High Brightness Circularly Polarized Organic Light-Emitting Diodes

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Letter Cite This: Org. Lett. 2019, 21, 439−443

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High Brightness Circularly Polarized Organic Light-Emitting Diodes Based on Nondoped Aggregation-Induced Emission (AIE)-Active Chiral Binaphthyl Emitters Xueyan Zhang,† Yu Zhang,† Haiping Zhang,† Yiwu Quan,*,† Yunzhi Li,‡ Yixiang Cheng,*,‡ and Shanghui Ye*,§

Org. Lett. 2019.21:439-443. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/18/19. For personal use only.

†

Key Laboratory of High Performance Polymer Material and Technology of Ministry of Education, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡ Key Lab of Mesoscopic Chemistry of MOE and Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China § Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, China S Supporting Information *

ABSTRACT: A pair of chiral binaphthyl enantiomers (S-/R-6) incorporating a tetraphenylethene (TPE) moiety as an aggregation-induced emission (AIE) active group exhibits bright yellow circularly polarized electroluminescence (CP-EL) emission with a remarkable gEL value, low turn-on voltage, and high brightness in the nondoped CP organic light emitting diodes (CP-OLEDs). This work provides a new strategy to develop doping-free CP-OLED materials.

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usually suffer from an aggregation-caused quenching (ACQ) problem, leading to the poor CPL behavior in the condensed phase.7 Since 2001, Tang’s group found some organic chromophores exhibited strong fluorescence emission in aggregated states, but nonemissive in dilute solutions, which was called an aggregation-induced emission (AIE) effect.8 In the past decade, a number of AIE-active molecules, especially tetraphenylethene (TPE) as a typical example, have been widely applied for various fields.9 Zheng’s group first reported the separation of M- and P- enantiomers of the fixed propellerlike TPE derivative and found a large luminescence dissymmetry factor (glum) value, which depends on the helicity of the enantiomer.10 Tang’s group developed some remarkable AIE-active CPL small emitters based on TPE groups and chiral amino acid derivative units.11 Over the past several years, our group designed and synthesized a series of CPL materials containing chiral 1,1′-binaphthol (BINOL) and TPE groups.12 Hence, incorporating AIE-active dyes into chiral moieties can be regarded as an efficient approach to construct CPL emission materials. However, the CP-OLED performances of these AIEactive CPL materials as mentioned above were not studied.

t is well-known that organic light-emitting diodes (OLEDs) have been becoming very popular display techniques due to their unique advantages on energy-saving, low driving voltage, high brightness, and high contrast display for full-color emission.1 For the past few years, circularly polarized luminescence (CPL) showed potential applications in 3D displays, optical storage devices as well as sensors, and liquidcrystal optical technology.2 Currently, increasing attention has been focused toward circularly polarized OLEDs (CP-OLEDs) based on chiral organic chromophores. In 2013, Fuchter and co-workers designed and fabricated a kind of OLED by introducing chiral 1-aza[6]helicene into the achiral poly[9,9dioctylfluorene-co-benzothiadiazole] (F8BT) fluorescence polymers, and the electroluminescence dissymmetry factor (gEL) value reached as high as 0.2.3 Friend’s group reported two single-layer donor−acceptor (D−A) typed chiral polyfluorene OLEDs by using a thermal annealing polymer to self-assemble as a multidomain cholesteric film, giving rise to a high gEL value up to −0.8.4 However, there have been very few report on CPOLEDs by using chiral organic fluorescence dyes as emitters except chiral lanthanide(III) complexes,5 transition metal Ir(III) and Pt(II) complexes.6 To date, a variety of chiral organic fluorescence dyes have been designed for CPL materials, but most of these dyes © 2018 American Chemical Society

Received: November 13, 2018 Published: December 27, 2018 439

DOI: 10.1021/acs.orglett.8b03620 Org. Lett. 2019, 21, 439−443

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Organic Letters Therefore, developing CP-EL devices based on chiral pure organic small emitters is of great significance. To date, there have been very few reports on chiral AIEactive dyes for CP-OLEDs.13 In 2018, Chen and his coworkers first reported CP-OLEDs based on a couple of chiral aromatic imide enantiomers with thermally activated delayed fluorescence (TADF) properties, which exhibited a high external quantum efficiency (EQE, 19.8%) and gEL value (−1.7 × 10−3 for the (+)-(S,S)-enantiomer and +2.3 × 10−3 for the (−)-(R,R)-enantiomer).13a Tang’s group also reported several AIE-active small fluorophores linked with the BINOL group as excellent CPL and TADF emitters for CP-OLEDs.13b In our previous work, we found BINOL with a high CD absorption dissymmetry factor (gCD) value can be used as prominent axial chiral molecules for designing outstanding CPL materials.14 Herein, we synthesized a pair of AIE-active CPL dyes by incorporating TPE groups into the chiroptical BINOL moieties. Moreover, we further studied the CP-OLEDs performances of these chiral AIE-active binaphthyl enantiomers. Interestingly, bright yellow CP-EL emission was clearly observed with low turn-on voltage in the nondoped CPOLED devices. The synthesis procedures of S-/R-6 are outlined in Scheme 1. TPE derivative 4 was obtained from benzophenone and 4,4′Scheme 1. Synthesis Procedures of Enantiomers S-/R-6

Figure 1. (a) UV−vis absorption of S-6 in THF, aggregated, and spincoated film; fluorescence emission spectra in aggregated and spincoated film; (b) fluorescence emission spectra of S-6 in THF−H2O mixtures with different water fractions (f w) at a fixed concentration (1 × 10−5 M, λex = 365 nm). Inset curve: plot of (I/I0) values versus the compositions of the aqueous mixtures; inset color images: photograph of S-6 in THF−H2O mixed solvents under UV illumination (365 nm).

nm of the long wavelength region can be regarded as the extended conjugated structure between binaphthyl and AIEactive TPE units. Meanwhile we also observed similar absorption behaviors in both the aggregate state and spincoated film. As is evident from Figure 1b, almost no fluorescence emission of S-6 was detected in pure THF solution. Upon addition of poor solvent water, almost no change was observed until the water fraction (f w) reached 80%. With the further increase of the water fraction up to 99%, bright yellow fluorescence emission appears at 532 nm (insert color image of Figure 1b) with 262-fold enhancement and an absolute fluorescence quantum yield (φF) = 32.1% (fluorescence lifetime: τ = 1.29 ns) (inset of Figure 1b), which demonstrates that S-6 can act as an excellent AIE-active emitter in the aggregated state. Meanwhile, we also measured the fluorescent spectra (at 532 nm) and the absolute fluorescence quantum yield (φF = 39.8%, τ = 1.37 ns) in the spin-coated film of S-6 (Figure 1a and Figure S2), which is very similar to S-6 in the aggregated state. The cyclic voltammetry (CV) of S-6 was performed in CH2Cl2 solution (1 × 10−5 M). The detailed measurement conditions and results were collected in Table S1 and Figure S3. The HOMO energy level is calculated to be −4.88 eV according to the onset oxidation potential at 0.18 V, and the energy level gap (Eg) is estimated to be 2.71 eV using the onset

bis(diethylamino)benzophenone by McMurry and Miyaura reactions in sequence.15 AIE-active CPL emitters S-/R-6 were synthesized by Pd-catalyzed Suzuki coupling reaction of enantiomers (S- or R-)-2,2′-dibutoxy-3,3′-diiodo-1,1′-binaphthalene (S-/R-5) with compound 4 in about 53% yield. The detailed procedures and characterizations are described in the Supporting Information (sections 2−9). The resultant compounds S-/R-6 show good solubility in general organic solvents. As shown in the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of Figure S1, the degradation temperature (Td) and the glass-transition temperature (Tg) of S-6 appear at 405 and 126 °C, respectively, demonstrating the excellent thermal and morphological stability. The high thermal properties are beneficial to the preparation of stable and long lifespan CP-OLEDs. The UV−vis absorption and emission spectra of S-6 were measured in THF solution at a fixed concentration (1 × 10−5 M). Three main absorption bands appear at 285, 329, and 387 nm in Figure 1a. The high energy absorption at 285 and 329 nm can be assigned to the π−π* electronic transitions of the binaphthyl moiety, and the low energy absorption band at 387 440

DOI: 10.1021/acs.orglett.8b03620 Org. Lett. 2019, 21, 439−443

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Organic Letters absorption wavelength of S-6. The LUMO energy level of S-6 is calculated to be −2.17 eV according to the reported reference.16 It is important to note that the CV curves of S-6 remain unchanged after multiple successive potential scans, which further confirms that chiral AIE-active binaphthyl emitters also exhibit excellent electrochemical stability.17 To gain deep insight into the optical behaviors of S-6, its geometrical and electronic properties were optimized at the TD-B3LYP/6-311+G** level basis set using the Gaussian 09 program package. The calculated HOMO energy level, LUMO energy level, and energy gap are −4.78, −1.45, and 3.33 eV, respectively (Table S1). Figure 2 shows the most optimized

Figure 2. Optimal structure for HOMO and LUMO distributions of S-6.

structures and almost complete separation for the HOMO and LUMO energy orbital distributions of S-6. The electron density of the HOMO is localized on the AIE-active TPE units, while the electron density of the LUMO is mainly located on the chiral binaphthyl core. The clear separation between the HOMO and LUMO levels is benefial for efficient carrier injection and transport.18 In order to investigate the chiroptical properties of the enantiomers S-/R-6, we measured CD absorption spectra of S-/R-6. As shown in Figure 3a, it is clearly observed that S-/R6 both exhibit good mirror-image CD bands both in the aggregation state and in pure THF solution. The Cotton effects situated at 263 and 287 nm can be assigned to the feature absorption of the binaphthyl unit.19 The long wavelength CD signals (gCD = ± 3 × 10−4) at 400 nm can be regarded as the extended π−π conjugated structure between binaphthyl and TPE units, which indicates the effective chirality transfer from the chiral binaphthyl unit to the AIEactive TPE chromophore.20 Compared with the corresponding CD absorption spectra in THF solution, the aggregationinduced CD (AI-CD) spectra of S-/R-6 in both aggregation and spin-coated film showed much stronger signals (gCD = ± 7.8 × 10−4) in the longest wavelength region and about 12−16 nm red shifts, which demonstrates that AIE-active chiral binaphthyl dyes can maintain strong chirality in the aggregated state.21 Meanwhile, the AI-CD signals of S-/R-6 in the aggregation state are very similar to those in the spin-coated film. Herein, we further investigated the CPL property of S-/R6 enantiomers. Almost no CPL emission of S-/R-6 were detected at f w < 95%. Interestingly, the aggregation-induced CPL (AI-CPL) signals with the clear mirror image were observed at 532 nm until f w = 99% (Figure S4), and the glum values correspond to +2.8 × 10−3 for S-6 and −2.7 × 10−3 for R-6, respectively. As is shown in Figure 3b, S-/R-6 in the spincoated films also emit obvious mirror-image AI-CPL signals

Figure 3. (a) CD spectra of S-/R-6 in spin-coated film and mixed solutions (f w = 0% and f w = 99%); (b) CPL spectra of S-/R-6 in spincoated film.

with +3.6 × 10−3 for S-6 and −3.2 × 10−3 for R-6 at around 532 nm. In this paper we further chose S-/R-6 as CP-OLEDs emitters and designed nondoped OLEDs with the configuration of ITO/PEDOT:PSS (25 nm)/(S-6 or R-6 (35 nm))/ TPBi (35 nm)/Ca (10 nm)/Ag (100 nm). The current density−voltage−luminance curves (J−V−L) and current efficiency−luminance curves (CE−L) of device A and B are depicted in Figure 4. The electroluminescence spectra of doping-free devices based on S-6 for device A and R-6 for device B are presented in Figure S5; bright yellow fluorescence emissions are detected at around 534 nm with CIE coordinates (0.33, 0.52) and (0.31, 0.52), respectively. It should be noted that almost no change was found for the spectral profiles and emission wavelengths for S-6 and R-6 by varying the driving voltages from 4 to 8 V, demonstrating the highly stable electroluminescence performance. The electroluminescence characteristic data for device A and B are collected in Table S2. The turn-on voltage (Von), maximum luminance (Lmax), maximum external quantum efficiency, and maximum current efficiency (CEmax) of device A vs device B are 3.18 V, 8061 cd m−2 (at 8.56 V), 0.48%, and 1.32 cd A−1 (at 4.56 V), vs 3.24 V, 7946 cd m−2 (at 8.45 V), 0.45%, and 1.26 cd A−1 (at 4.7 V), respectively. Both of these two devices A and B exhibit high brightness that is bright enough for practical applications in solid-state displays (generally 100−1000 cd m−2). As is evident from the current efficiency−luminance curves of Figure 4b, both devices A and B show very stable efficiency CE > 1 cd A−1 at the brightness range from 312 to 1000 cd m−2. Furthermore, 441

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yiwu Quan: 0000-0001-6017-1029 Yixiang Cheng: 0000-0001-6992-4437 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21474048, 21674046, 51673093, and 61106017).

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Figure 4. (a) Current density−voltage−luminance curves and (b) current efficiency−luminance curves of CP-OLEDs based on S-6 and R-6. Inset curve: CP-EL spectra vs fluorescence luminescence of devices based on S-/R-6.

we also measured the CP-EL performances of the nondoped devices A and B. The CP-OLEDs A and B appear as stable mirror-image CP-EL emission at about 534 nm (Figure 4b and Figure S6), which well coincide with the PL spectra in the aggregated state and spin-coated film. The calculated gEL values are +3.2 × 10−3 for device A (S-6) and −3.0 × 10−3 for device B (R-6) very similar to AI-CPL signals of S-/R-6 in the aggregation state and in spin-coated film. In conclusion, a pair of AIE-active CPL materials were synthesized, which exhibit excellent chiroptical properties including mirror-image CD and CPL signals in aggregation states and in spin-coated films. High brightness yellow CP-EL performances were achieved from the nondoped OLEDs by using AIE-active chiral binaphthyl as emitters. Our work provides a new strategy to develop promising CP-EL materials based on AIE-active chiral fluorescence dyes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03620. Experimental details, spectra of compounds, and additional results (PDF) 442

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