NPBI 3.2 449 2657 3.26/2.97/2.85 3.20/1.78/1.05 4.66/4.22/3.58 (0.15, 0.07)
C PBI-PPI-TPA 2.8 451 10[thin space (1/6-em)]612 5.90/5.90/5.37 5.77/4.99/3.16 6.88/6.88/6.48 (0.15, 0.09)
D NPBI-PPI-TPA 3.0 429 3366 2.60/2.46/1.92 2.72/1.97/1.02 5.59/5.33/4.18 (0.15, 0.05)


Comparing the four devices, we find that the substituent at the C2 position of the PI core has a significant influence on the emission wavelength and CIE coordinates, which agrees with the photophysical results. Meanwhile, the isomers with PBI/NPBI connected at the C2 position exhibit better device performance than the isomers with that connected at the N1 position. Inspired by the above results, we provided a design strategy where modifying the electron acceptor at the C2 position and the electron donor at the N1 position of the PI core will be an effective way to fabricate high-performance PI-based bipolar emitters.

Conclusions

In this work, we designed four PI-based isomers for high efficiency deep-blue organic light-emitting diodes. The structure–property relationship is systematically studied. Theoretical calculations suggested that C2 and N1 positions of PI are electronically different owing to the electronic preference effect; meanwhile, the electron donating/accepting properties and geometry of substituents co-control the HOMO/LUMO distribution and ICT properties. Devices based on TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA achieved deep-blue emissions with CIE coordinates of (0.15, 0.07), (0.15, 0.07), (0.15, 0.09) and (0.15, 0.05), respectively; and EQEmax of 4.12, 4.66, 6.88 and 5.59%, respectively. In particular the PBI-PPI-TPA based device C exhibited negligible efficiency roll-off with an EQE of 6.25, 6.88, and 6.48% at 10, 100, and 1000 cd m−2, respectively. Moreover, the EQE is still over 5% at a high brightness of 10[thin space (1/6-em)]000 cd m−2. The HOMO/LUMO distribution of NPBI-PPI-TPA inverts to traditional HLCT strategy designed TPA-PPI-PBI. However, this special distribution displays better device performance. Comparing the four isomers, we found that the substituent at the C2 position of the PI core controls the emission wavelength and CIE coordinates in a device. Furthermore, we provided a design strategy where modifying an electron acceptor (A) at the C2 position and an electron donor (D) at the N1 position of PI core will be an effective way to fabricate high-performance PI-based bipolar emitters.

Experimental section

General information

1H and 13C NMR measurement was recorded with a Bruker 400/500 MHz NMR spectrometer. Mass spectra were recorded on a Bruker MALDI-TOF autoflex spectrometer. The decomposition temperature (Td) was measured with Shimadzu TA50 at a heating rate of 10 °C min−1 under a nitrogen atmosphere. The glass transition temperature (Tg) and melting point (Tm) were determined on a Shimadzu DSC60. Absorption and photoluminescence spectra were measured with a Shimadzu UV2600 UV-Vis spectrophotometer and a Hitachi F7000 Luminescence spectrophotometer, respectively. Cyclic voltammetry (CV) was performed on a CHI 600E electrochemical analyzer with a three-electrode system (a glassy carbon electrode as the working electrode, a platinum wire as the auxiliary electrode, and an Ag/AgCl electrode as the pseudo-reference electrode with Fc/Fc+ as the internal standard, which has an absolute HOMO level of −4.80 eV). Nitrogen saturated DCM was used as a solvent with 0.1 mol L−1 tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The HOMO and LUMO were predicated by the 6-31G(d,p) basis set with the Gaussian 09 program package. The PL lifetime was measured on an EDINBURGH FLS980 fluorescence spectrophotometer. The absolute fluorescence quantum yields of neat films were measured on a Hamamatsu C11347-11.

Pre-patterned indium tin oxide (ITO) glass substrates with a sheet resistance of 15 Ω m−2 were routinely cleaned with organic solvents and deionized water. The substrates were dried under N2 flow and then stored in an oven at 120 °C before use. After a 15 min UV-ozone treatment, the ITO substrates were immediately transferred into a deposition chamber with a base pressure of 5 × 10−7 Torr for organic and cathode depositions. The deposition of the organic layers was monitored with a quartz oscillating crystal and controlled at 1 Å s−1. The cathodes were prepared by deposition of Liq at a rate of 0.1–0.2 Å s−1 and then Al (5 Å s−1, 150 nm) through a shadow mask. Electroluminescence (EL) spectra and the corresponding CIE coordinates were measured with a Spectra scan PR650 photometer. Current–voltage–luminance (J–V–L) characteristics were recorded with a Keithley 2400 source meter under an ambient atmosphere without device encapsulation.

Synthesis of compounds

All chemicals and solvents are received from commercial sources and used without further purification. Column chromatography was carried out using 200–300 mesh silica gel. Scheme 1 presents the synthetic routes of the new compounds. Two steps of reactions are involved in the synthesis process (Scheme 2). First, a “one-pot” reaction49 of aryl carbaldehyde, phenanthrene-9,10-dione, and 4-bromoaniline proceeded to form the corresponding intermediates TPA-PPI-Br, PBI-PPI-Br and NPBI-PPI-Br. Then through the Suzuki-coupling reaction,50 intermediates react with boric acids to form target compounds. After that, the products were isolated with moderate yields and their molecular structures were confirmed by NMR and mass spectrometry.
image file: c9tc06658f-s2.tif
Scheme 2 Synthetic routes of the new compounds (i) one-pot reaction, aryl carbaldehyde, 4-bromoanilines and ammonium acetate in CH3COOH, refluxed for 2 h, yield: >80%; (ii) Suzuki coupling reaction, aryl bromide, aryl boronic acid in toluene[thin space (1/6-em)]:[thin space (1/6-em)]ethanol = 5[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v, Pd(PPh3)4 (5 mol%), 2 M sodium carbonate refluxed for 12 h under argon, yield: >70%).
Synthetic routes of intermediates and target compounds.
4′-(1-(4-Bromophenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1′-biphenyl]-4-amine (TPA-PPI-Br). A mixture of 5 mmol (1.75 g) 4′-(diphenylamino)-[1,1′-biphenyl]-4-carbaldehyde (TPA-CHO), 5 mmol (0.85 g) 4-bromoanilines, 5 mmol (1.04 g) 9,10-phenanthraquinone, and 50 mmol (3.85 g) ammonium acetate was dissolved in 150 mL acetic acid, and then refluxed under an argon atmosphere. After 10 h, as the mixture cooled to room temperature, 100 mL ethanol was added, and precipitates were collected and dried under vacuum. The residue was purified by column chromatography (petroleum ether[thin space (1/6-em)]:[thin space (1/6-em)]CH2Cl2 = 2[thin space (1/6-em)]:[thin space (1/6-em)]1) to give a light-yellow powder, yield 85%. 1H NMR (500 MHz, CD2Cl2) δ 8.87 (s, 1H), 8.77 (dd, J = 31.0, 8.3 Hz, 2H), 7.78 (dd, J = 13.9, 7.9 Hz, 3H), 7.72–7.67 (m, 1H), 7.64 (d, J = 8.1 Hz, 2H), 7.61–7.41 (m, 7H), 7.35 (t, J = 7.5 Hz, 1H), 7.28 (dd, J = 17.3, 9.4 Hz, 5H), 7.12 (dd, J = 7.9, 3.8 Hz, 6H), 7.06 (t, J = 7.3 Hz, 2H). m/z (MALDI-TOF) 691.283.
1-(4-Bromophenyl)-2-(4′-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (PBI-PPI-Br). The synthetic route of PBI-PPI-Br was the same as that of TPA-PPI-Br, except that aryl carbaldehyde was changed to 4′-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1′-biphenyl]-4-carbaldehyde (PBI-CHO).

1H NMR (500 MHz, CD2Cl2) δ 8.80 (t, J = 7.9 Hz, 2H), 8.73 (d, J = 8.3 Hz, 1H), 7.85 (d, J = 7.9 Hz, 1H), 7.82–7.73 (m, 3H), 7.71–7.62 (m, 5H), 7.62–7.50 (m, 8H), 7.45 (d, J = 6.7 Hz, 2H), 7.35 (dd, J = 20.2, 7.8 Hz, 4H), 7.28 (dd, J = 17.5, 8.0 Hz, 3H). m/z (MALDI-TOF) 717.012.


1-(4-Bromophenyl)-2-(4′-(2-phenyl-1H-benzo[d]imidazol-1-yl)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (NPBI-PPI-Br). The synthetic route of PBI-PPI-Br was the same as that of TPA-PPI-Br, except that aryl carbaldehyde was changed to 4′-(2-phenyl-1H-benzo[d]imidazol-1-yl)-[1,1′-biphenyl]-4-carbaldehyde (NPBI-CHO). 1H NMR (400 MHz, CD2Cl2) δ 8.85 (t, J = 6.5 Hz, 2H), 8.78 (d, J = 8.3 Hz, 1H), 7.91 (d, J = 7.8 Hz, 1H), 7.82 (dd, J = 15.6, 8.4 Hz, 5H), 7.77–7.66 (m, 7H), 7.60 (t, J = 8.3 Hz, 1H), 7.52 (d, J = 8.6 Hz, 2H), 7.41 (ddd, J = 15.0, 10.6, 6.0 Hz, 9H), 7.31 (d, J = 8.3 Hz, 1H) 717.021.
N,N-Diphenyl-4′-(1-(4′-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1′-biphenyl]-4-yl)-1H-phenanthrol[9,10-d]imidazol-2-yl)-[1,1′-biphenyl]-4-amine (TPA-PPI-PBI). A mixture of 1 mmol (0.69 g) TPA-PPI-Br, 1.5 mmol (0.47 g) (4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)boronic acid, 0.20 g tetrakis-(triphenylphosphine)-palladium(0) (Pd(PPh3)4) and 15 mL aqueous K2CO3 (2 M) with 10 mL ethanol and 40 mL toluene was added into a 150 mL degassed three-necked flask and refluxed under an argon atmosphere. After 12 h, as the mixture was cooled to room temperature, the organic phase was washed with 20 mL water twice, then extracted with dichloromethane and dried over anhydrous MgSO4 before removing the solvent. Finally, the residue was purified by column chromatography (petroleum ether[thin space (1/6-em)]:[thin space (1/6-em)]CH2Cl2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) to give a pure white powder. Yield: 75%. 1H NMR (400 MHz, CD2Cl2) δ 8.80 (d, J = 9.1 Hz, 1H), 8.75 (d, J = 8.1 Hz, 1H), 7.90 (d, J = 8.2 Hz, 3H), 7.76 (s, 5H), 7.67 (dd, J = 15.4, 8.3 Hz, 5H), 7.56 (t, J = 6.8 Hz, 6H), 7.50 (d, J = 8.6 Hz, 2H), 7.45–7.36 (m, 3H), 7.29 (dd, J = 16.2, 8.9 Hz, 8H), 7.11 (d, J = 7.4 Hz, 6H), 7.05 (t, J = 8.0 Hz, 2H). 13C NMR (126 MHz, CDCl3) δ 151.75, 150.84, 147.58, 143.09, 141.31, 140.81, 140.18, 138.32, 137.67, 137.45, 137.09, 133.86, 130.09, 130.05, 129.80, 129.77, 129.57, 129.32, 128.83, 128.79, 128.54, 128.31, 128.18, 127.64, 127.56, 127.32, 127.25, 127.01, 126.36, 126.29, 125.66, 124.92, 124.53, 124.18, 123.72, 123.59, 123.19, 123.13, 123.09, 123.05, 122.82, 120.87, 119.95, 110.52. m/z (MALDI-TOF) 881.551.
N,N-Diphenyl-4′-(1-(4′-(2-phenyl-1H-benzo[d]imidazol-1-yl)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1′-biphenyl]-4-amine (TPA-PPI-NPBI). The synthetic route of TPA-PPI-NPBI was the same as that of TPA-PPI-PBI, except that boronic acid was changed to (4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl)boronic acid. 1H NMR (500 MHz, CD2Cl2) δ 8.83 (d, J = 8.5 Hz, 1H), 8.76 (d, J = 8.5 Hz, 1H), 8.01–7.92 (m, 5H), 7.79 (t, J = 6.3 Hz, 2H), 7.72 (t, J = 8.3 Hz, 6H), 7.58 (t, J = 8.4 Hz, 4H), 7.51 (d, J = 8.5 Hz, 4H), 7.47–7.33 (m, 8H), 7.28 (t, J = 7.9 Hz, 4H), 7.17–7.09 (m, 6H), 7.06 (t, J = 7.4 Hz, 2H). 13C NMR (126 MHz, CDCl3) δ 152.43, 150.88, 147.66, 147.56, 143.19, 140.93, 140.85, 139.54, 138.54, 137.73, 137.17, 136.99, 133.77, 130.00, 129.81, 129.77, 129.61, 129.58, 129.34, 128.83, 128.64, 128.59, 128.48, 128.33, 128.18, 127.99, 127.63, 127.38, 127.26, 126.35, 126.30, 125.74, 124.97, 124.57, 124.26, 123.69, 123.54, 123.21, 123.15, 123.05, 122.85, 120.84, 120.09, 110.40. m/z (MALDI-TOF) 881.670.
N,N-Diphenyl-4′-(2-(4′-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)-[1,1′-biphenyl]-4-amine (PBI-PPI-TPA). The synthetic route of PBI-PPI-TPA was the same as that of TPA-PPI-PBI, except that the starting materials were changed to PBI-PPI-Br and (4-(diphenylamino)phenyl)boronic acid. 1H NMR (500 MHz, CD2Cl2) δ 8.80 (d, J = 8.3 Hz, 1H), 8.75 (d, J = 8.3 Hz, 1H), 7.91 (d, J = 8.6 Hz, 1H), 7.86 (d, J = 6.6 Hz, 2H), 7.77 (dd, J = 12.8, 7.8 Hz, 3H), 7.68 (dd, J = 18.5, 7.1 Hz, 6H), 7.64–7.53 (m, 10H), 7.42–7.36 (m, 4H), 7.36–7.25 (m, 7H), 7.17 (dd, J = 16.2, 8.1 Hz, 6H), 7.09 (t, J = 7.4 Hz, 2H). 13C NMR (126 MHz, CDCl3) δ 151.97, 150.46, 148.18, 147.49, 143.11, 141.86, 141.03, 140.16, 137.63, 137.40, 137.22, 137.10, 132.76, 129.98, 129.93, 129.85, 129.76, 129.44, 129.40, 129.34, 129.13, 128.69, 128.37, 128.34, 127.92, 127.83, 127.52, 127.33, 127.27, 126.79, 126.38, 125.68, 124.98, 124.71, 124.16, 123.71, 123.44, 123.37, 123.17, 123.09, 122.81, 120.97, 119.88, 110.47. m/z (MALDI-TOF) 881.823.
N,N-Diphenyl-4′-(2-(4′-(2-phenyl-1H-benzo[d]imidazol-1-yl)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)-[1,1′-biphenyl]-4-amine (NPBI-PPI-TPA). The synthetic route of NPBI-PPI-TPA was the same as that of TPA-PPI-PBI, except that the starting materials were changed to NPBI-PPI-Br and (4-(diphenylamino)phenyl)boronic acid. 1H NMR (500 MHz, CD2Cl2) δ 8.82 (d, J = 8.4 Hz, 1H), 8.76 (d, J = 8.2 Hz, 1H), 7.89 (d, J = 8.4 Hz, 2H), 7.81 (dd, J = 15.3, 7.9 Hz, 6H), 7.70 (dd, J = 22.5, 8.2 Hz, 7H), 7.64 (d, J = 8.4 Hz, 2H), 7.57 (t, J = 7.2 Hz, 1H), 7.51–7.34 (m, 10H), 7.34–7.28 (m, 5H), 7.19 (d, J = 8.6 Hz, 2H), 7.15 (d, J = 7.6 Hz, 4H), 7.09 (t, J = 7.3 Hz, 2H)

13C NMR (126 MHz, CDCl3) δ 152.41, 150.37, 148.24, 147.46, 143.13, 141.92, 140.35, 139.75, 137.66, 137.22, 137.18, 136.40, 132.69, 130.20, 129.99, 129.93, 129.52, 129.44, 129.38, 128.40, 128.36, 128.30, 127.94, 127.82, 127.71, 127.36, 127.26, 126.87, 126.41, 125.73, 125.04, 124.73, 124.18, 123.65, 123.42, 123.19, 123.09, 122.79, 120.98, 119.98, 110.48. m/z (MALDI-TOF) 881.733.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (No. 51673113 and 51973107), the key project of Department of Education of Guangdong Province (No. 2018KZDXM032), and the Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme 2019 (GDUPS2019) for the financial support.

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Footnotes

Electronic supplementary information (ESI) available. CCDC 1948948 and 1948950. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9tc06658f
Jie-Ji Zhu and Yuwen Chen contributed equally to this work.

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