Solution processible distyrylarylene-based fluorescent dendrimers: Tuning of carbazole-dendron generation leads to nondoped deep-blue electroluminescence

Organic Electronics 53 (2018) 43–49

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Solution processible distyrylarylene-based fluorescent dendrimers: Tuning of carbazole-dendron generation leads to nondoped deep-blue electroluminescence

T

Lei Zhao, Shumeng Wang, Junqiao Ding∗, Lixiang Wang∗∗ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Distyrylarylene Fluorescent dendrimers Oligocarbazole Nondoped OLEDs Deep-blue

A series of solution processible fluorescent dendrimers named PVCt1, PVCt2 and PVCt3 have been designed and synthesized by using 4,4′-distyryl-1,1′-biphenylene as the core and oligocarbazole as the dendron. When the dendron generation grows, the conjugation extension is limited due to the enhanced torsion between core and dendron, and simultaneously the intermolecular interactions can be weakened owing to the dendron encapsulation. As a result, a considerable hypsochromic shift is observed for both the photoluminescence and electroluminescence spectra in nondoped solid states, and the corresponding CIE coordinates move from (0.16, 0.21) of PVCt1 to (0.16, 0.15) of PVCt2 and (0.16, 0.10) of PVCt3. Moreover, compared with the sky-blue PVCt1 (0.82%), the nondoped device based on PVCt3 emits a deep-blue light with an improved external quantum efficiency of 2.81%. These results indicate that the tuning of carbazole-dendron generation can lead to deep-blue distyrylarylene-based fluorescent dendrimers used for efficient nondoped OLEDs.

1. Introduction Besides small molecules and polymers, dendrimers that are composed of core, dendrons and surface substituents have also attracted much interest in organic light-emitting diodes (OLEDs) [1,2]. They possess not only the well-defined structures of small molecules for ease purification to favor the synthesis and device reproducibility, but also the good solution processibility of polymers to ensure the cost-effective fabrication via wet methods [3–6]. According to the used core, they can be generally divided into phosphorescent and fluorescent dendrimers. On one hand, phosphorescent dendrimers containing triplet emitter as the core have been widely and extensively investigated by many groups [7–23]. For example, by tuning the central core (e.g. Ir [7–19], Pt [20] and Au complexes [21]) or the peripheral dendron (e.g. Müllen- [7,8], Fréchet- [9], phenylene- [10,11], carbazole- [12–18], and arylaminebased dendrons [19] as well as hybridized [22] and bipolar dendrons [23]), highly efficient solution-processed phosphorescent OLEDs with emissions in the whole visible region have been successfully reported, whose performance could well compete with those of vacuum-deposited small molecules. On the other hand, the singlet-based fluorescent dendrimers [24–29], especially blue-emitting ones, remain less attention even though the dendritic blue fluorescent emitters are

∗

believed to show better device lifetime than the blue phosphorescent counterparts. In this article, we demonstrate a series of solution processible fluorescent dendrimers named PVCt1, PVCt2 and PVCt3 by using 4,4′distyryl-1,1′-biphenylene as the core and oligocarbazole as the dendron (Scheme 1). With the increasing dendron generation, the conjugation extension is limited due to the enhanced torsion between core and dendron, and the intermolecular interactions can be effectively prevented at the same time due to the dendron encapsulation. Consequently, the photoluminescence (PL) in solid states is found to be blueshifted from 461/484 nm of PVCt1 to 439/452 nm of PVCt3 accompanied by the gradually enhanced PL quantum yields (PLQYs) from 40% to 59%. The corresponding solution-processed device based on PVCt3 as the nondoped emitting layer obtains a deep-blue electroluminescence (EL) with Commission International De L'Eclairge (CIE) coordinates of (0.16, 0.10) and a peak external quantum efficiency (EQE) of 2.81%. Compared to the sky-blue emissive PVCt1 (CIE: (0.16, 0.21); EQE: 0.82%), the improved blue color purity and device efficiency clearly indicate that the tuning of the carbazole-dendron generation is a promising strategy to realize efficient nondoped deep-blue EL in fluorescent dendrimers.

Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Ding), [email protected] (L. Wang).

∗∗

http://dx.doi.org/10.1016/j.orgel.2017.11.003 Received 28 August 2017; Received in revised form 1 November 2017; Accepted 2 November 2017 Available online 03 November 2017 1566-1199/ © 2017 Elsevier B.V. All rights reserved.

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Scheme 1. Synthetic route of the distyrylarylene-based fluorescent dendrimers PVCt1 ∼ PVCt3. Reagents and conditions: (i) CuI, (+/−)-trans-1,2-diaminocyclohexane, K3PO4, toluene, 110 °C; (ii) t-BuOK, THF, 0 °C.

2. Results and discussion

Table 1. As can be clearly seen in Fig. 2a, the low-energy absorption bands in the range of 310–425 nm can be safely ascribed to the π-π∗ transitions of the distyrylarylene-based core, while the high-energy absorption located at about 298 nm are from the peripheral carbazole dendrons. Moreover, an obvious hypsochromic shift is observed for the absorption onset from PVCt1 to PVCt3, corresponding to the elevated optical bandgap from −2.95 eV to 3.02 eV. Accordingly, their maximum emissions in toluene solutions also show a small blue-shift of 8 nm. These observations suggest that the conjugation length turns out to be shorter after the introduction of the large-size oligocarbazole. To further study this point, we performed density functional theory (DFT) simulations for the geometrical structures of PVCt1 ∼ PVCt3. As depicted in Fig. 3, the torsion angle θ1 between two diphenylethylene moieties in the core keeps nearly unchanged. However, the torsion angle θ2 between core and dendron is gradually up from 52.2° of PVCt1 to 55.6° of PVCt2 and 57.8° of PVCt3, which may be responsible for the limitation of the conjugation extension and thus the gentle blue-shifted absorption and PL spectra observed in solutions. On going from solutions to films, the PL spectra become structureless and red-shifted (Fig. 2b), indicative of aggregation to some extent. Despite such a bathochromic trend, the PL does moved from 461/ 484 nm of PVCt1 to 439/452 nm of PVCt3. Compared with the solution counterparts, a larger blue-shift of 22 nm is found in the film PL spectra. Besides the above-mentioned torsion between core and dendron, the weakened intermolecular interactions also contribute to the difference for the long-axis distance is up from 32 Å to 52 Å with the increasing generation number (Fig. 3). This is further verified by the film PLQYs, which show a significant enhancement from 40% of PVCt1 to 59% of PVCt3.

2.1. Synthesis and characterization Scheme 1 outlines the synthetic route for the distyrylarylene-based fluorescent dendrimers PVCt1 ∼ PVCt3. With the first, second and third generation carbazole dendrons D1 ∼ D3 in hand, at first, they were treated with 4-bromobenzaldehyde via a CuI-catalyzed Ullmann reaction to afford the key intermediates MD1 ∼ MD3. Finally, a HornerWadsworth-Emmons reaction between MD1 ∼ MD3 and 4,4′-Bis(diethylphosphonomethyl)biphenyl was carried out to give the desired dendrimers PVCt1 ∼ PVCt3 in an acceptable moderate yield of 62–71%. All the dendrimers could be easily and satisfactorily purified by column chromatography, and their molecular structures were fully characterized and confirmed by 1H NMR spectra, MALDI-TOF mass spectroscopy and elemental analysis. Moreover, they were readily soluble in common organic solvents including chloroform, toluene, chlorobenzene, o-dichlorobenzene and mesitylene, ensuring the formation of high-quality films prepared via spin-coating. 2.2. Thermal properties The thermal properties of the dendrimers PVCt1 ∼ PVCt3 were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) at a scanning rate of 10 °C min−1 under a nitrogen atmosphere. As shown in Fig. 1, the decomposition temperature (Td) corresponding to a 5% weight loss is considerably up from 484 °C of PVCt1 to 503 °C of PVCt2 and 537 °C of PVCt3. Meanwhile, a distinct endothermic peak related to the glass transition (Tg) appears at 293, 338 and 371 °C for PVCt1, PVCt2 and PVCt3, respectively. The obtained high Td and Tg for PVCt1 ∼ PVCt3 imply that these distyrylarylene-based fluorescent dendrimers may have excellent thermal and morphological stability, a favorable feature for the realization of long-term OLEDs.

2.4. Electrochemical properties Cyclic voltammetry (CV) was performed to probe the electrochemical properties of PVCt1 ∼ PVCt3. During the anodic sweeping in dichloromethane, they all exhibit multiple reversible oxidation processes, whereas no reduction signals are detected upon the cathodic scan (Fig. 4). When the dendron generation grows, the first oxidation wave is found to be down-shifted to a negative potential resulting from the much richer electron-cloud density of the high-generation

2.3. Photophysical properties Fig. 2 presents the UV–Vis and PL spectra both in solutions and films for the dendrimers PVCt1 ∼ PVCt3, and the data are summarized in 44

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Fig. 1. TGA (a) and DSC traces (b) for the dendrimers PVCt1 ∼ PVCt3.

oligocarbazole. With ferrocene/ferrocenium (Fc/Fc+) as the standard (4.8 eV under vacuum), the determined highest occupied molecular orbital (HOMO) energy level is accordingly increased from PVCt1 (−5.46 eV) to PVCt2 (−5.37 eV) and PVCt3 (−5.35 eV). Combined with the optical bandgap, the lowest unoccupied molecular orbital (LUMO) level is estimated to be −2.51, −2.38 and −2.33 eV for PVCt1, PVCt2 and PVCt3, respectively.

PVCt3. For instance, PVCt2 achieves a blue emission with a maximum luminance of 6384 cd/m2, a peak current efficiency of 3.72 cd/A and a peak EQE of 2.76%, and PVCt3 realizes a deep-blue emission with a maximum luminance of 4729 cd/m2, a peak current efficiency of 2.89 cd/A and a peak EQE of 2.81%. The results demonstrate that, by simply varying the dendron generation of fluorescent dendrimers, both the blue color purity and device efficiency could be well tailored in order to achieve high-performance solution-processed nondoped OLEDs with deep-blue emissions.

2.5. EL properties

3. Conclusions

To evaluate the EL properties of PVCt1 ∼ PVCt3, their solutionprocessed OLEDs were fabricated with a nondoped device configuration of ITO/PEDOT:PSS/dendrimer/TPBI/LiF/Al (Fig. 5a). Here the emitting layer (EML) is independently made of PVCt1, PVCt2 or PVCt3 and free of host; TPBI stands for 1,3,5-tris(1-phenyl-benzimidazol-2-yl) benzene and acts as the electron transporting layer. Consistent with their PL counterparts (Fig. 5b), the EL spectra are found to be intensively dependent on the dendron generation. From PVCt1 to PVCt2 and PVCt3, an obvious blue shift is observed, giving CIE coordinates of (0.16, 0.21), (0.16, 0.15) and (0.16, 0.10), respectively (Table 2). Fig. 5c and d portray the current density-voltage-luminescence characteristics and the luminescence dependence on the current efficiency and EQE. Among these distyrylarylene-based fluorescent dendrimers, PVCt1 displays the worst nondoped device performance with a sky-blue emission and poor EQE of 0.82% (1.43 cd/A). With the enlarged carbazole dendron, as discussed above, the intermolecular interactions could be weakened to reduce the aggregation-induced luminescence quenching because of the shielding effect. Therefore, an improved efficiency can be reasonably anticipated for PVCt2 and

In summary, a series of distyrylarylene-based fluorescent dendrimers have been designed and synthesized for the application in solution-processed nondoped OLEDs. Attributable to the enhanced torsion between core and core as well as the weakened intermolecular interactions, both the PL and EL spectra are found to be significantly blueshifted with the increasing carbazole-dendron generation. As a result, a deep-blue emission is realized for the third-generation dendrimer PVCt3, revealing a peak EQE of 2.81% and CIE coordinates of (0.16, 0.10). This work, we believe, will shed light on the development of efficient deep-blue emitters based on a dendritic fluorescent system. 4. Experimental section 4.1. General information 1

H NMR spectra were recorded with a Bruker Avance 400 NMR spectrometer. MALDI-TOF mass spectra were obtained on an AXIMA

Fig. 2. Photophysical properties of PVCt1 ∼ PVCt3: (a) UV–Vis absorption and PL spectra in toluene solutions; (b) PL spectra in solid films.

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Table 1 Photophysical, electrochemical and thermal properties of PVCt1 ∼ PVCt3. Dendrimer

λabsa [nm]

λema [nm]

ФPLb [%]

λemc [nm]

ФPLd [%]

Ege [eV]

HOMO/LUMOf [eV]

Td [°C]

Tg [°C]

PVCt1 PVCt2 PVCt3

377, 298 371, 298 353, 298

426, 449 423, 445 418, 441

91 93 94

461, 484 451, 458 439, 452

40 45 59

2.95 2.99 3.02

−5.46/-2.51 −5.37/-2.38 −5.35/-2.33

484 503 537

293 338 371

Measured in toluene at a concentration of 10−6 M. Measured in toluene using 9,10-diphenylanthracene as the reference (ΦPL = 0.90 in cyclohexane). c Measured in films. d Measured in films using an integrating sphere. e Optical bandgap estimated from the absorption onset. f HOMO = -e(Eox + 4.8 V), LUMO = HOMO + Eg, where Eox is the onset value of the first oxidation. a

b

9,10-diphenylanthracene (ΦPL = 0.90 in cyclohexane) as the standard and excited at 350 nm. Thin films for spectroscopic measurements were prepared by spin-coating on quartz. The PLQYs in solid films were recorded by measuring the total light output in all directions in an integrating sphere (C9920-02, HAMAMATSU). CV measurements were obtained in anhydrous dichloromethane with 0.1 M n-Bu4NClO4 as the electrolyte on a CHI660a electrochemical analyzer under a scan rate of 100 mV s−1. A glass carbon electrode, an Ag/AgCl electrode and a Pt

CFR MS apparatus (COMPACT). Elemental analyses of carbon, hydrogen and nitrogen were performed on a Bio-Rad elemental analysis system. TGA and DSC were performed on Perkin Elmer-TGA 7 and Perkin Elmer-DSC 7 systems under nitrogen atmosphere at a heating rate of 10 °C min−1. The UV–Vis absorption and PL spectra were measured by a Perkin-Elmer Lambda 35 UV/vis spectrometer and a Perkin-Elmer LS 50B spectrofluorometer, respectively. The PLQYs of the fluorescent dendrimers in toluene solutions were measured with

Fig. 3. Geometrical structures of PVCt1 ∼ PVCt3 optimized by DFT calculations.

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Table 2 Nondoped device performance of PVCt1 ∼ PVCt3. Device

Vona (V)

Lmax (cd m−2)

ηc, max (cd A−1)

ηp, max (lm W1)

EQEb (%)

CIEc (x, y)

PVCt1

3.2

3842

1.43

1.11

0.82

PVCt2

3.6

6384

3.72

3.05

2.76

PVCt3

3.6

4729

2.89

2.37

2.81

(0.16, 0.21) (0.16, 0.15) (0.16, 0.10)

Turn-on voltage at a brightness of 1 cd m−2. Data at maximum. c CIE coordinates at 5 V. Lmax, ηc, max and ηp, max are the maximum luminance, current efficiency and power efficiency, respectively. a

b

Batron- P4083, Bayer AG) was spin-coated on the top of ITO at a speed of 5000 rpm for 60 s and then baked at 120 °C for 45 min. Then solutions of the dendrimers in o-dichlorobenzene (10 mg/mL) were filtered through a filter (0.45 μm) and spin-coated on PEDOT:PSS as the EML at a speed of 1500 rpm for 60 s. Under this concentration, the corresponding viscosity of PVCt1, PVCt2 and PVCt3 is 1.32, 1.10 and 0.75 mPa s, respectively. After annealed at 100 °C for 30 min inside a nitrogen-filled glove box and transferred to a vacuum evaporator, a 60 nm thick film of TPBI was thermally deposited onto the EML at a pressure of 4.0 × 10−4 Pa. Finally, 0.5 nm LiF and 100 nm Al were deposited as the cathode through a shadow mask with an array of 14 mm2 openings. The EL spectra and CIE coordinates were measured using a PR650 spectra colorimeter. The current−voltage and brightness−voltage curves were measured using a Keithley 2400/2000 source meter and a calibrated silicon photodiode. The EQE was calculated from the luminance, current density and EL spectra assuming a Lambertian distribution. All the measurements were carried out at room temperature under ambient conditions.

Fig. 4. CV curves for the dendrimers PVCt1 ∼ PVCt3.

wire were used as the working electrode, the reference electrode and the counter electrode, respectively. Molecular geometry optimizations were carried out by the Gaussian 03 program with DFTcalculations, in which the Becke's three parameter functional combined with Lee, Yang, and Parr's correlation functional (B3LYP) hybrid exchange-correlation functional with the 6-31G* basic set were used [30].

4.2. Device fabrication and measurement Patterned glass substrates coated with indium tin oxide (ITO) were cleaned with acetone, detergent, distilled water and then in an ultrasonic solvent bath. After baking in a heating chamber at 120 °C for 8 h, the ITOglass substrates were treated with O3 plasma for 30 min. Subsequently, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS,

Fig. 5. Solution-processed nondoped device performance for PVCt1 ∼ PVCt3: (a) device configuration; (b) EL spectra at 5 V; (c) current density–voltage–luminance characteristics; (d) current efficiency and EQE as a function of luminance. Inset: CIE chromaticity diagram.

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4.3. Synthesis

91333205).

All solvents for chemical synthesis were refined according to the standard procedures. Oligocarbazole dendrons including D1, D2 and D3 were synthesized according to the previously-reported literature [17,29]. MD1: A mixture of D1 (5.5 g, 20.0 mmol), 4-bromobenzaldehyde (5.6 g, 30.0 mmol), CuI(0.4 g, 2.0 mmol), (+/−)-trans-1,2-diaminocyclohexane (0.5 mL, 4 mmol) and K3PO4 (8.0 g, 40.0 mmol) in toluene (150 mL) was heated at 110 °C for 24 h under argon. After cooling to room temperature, the mixture was poured into water, and extracted with dichloromethane. The organic layer was carefully washed with water and dried with Na2SO4. The crude product was purified by chromatography on silica gel with hexane/dichloromethane (3:1) as the eluent to furnish a white powder MD1 (6.5 g, 85.0%). 1H NMR (400 MHz, CDCl3) δ 10.10 (s, 1H), 8.14 (d, J = 1.3 Hz, 2H), 8.13–8.07 (m, 2H), 7.82–7.76 (m, 2H), 7.51–7.46 (m, 2H), 7.47 (s, 2H), 1.52–1.43 (m, 18H). MD2: This compound was prepared in a yield of 74.0% according to the procedure for the synthesis of MD1 by using D2 instead of D1. 1H NMR (400 MHz, CDCl3) δ 10.16 (s, 1H), 8.33–8.20 (m, 4H), 8.18 (s, 4H), 7.96 (d, J = 8.3 Hz, 2H), 7.74 (d, J = 8.7 Hz, 2H), 7.65 (dd, J = 8.7, 1.9 Hz, 2H), 7.48 (dd, J = 8.6, 1.8 Hz, 4H), 7.35 (d, J = 8.6 Hz, 4H), 1.48 (s, 36H). MD3: This compound was prepared in a yield of 45.0% according to the procedure for the synthesis of MD1 by using D3 instead of D1. 1H NMR (400 MHz, CDCl3) δ 10.23 (s, 1H), 8.56 (s, 2H), 8.31 (d, J = 8.5 Hz, 2H), 8.27 (d, J = 1.5 Hz, 4H), 8.15 (d, J = 1.7 Hz, 8H), 8.04 (d, J = 8.4 Hz, 2H), 7.87 (s, 4H), 7.68–7.57 (m, 8H), 7.45 (dd, J = 8.7, 1.9 Hz, 8H), 7.34 (d, J = 8.6 Hz, 8H), 1.45 (s, 72H). PVCt1: A mixture of MD1 (2.7 g, 7.0 mmol) and 4,4′-bis(diethylphosphonomethyl)biphenyl (1.4 g, 3.0 mmol) were dissolved in dry tetrahydrofuran (70 mL) under argon and cooled to 0 °C. Then the tetrahydrofuran (30 mL) solution of t-BuOK (3.4 g, 30.0 mmol) was slowly added under vigorous stirring. Subsequently the mixture was warmed to room temperature and stirred for another 8 h. After reaction, 100 mL dichloromethane was added to the solution. The organic layer was washed with water and dried with Na2SO4. The crude product was purified by chromatography on silica gel with hexane/dichloromethane (6:1) as the eluent to furnish a white powder PVCt1 (3.8 g, 71.0%). 1H NMR (400 MHz, CDCl3) δ 8.15 (d, J = 1.5 Hz, 4H), 7.75 (d, J = 8.5 Hz, 4H), 7.67 (q, J = 8.6 Hz, 8H), 7.57 (d, J = 8.5 Hz, 4H), 7.48 (dd, J = 8.7, 1.9 Hz, 4H), 7.40 (d, J = 8.7 Hz, 4H), 7.26 (s, 4H), 1.48 (s, 36H). MS (MALDI-TOF) m/z: 912.5 [M+]. Anal. Calcd. for C68H68N2: C, 89.43; H, 7.50; N, 3.07. Found: C, 89.94; H, 7.28; N, 3.03. PVCt2: This compound was prepared in a yield of 67.0% according to the procedure for the synthesis of PVCt1 by using MD2 instead of MD1. 1H NMR (400 MHz, CDCl3) δ 8.28 (s, 4H), 8.20 (d, J = 1.9 Hz, 8H), 7.89 (d, J = 8.5 Hz, 4H), 7.76 (d, J = 16.8 Hz, 4H), 7.71 (s, 10H), 7.65 (dd, J = 8.7, 1.9 Hz, 5H), 7.50 (dd, J = 8.6, 1.9 Hz, 8H), 7.38 (d, J = 8.6 Hz, 4H), 7.35 (s, 6H), 1.50 (s, 72H). MS (MALDI-TOF) m/z: 1798.0 [M+ + 1]. Anal. Calcd. for C132H128N6: C, 88.15; H, 7.17; N, 4.67. Found: C, 87.94; H, 7.28; N, 4.43. PVCt3: This compound was prepared in a yield of 67.0% according to the procedure for the synthesis of PVCt1 by using MD3 instead of MD1. 1H NMR (400 MHz, CDCl3) δ 8.55 (s,4H), 8.27 (d, J = 1.3 Hz, 8H), 8.16 (d, J = 1.5 Hz, 16H), 7.94 (d, J = 8.4 Hz, 4H), 7.87–7.80 (m, 12H), 7.73 (s, 8H), 7.62 (dt, J = 8.8, 5.2 Hz, 16H), 7.45 (dd, J = 8.7, 1.7 Hz, 16H), 7.36 (m, 20H), 1.46 (s, J = 4.7 Hz, 144H). MS (MALDITOF) m/z: 3566.0 [M+]. Anal. Calcd. for C260H248N14: C, 87.53; H, 6.95; N, 5.52. Found: C, 87.94; H, 7.18; N, 4.88.

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Acknowledgements This work is supported by the 973 Project (2015CB655001) and the Natural Science Foundation of China (No. 21604083, 51573183 and 48

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