“ Simple ” Aggregation-Induced Emission Luminogens for Nondoped Solution-Processed Organic Light-Emitting Diodes with Emission Close to Pure Red in the Standard Red, Green, and Blue Gamut

Development of simple and ef ﬁ cient red emissive luminogens is desirable yet challenging for optoelectronic devices due to the limited molecular design and the dif ﬁ culties of synthesis. Red emitting molecules possess large π -conjugated systems, which permit quenching in the solid state due to π – π stacking and are detrimental to the performance of devices. Furthermore, traditional red emitters usually exhibit emission far from pure red in the standard red, green, and blue (sRGB) gamut. Herein, two red luminogens, DCMa and DCIs, with aggregation-induced emission (AIE) characteristics based on simple donor – acceptor (D – A) structures are explored. They show high ﬂ uorescence quantum yields (QYs) of 13.2% and 7.8% in the ﬁ lm state. Ef ﬁ cient nondoped solution-processed organic light emitting diodes (OLEDs) with a con ﬁ guration of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/poly[(9,9-dioctyl ﬂ uorenyl-2,7-diyl)-co-(4,40-(N-(4-sec-butylphenyl)diphenylamine)] 2 plasma cleaning for 30 min. PEDOT:PSS (Al 4083) was ﬁ ltered by a 0.45 μ m polyvinylidene ﬂ uoride ﬁ lter and spin coated at 3500 rpm for 40 s onto the ITO substrate and then annealed at 150 (cid:4) C for 20 min in ambient conditions to yield a 40 nm thick thin ﬁ lm. After the annealing of PEDOT:PSS, the following processes were all performed under nitrogen atmosphere. A solution of TFB in anhydrous toluene with a concentration of 12 mg mL (cid:3) 1 was spin coated at 3000 rpm for 40 s onto PEDOT:PSS layer and then annealed at 200 (cid:4) C for 30 min to form a 20 nm thick layer. Emissive layer of DCMa and DCIs with a concentration of 10 mmol mL (cid:3) 1 in anhydrous THF was spin coated at 3000 rpm for 40 s onto TFB layer, followed by annealing at 100 (cid:4) C for 20 min to form an emis- sive layer of around 80 and 90 nm, respectively. Next, TPBi (40 nm), LiF (1 nm), and Al (100 nm) were deposited successively onto the sample through a shadow mask by thermal evaporation under a high vacuum of 1.5 (cid:1) 10 (cid:3) 6 Torr. After assembly of electrical contacts, OLEDs with a device area of 6 mm 2 were ready for characterization. All measurements were conducted in ambient air. The active device area of each device was 2.0 (cid:1) 3.0 mm 2 . A Keithley 2401 SMU instrument was used as a power source for the OLEDs. All the OLEDs were characterized using a QE Pro Ocean Optics spectrometer connected with a 400 μ m optical ﬁ ber (QP400-2-SR-BX). The optical ﬁ ber was carefully calibrated with an Ocean Optics HL-3 plus vis – NIR light source.


Introduction
Organic light-emitting diodes (OLEDs) have emerged as one of the most competitive types of optoelectronic devices for nextgeneration flexible displays and solid-state lighting. [1,2] To achieve full-color devices, light-emitting materials with the three primary colors of blue, green, and red, are needed. For OLED devices, the design of red emitters still has challenges on various fronts, such as the limited options of emissive materials, complicated processing technologies, stagnant device performance, and reduced color fidelity.
To date, most of the reported red emitters are based on lanthanide or transitionmetal complexes. [3][4][5][6] Although these molecules have been at the forefront of many achievements in OLEDs, with their long wavelengths and even near-IR emission, the application of transition metalligand complexes suffers from several problems, such as high cost, and limited options for transition-metal cores and, thus, a dearth of candidates for improvement. Furthermore, the electroluminescence (EL) of transition-metal complexes-based OLEDs is phosphorescent in nature, where the long-lived triplet-state excitons are unstable and are easily quenched by different external factors, such as air, moisture, and impurities. These sensitivities also restrict their mass production and inhibit further commercialization. Metal-free organic fluorescent materials, on the other hand, possess relatively higher stability and structural variation, both of which could theoretically provide more possibilities for materials development and performance improvement. [7,8] Most of the organic emitters prepared for OLEDs devices so far exhibit emission at shorter wavelengths (630 nm) and are, thus, not ready for use in commercial applications. Conventional red luminogens can be constructed by extending the π-conjugation with planar macrocyclic units. However, such an approach involves some difficult synthesis steps and demanding device fabrication technology due to the poor processability of these molecules. Introduction of strong intramolecular donor-acceptor (D-A) interaction is another commonly used strategy for narrowing the energy gap to impart red emission without the need of synthesizing complex molecular structures by harsh organic reactions. [9] Similar to transition-metal complexes, organic D-A molecules cannot be easily altered due to a restricted choice of electron acceptors. Therefore, it is highly desirable to explore a simple strategy for easy synthesis of efficient D-A emitters for OLED applications.
Another problem associated with conventional organic red emitters is that their strong emission in solution dramatically decreases and can be quenched in concentrated solution or in the solid state. That is due to strong intramolecular charge transfer (ICT) or serious intermolecular π-π stacking, which causes excitons to decay through the nonradiative relaxation channel. Such an aggregation-caused quenching (ACQ) effect substantially limits their roles and the practical applications as solid-state emitters. To solve this intractable problem in EL devices, several strategies are proposed, such as the incorporation of steric hindered moieties or doping with host matrix to hamper the intermolecular interaction; however, new troubles emerge, such as inefficient charge transportation, unavoidable performance degradation due to phase separation upon thermal process, complicated device fabrication technology due to the uncertain ratio control of dopant addition, and high cost of mass production. [10][11][12] In terms of device fabrication, it is well established that the solution processing is the most advantageous fabrication method for OLEDs compared with the evaporative deposition. It shows the merits of low cost, dosage saving, simple device fabrication, and large-area applications. Organic conjugated polymers are once reported to be overwhelming candidates to possess good solution processability and are demonstrated as a cost-effective approach for mass production. [2,12] However, their relatively low quantum yield (QY) and exciton annihilation resulting from kinks in the molecular backbone and strong intermolecular interaction are both problems that have yet to be solved. Hence, how to overcome the dilemma between the fabrication of simple nondoped OLEDs and the prevention of ACQ effect of traditional red-emissive materials during solution processing has become a tough question to be solved.
In 2001, Tang and co-workers discovered a group of propellerlike molecules containing rotor structures, such as silole and tetraphenylethene, which are nonemissive in a dilute solution state but show intense emission upon aggregates formation. [13,14] This interesting phenomenon was coined as aggregation-induced emission (AIE). After many studies, the restriction of intermolecular motion has been proposed as the most commonly accepted interpretation for the AIE effect. Since then, many research groups have been devoted to developing the AIE materials and proposing various mechanisms. [15][16][17] Because AIE luminogens (AIEgens) exhibit enhanced emission in the solid state, they are regarded as promising and widespread materials for fabricating simple nondoped OLEDs. Recently, more and more red AIEgens have been reported through unremitting efforts, and some of them have been applied in OLEDs and other display systems (Scheme 1). [18][19][20][21][22][23][24][25] Unfortunately, although these molecules show long-wavelength EL and high device performance, they need to be prepared via multistep reaction routes. Furthermore, the design strategies of red-emissive AIEgens are mostly based on the large D-π-A-π-D molecular structures, in which different kinds of electron donors with typical AIE units (e.g., tetraphenylethene) are incorporated to guarantee the performance in solid phase. However, a restrict choice of electron acceptor cores (mainly are benzothiadiazole [BTD]) for long emission wavelength limits the structural variation and further development of new emitting materials for OLEDs. Therefore, it is highly desirable to explore a new and more efficient molecular system for producing small and "simple" molecules with red emission and AIE characteristics for nondoped, solutionprocessed OLEDs.
In this work, the photophysical properties of two facilely synthesized red emissive AIEgens (DCMa and DCIs) with simple structures and their promising application in nondoped solution-processed OLEDs were explored. DCMa and DCIs are AIE-active and emit a bright red emission at 644 and 700 nm in the film state with the high fluorescence QYs of 13.2% and 7.8%, respectively. Nondoped OLEDs using these luminogens as emitters show the EL maximum at 652 and 711 nm with International Commission on Illumination (CIE) coordinates of (0.63, 0.36) and (0.64, 0.35), respectively, both of which are close to the value of primary red color (0.63, 0.34) according to the digital television standard red, green, and blue (sRGB) that is the most widely used color gamut standards for television monitors, the internet, digital photography, and so on. [26] Compared with most of the previously reported red AIE emitters, DCMa and DCIs are constructed with a simple D-π-A structure using a diphenylamine group and an electron-rich carbazolyl ring as electron donors and malononitrile or 2-(3,5,5-trimethylcyclohex-2-enylidene)-malononitrile as the electron acceptor, respectively. The associated synthetic routes are much easier and show great potential for mass production. The successful application of small molecule DCMa and DCIs for nondoped solution-processed OLEDs will point to a method for the development of long-wavelength emitters for OLEDs.

Photophysical Property
DCMa and DCIs were prepared according to previously published procedures (Figure 1a). [27] Both compounds enjoy good www.advancedsciencenews.com www.adpr-journal.com solubility in common organic solvents, such as dichloromethane, tetrahydrofuran (THF), acetone, and dimethylformamide (DMF), which makes them suitable candidates for producing solutionprocessed OLEDs. The absorption spectra of DCMa and DCIs in DMF show peaks at 468 and 484 nm with the high molar absorptivities of 3.06 Â 10 4 and 3.43 Â 10 4 , respectively ( Figure S1, Supporting Information). The AIE characteristics of the molecules were further confirmed by studying their emission behaviors in DMF/H 2 O mixtures where water is the poor solvent. Taking DCMa as an example, gradual addition of water (≤40%) into its dilute solution had weakened its photoluminescence (PL) intensity, as shown in Figure 1b. That is because DCMa possesses an electron donor and acceptor in its structure, the enhancement of the solvent polarity upon water addition may strengthen the twisted ICT (TICT) effect, resulting in emission annihilation. [9,28] Their PL intensities were gradually enhanced when water fraction increased over 50%. In this case, the aggregates of DCMa were formed due to its poor water solubility.
Because of the restriction of intramolecular motions (RIM) mechanism, intramolecular motions of DCMa are restricted at aggregate states, which decrease the loss of energy through the nonradiative relaxation pathway and bring back the strong fluorescence. Similar to DCMa, the DCIs displayed obvious AIE property but with a more obviously increasing α AIE (α AIE ¼ I/I 0 , I ¼ emission intensity in a DMF/water mixture and I 0 ¼ emission intensity in DMF solvent) value of more than 100 times, confirming that they show properties consistent with AIE ( Figure 1c and Figure S2, Supporting Information). www.advancedsciencenews.com www.adpr-journal.com Then, the PL spectra of the molecules in the crystalline and film states were recorded, as shown in Figure 1d. In the crystalline state, a red emission at 652 and 660 nm with a high QY (Φ F ) of 10.0% and 8.8% was observed for DCMa and DCIs, respectively. In the film state, DCMa emitted at 644 nm with a Φ F of 13.2%, whereas DCIs emitted at 700 nm with a good Φ F of 7.8%. Compared with the emission peak of AIEgens in the crystalline state, that of DCMa showed a hypochromatic shift in the film state, whereas a bathochromic shift was observed for DCIs ( Table 1). That could be attributed to their different molecular conformation and packing mode and their effect on the two states. The fluorescence decays of DCMa and DCIs in the crystalline crystals and thin films were also studied, revealing the lifetime values of 0.89-4.58 ns for two compounds (Table 1 and Figure S3, Supporting Information). The good photophysical properties of two AIEgens in crystals and film states suggested that further investigation of their potential in OLEDs was warranted. It was hoped here that they may solve the tradeoff between high-performance solid-state devices and simplified (e.g., undoped) thin-film design.

Theoretical Calculation
To gain deeper insight into the photophysical properties of the molecules, density functional theory (DFT) calculations were carried out at a PBE0/6-311G(d) level with the solvent model called the polarizable continuum model (PCM) (solvent ¼ acetonitrile) using a suite of Gaussian 09 programs (Figure 2). The DFT calculations were performed based on the crystal structures of DCMa and DCIs for obtaining the reliable results accord with the practice. As illustrated, the crystal structures adopt nonplanar conformations, as they are constructed from the twisted diphenylamino unit. Such molecular structures should effectively avoid compact intermolecular π-π packing in the packed state, thus endowing DCMa and DCIs with high Φ F in the crystalline and film states. [29] The molecular orbital density in the highest  occupied molecular orbitals (HOMO) is mainly distributed on the donor fraction diphenylamino group and the central carbazole ring, whereas it is primarily distributed on the acceptor part for the lowest unoccupied molecular orbitals (LUMO). Such an orbital distribution suggests strong ICT characteristics of the luminogens. The calculated HOMO and LUMO energy levels are À5.57 and À2.81 eV for DCMa and À5.43 and À2.93 eV for DCIs, respectively. In addition, their electrochemical properties were further investigated by cyclic voltammetry (CV). As shown in Figure 2b,c, HOMO energy levels obtained from the CV diagrams are À5.31 and À5.19 eV for DCMa and DCIs, respectively. Such values are close to that of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4 0 -(N-(4-sec-butylphenyl)diphenylamine)] (TFB), which is indicative of their good hole-transporting properties. The energy gaps (E g ) of DCMa and DCIs estimated from the onset wavelengths of their UV spectra were obtained to be 2.29 and 2.17 eV, respectively. Their LUMO can be obtained using the equation: E LUMO ¼ E HOMO þ E g that are equal to À3.02 eV. The low LUMO energy level of the molecules suggests that they possess the narrow bandgap, which is consistent with their red and NIR emission.

Device Fabrication
Prior to device fabrication, the thermal properties of DCMa and DCIs were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen at a heating rate of 10 C min À1 . As shown in Figure S4 and S5 and Table S1 (Figure 3a and Figure S6 and S7, Supporting Information). In these devices, the neat films of DCMa or DCIs serve as an EML, whereas PEDOT:PSS functions as a hole-injection layer, TFB works as a hole-transport layer (HTL), and TPBi works as an electron-transport layer (ETL) as well as the hole-blocking layer due to their much lower HOMO values (with the large energy barrier exists between the EML and ETL layers) than that of DCMa and DCIs. The performances of the devices are shown in Figure 3 and Figure S8 and S9, Supporting Information. Two devices were turned on at a bias of 5.0 and 6.6 V, emitting a deep red and NIR EL at 652 and 711 nm, respectively, with good stabilities under varying voltage, as shown in Figure S6, Supporting Information. The maximum external quantum efficiency (EQE) values were calculated to be 1.95% and 0.58% for devices incorporating DCMa and DCIs, respectively (Table S2, Supporting Information). There is a difference of performance between DCMa and DCIs even they used the same device configuration. That is because 2-(3,5,5trimethylcyclohex-2-enylidene)-malononitrile was incorporated in DCIs as an electron acceptor, which could enhance the ICT effect and extend the conjugation of DCIs compared with DCMa. Therefore, DCIs showed a more obvious bathochromic shift in EL emission wavelength and lower QY, which may result in a lower EQE compared with that of DCMa. The CIE coordinates of the EL emission are (0.63, 0.36) and (0.64, 0.35), respectively, which are very close to the primary red color of (0.63, 0.34) (sRGB/Rec. 601/Rec. 709) for digital standard definition television. Their performance in expressing the red primary chromaticity makes them the suitable candidates for commercial digital displays and among most of the previously reported molecular www.advancedsciencenews.com www.adpr-journal.com red emitters ( Figure 4 and Table S3, Supporting Information). [19,22,23,25,[30][31][32][33][34][35][36][37][38][39][40] The high performance of the devices thanks to not only the excellent photophysical properties of DCMa and DCIs in film states but also their good holetransporting as well as electron-transporting properties.

Conclusion
In summary, two red and NIR AIEgens DCMa and DCIs with simple D-π-A structures were found to be efficient nondoped red emitters for solution-processed OLEDs. DCMa and DCIs exhibit a high QY of 13.2% and 7.8% in the thin-film states as well as good thermal stability in the solid crystalline states. Simple and efficient nondoped OLEDs were fabricated and exhibited a long EL wavelength at 625 and 711 nm, respectively. They display good device performance with a maximum EQE of up to 1.95%. In addition, devices using these materials show red emission with CIE coordinates of (0.63, 0.36) and (0.64, 0.35), both of which are close to primary red color (0.63, 0.34) according to the digital television standard. The successful application of small molecules DCMa and DCIs for nondoped solutionprocessed OLEDs devices points to a new strategy for the development of long-wavelength OLEDs emitters. Next generation of AIEgens with a simple D-A structure and good film-   Table S3, Supporting Information, and references for previously reported device data).
www.advancedsciencenews.com www.adpr-journal.com forming properties are thereby expected to be explored for future OLED devices with better performance.

Experimental Section
Materials and Characterization: Solvents were all purchased from J&K, Sigma, Meryer, and AIEgen Biotech and used as received without further purification. Toluene was distilled using sodium as a drying agent and benzophenone as an indicator under nitrogen prior to use. PEDOT:PSS (Al 4083) was purchased from Ossila. TFB and TPBi were purchased from Luminescence Technology Corp. Lithium fluoride (LiF, ≥99.99%) was purchased from Sigma-Aldrich.
UV-vis spectra were measured on a Milton Roy Spectronic 3000 Array spectrophotometer. Steady-state PL spectra were measured on a PerkinElmer LS 55 spectrofluorometer. QY was determined by a Quanta-φ integrating sphere. Fluorescence lifetimes were measured with Hamamatsu Quantaurus-Tau C11367-11 at room temperature. Electrochemical measurements were recorded on a CHI610D electrochemical workstation in a three-electrode cell using a platinum button as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode as the reference electrode in acetonitrile (CH 3 CN) with 0.1 M Bu 4 NPF 6 at a scan rate of 100 mV s À1 and ferrocene as the internal standard. TGA measurements were carried out on a TA Q5000 instrument under a dry nitrogen flow at a heating rate of 10 C min À1 . Optimized geometric structures in the gas phase of DCMa and DCIs were calculated at the PBE0/6-311G(d) level with solvent model of PCM (solvent ¼ acetonitrile) using Gaussian 09 program. Scanning electron microscopy (SEM) images were collected on a fieldemission scanning electron microscope (FESEM; JEOL-7100F).
Device Fabrication: OLEDs were fabricated on glass substrates coated with patterned ITO. The ITO substrates were ultrasonically cleaned with 2% micro-90, deionized water, acetone, and isopropanol for 20 min each, followed by O 2 plasma cleaning for 30 min. PEDOT:PSS (Al 4083) was filtered by a 0.45 μm polyvinylidene fluoride filter and spin coated at 3500 rpm for 40 s onto the ITO substrate and then annealed at 150 C for 20 min in ambient conditions to yield a 40 nm thick thin film. After the annealing of PEDOT:PSS, the following processes were all performed under nitrogen atmosphere. A solution of TFB in anhydrous toluene with a concentration of 12 mg mL À1 was spin coated at 3000 rpm for 40 s onto PEDOT:PSS layer and then annealed at 200 C for 30 min to form a 20 nm thick layer. Emissive layer of DCMa and DCIs with a concentration of 10 mmol mL À1 in anhydrous THF was spin coated at 3000 rpm for 40 s onto TFB layer, followed by annealing at 100 C for 20 min to form an emissive layer of around 80 and 90 nm, respectively. Next, TPBi (40 nm), LiF (1 nm), and Al (100 nm) were deposited successively onto the sample through a shadow mask by thermal evaporation under a high vacuum of 1.5 Â 10 À6 Torr. After assembly of electrical contacts, OLEDs with a device area of 6 mm 2 were ready for characterization. All measurements were conducted in ambient air. The active device area of each device was 2.0 Â 3.0 mm 2 . A Keithley 2401 SMU instrument was used as a power source for the OLEDs. All the OLEDs were characterized using a QE Pro Ocean Optics spectrometer connected with a 400 μm optical fiber (QP400-2-SR-BX). The optical fiber was carefully calibrated with an Ocean Optics HL-3 plus vis-NIR light source.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.