Unconventional Three-Armed Luminogens ... - ACS Publications

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Unconventional Three-armed Luminogens Exhibiting Both Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence Resulting in High-Performing Solution-Processed OLEDs Seo Yeon Park, Suna Choi, Gi Eun Park, Hyung Jong Kim, Chiho Lee, Ji Su Moon, Si Woo Kim, Sungnam Park, Jang Hyuk Kwon, Min Ju Cho, and Dong Hoon Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19681 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Applied Materials & Interfaces

Unconventional Three-armed Luminogens Exhibiting Both Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence Resulting in High-Performing SolutionProcessed OLEDs Seo Yeon Park,† Suna Choi,† Gi Eun Park,† Hyung Jong Kim,† Chiho Lee, † Ji Su Moon,‡ Si Woo Kim, ‡ Sungnam Park,† Jang Hyuk Kwon, ‡ Min Ju Cho*,† and Dong Hoon Choi*,†

†

Dept. of Chemistry, Research Institute for Natural Sciences, Korea University, 5 Anam-

dong, Sungbuk-gu, Seoul 136-701, Korea ‡

Dept. of Information Display Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu,

Seoul, 02447, Korea.

KEYWORDS: Three-armed luminogen, aggregation-induced emission, solution-process, thermally activated delayed fluorescence, organic light-emitting diodes *Corresponding authors: [email protected], [email protected]

ABSTRACT In this work, three-armed luminogens IAcTr-out and IAcTr-in were synthesized and used as emitters bearing triazine and indenoacridine moieties in thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). These 1

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molecules could form a uniform thin film via the solution process, and also allowed the subsequent deposition of an electron transporting layer either by vacuum deposition or by an all-solution coating method. Intriguingly, the new luminogens displayed aggregation-induced emission (AIE), which is a unique photophysical phenomenon. As a non-doped emitting layer, IAcTr-in showed external quantum efficiencies (EQEs) of 11.8% for hybrid-solution processed OLED and 10.9% for all-solution processed OLED with a low efficiency roll-off. This was evident by the higher photoluminescence quantum yield and higher rate constant of reverse intersystem crossing of IAcTr-in, as compared to IAcTr-out. These AIE luminogens were used as dopants and mixed with the well-known host material 1,3-bis(Ncarbazolyl)benzene (mCP) to produce a high-efficiency OLED with a two-component emitting layer (EML). The maximum EQE of 17.5% was obtained when using EML with IAcTr-out doping (25 wt%) into mCP, and the OLED with EML bearing IAcTr-in and mCP showed a higher maximum EQE of 18.4% as in the case of the non-doped EML-based device.

INTRODUCTION Among the currently employed methods for the development of organic light-emitting diodes (OLEDs), the device fabrication technology of vacuum evaporation has a lot of advantages in terms of reliability and reproducibility, even though there are still many aspects that need to be improved in terms of manufacturing cost and process complexity.1-4 On the other hand, the solution processing technology is attractive for simple, low cost, and large area display panel manufacturing. Solution-processable fluorescent and phosphorescent materials that can be applied to OLEDs have been studied for a long time.5-9 However, it is 2


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known that the efficiency and lifetime of the OLEDs fabricated by solution processes is generally lower than that of the devices fabricated by vacuum deposition, largely owing to the presence of possible charge traps or the nonuniformity of the dopant distribution in the host materials.10, 11 Since they were first described in 2009, thermally-activated delayed fluorescence organic light-emitting diodes (TADF OLED) devices have emerged as next-generation OLEDs.12 These devices are also primarily manufactured by vacuum deposition and exhibit excellent performance. In fact, much research has been conducted on the characteristics of TADF OLEDs fabricated by vacuum deposition technique. Recently, many research papers have been published on the fabrication of devices by solution processes and their characterization.13-15 Furthermore, it is very difficult to prepare all the layers in multi-layered OLEDs via solution processing because the lower layer can be dissolved during the deposition of the upper layer. Therefore, after preparing an emitting layer (EML) by spincoating, the electron transport layer (ETL) is still applied by vacuum deposition. This process is often termed as the hybrid-solution processed OLED fabrication method. When an all-solution process is used to fabricate a multi-layer OLED device, the problem of swelling or dissolution of the lower layer can be resolved when the upper layer is spincoated on the surface of the already applied lower layer. In general, the orthogonal solvent method is a well-established approach to overcome the problem of dissolution of the lower layer. Therefore, it is necessary to develop a material for the underlying layer such that either its solubility can be controlled by inducing crosslinking after film formation, or which has a higher molecular weight than that of the upper layer material.16

3



In order to eliminate the aggregation concentration quenching effect observed between single emitter materials, in general, an EML is fabricated by mixing the emitter as a dopant in a host material, which can smoothly transfer the absorbed energy. However, it may be difficult to uniformly distribute the dopant in the host while preparing the solution by dissolving the host and the emitting dopant in a specific solvent. This is because of the differences in the interactions (e.g., solubility) of the host and dopant with the solvent. As a result, the photoluminescence quantum yield (PLQY) of the EML becomes smaller and the contribution of the nonradiative decaying processes becomes larger, which in turn affects the efficiency of the OLED device. The phase separation of two physically mixed materials can also prove detrimental to the lifetime of the device. In order to overcome these possible disadvantages, a method of manufacturing the EML as a single material would be preferable, rather than the method of physically mixing two materials. For this purpose, two types of material design strategies have been considered. The first method involves the design and synthesis of a dendritic large molecule in which the emitter core is surrounded by conventional host moieties placed as the peripheral dendrons. This strategy is quite useful as it can induce an isolation effect between the emitters located at the center, eliminate the concentration emission quenching effect, and induce effective energy transfer phenomenon from a specific host moiety to the core unit.16,17 The second strategy is to apply a single molecule that exhibits high emission intensity in the film state to EML. In general, molecules that do not exhibit aggregation-induced emission quenching observed in the film state are called AIE luminogens, which can be utilized for highly efficient non-doped OLEDs with low efficiency roll-off.18 If such AIE luminogens have excellent film forming properties, they are good candidates for use in EML. 19-25

4


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Among the two strategies mentioned above, the dendrimer with host moieties at the periphery is produced via a complicated synthesis; the core emitter must have several reactive sites to be able to host the moieties either via a convergent or divergent synthesis. In particular, the difficulty in changing the composition between the dopant and the host material is a crucial demerit. AIE luminogens corresponding to the second strategy typically have a conjugated molecular structure with photoluminescence (PL) characteristics. However, they often also have a geometrically twisted molecular conformation and disturbed intermolecular π–π interactions with the neighboring molecules. For this reason, such molecules have the advantage that the exciton quenching effect, which is normally observed in linear or two-dimensional rigid molecules, can almost be excluded. Owing to these attractive characteristics, AIE luminogens have been well-studied and used for developing OLED devices with a stable and efficient single-component EML.18,20,26 Nonetheless, since materials with low molecular weight are not very good at film formation and have poor thermal stability, they appear to have some problems with device applications containing single component EMLs.27,28 In order to overcome the aforementioned shortcomings, a new AIE dendritic luminogen with a relatively high molecular weight and monodispersity could be advantageous to achieve a high performance TADF OLED with non-doped EML.29 In this work, we designed and synthesized two three-armed TADF emitters, IAcTr-out and IAcTr-in, which are composed of 2,4,6-triphenyl-1,3,5-triazine and 7,7-dimethyl-13,13diphenyl-7,13-dihydro-5H-indeno[1,2-b]acridine in two different geometries. These two molecules exhibit AIE, which makes them suitable for fabricating the non-doped EML in OLEDs. Moreover, these molecules have been found to have a small singlet-triplet energy gap (∆EST), which can display TADF behavior commonly. TADF OLEDs with non-doped EML bearing IAcTr-in exhibited a maximum external quantum efficiency (ηext) of 11.75% 5



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for the hybrid-solution processed device and 10.93% for the all-solution processed device, and the efficiency roll-offs of the devices were found to be remarkably small. In view of its higher PLQY, higher rate constant of reverse intersystem crossing (RISC), and a smaller nonradiative rate constant value of the triplet state in comparison to those of the IAcTr-out emitter, it was concluded that the IAcTr-in emitter exhibits excellent TADF OLED device performance. These three-armed AIE luminogens can function as a dopant that can be blended into an appropriate host, 1,3-bis(N-carbazolyl)benzene (mCP), with a high triplet energy. The corresponding two component EML system in TADF OLEDs produced a high efficiency of around over 17%. In particular, the OLED device using IAcTr-in as a dopant showed a higher EQE of 18.4% compared to the device with IAcTr-out with a low efficiency roll-off, which is the same trend as that in the non-doped EML-based OLEDs

N N

N

N

N N

N N

N N

N N

N

N

N

N N

N

IAcTr-out

IAcTr-in

Figure 1. Molecular structures of IAcTr-out and IAcTr-in.

EXPERIMENTAL SECTION 6


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Materials. All chemical reagents were purchased from either Sigma-Aldrich, TCI, or Alfa Aesar and were used as received. All reactions were conducted under an inert N2 atmosphere. Methyl 2-((9,9-dimethyl-9H-fluoren-2-yl)methyl)benzoate was prepared by following a previously reported procedure.30 Synthesis of 7,7-Dimethyl-13,13-diphenyl-7,13-dihydro-5H-indeno[1,2-b]acridine (1): n-BuLi (12 mL, 0.13 mol) was slowly added dropwise into a solution of bromobenzene (5.00 g, 0.032 mol) in THF (20 mL) under nitrogen at –78 °C. After stirring for 1 h, methyl 2-((9,9dimethyl-9H-fluoren-2-yl)methyl)benzoate (2.50 g, 0.007 mol) in THF (10 mL) was added to the reaction mixture at –78 °C. After stirring overnight at room temperature, the reaction mixture was quenched with water and extracted with water and diethylether. The organic extract was dried over sodium sulfate, filtered, and concentrated. The crude product was dissolved in acetic acid (30 mL) and HCl (1 mL) was added to the acidic mixture at room temperature. The reaction mixture was stirred overnight. Then, water was poured into the cooled compound mixture for precipitation. The precipitates were dissolved in dichloromethane and dried with sodium sulfate. After filtration to remove sodium sulfate, the filtrate was evaporated by using rotary evaporator to obtain compound 1 as a white solid. Yield = 1.75 g (53%). 1H NMR (500 MHz, DMSO): δ (ppm) 9.23 (s, 1H), 7.47-7.45 (m, 1H), 7.31-7.27 (m, 5H), 7.25-7.22 (m, 2H), 7.19-7.15 (m, 3H), 7.06 (s, 1H), 7.03 (s, 1H), 6.976.92 (m, 5H), 6.83-6.79 (m, 1H), 6.63 (d, J = 7.6 Hz, 1H), 1.43(s, 6H). MS (MALDI-TOF) m/z: [M+H]+ calcd. for C34H27N, 449.2143; found 450.2240.

7



Synthesis of 5-(4-(4,6-Diphenyl-1,3,5-triazin-2-yl)phenyl)-7,7-dimethyl-13,13-diphenyl7,13-dihydro-5H-indeno[1,2-b]acridine (2): A mixture of 1 (0.67 g, 1.50 mmol), 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine (0.60 g, 0.79 mmol), potassium tert-butoxide (0.20 g, 1.80 mmol), palladium acetate (6 mg, 0.03 mmol), and tri-tert-butylphosphine (0.03 g, 0.12 mmol) in toluene was stirred overnight under N2 at 100 °C. Subsequently, the reaction mixture was allowed to cool to room temperature and then poured into methanol for precipitation. The precipitates were dissolved in dichloromethane and the solution was dried with sodium sulfate. After filtration, the collected dichloromethane filtrate was evaporated under vacuum. The crude product was purified by column chromatography on silica gel using dichloromethane and hexane as eluents to obtain pure compound 2) as a yellow solid. Yield = 0.34 g (85%). 1H NMR (500 MHz, CDCl3): δ (ppm) 8.98 (d, J = 8.2 Hz, 2H), 8.83 (d, J = 6.7 Hz, 4 H), 7.66-7.59 (m, 6H), 7.44 (d, J = 7.0 Hz, 1H), 7.34-7.29 (m, 10H), 7.24 (t, J = 7.35 Hz, 1H), 7.20 (t, J = 7.30, 1H), 7.09-7.06 (m, 5H), 6.92 (d, J = 4.25 Hz, 2H), 6.60 (s, 1H), 6.55 (d, J = 8.2 Hz, 1H), 1.33 (s, 6H). MS (MALDI-TOF) m/z: [M+H]+ calcd. for C55H40N4, 756.3253; found 757.3455.

Synthesis of 2-Bromo-5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-7,7-dimethyl-13,13diphenyl-7,13-dihydro-5H-indeno[1,2-b]acridine (3): A mixture of compound 2 (0.60 g, 0.79 mmol) and N-bromosuccinimide (NBS) (0.15 g, 0.84 mmol) in dichloromethane (20 mL) was stirred under N2. After stirring overnight, distilled water and ether was added to the reaction mixture for repeated extraction. Subsequently, the mixture was added to methanol for precipitation and dichloromethane was added to dissolve 8


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the precipitates. The dichloromethane solution was dried with sodium sulfate and after removing the drying agent by filtration, the filtrate was evaporated under vacuum. The crude product was purified by silica gel column chromatography using dichloromethane as the eluent to obtain pure compound 3 as a yellow solid. Yield = 0.42 g (70%). 1H NMR (500 MHz, CDCl3): δ (ppm) 8.99 (d, J = 8.55 Hz, 2H), 8.82 (d, J = 7.9 Hz, 4H), 7.65-7.59 (m, 6H), 7.43 (d, J = 6.1Hz, 1H), 7.34-7.29 (m, 9H), 7.23-7.16 (m, 4H), 7.06-7.04 (m, 5H), 6.59 (s, 1H), 6.42 (d, J =8.85 Hz, 1H), 1.33 (s, 6H). MS (MALDI-TOF) m/z: [M+H]+ cald. for C55H39BrN4, 834.2358; found 835.2256.

Synthesis of IAcTr-in (4): A mixture of compound 1 (0.40 g, 0.73 mmol), 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (1.02 g, 2.27 mmol), potassium carbonate (0.62 g, 4.47 mmol), palladium acetate (0.01 g, 0.015 mmol), and tri-tert-butylphosphine (0.02 g, 0.06 mmol) in toluene (20 mL) and distilled water (3 mL) was stirred overnight under N2 at 100 °C. Subsequently, the reaction mixture was allowed to cool to room temperature and distilled water and diethylether were added for extraction. The organic extracts were dried with anhydrous sodium sulfate and after filtration, the filtrate was evaporated under vacuum. The crude product thus obtained was purified by silica gel column chromatography by using dichloromethane and hexane as eluents to finally obtain pure compound 4 as a yellow solid. Yield = 0.12 g (90%). 1H NMR (500 MHz, CDCl3): δ (ppm) 9.01 (s, 6H), 7.45 (d, J = 7.35 Hz, 3H), 7.37 (d, J = 8.55 Hz, 6H), 7.34-7.28 (m, 24H), 7.25-7.19 (m, 6H), 7.12-7.06 (m, 15 H), 6.94-6.93 (m, 6H), 6.62 (s, 3H), 6.58 (d, J =8.55 Hz, 3H), 1.35 (s, 18H). 13C NMR (125 MHz, CDCl3): δ (ppm) 171.44, 153.18, 152.90, 146.38, 145.60, 142.10, 141.80, 139.41, 138.54, 135.54, 132.01, 131.36, 131.27, 130.50, 9



130.15, 129.46, 127.72, 126.93, 126.85, 126.45, 126.03, 122.37, 121.55, 120.64, 119.17, 114.53, 108.68, 57.21, 46.71, 27.37. MS (MALDI-TOF) m/z: [M+H]+ cald. for C123H90N6 , 1650.7227; found 1651.7314. Anal. Calcd for C123H90N6 : C, 89.42; H, 5.49; N, 5.09. Found: C, 89.41; H, 5.46; N, 5.09 %.

Synthesis of IAcTr-out (5): A mixture of compound 3 (0.40 g, 0.94 mmol), 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)benzene (0.07 g, 0.15 mmol), potassium carbonate (0.60 g, 0.793 mol), tris(dibenzylideneacetone) dipalladium(0) (3 mg, 0.003 mmol), and tri(o-tolyl)phosphine (4 mg, 0.012 mol) dissolved in toluene (20 mL), distilled water (3 mL) and methanol (3 mL) was stirred overnight under N2 at 100 °C. Subsequently, the reaction mixture was allowed to cool to room temperature and distilled water and ether were poured into the cooled reaction mixture for extraction. The organic extracts were dried with anhydrous sodium sulfate and after removing the drying agent by filtration, the filtrate was evaporated under vacuum. The crude product was purified by silica gel column chromatography using dichloromethane and hexane as the eluents to obtain pure compound 5 as a yellow solid. Yield = 0.11 g (79%). 1H NMR (500 MHz, CDCl3): δ (ppm) 9.01 (d, J =8.2 Hz, 6H,), 8.83 (d, J =7.9 Hz, 12H), 7.667.59 (m, 21H), 7.43 (d, J = 7.6 Hz, 3H), 7.38 (d, J = 8.55 Hz, 6H), 7.33 (d, J =6.7 Hz, 3H), 7.29-7.08 (m, 45H), 6.60-6.59 (m, 6H), 1.33(s, 18H). 13C NMR (125 MHz, CDCl3): δ (ppm) 171.88, 171.04, 153.19, 152.94, 146.17, 145.01, 141.90, 141.69, 141.81, 139.38, 136.09, 136.07, 133.45, 132.72, 132.02, 131.36, 131.28, 130.52, 130.45, 129.11, 129.02, 128.84, 128.73, 127.75, 126.79, 126.52, 125.98, 125.75, 122.34, 121.56, 119.14, 114.70, 108.54, 57.28, 46.70, 29.06, 27.33. MS (MALDI-TOF) m/z: [M+H]+ calcd. for C171H120N12, 10


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2340.9759; found 2341.9660. Anal. Calcd for C171H120N12, C, 87.66; H, 5.16; N, 7.17; Found C, 87.63; H, 5.15; N, 7.15 %.

Characterization. 1H and 13C NMR spectra of all compounds were obtained on Bruker 500 and 125 MHz spectrometers, respectively. Elemental analyses (CHN) were performed on a Thermo Scientific Flash 2000 (ThermoFisher Scientific) elemental analyzer. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (LRF20, Bruker Daltonics) was used to determine the mass of the compounds. The absorption spectra of the solutions and films were measured by using a UV-Vis absorption spectrophotometer (Agilent 8453, photodiode array = 190–1100 nm). The oxidation properties of the two materials were investigated by using a film of the respective small molecules in cyclic voltammetry (EA161, eDAQ) experiments. A 0.10 M solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile was used as the electrolyte. The reference and counter electrodes were Ag/AgCl and Pt wire (diameter 0.5 mm) electrodes, respectively. The fluorescence spectrophotometer (JASCO FP-8500) equipped with an integrated sphere was used to obtain the PL quantum yields and perform the transient PL measurements at room temperature under N2. The Quantaurus-Tau fluorescence lifetime spectrometer (C11367-03, Hamamatsu Photonics) was used to obtain the exciton lifetimes and transient PL intensities. Atomic force microscopy (XE-100 advanced scanning probe microscope, PSIA) in the tapping mode was used to investigate the surface morphologies of the spin-coated films.

Device fabrication and measurements. 11



OLED devices having a structure of ITO/PEDOT:PSS (50 nm)/PVK (30 nm)/emitting layer (50 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated. ITO-coated glass with a sheet resistance of 10 ohm/sq was used as the anode and an active pattern area of 2 × 2 mm2 was formed. It was sequentially washed in an ultrasonic bath with deionized water and isopropanol for 10 min each. The cleaned ITO-coated glass was then dried in a vacuum oven. The substrates were treated in a UV zone chamber for 20 min. A thin layer of PEDOT:PSS was spin-coated on the ITO-coated glass and annealed at 155 °C for 15 min. The 0.5 wt% poly(9-vinylcarbazole) (PVK) film was prepared from a chlorobenzene (CB) solution and spin-coated onto ITO/PEDOT:PSS. The emitting layer of the dopant and host material blends (25 and 35 wt% for IAcTr-out and IAcTr-in, respectively) or only dopant was prepared from a toluene solution. While a TPBi thin layer of 40 nm thickness was deposited under high vacuum in the hybrid-solution processed device, the 1 wt% films of TPBi were prepared from its ethanol solution and spin-coated onto the emitting layer in all-solution processed OLED devices. Finally, LiF and Al were deposited sequentially under high vacuum. The current density–voltage-luminance (J–V–L) characteristics of the OLED devices were measured with a Keithley SMU 236 instrument and a SpectraScan PR-655 colorimeter.

RESULT AND DISCUSSION Synthesis and characterization Figure 1 shows the two new three-armed molecules that exhibit AIE in TADF OLEDs. Scheme S1 depicts the synthetic procedure for IAcTr-out and IAcTr-in in detail. The precise synthetic procedures have been described in the experimental section. Each molecule (e.g., 12


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IAcTr-out and IAcTr-in) was designed to contain a different number of 7,7-dimethyl-13,13diphenyl-7,13-dihydro-5H-indeno[1,2-b]acridine and 2,4,6-triphenyl-1,3,5-triazine units in the structure. For preparing two new three-armed luminogens, compound 1 was synthesized by using (2-((9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)diphenylmethanol, which was prepared from methyl 2-((9,9-dimethyl-9H-fluoren-2-yl)amino)benzoate by following a previously reported method.30 IAcTr-in was obtained by the palladium-catalyzed Buchwald-Hartwig coupling reaction between three equivalents of compound 1 and 2,4,6-tris(4-bromophenyl)-1,3,5-triazine. Compound 2 was also prepared under similar reaction conditions as IAcTr-in, using compound 1 and 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine. IAcTr-out was successfully synthesized via the Suzuki coupling reaction between 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)benzene and compound 3, which was prepared by the bromination of compound 2. The molecular structures of the target molecules, including IAcTr-out and IAcTr-in, were confirmed by 1H and

13

C NMR spectroscopy, mass spectrometry, and

elemental analyses. In the structure of IAcTr-out, three 5-(4-(4,6-diphenyl-1,3,5-triazin-2yl)phenyl)-7,7-dimethyl-13,13-diphenyl-7,13-dihydro-5H-indeno[1,2-b]acridine

units

are

anchored to 1-, 3-, and 5-positions of the benzene ring at the core. In the structure of IAcTrin, the three 7,7-dimethyl-5,13,13-triphenyl-7,13-dihydro-5H-indeno[1,2-b]acridine donor units are attached to 1-, 3-, and 5-positions of triazine, which acts as an acceptor. The unique structural characteristic of IAcTr-in is the three donor moieties surrounding the triazine acceptor in the core. On the other hand, in the structure of IAcTr-out, the donor and acceptor moieties present in the structure of 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-7,7dimethyl-13,13-diphenyl-7,13-dihydro-5H-indeno[1,2-b]acridine are exposed to the outside. It can be conjectured that the overall structures of IAcTr-out and IAcTr-in are away from the 13



planar geometry of the core moiety and some single bonds allow the localized moiety to rotate freely in a single molecular state (e.g., dilute solution state). The thermal properties of the two molecules were analyzed by differential scanning calorimetry (DSC). The two molecules did not show any transition temperature up to 275 oC. (Figure S1). In order to investigate the relationship between the molecular orbitals and the properties of the materials and understand the electronic structures of IAcTr-out and IAcTr-in at the molecular level, time-dependent density functional theory (TD-DFT) calculations were performed at the B3LYP/6-31G* basis level. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions of the optimized molecular structures of two molecules are depicted in Figure 2. As shown in Figure 2, the HOMO and LUMO distributions of the two molecules are clearly separated. The HOMO of IAcTr-out is mainly delocalized on the indenoacridine moieties attached to the benzene core, whereas the LUMO is mainly localized on the triazine moiety. Meanwhile, the HOMO of IAcTr-in was localized over the indenoacridine moieties located at the periphery. The LUMO was mainly localized on the triazine acceptor core unit. Such a separation and distribution of HOMO and LUMO could produce effective hole- and electron-transporting channels, respectively, which allow intermolecular hopping of holes and electrons along their respective routes.31 Owing to their highly twisted 3D-geometries consisting of donor-acceptor (D-A) characters and a clear separation of the HOMO and LUMO energy levels, the two molecules showed small calculated ∆EST values of ~0.005 eV, which allowed for an efficient RISC. Unlike most of the well-known AIE materials in the literatures, the two molecules synthesized in this study have three arms around the central core. 20,21,27,32 It is a somewhat 14


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complicated structure, but there are several positions within the molecular structure that can be rotated due to local single bonds. (Figure S9). In addition to the non-planar molecular geometry, the above-mentioned possible large degree of rotational freedom may contribute to the AIE phenomenon by blocking the π–π interactions in the solid state.

Figure 2. Optimized geometries and calculated HOMO and LUMO density maps for IAcTrout and IAcTr-in obtained by DFT calculations [B3LYP; 6-31G(d)].

Photophysical properties of IAcTr-out and IAcTr-in UV-Vis absorption, PL, and low temperature PL spectra (LTPL) of IAcTr-out and IAcTr-in in toluene solutions and thin films are shown in Figures 3. The pertinent optical and photophysical data are listed in Table 1. As shown in Figure 3, the two compounds exhibit a high intensity absorption band centered at a wavelength of ~350 nm, which can be attributed to π–π* transitions. The characteristic band at around 400 nm corresponds to the intramolecular charge-transfer (ICT) transition from the indenoacridine donor moiety to the 15



triphenyltriazine acceptor moieties. Compared to IAcTr-out, the IAcTr-in compound displayed a more intense ICT absorption peak at 402 nm, which might be explained by the combination of three donors and one single acceptor moiety in the molecule. The optical bandgaps (Egopt) of the IAcTr-out and IAcTr-in, estimated from their absorption edges, are 2.65 and 2.58 eV, respectively. Owing to the different absorption characteristics of the two compounds, their PL spectra were also slightly different; the emission maxima for IAcTr-out and IAcTr-in in their toluene solutions were observed at 507 and 515 nm, respectively. Figure S10 also depicts the low-temperature PL (LTPL) spectra (i.e., fluorescence and phosphorescence spectrum measured at 77 K) of two molecules, which are almost overlapped in a shape. In the LTPL spectra, the T1 were determined to be 2.620 and 2.536 eV for IAcTr-out and IAcTr-in, respectively, as estimated from the onset of the wavelength in films at 77 K.5,33,34 The T1 levels of the two compounds are just high enough to give a small single-triplet energy gap (∆EST) of 0.052 eV and 0.069 eV for IAcTr-out and IAcTr-in, respectively, which was calculated by following the literature methods. 35,36

1.0 0.8

1.2 1.0 0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 300

400

500

600

(b)

0.0 700

1.2

(i) (ii) (iii) (iv)

1.0 0.8

1.2 1.0 0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 300

Wavelength (nm)

400

500

600

PL Intensity

(i) (ii) (iii) (iv)

Normalized absorbance (a.u.)

1.2

PL Intensity

(a) Normalized absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

0.0 700

Wavelength (nm)

Figure 3. UV-Vis absorption of and PL spectra of of (a) IAcTr-out and (b) IAcTr-in (i, iii) toluene solution and (ii, iv) thin film states. 16


Page 17 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Photophysical and electrochemical properties of IAcTr-out and IAcTr-in. max

λabs, [nm] Sol.

a)

a)

IAcTr-out

357

IAcTr-in

321

Film

max

λPL, [nm] Sol.

a)

Egopt [eV]

Eoxb) [V]

HOMOb) [eV]

LUMOc ) [eV]

S1d) [eV]

T1d) [eV]

∆ESTd) [eV]

-2.80

2.672

2.620

0.052

-2.84

2.605

2.536

0.069

Film

357

507

524

2.65

1.01

-5.45

344

515

525

2.58

0.98

-5.42

b)

c)

Toluene solution. The values were obtained from cyclic voltammograms. HOMO(eV) + Estimated from the onset of the fluorescence and phosphorescence spectra of the thin film.

Egopt(eV). d)

Photophysical properties for aggregation-induced emission behaviors It is well known that the planar aromatic structure of conjugated molecules enables them to undergo uniform and ordered packing in the aggregates. The involved π−π stacking interactions between the planar molecules promotes the formation of detrimental species such as excimers, which waste the energy via non-radiative decaying processes and are responsible for the observed aggregation-caused quenching (ACQ) effect.21 However, the three-armed luminogens used in this study displayed an intriguing AIE behavior, which is a unique photophysical phenomenon arising from chromophore aggregation. Under a nitrogen atmosphere, the PL quantum yields (PLQYs) for IAcTr-out were 32.8 and 47.7% in its toluene solution and film state, respectively, and 30.4 and 64.5% in the case of IAcTr-in, as shown in Table 2. The enhanced PLQYs from the solution to the film state indicate the AIE behavior of the two new compounds. In order to confirm the AIE activity, the PL behaviors of IAcTr-out and IAcTr-in in THF and water mixed solvents were investigated in detail. As described above, toluene was used to measure the PLQY of the solution, but THF was used in this experiment because of its 17



miscibility with water.37-39 As shown in Figure 4, in the pure THF solution, IAcTr-out and IAcTr-in exhibited yellow (λmax = 562 nm) and orange-red light (λmax = 580 nm), respectively, at a relatively low intensity. (inset image of Figures 4b and 4d) In its mixed solutions, the PL intensity of IAcTr-out remained low in the mixtures with fw less than 20% and the λmax was red-shifted from 562 to 579 nm. In the case of IAcTr-in, the PL phenomenon was quite similar to the behavior of IAcTr-out in the water fraction from 0 to 10%. The λmax of the IAcTr-in solution was red-shifted from 580 to 594 nm. The PL spectral shifts in the solution state of the two molecules result from the intensification of the ICT effect with increasing solvent polarity, which leads to the weak light emission. On the other hand, the PL intensity and their spectra were significantly increased and blue-shifted at

fw > 30% for IAcTr-out and fw > 20% for IAcTr- in. Therefore, it can be concluded that at a very low threshold concentration of a poor solvent, the PL is turned on and its intensity increases significantly as the concentration increases. These results clearly demonstrate the AIE property of both luminogenic materials, although there is a difference in the degree of AIE enhancement. These AIE characteristics of both IAcTr-out and IAcTr-in might be originated from their sterically hindered three-armed molecular geometry.35,36 The aggregates formation can restrict intermolecular motions owing to their unique geometries. Thus, the nonradiative decay processes can be prevented, which is prominently ascribed to the AIE effect.39,40 Another factor that needs to be included for consideration is that the aggregate formation excludes the majority of highly polar solvent molecules, and the fluorescent molecules experience less polarity in the aggregate form with a weakened ICT process. The dipoledipole interactions among the molecules also become weaker in comparison to those that 18


Page 18 of 40

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exist between the polar solvent molecules and the fluorescent molecules.41,42 In addition, the suppressed geometric relaxation could induce reduced Stokes shift for hypsochromic emission.43 Therefore, the materials show blue-shifted PL peaks and an increased PL intensity in their aggregates. The three-armed molecular conformation of both materials can obstruct the intermolecular π–π stacking interactions in the aggregated state to some extent. This can favorably alleviate the concentration quenching and/or exciton annihilation caused by the interactions between TADF molecules, which also play a constructive role for the OLEDs performance.44,45 The three-armed luminogens, which form molecular aggregates in a very small amount of non-solvent (or poor solvent) and exhibit AIE phenomenon, are interesting materials for non-doped OLED devices.

Figure 4. PL spectra in THF-water mixtures with different water fractions at room temperature for (a) IAcTr-out and (c) IAcTr-in, excitation wavelength = 400 nm, solution concentration = 0.5 mM. Plots of water volume fraction versus PL intensity and emission maximum wavelength of (b) IAcTr-out and (d) IAcTr-in, inset: emission images of the 19



corresponding THF-water mixtures with fw values of 0, 10, 50, and 90% under 365 nm UV irradiation.

Photophysical properties and kinetic parameters of the single-component films of IAcTr-out and IAcTr-in In order to evaluate the relationship between the chemical structure and TADF characteristics of a new luminogen, the transient PL spectra of the thin films of IAcTr-out and IAcTr-in were acquired. Figure 5a depicts the room-temperature transient PL decay characteristics for the IAcTr-out and IAcTr-in films, which can be generally fitted by the double exponential decay function as: I (t) = A1 exp(−t/τp ) + A2 exp(−t/τd), where A1 and A2 are the pre-factor constants, and τp (= 1/kp) and τd (= 1/kd) refer to the observed exciton lifetimes of the prompt and delayed decaying behaviors. As shown in Figure 5a, a prompt component (τp) was clearly observed for IAcTr-out and IAcTr-in about 12.9 and 26.8 ns, respectively, after excitation. The delayed exciton lifetimes (τd) of IAcTr-out and IAcTr-in were determined to be 1.3 and 1.6 µs for the single-component films, respectively. The calculated τp and τd for IAcTr-out and IAcTr-in are listed in Table 2. The two AIE luminogens synthesized in this study showed short delayed exciton lifetimes less than 2.0 µs. This results in suppression of exciton quenching, such as triplet-triplet annihilation (TTA), by reducing the triplet exciton density at high luminance or high current density when applying two luminogens as a single component EML to an OLED device. To confirm the TADF feature of IAcTr-out and IAcTr-in, their temperature-dependent transient PL decays at 100, 200 and 300 K were investigated (Figures 5c and 5d). It is very clear that the delayed fluorescence intensities are enhanced with the rising temperatures, 20


Page 20 of 40

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ascribed to the acceleration of the RISCs from the triplet to singlet excited states by thermal activation.

46-48

Accordingly, the prompt and delayed fluorescence spectra have been

illustrated in the insets of Figures 5c and 5d. The profiles of the prompt PL spectra are almost identical to that of the delayed PL spectra, which can be explained by the principle of RISC in TADF. The prompt (Φp) and delayed fluorescence (Φd) components of the ΦPL were estimated from the parameters obtained from the decay curve. The prompt (Φp) and delayed (Φd) fluorescence quantum yields were determined by the ratio of the emission area in the transient PL spectra, based on the total photoluminescence quantum yield (ΦPL).49,50 The Φp and Φd values determined for IAcTr-out and IAcTr-in are summarized in Table 2. To gain further insights into the TADF mechanism of the IAcTr-out and IAcTr-in molecules, the kinetic parameters of the non-doped films were estimated based on the experimental results of PLQY in a nitrogen atmosphere and transient PL characteristics (Table 2). The radiative decay rate constant of the singlet excited state (kSr), the rate constant for intersystem crossing (ISC) from the singlet excited state to the triplet excited state (kISC), the rate constant for RISC from the triplet excited state to the singlet excited sate (kRISC) and the non-radiative (nr) decay rate constant of the triplet excited state (kTnr) were calculated by using the equations (Equation S1) based on the assumption that the non-radiative decay rate constant of the singlet excited state (kSnr) is zero at room temperature.49 The respective value of kRISC for IAcTr-in (7.5 × 105 s–1) was estimated to be 1.79 times higher than that of IAcTr-out (4.2 × 105 s–1). These high kRISC values could be explained by the small ∆EST (~0.07 eV) of the IAcTr-in emitter, probably because of its stabilized S1 state as an emitter itself. The stabilization of the S1 state might originate from the intramolecular interactions of the indenoacridine donors and triphenyltriazine acceptor. As kRISC increases, 21



Page 22 of 40

the non-radiative rate constant value of the triplet state (kTnr) generally decreases according to the equation (4). In particular, the kTnr value of the IAcTr-in-based film (3.5 × 105 s–1) is observed to be lower than that of the IAcTr-out-based film (6.2 × 105 s–1). Hence, a high PLQY (~64.5%) of the IAcTr-in emitter was obtained in the non-doped film because of the high kRISC values, especially the low kTnr values for IAcTr-in.

Figure 5. (a) Transient decay spectra of IAcTr-out and IAcTr-in films at room temperature (300 K). (b)Major processes occurring in a TADF emitter after excitation. The temperaturedependent transient PL decay spectra of the (c) IAcTr-out and (d) IAcTr-in in the neat film from 100 to 300 K. Excitation wavelength : 377 nm, Detection wavelength : 510 nm. (inset: Prompt and delayed PL spectra)

Table 2. Photophysical properties and kinetic parameters of the single-component films of IAcTr-out and IAcTr-in. ࣎࢖ (ns)

࣎ࢊ (µs)

߶௉௅ a) (%)

߶௣ (%)

߶ௗ (%)

݇௣ (107) (s-1)

݇ௗ 5 (10 ) (s-1)

22


݇௥ௌ 7 (10 ) (s-1)

݇ூௌ஼ 7 (10 ) (s-1)

݇ோூௌ஼ 5 (10 ) (s-1)

் ݇௡௥ 5 (10 ) (s-1)

Page 23 of 40

IAcTr-out IAcTr-in

12.9 26.8

1.3 1.6

b)

35.2

12.5

7.8

7.7

2.7

5.1

4.2

6.2

b)

37.0

27.5

3.7

6.3

1.4

2.3

7.5

3.5

47.7/32.8

64.5/30.4

a) in a nitrogen atmosphere, b) Toluene solution.

Electrochemical properties The electrochemical behaviors of IAcTr-out and IAcTr-in were measured by cyclic voltammetry (CV) (Figure 6). The oxidation onset potentials versus Ag/Ag+ were determined to be +1.01 and +0.98 V for IAcTr-out and IAcTr-in, respectively. Accordingly, the HOMO energy levels of the two molecules were –5.45 and –5.42 eV, respectively. The LUMO energy levels were calculated by adding the optical bandgap to the HOMO level (Table 1). Owing to the same kind of donor and acceptor units, the two molecules did not show any significant differences in their HOMO and LUMO energy levels. For developing a solutionprocessable TADF molecule as an efficient single-component emitter, the frontier orbital energy level should be suitably matched with that of a nearby layer in order to facilitate the charge injection and transport. The appropriate HOMO and LUMO energy levels of IAcTrout and IAcTr-in will ensure an effective injection and recombination of holes and electrons in the EML, which is crucial to improving the device performance of OLEDs with a nondoped EML.

Current (µ A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


IAcTr-out

IAcTr-in Ferrocene

0.0

0.4 0.8 1.2 E(V) vs Ag/AgCl

1.6

23



Figure 6. Cyclic voltammograms of IAcTr-out and IAcTr-in.

Characterization of TADF OLEDs In order to investigate the TADF OLED performance of the two AIE luminogens, multilayered devices were fabricated by a solution process. The device configuration is as follows: indium tin oxide (ITO)/PEDOT:PSS (50 nm)/PVK (30 nm)/IAcTr-out or IAcTr-in (35 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (100 nm). In these devices, poly(3,4-ethylenedioxy thiophene):poly(styrenesulfonic acid) (PEDOT:PSS) and poly(9-vinylcarbazole) (PVK) were used as the hole-injecting and hole-transport layers, respectively, and 1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl)benzene (TPBi) was employed as an electron-transporting material. These devices contain an emission layer made only with a three-armed AIE luminogens without using any host molecules. The device parameters are summarized in Table 3. The function of IAcTr-out and IAcTr-in as efficient single-component emitters in the hybrid-solution processed OLEDs was investigated by fabricating two different devices with the structure shown in Figure 7a. In order to form the ETL, TPBi was deposited on top of single-component EML via vacuum evaporation. Figure 7b and 7c shows the current density–voltage–luminance (J–V–L) and EQE vs. J curves for the devices, respectively. The TADF OLED device with IAcTr-out showed a low maximum ηext of 3.80% with a poor current efficiency (CE) of 12.02 cd A–1. Meanwhile, the IAcTr-in–based device had a maximum ηext of 11.75% with a high CE of 39.31 cd A–1, which was significantly higher than that of the devices with IAcTr-out. The superior performance of the device with IAcTr-in could be explained by the strong AIE enhancement in the solid state (e.g., 64.5% of PLQY), which has been discussed in the previous section. 24


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In brief, the enhanced performance of the IAcTr-in-based device is attributed to the strong AIE, high PLQY, high kRISC, low kTnr, and efficient triplet harvesting of the TADF emitter. In addition to the factors mentioned above, IAcTr-in was shown to be more favorable for balance and transport of holes and electrons than IAcTr-out, based on the characterization of single carrier devices such as hole-only and electron-only devices. (Figure S20a of support information) These characteristics also help to explain the relatively high performance of the device using the IAcTr-in as an emitter. Both emitter-based devices exhibit relatively low roll-off behavior at a luminance of 1000 cd m–2, which might be attributed to a high PLQY for emission, the short lifetime, τd, weak interactions of the excitons in the film, the suppression of non-radiative exciton quenching process, and a small ∆EST for triplet harvesting.50-53 The two emitter-based OLED devices exhibit green EL spectra similar to those of the corresponding PL spectra of the films (Figure S17), confirming that EL has the same origin as radiative decay processes. The EL spectra of the devices using IAcTr-out and IAcTr-in exhibit the emission maxima at 524 and 532 nm, respectively. Besides the hybrid-solution processed devices, all-solution processed OLEDs are more desirable for simplifying the fabrication process. Since this method involves putting a solution on top of the emitting layer, the lower layer should be resistant to the solvent for the upper layer as a sequential process. Thus, the high solvent resistance of IAcTr-out and IAcTr-in was examined by absorption spectroscopy. Figures S19a and S19b show the changes in the absorption spectra of IAcTr-out and IAcTr-in before and after spin-rinsing with ethyl alcohol, which was used for processing the TPBi as the upper ETL. The absorption spectra of neat films prepared with IAcTr-out and IAcTr-in did not change after rinsing with 25



ethanol. This demonstrates the possibility of applying TPBi–based ETL as an ethanol solution on top of EML made with two emitter materials, one that had a moderately high threshold molecular weight, and was compatible with the subsequent solution process involving the orthogonal solvent (e.g., ethanol). In order to evaluate IAcTr-out and IAcTr-in as efficient single-component emitters in the all-solution processed OLEDs, the devices were fabricated with the structure shown in Figure 7d. In the multi-layered geometry of the all-solution processed multi-layer OLEDs, the TPBi solution in ethanol as the ETL was successfully spin-coated on the EML. The J−V−L and EQE-J curves for the devices are shown in Figures 7e and 7f. The related device data are also given in Table 3. As shown in Figure 7f, the all-solution processed devices based-on IAcTr-out and IAcTr-in show a maximum ηext of 4.10% and 10.93% respectively, which are very similar to those achieved by the hybrid-solution processed OLEDs. Generally, the high luminance is attributed to the high concentration of excitons, which indicates that the AIE can effectively suppress the exciton quenching behavior of the TADF emitter in the solid state. To the best of our knowledge, this is the first time a non-doped emitting layer bearing a new AIE luminogen was used to construct the all-solution processed multi-layer OLEDs.

26


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Figure 7. (a) Device configurations and fabrication process, (b) current density-voltageluminance (J–V–L) characteristics, (c) external quantum efficiency versus J curves of hybridsolution processed (a–c) and all-solution processed (d–f) devices made with IAcTr-out and IAcTr-in.

Table 3. Device performances of the hybrid-solution processed and all-solution processed TADF OLEDs. Von

c)

[V]

IAcTr-out

a)

IAcTr-in a) IAcTr-out IAcTr-in

b)

b)

CEmax /PEmax /EQEmax

CE100 /PE100 /EQE100

[cd A /Im W /%]

[cd A /Im W /%]

–1

–1

d)

–1

–1

e)

CE1000/PE1000/EQE1000 –1

–1

[cd A /Im W /%]

f)

g)

CIE [x,y]

3.6

12.02/8.39/3.8

11.90/8.26/3.71

11.05/6.23/3.55

(0.33, 0.56)

3.8

39.31/27.45/11.75

36.69/24.41/10.90

29.91/17.74/9.06

(0.36, 0.58)

3.6

12.98/6.86/4.10

13.25/6.28/2.61

13.41/5.12/4.09

(0.35, 0.57)

3.3

35.88/18.79/10.93

10.01/12.52/4.95

14.27/13.86/10.28

(0.38, 0.57)

a)

Hybrid-solution processed device, b) All-solution processed device, c) Turn-on voltage at 1 cd m–2, d) CEmax = maximum current efficiency, PEmax = maximum power efficiency, EQEmax = maximum external quantum efficiency, EQEave = average external quantum efficiency obtained from more than ten devices, e) CE, PE, and EQE at 100 cd m–2, f) CE, PE, and EQE at 1000 cd m–2, g) At 1000 mA cm–2.

Finally, we evaluated the performance of the two devices with EML made of mCP host material with high excited triplet energy (T1= 2.9 eV) and high hole affinity (Figure 8 and 27



Table 4). The UV-Vis absorption spectra and PL spectra of the doped films are shown in Figure S13 for comparison with single-component films. Their transient decay spectra and photophysical parameters are also shown in Figure S15 and Table S2. IAcTr-out:mCP and IAcTr-in:mCP showed maximum EQE values at doping concentrations of 25 wt% and 35 wt%, respectively (Figure S18, Table S3). In this two component EML fabrication, allsolution processed devices was impossible due to the dissolution of mCP in ethanol solvent, which was confirmed by absorption spectroscopy. (Figures S19c and S19d) Therefore, only hybrid-solution processed devices could be fabricated and operated. The emitter:mCP–based devices showed a maximum ηext of 17.53% in the case of IAcTrout:mCP and 18.43% for IAcTr-in:mCP, which are noticeably high values achieved in the solution-processed TADF OLEDs. Compared to the EQEs of a non-doped EML based devices, enhanced EQEs are probably a result of the suppression of triplet–triplet and/or singlet–triplet annihilations in the blend films of host and guest molecules. It should be pointed out that the efficiency roll-off in the doped EML-based device was remarkably low, which is identical to the phenomenon observed in the non-doped EML-based devices characterized before. Consequently, as was observed in the case of non-doped EML-based devices, the device with IAcTr-in displayed a superior efficiency compared to that with IAcTr-out, although the performances of both the devices were enhanced. To summarize, unlike the single-component EML device in which the emission is retarded by the interactions between the molecules, in the case of IAcTr-out, it might be uniformly distributed in the mCP host and the OLED device exhibited similar performance aided by efficient energy transfer to IAcTr-out compared to the devices with IAcTr-in:mCP based emitting layer. This result can also be supported by the J-V characteristics of hole-only and

28


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electron-only devices fabricated with the blend films of two luminogens and mCP. (Figure S20b in the supporting information)

Figure 8. (a) Device configurations and energy level, (b) Current density-voltage-luminance (J-V-L) characteristics, (c) External quantum efficiency versus J, (d) Current efficiency-Jpower efficiency (CE-J-PE), (e) EL spectrum of hybrid-solution processed device of IAcTrout:mCP (25 wt%) and IAcTr-in:mCP (35 wt%).

Table 4. Device performances of the hybrid-solution processed OLEDs of IAcTr-out:mCP (25 wt%) and IAcTr-in:mCP (35 wt%). Von

a)

IAcTr-out:mCP b)

IAcTr-in:mCP

c)

CEmax /PEmax /EQEmax -1

-1

d)

CE100 /PE100 /EQE100 -1

-1

[cd A /Im W /%]

[V]

[cd A /Im W /%]

3.9

54.36/37.95/17.53

52.25/32.86/16.82

4.0

60.70/37.16/18.43

60.31/35.57/18.31

a)

CE1000/PE1000/EQE1000

e)

-1

-1

[cd A /Im W /%]

b)

f)

g)

CIE [x,y]

44.52/22.54/14.05

(0.29, 0.54)

53.59/25.89/16.22

(0.33, 0.58)

Hybrid-solution processed device (doping concentration : 25 wt%), Hybrid-solution processed device (doping concentration : 35 wt%), c) Turn-on voltage at 1 cd/m2, d) CEmax = maximum current efficiency, PEmax = maximum power efficiency, EQEmax = maximum external quantum efficiency, EQEave = average external quantum efficiency obtained from more than ten devices, e) CE, PE, and EQE at 100 cd m-2, f) CE, PE, and EQE at 1000 cd m-2, g) At 1000 mA cm-2

29



Page 30 of 40

CONCLUSIONS In this work, solution processable, three-armed luminogens as a single component emitters bearing triazine and indenoacridine moieties were synthesized and applied to TADF OLEDs. These new luminogens displayed AIE in both solution and film states, which is a unique photophysical phenomenon that exists in chromophore aggregation states. Compared to the poor device performance of the TADF OLED with the IAcTr-out emitter, the maximum ηext of 11.75 and 10.93% was observed, respectively, for the hybrid-solution and the all-solution processed devices based on IAcTr-in. The fairly low efficiency roll-off of the two types of devices was mainly attributed to the delayed fluorescence exciton life-time (1.3–1.6 µs) and the suppression of the exciton annihilation. These three-armed AIE luminogens were also employed as a dopant and blended with mCP to produce a high efficiency OLED with a twocomponent EML. In particular, the OLED device using IAcTr-in as a dopant showed a higher EQE of 18.43%. Based on their inherent AIE capability and TADF characteristics, the two synthesized luminogens displayed very intriguing OLED device characteristics. We believe that these results encourage the implementation of a “simple TADF OLED” device instead of a device using a conventionally doped EML-based device.

█

ASSOCIATED CONTENT

Supporting Information Synthetic procedure, NMR spectra, DSC thermogram, DFT calculation, UV-Vis absorption spectra,

fluorescence

and

phosphorescence

spectra,

transient

PL

decay

spectra,

prompt/delayed PL spectra, table for key physical property, AFM images, OLED device 30


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performance, J-V characteristics of hole-only device and electron-only devices. This material is available free of charge via the Internet at http://pubs.acs.org.

█

AUTHOR INFORMATION

Corresponding Authors *Corresponding authors:

Dr. Min Ju Cho, Prof. Dong Hoon Choi,

E-mail: [email protected], and [email protected]

Notes The authors declare no competing financial interest. █

ACKNOWLEDGMENTS

This research was supported by National Research Foundation of Korea (NRF20100020209).

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REFERENCES

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Table of Contents

aggregate

IAcTr-in

IAcTr-out

Aggregationinduced Emission (AIE)

IAcTr-in

IAcTr-out

solution

101 N N N

100

EQE (%)

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

N

N

N

N

0

10

N N

N

N N N N

N

N N N

10-1 -2 10

10-1

100

101

2

Current Density (mA/cm )

102

10-1 -2 10

10-1

100

101

2

Current Density (mA/cm )


102

Unconventional Three-Armed Luminogens ... - ACS Publications

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