StructureâPerformance Investigation of Thioxanthone Derivatives for

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Structure-Performance Investigation of Thioxanthone Derivatives for Developing Color Tunable Highly Efficient Thermally Activated Delayed Fluorescence Emitters Zhiheng Wang, Yunchuan Li, Xinyi Cai, Dongcheng Chen, Gaozhan Xie, Kunkun Liu, Yuan-Chun Wu, Chang-Cheng Lo, A. Lien, Yong Cao, and Shi-Jian Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12559 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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ACS Applied Materials & Interfaces

Structure-Performance Investigation of Thioxanthone Derivatives for Developing Color Tunable Highly Efficient Thermally Activated Delayed Fluorescence Emitters Zhiheng Wang,a# Yunchuan Li,a# Xinyi Cai,a Dongcheng Chen,a Gaozhan Xie,a KunKun Liu,a Yuan-Chun Wu,b Chang-Cheng Lo,b A. Lien,b Yong Cao,a and Shi-Jian Sua* a

State Key Laboratory of Luminescent Materials and Devices and Institute of Polymer

Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China b

Shenzhen China Star Optoelectronics Technology Co., Ltd., Shenzhen 518132, China

#

These authors contributed equally to this study.

KEYWORDS: thioxanthone derivatives, intramolecular charge transfer, color tunable, thermally activated delayed fluorescence, organic light-emitting diodes

ABSTRACT: Thioxanthone derivatives consisting of undecorated carbazole as an electron donor and thioxanthone (TXO) or 9H-thioxanthen-9-one-S,S-dioxide (SOXO) as an electron acceptor in

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a donor-acceptor (D-A) or donor-acceptor-donor (D-A-D) structure were developed as thermally activated delayed fluorescence (TADF) emitters to fabricate highly efficient fluorescent OLEDs. Their emission color was successfully tuned from blue to yellow by changing the sulfur atom valence state of the thioxanthone unit to tune intramolecular charge transfer (ICT) effect. Their thermal, electrochemical, photophysical, and electroluminescent properties, and theoretical calculations were systematically investigated to illustrate the molecular structure and property relationships. Maximum external quantum efficiency (EQE) of 13.6% with Commission Internationale de L’Eclairage (CIE) coordinates of (0.37, 0.57) was achieved for green light emission CzSOXO consisting of SOXO and carbazole in a D-A structure. Blue light emission CzTXO and DCzTXO consisting of TXO and carbazole in a D-A and D-A-D structure could also give EQE values exceeding 11%. Their efficiency roll-off with increasing current density was simulated by adopting triplet-triplet annihilation model, indicating that the TXO derivatives suffer more severe efficiency roll-off because of their relatively long delayed fluorescence lifetime (τD).

INTRODUCTION In the last few decades, organic light emitting diodes (OLEDs) were paid close attention as a promising technology to fabricate large area, energy-efficient, high-resolution flexible panel displays and light sources.1,2,3 Traditional fluorescent emitters are regarded as the first generation light emitting materials. However, based on quantum statistics, only 25% of singlet excitons can be harvested for fluorescent emission and result in limited efficiency. In order to overcome this triplet spin forbidden in conventional fluorophors, researchers developed noble-metal based organometallic phosphors to collect both 25% singlet and 75% triplet excitons by heavy atom


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enhanced intersystem crossing (ISC). Despite of achieving 25% external quantum efficiency (EQE) for optimized sky blue phosphorescent OLEDs, expensive material cost, severe efficiency roll-off, and short practical lifetime obstruct their extension in application fields.4 Fortunately, in 2012, Adachi et al reported nearly 100% internal quantum yield for thermally activated delayed fluorescence (TADF) molecules like green light emitting 4CzIPN ((4s,6s)-2,4,5,6-tetra(9Hcarbazol-9-yl)isophthalonitrile)5 and sky blue light emitting DMAC-DPS (10,10'-(sulfonylbis(4,1phenylene))bis(9,9-dimethyl-9,10-dihydroacridine))6. Recently, we also reported evaporation- and solution-process-feasible highly efficient green light emitting ACRDSO2 (2-[4-(9,9-dimethyl9,10-dihydroacridine)phenyl]thianthrene-9,9’,10,10’-tetraoxide) and PXZDSO2

yellow light emitting

(2-(4-phenoxazinephenyl)thianthrene-9,9’,10,10’-tetraoxide)

molecules.7

These

TADF emitters realized ~20% EQE due to their small singlet-triplet splitting energy (∆EST) and thus effective reverse intersystem crossing (T1→S1, RISC). As TADF emitters could utilize all singlet and triplet excitons and realize high-efficiency without assistance of noble-metal, there is high potentiality to fabricate much lower cost but more promising OLED devices. For achieving ~100% internal quantum yield, TADF materials should actualize an efficient RISC process to capture 75% non-luminescent triplet excitons at room temperature. Therefore, a small ∆EST, usually 0.2-1.0 eV,5 is required as RISC rate constant kRISC∝exp(-∆EST/T). According to such requirement, the most feasible route to attain efficient TADF fluorophors is constructing donor-acceptor (D-A) charge transfer (CT) state molecules, which can effectively separate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). On the other hand, it is also expected to design a molecule with a large twist angle between the donor and acceptor planes, which is usually adopted to further insulate HOMO and LUMO. As the back of a coin, such orbital separation may also trigger efficiency loss because of simultaneously


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increased non-radiative transition.8 After all, a balance between increasing twist angle and keeping high efficiency should be properly controlled by molecular engineering. In previous studies, donor parts were mostly chosen from aromatic amines, such as triphenylamine,9 phenoxazine,10,11 acridine,6,12 carbazole,5,13,14 etc. Among these amines, carbazole is much more stable than the other amines and could also maintain high-efficiency with small ∆EST and enough insulation of HOMO and LUMO. To carry forward TADF materials to practical operations, materials with stable and simple carbazole donors may play an irreplaceable role to promote OLED technology for fullcolor display and white lighting applications. The energy gap between the first singlet and triplet excited states of thioxanthone (TXO) was reported lower than 0.3 eV.15,16 As it is known, the TXO core contains a ketone moiety which is alone able to produce delayed fluorescence.17,18 In this article, undecorated carbazole donor was connected to the meta-position of the ketone of TXO to control the intramolecular charge transfer (ICT) and thus to produce short wavelength emitters 2-cabazole-9H-thioxanthen-9-one (CzTXO) and 2,7-dicabazole-9H-thioxanthen-9-one (DCzTXO). The sulfane could be further oxidized to sulfonyl to give 2-cabazole-9H-thioxanthen-9-one-S,S-dioxide (CzSOXO) and 2,7-dicabazole9H-thioxanthen-9-one-S,S-dioxide (DCzSOXO) based on the 9H-thioxanthen-9-one-S,S-dioxide core (SOXO) (Scheme 1). With the aid of strong electron-withdrawing sulfonyl unit and further increased electron-withdrawing ability of the core, more stabilized CT can be achieved due to strengthened ICT by introduction of the sulfonyl unit. Linking the carbazole donor to the paraposition of the sulfonyl may further increase ICT effect to tune the emission to the longer wavelength. In addition, the stronger ICT effect may also induce a narrower ∆EST, which could do a favor for a more effective RISC process and thus probably give assistance in efficiency of TADF devices. In addition, efficiency roll-off of the TADF devices containing TXO or SOXO emitters


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were also discussed according to the photophysical data and built-up triplet-triplet annihilation (TTA) models.

Scheme 1 Chemical structures and synthetic routes of the thioxanthone derived compounds CzTXO, DCzTXO, CzSOXO, and DCzSOXO. (a) carbazole, 1,3-dimethyl-3,4,5,6-tetrahydro-2pyrimidone (DMPU), 18-crown-o-6, cuprous iodide, 160°C; (b) dichloromethane, acetic acid, 30% hydrogen peroxide, 80°C.

EXPERIMENTAL SECTION General. 1H and 13C NMR spectra were recorded on a Bruker NMR spectrometer operating at 600 and 150 MHz, respectively, in deuterated chloroform (CDCl3) solution at room temperature. Differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 209 under a N2 flow at a heating and cooling rate of 10 °C min-1. Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 under a N2 flow at a heating rate of 10 °C min-1. Cyclic voltammetry (CV) was performed on a CHI600D electrochemical workstation with a Pt working


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electrode and a Pt wire counter electrode at a scanning rate of 100 mV s-1 against a Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) reference electrode with a nitrogen-saturated anhydrous acetonitrile and dichloromethane (DCM) solution of 0.1 mol L-1 tetrabutylammonium hexafluorophosphate. UVvis absorption spectra were recorded on a HP 8453 spectrophotometer. Photoluminescence (PL) spectra were measured using a Jobin-Yvon spectrofluorometer. Organic films for optical measurements were evaporated onto pre-cleaned quartz substrates by thermal evaporation under high vacuum ~5×10−4 Pa. Photoluminescence quantum yields (PLQYs) of films were recorded at room temperature by using a photoluminescence quantum yield measurement system Hamamatsu Photonics C9920-02. For triplet states measurement, fluorescence and phosphorescence spectra were observed in THF solutions of the investigated organics (10-5 M) under atmospheric pressure at 77 K. Transient photoluminescence characteristics of the evaporation deposited films were collected by Hamamatsu Photonics C4334 at 300 K. Device preparation and measurements. Glass substrates pre-coated with a 95 nm thin layer of indium tin oxide (ITO) with a sheet resistance of 15-20 Ω sq-1 were thoroughly cleaned by ultrasonic of various detergents and treated with O2 plasma. OLED devices were fabricated onto the cleaned ITO-coated glass substrates by thermal evaporation under high vacuum (~5×10−4 Pa). Deposition rates are 0.5~2 Å/s for organic materials, 0.1 Å/s for LiF layer, and 5~6 Å/s for Al film, respectively. The current density-luminance-voltage (J-V-L) characteristics were measured by Keithley 2400 and Konica Minolta CS-200 electroluminescence measurement system. Electroluminescence (EL) spectra of the devices were recorded by spectrometer PR705 and power supply Keithley 2400. Theoretical Calculation. Density functional theory (DFT) calculations were performed on B3LYP/6-31G(d) basis set based on Gaussian suite of programs (Gaussian 03W) for ground state


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optimization. No negative mode was found, and it indicates that the optimized structures are in accordance with the minimum energy structures. B3LYP/6-31G(d, p) basis set were performed for excited state calculation. Based on the optimized structures of the excited states, the excited singlet energy (ES) and triplet energy (ET) were acquired to investigate the splitting energy between ES and ET, which is generally called ΔEST. Materials. All solvents and materials were used as received from commercial suppliers without further purification. 2-Bromo-9H-thioxanthen-9-one (1)19 and 2,7-dibromo-9H-thioxanthen-9-one (2)20 were synthesized according to literatures. Synthetic routes of CzTXO, DCzTXO, CzSOXO, and DCzSOXO are outlined in Scheme 1. All the developed materials were further purified by repeated temperature gradient vacuum sublimation. 2-cabazole-9H-thioxanthen-9-one (3, CzTXO). Under atmosphere, 1 (1.75 g, 6 mmol), 20 mL of 1,3-dimethyl-3,4,5,6-tetrahydro-2-pyrimidone (DMPU), CuI (0.167 g), K2CO3 (0.600 g), carbazole (1.20 g, 1.2 equ.), and 18-crown-o-6 (0.36 g) was added into a 100 mL three-necked bottle. The reaction mixture was stirred at 160°C for 24h. After termination of the reaction, the mixture was extracted with DCM. The solvent was removed by vacuum distillation. The oil residue was precipitated in methanol and the precipitation was further purified by column chromatography to afford a yellow powder (1.09 g, yield 48%). 1H NMR (600 MHz, CDCl3) δ 8.86 (d, J = 1.8 Hz, 1H), 8.64 (d, J = 8.0 Hz, 1H), 8.15 (d, J = 7.7 Hz, 2H), 7.84 (dd, J = 8.4, 1.9 Hz, 1H), 7.79 (d, J = 8.4 Hz, 1H), 7.69 – 7.61 (m, 2H), 7.52 (t, J = 7.4 Hz, 1H), 7.48 – 7.37 (m, 4H), 7.31 (t, J = 7.3 Hz, 2H).

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C NMR (151 MHz, CDCl3) δ 177.85, 142.80, 141.04, 139.75, 138.46, 219.44, 136.26,

134.97, 134.17, 136.26, 133.26, 136.26, 136.26, 125.81, 124.21, 123.66, 121.37, 120.67, 109.44. C25H15NOS 377.09, EI-MS (m/z): 378.2 (M+). Anal. calcd for C25H15NOS: C, 79.55; H, 4.01; N, 3.71; O, 4.24; S, 8.49. Found: C, 79.25; H, 4.37; N, 3.91; S, 8.12.


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2-cabazole-9H-thioxanthen-9-one-S,S-dioxide (5, CzSOXO). 3 (0.85 g, 2.25 mmol), 30 mL of DCM, 15 mL of AcOH, and 2.5 mL of H2O2 (22.5 mmol, 10 equ.) was stirred under atmosphere at 80°C for 24h. The reaction mixture was extracted with DCM and further purified by column chromatography to afford CzSOXO as a brown solid (0.81 g, yield 88%). 1H NMR (600 MHz, CDCl3) δ 8.58 (d, J = 2.2 Hz, 1H), 8.41 (d, J = 8.3 Hz, 1H), 8.37 (dd, J = 7.8, 1.1 Hz, 1H), 8.24 (dd, J = 7.8, 0.8 Hz, 1H), 8.14 (d, J = 7.8 Hz, 2H), 8.11 (dd, J = 8.3, 2.2 Hz, 1H), 7.92 (td, J = 7.7, 1.2 Hz, 1H), 7.82 (td, J = 7.7, 1.1 Hz, 1H), 7.46 (ddd, J = 13.5, 9.3, 4.6 Hz, 4H), 7.38 – 7.31 (m, 2H).

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C NMR (151 MHz, CDCl3) δ 177.85, 142.80, 141.04, 139.75, 138.46, 219.44, 136.26,

134.97, 134.17, 136.26, 133.26, 136.26, 136.26, 125.81, 124.21, 123.66, 121.37, 120.67, 109.44. C25H15NO3S 409.08, EI-MS (m/z): 410.3 (M+). Anal. calcd for C25H15NO3S: C, 73.33; H, 3.69; N, 3.42; O, 11.72; S, 7.83. Found: C, 73.23; H, 3.86; N, 3.41; S, 7.68. 2,7-dicabazole-9H-thioxanthen-9-one (4, DCzTXO). 4 was synthesized as a yellow solid in a similar manner of 3 with 2 instead of 1 (1.44 g, yield 64%), and carbazole was added with two times amount. 1H NMR (600 MHz, CDCl3) δ 8.88 (d, J = 2.3 Hz, 1H), 8.15 (d, J = 7.8 Hz, 2H), 7.89 (dd, J = 8.5, 2.3 Hz, 1H), 7.84 (d, J = 8.5 Hz, 1H), 7.46 (d, J = 8.2 Hz, 2H), 7.44 – 7.39 (m, 2H), 7.34 – 7.28 (m, 2H). 13C NMR (151 MHz, CDCl3) δ 178.78, 140.52, 136.61, 135.55, 131.24, 130.23, 127.94, 127.78, 126.23, 123.70, 120.50, 109.50. C37H22N2OS 542.15, EI-MS (m/z): 543.35 (M+). Anal. calcd for C37H22N2OS: C, 81.89; H, 4.09; N, 5.16; O, 2.95; S, 5.91. Found: C, 81.79; H, 4.28; N, 4.96; S, 5.65. 2,7-dicabazole-9H-thioxanthen-9-one-S,S-dioxide (6, DCzSOXO). 6 was synthesized as a yellow solid in a similar manner of 5 with 4 instead of 3 (1.24 g, yield 84%). 1H NMR (600 MHz, CDCl3) δ 8.62 (d, J = 2.1 Hz, 3H), 8.62 (d, J = 2.1 Hz, 3H), 8.47 (d, J = 8.4 Hz, 3H), 8.47 (d, J = 8.4 Hz, 3H), 8.19 – 8.13 (m, 10H), 8.20 – 8.12 (m, 10H), 7.51 (d, J = 8.2 Hz, 7H), 7.54 – 7.43 (m, 14H),


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7.46 (dd, J = 11.4, 3.9 Hz, 7H), 7.36 (t, J = 7.4 Hz, 7H), 7.36 (t, J = 7.4 Hz, 7H), 7.25 (s, 4H). 13C NMR (151 MHz, CDCl3) δ 177.44, 143.00, 139.73, 138.43, 132.59, 132.27, 126.63, 125.92, 124.27, 121.46, 120.71, 109.41. C37H22N2O3S 574.14, EI-MS (m/z): 575.3 (M+). Anal. calcd for C37H22N2O3S: C, 77.33; H, 3.86; N, 4.87; O, 8.35; S, 5.58. Found: C, 77.38; H, 4.03; N, 4.97; S, 5.45.

RESULTS AND DISCUSSION To understand the electronic and geometrical structure of these four thioxanthone derived materials, UV-vis absorption and PL spectra were measured as shown in Fig. 1c. The absorption band at ~420 nm in neat film could be attributed to ICT from the carbazole donor to the acceptor core. A wide wavelength scale of CT bands was found in all of these compounds, indicating strong ICT effect exists because of strong electron-withdrawing ability of both SOXO and TXO acceptors. Comparing the UV-vis absorption spectra of CzTXO in solution and neat film, disappearance of absorption subtle structures and existence of a slight emission band at ~570 nm indicate the existence of molecular aggregation induced excimer in neat film. Further evidence can be found in C-S bond length and optimized molecular conformation simulation (Fig. S3, Supporting Information) that TXO in the middle six-member ring may have a tendency to form more regular planar. So it is difficult to compare the ICT effect between CzTXO and other SOXO compounds base on their solid film absorption spectra. To further investigate their ICT abilities, photoluminescence spectra of their neat films were recorded. Comparing PL spectra of bilateral dicarbazole compounds DCzTXO and DCzSOXO and unilateral carbazole compounds CzTXO and CzSOXO, respectively, an obvious spectrum red-shift and larger full width at half maximum


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(FWHM) was found for the SOXO molecules, which is induced by the SOXO core with stronger electron-withdrawing ability and enhanced ICT effect than the TXO core. The PL spectra FWHM of CzSOXO, DCzSOXO, CzTXO, and DCzTXO in toluene matrix are 100, 99, 59, and 61 nm. As shown in Fig. 1c, PL spectra of CzSOXO, DCzSOXO, DCzTXO, and DCzTXO peak are at 448, 505, 541, and 572 nm, respectively, covering a wide range from blue to yellow. It is of interest diversified emission color can be realized based on just one material system by solely changing the valence states of the sulfur atom. In addition, a noticeable long tail and red-shifted peak were found for the neat films of CzTXO and DCzTXO, probably originating from aggregation induced intermolecular accumulation and excimer formation. To distinguish excited states of the emitters, PL spectra of the emitters were measured in solvents with different polarity (Fig. S1, Supporting Information). All the compounds display obvious solvation chromaticity such that the emission spectrum exhibits a clear bathochromic shift from nonpolar hexane to higher polar N,N-dimethylformamide. Meanwhile, CzSOXO and DCzSOXO consisting of sulfonyl show further bigger solvation shift compared with the corresponding sulfane containing emitters CzTXO and DCzTXO in different solvents. This increase of dipole moment from the ground state to the excited state was calculated by Lipper−Mataga calculation. As shown in Supporting Information, large slopes of 7086.8 (R=0.70), 7921.8 (R=0.73), 14404 (R=0.88), and 15123 (R=0.87) were found for CzTXO, DCzTXO, CzSOXO, and DCzSOXO, respectively. In addition, increase of dipole moments for CzTXO (12.1 D), DCzTXO (15.3 D), CzSOXO (17.9 D), and DCzSOXO (21.8 D) was obtained (Fig. S2, Supporting Information), indicating that all the four compounds are CT state emitters, meanwhile the CT trend is strengthened when sulfane is oxidized to sulfonyl. Besides the increase of dipole moments, the decrease of emission intensity in stronger polar solvent may also be helpful to


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explain the CT character of the four compounds, especially for CzSOXO and DCzSOXO (Fig. S1, Supporting Information).

Fig. 1 (a) Molecular structures and HOMO/LUMO distributions of the SOXO and TXO compounds (based on DFT/B3LYP/6-31G(d) level theory). (b) Cyclic voltammograms of the materials. (c) UV-vis absorption and PL spectra of the SOXO and TXO compounds (toluene solution: , neat film: ).

Based on molecular orbital distributions via DFT/B3LYP/6-31G (d) level theory (Fig. 1a), the C-S bond lengths of the TXO and SOXO molecules are 1.76 and 1.79 Å, respectively. The


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increase of the C-S bond length in the middle six-member ring may induce a more irregular planar in the SOXO unit. So compared with the SOXO compounds, the TXO molecules will be more inclined to aggregate and then cause aggregation-caused quenching (ACQ) problem. For avoidance, the TXO derived emitters should be dispersed in an appropriate host material during device fabrication. Besides, both the SOXO and TXO compounds show pretty large twisted angle between the donor and acceptor planes. The large twisted angle can further insulate HOMO and LUMO to generate a negligible ∆EST. To experimentally evaluate ∆EST values of the developed compounds, their fluorescence and phosphorescence spectra in THF solutions were measured at 77 K. S1 and T1 are estimated from the onset of the fluorescence and phosphorescence spectra, respectively. According to Fig. S4 (Supplementary Information), ∆EST values of CzSOXO and DCzSOXO in THF matrix are estimated to be 0.05 and 0.01 eV, in contrast with 0.10 and 0.06 eV for CzTXO and DCzTXO, respectively. On account of better separation of HOMO and LUMO in the SOXO emitters, the SOXO compounds display a bit smaller ∆EST than the TXO compounds, which are consistent with the corresponding theoretical values obtained by M06-2X/6-31G* (Table 1). The smaller ∆EST gives an enhancement in reversed intersystem crossing rate constant kRISC and finally helps to improve electroluminescence efficiency of TADF devices possibly. Furthermore, it seems that the bilateral donor-acceptor-donor (D-A-D) structures own smaller ∆EST than unilateral donor-acceptor (D-A) structures. Summary of thermal stability and optical properties of the TXO and SOXO compounds are demonstrated in Table 1. It can be clearly seen that the bilateral D-A-D compounds show much higher decomposition temperature (Td) and glass transition temperature (Tg) than the unilateral DA compounds, and their thermal stability could be further improved by oxidation of sulfane to sulfonyl, due to improved molecular rigidity and intermolecular interaction.21 As estimated by


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oxidation potentials of these compounds, both the SOXO and TXO compounds have similar HOMO levels of ~-5.7 eV, the bilateral D-A-D compounds shows a lower-lying LUMO level maybe due to its one more electron-donating carbazole unit, compared to the D-A molecules. When sulfane is oxidized to give sulfonyl, lower-lying LUMO levels were obtained for the SOXO compounds CzSOXO and DCzSOXO due to the increase of electron-withdrawing ability of the SOXO core and further stronger ICT effect. Relatively lower-lying LUMO levels of -3.42 and 3.49 eV were found for CzSOXO and DCzSOXO, which may easily cause electroplex interactions with shallow HOMO level of the adjacent hole-transporting layer.

Table 1 Thermal stability and physical properties of CzTXO, DCzTXO, CzSOXO, and DCzSOXO Compounds

Td/ Tg(℃)

HOMO/ LUMO* (eV)

Theoretical values**

Measured values***

S1

T1

∆EST

S1

T1

∆EST

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

CzSOXO

358.1/ 98.2

-5.76/ -3.42

2.39

2.25

0.14

2.65

2.60

0.05

DCzSOXO

430.8/ 157.8

-5.75/ -3.49

2.30

2.16

0.14

2.56

2.55

0.01

CzTXO

332.4/ 73.9

-5.65/ -2.83

2.84

2.57

0.27

2.76

2.66

0.10

DCzTXO

427.1/ 141.9

-5.63/ -2.92

2.71

2.42

0.29

2.63

2.58

0.06

* Estimated

from the empirical formula: EHOMO = −(Eox+4.29) eV, ELUMO = −(Ere+4.29) eV;

** Theoretical *** Measured

values of S1, T1, and ∆EST simulated by M06-2X/6-31G*;

values of S1, T1, and ∆EST estimated from the fluorescence and phosphorescence spectra of the

investigated organics in THF solution at 77 K.

In order to demonstrate TADF phenomenon and further understand the structure-property relationships of these TADF emitters, transient photoluminescence decays of their solid films were also measured. As shown in Fig. 2, delayed fluorescence appeared in both the doped and neat films


13


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of the SOXO compounds, suggesting the SOXO emitters can utilize T1 excitons through RISC process for delayed fluorescence emission. Based on optimized exponential curve fitting, we obtain multi delayed lifetimes in various proportions of τD1 = 0.982 μs (delayed lifetime proportion ηD1 = 0.40) and τD2 = 14.31 μs (ηD2 = 0.14) for CzSOXO, and τD1 = 1.457 μs (ηD1 = 0.16) and τD2 = 10.09 μs (ηD2 = 0.45) for DCzSOXO. The short delayed lifetimes of microsecond scale mean triplet excitons will rapidly transfer from T1 state to S1 state, and thus reduce triplet exciton aggregation and TTA annihilation. The more easily detected and short delayed fluorescence lifetimes for the SOXO emitters could be attributed to their smaller ∆EST and realize triplet to singlet up-conversion more easily. For the neat film of the unilateral D-A structure CzSOXO, the proportion of the delayed fluorescence enhances to 0.78, compared with its doped film (0.54). However, the delayed fluorescence proportion reduces to 0.08 for the neat film of the bilateral DA-D structure DCzSOXO. It is supposed that the symmetrical DCzSOXO is more facile to induce intermolecular interaction and exciton quenching. RISC rate constant (kRISC) and RISC efficiency (ϕRISC) of CzSOXO and DCzSOXO are 1.2×105 s-1 (ϕRISC = 0.24) and 3.7×104 s-1 (ϕRISC = 0.02), respectively. An enhanced delayed fluorescence proportion of the CzSOXO neat film indicates its application potential for nondoped TADF devices with a simplified device structure. Compared with the SOXO compounds, delayed fluorescence lifetimes of 1.86 and 0.39 ms are obtained for the sulfane containing TXO compounds CzTXO and DCzTXO, respectively. As the thioxanthone core possesses weaker electron-withdrawing ability and relatively larger ∆EST, it should be more difficult for RISC process from T1 to S1. As a result, the non-radiative decay constants of T1 (kTnr ) will increase, and the millisecond scale delayed fluorescence lifetime probably lead to severe efficiency roll-off as concentrated triplet excitons will cause emission quenching in the emission layer.


14

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Fig. 2 Transient photoluminescence decays of the neat and doped films of (a) DCzSOXO and (b) CzSOXO and the doped films of (c) CzTXO and DCzTXO in nitrogen under 300 K (doped films: CzSOXO and DCzSOXO doped into mCP, 3 wt%; CzTXO and DCzTXO doped into DPEPO, 3wt%).


15


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PLQYs of the investigated compounds are measured in solid films at room temperature, as summarized in Table 2. PLQYs of the CzSOXO and DCzSOXO doped films in mCP (1,3-di(9Hcarbazol-9-yl)benzene) are 0.512 and 0.412, respectively. Considering the relatively high triplet energy

of

the

TXO

compounds

CzTXO

and

DCzTXO,

DPEPO

(bis(2-

(diphenylphosphino)phenyl)ether oxide) was utilized as their host, and their doped films show PLQYs of 0.296 and 0.348, respectively. Compared with the doped films, obviously reduced PLQYs are obtained as 0.117, 0.102, 0.290, and 0.193 for the neat films CzTXO, DCzTXO, CzSOXO, and DCzSOXO, respectively. Consistent with the results of PL spectra, existence of intermolecular aggregation and excimer quenching in the TXO neat films result in poorer PLQYs. According to the molecular orbital distribution calculation shown in Fig. 1a, the planar carbazole donors probably induce intermolecular π-π stacking in neat films, which will be restricted in doped films. The intermolecular π-π stacking between the carbazole donors may form triplet trap species and thus emission quenching of the films.22 Therefore, it might be difficult to maintain efficient quantum yields compared with the doped films. Besides, obvious red-shift of PL spectrum was also observed for CzSOXO from 506 nm of the doped film to 541 nm of the neat film (Fig. 4b), as well as the EL spectra in multilayer devices. This phenomenon is mainly connected to the intermolecular interactions from π-π stacking between the carbazole units. It is supposed intermolecular π-π stacking may equally occur in the bilateral DCzSOXO molecules.


16

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Table 2 Photoluminescence quantum yields (PLQYs) of the neat and doped films of CzTXO, DCzTXO, CzSOXO, and DCzSOXO ηPL*/ %

CzSOXO

DCzSOXO

CzTXO

DCzTXO

neat film

29.0

19.3

11.7

10.2

doped film**

51.2

41.2

29.6

34.8

* PLQYs **

of the thin solid films are measured in air at room temperature;

Doped films consist of 10 wt% CzTXO doped in DPEPO, 10 wt% DCzTXO doped in DPEPO, 3 wt%

CzSOXO doped in mCP, and 3 wt% DCzSOXO doped in mCP, respectively.

To investigate their potential as a fluorescent emitter, multilayer devices were fabricated by doping the developed compounds into an appropriate host according to the formerly discussed photoluminescence measurements. For the sulfonyl containing SOXO compounds CzSOXO and DCzSOXO, multilayer devices were fabricated with following structure: ITO (95 nm)/ HATCN (5 nm)/ 1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane (TAPC) (20 nm)/ emission layer (EML, 25 nm)/ 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) (55 nm)/ LiF (1 nm)/ Al (80 nm) (Device A). Here, TAPC (T1 = 2.87 eV)23 and TmPyPB (T1 = 2.75 eV)24 are utilized as holetransporting layer (HTL) and electron-transporting layer (ETL) with perfect carrier transportation and higher T1 energy level than the doped guests to confine triplet excitons within the EML, as described in Fig. 3a. EML is consisting of CzSOXO or DCzSOXO (3 wt%) doped in mCP with a thickness of 25 nm. For the sulfane containing TXO compounds CzTXO and DCzTXO, multilayer devices have been fabricated in a structure of ITO (95 nm)/ NPB (30 nm)/ TCTA (20 nm)/ CzSi (10 nm)/ CzTXO or DCzTXO (10 wt%): DPEPO (25 nm)/ DPEPO (10 nm)/ TPBi (30 nm)/ LiF (1 nm)/ Al (80 nm) (Device B). CzSi (T1 =3.1 eV) and DPEPO (T1 =3.4 eV) thin films adjacent to the EML are exciton-blocking layers (EBL) with enough high T1 level to confine triplet excitons within the EML and reduce efficiency roll-off.


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Page 18 of 30

Fig. 3 (a) Schematic diagrams of the devices based on the developed TXO (Device A) and SOXO (Device B) emitters and chemical structures of the materials used. (b) External quantum efficiency (EQE) versus current density characteristics of the devices based on the TXO (Device A) and SOXO (Device B) compounds (insert: electroluminescence spectra at 1 mA/cm2). (c) Current density and luminance versus voltage (J-V-L) characteristics of the devices based on the TXO (Device A) and SOXO (Device B) compounds.


18

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According to the electroluminescence results shown in Fig. 3b and 3c, yellow-greenish CzSOXO and DCzSOXO with short delayed fluorescence lifetime τD reach favorable maximum EQEs of 13.6% and 10.3%, and remain 8.8 % and 6.5 % at 1000 cd m-2. The limited efficiency roll-off are benefited from their rapid RISC process. According to the calculations, kRISC are described to be 2.4×105 s-1 for CzSOXO, and 3.2×104 s-1 for DCzSOXO.25 Meanwhile, the SOXO compounds may have a bipolar transporting advantage due to the electron-withdrawing sulfonyl, and it may induce an improved carrier balance in a wide current density range. For giving the more completed trend in variation of acceptor, the carbonyl in CzSOXO is further replaced by sulfonyl to give CzDSO2 (Fig. S5a, Supporting Information). The acceptor of CzDSO2 owns the moderate electron-withdrawing ability and ICT effect. For the similar unilateral D-A compounds with the same carbazole donor but different acceptors, their HOMO levels are almost consistent. However, their LUMO levels are lower-lying in the order of CzTXO< CzDSO2< CzSOXO because of the increased electron-withdrawing ability of the acceptor. As a result, greenish-blue emission with a relatively low EQE of 8.6% was obtained for the device based on CzDSO2 (10 wt%): mCP, and the emission wavelength from CzDSO2 is between the emissions from CzTXO and CzSOXO (Fig. S5b, Supporting Information). In spite of relatively low PLQYs, maximum EQEs of 11.2 % and 11.5 % could still be achieved by blue emitters CzTXO and DCzTXO with CIE coordinates of (0.16, 0.20) and (0.18, 0.30), respectively. Electroluminescence properties of the TADF devices based on CzTXO, DCzTXO, CzSOXO, and DCzSOXO are summarized in Table 3. Similar to their PL spectra, ultra-wide color scope from blue to yellow radiation could be realized by changing the sulfur atom valence states of thioxanthone. Compared with the devices based on the SOXO compounds, the devices based on the TXO compounds suffer from a severe


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Page 20 of 30

efficiency roll-off, which could be attributed to their long period of delayed fluorescence lifetimes. Such millisecond scale lifetime may easily trigger triplet excition concentration, then result in triplet-triplet annihilation. Although the significant difference in efficiency roll-off may also partially originate from different carrier balance in the current devices, the serious efficiency rolloff could be mainly attributed to coefficient of TTA, as the increase of current density.26 As aforementioned, delayed fluorescence was also found for the neat films of the SOXO compounds. Simplified nondoped devices were also fabricated by using a neat film of CzSOXO as an emission layer in a structure of ITO (95 nm)/ HATCN (5 nm)/ TAPC (20 nm)/ mCP (20 nm)/ CzSOXO (20 nm)/ TmPyPB (50 nm)/ LiF (1 nm)/ Al (80 nm). In consideration of deep LUMO level of CzSOXO (-3.42 eV) and shallow HOMO level of adjacent TAPC (-5.42 eV), the red-NIR emission observed for the device without thin mCP layer (Fig. S6, Supporting information) should be contributed to electroplex that only appears under the electric excitation.27 Therefore, a wide bandgap and triplet exciton limitation mCP layer was inserted between TAPC and EML to avoid red-NIR electroplex emission. Fig. 4a shows the EQE versus current density characteristics of the nondoped devices. A maximum EQE of 5.7 % with further reduced efficiency roll-off was achieved to give pure yellow light emission with CIE coordinates of (0.49, 0.50) (at 100 cd m-2). Similar to its PL spectra, EL spectra of the nondoped devices locate at 580 nm, compared with greenish 530 nm in the doped ones. As aforementioned, by inserting a wide bandgap (HOMO = -5.9 eV, LUMO = -2.4 eV) thin mCP layer, the red-NIR electroplex emission can be successfully avoided. For further improvements, molecular engineering like introducing a bulky or twisted donors may be beneficial for suppressing emission quenching induced by intermolecular interactions.28


20

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Fig. 4 (a) External quantum efficiency (EQE) versus current density characteristics of the nondoped device based on CzSOXO; (b) Comparison of the photoluminescence spectra of the doped and neat films of CzSOXO and electroluminescence spectra of the doped and nondoped devices at 1 mA cm-2 (dash lines: photoluminescence spectra, solid lines: electroluminescence spectra).

Table 3 Summary of the electroluminescence properties of CzTXO, DCzTXO, CzSOXO, and DCzSOXO with doped or nondoped EMLs Compounds CzSOXO DCzSOXO CzSOXO CzTXO DCzTXO

Host

mCP nondoped DPEPO

VON*

Max. Efficiency

EQE1000*

Lmax

CIE (x, y)

(%)

(cd m-2)

(@ 100 cd m-2)

13.6

8.8

8466

(0.37, 0.57)

31.8

10.3

6.5

8054

(0.42, 0.55)

4.2

13.6

5.7

4.0

6115

(0.49, 0.50)

5.9

18.0

11.2

-

183

(0.16, 0.20)

5.4

26.1

11.5

-

627

(0.18, 0.37)

CEmax

EQEmax

(cd/A)

(%)

3.8

43.1

3.8

(V)

* VON and EQE1000 are obtained at 1 and 1000 cd m-2 respectively.

Since TADF emitters can realize high efficiency through radiative utilization of triplet excitons via RISC process. Based on their photoluminescent characteristics, theoretical external quantum efficiency (Cal. EQE) could be estimated by following equation:29


21


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𝐶𝑎𝑙. 𝐸𝑄𝐸 = 𝛾 [0.25 𝜂PL + 0.75

𝜙𝐷𝐹 ]𝜂 1− (𝜂PL − 𝜙𝐷𝐹 ) 𝑜𝑢𝑡

Page 22 of 30

(1)

In equation (1), γ is the ratio of charge recombination to the electron and hole transportation. Generally, γ is assumed to be 1. ηPL is the photoluminescence efficiency, ΦDF is the photoluminescence quantum yield of the delayed component, and ηout is the light output coefficient. Here, ηout is assumed to be 0.30. For the SOXO emitters, Cal. EQEs of the doped devices are estimated to be 12.0 % (CzSOXO) and 9.8 % (DCzSOXO). The calculation results are highly consistent with the experimental EQEs under electrical excitation, which mean the fabricated devices are under perfect carrier balance as we assumed before. Despite of the similar ΦDF, the higher PLQY of 0.512 for CzSOXO leads to better efficiency performance compared with DCzSOXO. On the other hand, Cal. EQEs of the nondoped devices are highly concerned by delayed fluorescence enhancement. Although the PLQY of the CzSOXO neat film is only 0.290, its ηD reaches a high value of 0.775. Theoretical EQEs for the CzSOXO based nondoped device still exceed fluorophore restriction (7.6%). The experimental EQE value of 5.7% is also very close to the theoretical one. Among those TADF devices, most of them are mainly suffered from concentration quenching and triplet-triplet annihilation, which will mainly result in efficiency roll-off and the decline of device luminance. Similarly, singlet-triplet annihilation (STA) and other mechanism may also contribute to the roll-off. In order to simplify the variable parameters in efficiency roll-off simulations, TTA annihilation process is discussed here for understanding the device efficiency roll-off. Considering doping concentration and film thickness are well optimized and the experimental EQE values are almost consistent with the theoretical ones, carrier balance factor is assumed to be 1 in TTA model (equation (2)):30


22

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𝜂 𝜂0

=

𝐽 𝐽0

𝐽

[√1 + 8 𝐽 − 1] 0

(2)

where η, ηo and Jo represent EQE in the presence of TTA, the highest EQE value of OLEDs and the current density at the half maximum of the EQE. Simulation results are described in Fig. S7 and Table S3 (Supporting Information). Based on minimum error principles, Jo values of the doped devices based on CzSOXO and DCzSOXO are 10.8 and 13.1 mA cm-2, compared with those of 0.09 and 0.16 mA cm-2 for the doped devices based on CzTXO and DCzTXO, respectively. In spite of different device structures, the devices based on the TXO compounds show about 10 times smaller Jo than those of the SOXO compounds, which indicate the TXO compounds suffer from more severe triplet-triplet annihilation and efficiency roll-off as the increase of current density. The smaller Jo is mainly result from the longer delayed fluorescence lifetime (several millisecond seconds) and more likely to induce concentrated triplet annihilation. Furthermore, for the devices based on the TXO molecules, the TTA fitting lines deviate from the experimental EQE values at large current density, which suggests STA may take control as increase of current density and are agreed with the recent TADF exciton annihilation works.25,31, 32

CONCLUSIONS In summary, thioxanthone derivatives of CzTXO and CzSOXO with a D-A structure and DCzTXO and DCzSOXO with a D-A-D structure were developed as fluorescent emitters to fabricate efficient OLEDs. Blue to yellow emission was successfully achieved based on one material system with tunable ICT effect and π-conjugation by combining the thioxanthone core with undecorated carbazole. Among them, maximum EQE of 13.6% was achieved for the green light emission


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Page 24 of 30

CzSOXO with CIE coordinates of (0.37, 0.57). Blue emission CzTXO and DCzTXO could also give EQEs exceeding 11%, which indicate an effective energy transfer from the host to the TADF dopant. However, both of these compounds suffered from severe efficiency roll-off because of long delayed fluorescence lifetimes. According to TTA evaluations, both the devices based on the TXO and SOXO compounds are dominated by TTA model. It is of interest delayed fluorescence was still observed in the neat film of CzSOXO. Benefited from its high ηD value of 0.775, a maximum EQE of 5.7 % was still achieved for the simplified nondoped devices of CzSOXO with pure yellow emission and further reduced efficiency roll-off. The current contribution gives a platform for thorough understanding of molecular structures and their relationships with photophysical property and device performance.

ASSOCIATED CONTENT Supporting Information Lippert-Mataga calculation and solvatochromic characteristics, optimized molecular conformation simulations, transient decay behaviors of CzSOXO, DCzSOXO, CzTXO, and DCzTXO, electroplex formation energy level diagram, and external quantum efficiency calculation and TTA efficiency roll-off simulations of the fabricated devices. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


24

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* E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors greatly appreciate the financial support from the Ministry of Science and Technology (2015CB655003 and 2014DFA52030), the National Natural Science Foundation of China (91233116 and 51573059), and Guangdong Provincial Department of Science and Technology (2014B090901048). The authors are grateful to Shitong Zhang for his help in low temperature phosphorescence measurements.

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TOC GRAPHIC

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