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Synthesis and properties of ipsilateral double substituted diphenylsulfone thermally activated delayed fluorescent materials
Journal Pre-proof Synthesis and properties of ipsilateral double substituted diphenylsulfone thermally activated delayed fluorescent materials Di Zhang, Huaixin Wei, Yan Wang, Guoliang Dai, Xin Zhao PII:
S0143-7208(19)31752-8
DOI:
https://doi.org/10.1016/j.dyepig.2019.108028
Reference:
DYPI 108028
To appear in:
Dyes and Pigments
Received Date: 27 July 2019 Revised Date:
10 October 2019
Accepted Date: 5 November 2019
Please cite this article as: Zhang D, Wei H, Wang Y, Dai G, Zhao X, Synthesis and properties of ipsilateral double substituted diphenylsulfone thermally activated delayed fluorescent materials, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.108028. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Two novel donor-acceptor (D-A) type materials containing ipsilateral double substituted diphenylsulfone were designed and synthesized for application in thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs).
Synthesis and Properties of Ipsilateral Double Substituted Diphenylsulfone Thermally Activated Delayed Fluorescent Materials
Di Zhang1, Huaixin Wei1, 2*, Yan Wang1, Guoliang Dai1, Xin Zhao1* 1. College of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, Suzhou, Jiangsu, 215009, China. 2. Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft
Materials (FUNSOM), Soochow University, Suzhou，Jiangsu, 215123, China.
Abstract Two novel donor-acceptor (D-A) type thermally activated delayed fluorescent (TADF) materials 2PXZDPS and 2DMACDPS are synthesized by attaching two donors phenoxazine (PXZ) or 9,9-dimethyl-9,10-dihydroacridine(DMAC) to the ipsilateral side of the acceptor diphenylsulfone group. The two materials show high yield, good thermal properties, suitable photophysical properties, high triplet energy and small energy difference between singlet and triplet states, leading to feasible thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). Both 2PXZDPS and 2DMACDPS show excellent performance in OLED. 2PXZDPS based OLED give the maximum external quantum efficiency (EQEmax) of 20.7% and low turn-on voltage of 2.2V. Keywords：organic light-emitting diodes, diphenylsulfone, thermally activated delayed fluorescent, photoelectric performance Corresponding author: Huaixin Wei, Address: School of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, 215009 Suzhou, China, Email: [email protected]
Xin Zhao, Address: School of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, 215009 Suzhou, China, Email: [email protected]
Introduction In recent years, organic light emitting diodes (OLEDs) have attracted more and more attention due to their potential application in flat-panel displays and large-area solid state lighting
[1-3]
. Although dramatic breakthroughs have been achieved through many
researchers’ efforts, there still exist some difficulties hampering the future development of OLED.
It is well known that only singlet exciton can decay radiatively in traditional
fluorescent materials according to the spin statistics. Approximately 75% of the triplet excitons are wasted in nonradiative processes, leading to an upper limit of the internal quantum efficiency (IQE) of 25%, which is not suitable for the application of Industrial production [4]. In contrast, phosphorescent materials can approach 100% IQE than traditional fluorescent materials due to spin-orbit coupling of the heave atom effect. However, the choice of precious metals used in phosphorescent materials will increase high cost, in addition, the triplet-triple annihilation under the condition of high concentration in phosphorescent materials are remain to be solved [5-9]. how to make pure organic fluorescent materials to achieve a comparable high EQE with those of high efficiency PHOLEDs have attracted much interest by researchers. In recent years, thermally activated delayed fluorescence (TADF) materials obtained more and more attention due to its ability to increase the singlet exciton yield of fluorescent OLEDs [10-11]
. The common method at present is to design bipolar molecules containing a
pre-twisted donor-acceptor (D-A) type structure with intramolecular charge transfer (ICT) effect. In the pre-twisted D-A molecule, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are located on the electron donor group (D)
and the electron acceptor group (A), respectively. HOMO and LUMO of the molecule are easily separated, resulting in a smaller energy difference (∆EST) between the S1 and T1 states [12-13]
. Small ∆EST is conducive to the reverse intersystem crossing (RISC) and TADF
performance. Meanwhile, the limited overlap of HOMO and LUMO of the D-A molecule is favorable for obtaining high PLQYs by choosing suitable donor and acceptor. It is well known that Diphenyl sulfone (DPS) units are the more typical electron-transporting
groups
in
TADF
materials
because
of
their
excellent
electron-withdrawing ability. Phenoxazine (PXZ) and 9,9-dimethyl-9,10-dihydroacridine (DMAC) were selected as the electron donor to connect the ipsilateral side of the diphenylsulfone (DPS) phenyl. According to previous studies, steric hindrance of the space occurs when two donors are connected on the ipsilateral side. The twist angle between the donor and the acceptor will increase under the steric hindrance effect which can effectively separating HOMO and LUMO. Finally, a smaller energy difference (∆EST) was obtained between the S1 and T1 states. In 2014, Zhang[15]et.al reported two novel TADF materials by connecting two PXZ donors or DMAC donors on the opposite side of DPS. The two compounds exhibited superior TADF properties and the performance of corresponding OLED devices achieved the maximum EQE of 19.5% and 17.5% with the turn-on voltage values of 2.7 and 3.7 V, respectively. In the same year, Michael P. Gaj[14] et al. designed and synthesized the TADF material mCPSOB which with two carbazole units at the two meta positions of the benzene ring on the diphenylsulfone side, the corresponding devices show the maximum external quantum efficiency EQE as high as 26.5%, which surpasses all OLEDs at that time. It is maybe the increased steric hindrance effect by connecting the two
electron donors on the same side, resulting in raised dihedral angle between donor (D) and acceptor (A), reduced small ∆EST and improved device efficiency. According to previous reports, it is clearly that diphenyl sulfone modified by a strong electron-withdrawing group with a twist angle will show enhanced fluorescent. However, the molecules reported are mostly symmetrical structure, asymmetric structure with donors in one side are still scarce, and it is interest to clarify the properties of asymmetric molecule for the understanding and optimization of TADF materials. In this paper, we report two novel efficient diphenylsulfone TADF materials, 2PXZDPS and 2DMACDPS (as shown in Fig.1).
The widely used hole-transporting moiety,
phenoxazine (PXZ) and 9,9-dimethyl-9,10-dihydroacridine (DMAC) were selected as the electron donor to connect the same side of the diphenylsulfone (DPS) phenyl for increasing the distortion of the molecular structure as a design center. 2PXZDPS and 2DMACDPS exhibit higher photoluminescence quantum yields (PLQYs) and obvious TADF phenomena. The∆EST values are 0.02eV and 0.08eV, respectively. Besides, the maximum external quantum efficiency values of the corresponding OLEDs based on 2PXZDPS and 2DMACDPS were as high as 20.7% and 19.2%, respectively. In addition, the turn-on voltage values of the two TADF OLEDs based on 2PXZDPS and 2DMACDPS were as low as 2.2V and 3.2V. To the best of our knowledge, performance of the devices are among the best of OLEDs with diphenylsulfone as emitters. Results and discussion The TADF molecules 2PXZDPS and 2DMACDPS were synthesized by connecting two phenoxazine (PXZ) donors or 9,9-dimethyl-9,10-dihydroacridine(DMAC) donors to the
same side of the diphenylsulfone (DPS). The detailed synthetic routes and chemical structure of 2PXZDPS and 2DMACDPS are shown in Fig.1 and supporting information. The melting point of 2PXZDPS is around 169 ℃ and the decomposition temperature (mass loss 5%) Td is 408 ℃ (Fig.S4). The melting point of 2DMACDPS is around 215℃ and the decomposition temperature (mass loss 5%) Td is 388 ℃ (Fig.S4). Both molecules exhibit higher thermal stability because of the introduction of two donor units, which increased spatial distortion of the compound. In addition, it is clearly that the molecular weight of the compound was increased by adding the sulfone unit result in prominent thermal stability. Theoretical calculation of 2PXZDPS and 2DMACDPS was carried out by using the Gaussian-03 program. The structures of two compounds were determined and optimized by density functional theory (DFT) B3LPY/6-31G(d) [17-18]. Optimized structures were used for the calculation of HOMO and LUMO energy levels. As shown in Fig. 2, the two compounds possessed larger dihedral angles between the donor and the acceptor unit and larger spatial distortion in the optimized geometries. Moreover, the HOMOs and LUMOs of the two compounds are almost completely separated, in which the HOMOs are mainly localized at the electron-donating PXZ or DMAC units with a slight extent on the adjacent benzene ring but the LUMOs are mainly localized at DPS moiety. The almost completely separation and limited overlap of the frontier molecular orbitals (FMOs) will endow the two compounds small ∆ESTs and high PLQYs [19]. The experimental HOMO and LUMO energy levels of the two molecules were measured by cyclic voltammetry (CV). As shown in Fig.S5, The initial positions of oxidation peaks of 2PXZDPS and 2DMACDPS are 0.95 and 1.05 V, respectively. The oxidation peak starting position of the internal standard ferrocene is
located at 0.44 and 0.43 V, respectively. The HOMO and LUMO energy levels of the compounds 2PXZDPS and 2DMACDPS were -5.31, -5.42 eV and -2.42, -2.41 eV, respectively. In order to verify the theoretical calculation, cyclic voltammetry (CV) measurement were carried out to analyze the value of HOMO and LUMO energy levels. As shown in Fig.S5, it is clearly that the first oxidation peak of 2PXZDPS and 2DMACDPS are at 0.96 eV and 0.97 eV. The starting position of the internal standard ferrocene oxidation peak is located at 0.34 eV and 0.34 eV, respectively. According to the photophysics properties shown in Fig. 3, The HOMO and LUMO of compound 2PXZDPS were -5.42 eV and -2.43 eV, respectively and 2DMACDPS were -5.43 eV and -2.42 eV, respectively. Fig.3 shows the normalized spectrum of UV-Vis, PL and Low Tem Phos of 2PXZDPS and 2DMACDPS. It is clearly to see that the UV absorption of 2PXZDPS is at 238 and 317 nm in dilute solution of 10-5 mol/L-1 in CH2Cl2, which attribute to the π-π * charge transition in phenoxazine and the π-π * electronic transition. The optical bandgap (Eg) was calculated to be 2.89 eV based on the absorption band edge of the 2PXZDPS (429 nm). 2DMACDPS shows three absorption peaks at 228 nm, 280 nm and 364 nm, respectively. The three peaks corresponding to the π-π * transitions of benzene ring and sulfone group, respectively. The optical band gap (Eg) was calculated to be 3.01 eV based on the absorption edge (411 nm) of 2DMACDPS. The fluorescence emission peaks of 2PXZDPS and 2DMACDPS in the dilute solution of 10-5 mol/L-1 CH2Cl2 are located at 590 nm and 531 nm at room temperature, respectively. The low temperature phosphorescence emission curves of 2PXZDPS and 2DMACDPS
were also measured at 77K by using 2-methyltetrahydrofuran as the solvent. As shown in Fig.3, the peak position of the fluorescence emission peak of 2PXZDPS was at 444 nm. The calculated singlet energy level (ES) was 2.79 eV, and the peak of the phosphorescence emission peak was at 448 nm. The calculated triplet level (ET) is 2.77 eV, and its singlet-triplet energy level difference (∆EST) is 0.02 eV. The peak position of the fluorescence emission peak of 2DMACDPS was at 414 nm. The calculated singlet energy level (ES) was 3.00 eV, and the peak position of the phosphorescence emission peak was at 425 nm. The calculated triplet level (ET) is 2.92 eV, and the singlet-triplet energy level difference (∆EST) is 0.08 eV. It is apparently to see that both 2PXZDPS and 2DMACDPS have a small ∆EST, which is favorable for reverse intersystem crossing (RISC) of triplet excitons and improvement of delayed fluorescence properties. The detailed performance data of 2PXZDPS and 2DMACDPS are listed in Table 1. In order to investigate the fluorescence properties of 2PXZDPS and 2DMACDPS, the fluorescence profiles of the two materials were studied in the presence of oxygen and without oxygen (Fig.S6). It is clearly that the fluorescence peaks of the two materials are obvious higher than that in oxygen. The fluorescence quantum yields of 2PXZDPS and 2DMACDPS are 60.1% and 85.8% after deoxygenation with nitrogen bubbling. It is well known that oxygen can quench the triplet exciton and reduce the fluorescence quantum yield of TADF molecules, thus the fluorescence quantum yields of the two compounds are increased by 6.6% and 8.2% after deoxygenation, respectively. The transient PL decay curves of 2PXZDPS and 2DMACDPS in toluene solution were also measured to certify the delayed fluorescence performance. As shown in Fig.4, The two
compounds both show a double exponential decay, indicating that a normal fluorescence decay process and a delayed fluorescence emission process, which is the most important property of the delayed fluorescent material. Therefore, 2PXZDPS and 2DMACDPS are well worth considering as delayed fluorescent materials. The instantaneous fluorescence lifetime of 2PXZDPS is 88 ns and the delayed fluorescence lifetime is 0.40 µs; 2DMACDPS has a transient fluorescence lifetime of 51 ns and a delayed fluorescence lifetime of 0.21 µs. Therefore, both 2PXZDPS and 2DMACDPS have explicit delayed fluorescence performance. In order to assess the device performances of 2PXZDPS and 2DMACDPS, multilayer OLEDs devices were fabricated by doping the two TADF emitters into the mCBP host as the emissive layer (EML). As shown in Fig.5, the device structure is ITO/MoO3(5 nm)/TAPC(40 nm)/TCTA(10 nm)/mCBP: TADF(15%, 20 nm)/TmPYPB(35 nm)/LiF(1 nm)/Al(100 nm)[20]. In the device, TCTA was selected as the electron blocking layer (EBL)[21], MoO3 and LiF acted as hole injection layer (HIL) and electron injection layer (EIL), respectively. TAPC and TmPYPB served as hole transport layer (HTL) and electron transport layer (ETL)[22], respectively. The energy-level diagram, EL spectra, current density-voltage-luminance
(J-V-L),
current
efficiency-luminance
(CE-L),
EQE-luminance-power efficiency (EQE-L-PE) characteristics of these devices are shown in Fig.6. it is clearly from the spectra that the devices based on 2PXZDPS and 2DMACDPS exhibited green and bright sky-blue EL emission with peaks at 550 nm and 490 nm, respectively. The green TADF OLED based on 2PXZDPS presented the higher maximum external quantum efficiency (EQEmax) value of 20.7% with lower turn-on voltages (Von) of
2.2V. In contrast, The sky-blue TADF OLED based on 2DMACDPS exhibited the maximum external quantum efficiency (EQEmax) value of 19.2% with low turn-on voltages (Von) of 3.2V. Among the two TADF devices, the 2PXZDPS device achieved the maximum power efficiency value (PEmax) and the maximum current efficiency value (CEmax) were 20.7 lm/W-1 and 20.9 cd/A-1, respectively. The maximum luminance of the 2PXZDPS device was 12670 cd m-2. For the 2DMACDPS device, the PEmax and CEmax were 27.9 lm/W-1 and 21.8 cd/A-1, respectively. It should be noted that the maximum luminance of the 2DMACDPS device was 2398 cd m-2 (Table 2). When the luminance increased in the range of 100/cd m-2 and 1000/cd m-2, the EQE, PE and CE values of both TADF devices showed a negligible reduction. The device performances of 2PXZDPS exhibited a lower EQE value of 10.7 %, PE of 15.1 cd A-1 and CE of 18.7 lm W-1 at 100 cd m-2, EQE of 5.6 %, PE of 8.8 cd A-1 and CE of 16.5 lm W-1 at 1000 cd m-2. Similarly, for the 2DMACDPS device, the EQE, PE and CE were 7.0 %, 13.0 cd A-1 and 16.9 lm W-1 at 100 cd m-2 compared to those of 3.8 %, 3.8 cd A-1 and 8.1 m W-1 at 1000 cd m-2, respectively. Commission Internationale de L’Eclairage (CIE) coordinates of 2PXZDPS and 2DMACDPS were (0.32, 0.57) and (0.22, 0.38) at 100 cd m-2, respectively. Conclusion In summary, two novel TADF materials 2PXZDPS and 2DMACDPS were designed and synthesized by connecting two donors PXZ or DMAC to the ipsilateral side of the acceptor diphenylsulfone. Quantum computational simulations suggest that both compounds have a distorted D-A structure, which allows the material to have good orbital separation
while allows partial orbital overlap. So 2PXZDPS and 2DMACDPS have not only smaller ∆EST (0.02 and 0.08 eV), but also higher photoluminescence quantum yields (PLQYs). The OLEDs based on the two emitters show well electroluminescence performance. The TADF-OLEDs based on 2PXZDPS and 2DMACDPS achieve the maximum external quantum efficiency (EQEmax) of 20.7% and 19.2% with the turn-on voltage of 2.2 and 3.2 V, respectively. Acknowledgements The work described in this paper was supported by National Natural Science Foundation of China (No. 61705154); The Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 17KJB140022); China Postdoctoral Science Foundation (No. 2018M632358); Suzhou University of Science and Technology, China (No. 331512301).
Fig. 1 Synthetic routes and chemical structure of 2PXZDPS and 2DMACDPS
Fig.2 a) Chemical structures, b) Molecular structure models of 2PXZDPS and 2DMACDPS, and c) HOMO and LUMO distributions of 2PXZDPS and 2DMACDPS
Fig.3 UV-Vis absorption spectra, fluorescence spectra (both measured at room temperature) and phosphorescence spectra (measured at 77 K) of the two emitters in CH2Cl2 solution and 2-methyltetrahydrofuran solution
Fig.4 Transient PL decay of of 2PXZDPS and 2DMACDPS in toluene
Fig.5 (a) The energy level diagrams, (b) Commission Internationale de L’Eclairage (CIE) coordinates of 2PXZDPS and 2DMACDPS, and (c) the chemical structure of the materials used in the TADF devices
Fig. 6 (a) EL spectra of the TADF OLEDs at 10 V. (b) Current density and luminance versus the voltage (J–V–L) characteristics.(c) Current efficiency–luminance. (d) External quantum efficiency (EQE)–luminance–power efficiency
Fig. 1 Synthetic routes and chemical structure of 2PXZDPS and 2DMACDPS
Fig.2 Molecular structure, optimized structure, HOMO and LUMO distributions of a) 2PXZDPS and b) 2DMACDPS, respectively.
Fig.3 UV-Vis absorption spectra, fluorescence spectra (both measured at room temperature) and phosphorescence spectra (measured at 77 K) of the two emitters in CH2Cl2 solution and 2-methyltetrahydrofuran solution.
Fig.4 Transient PL decay of of 2PXZDPS and 2DMACDPS in toluene
Fig.5 a) The energy level diagrams, b) Commission Internationale de L’Eclairage (CIE) coordinates of 2PXZDPS and 2DMACDPS, and c) the chemical structure of the materials used in the TADF devices
Fig. 6 a) EL spectra of the TADF OLEDs at 10 V. b) Current density and luminance versus the voltage (J– V–L) characteristics.
c) Current efficiency–luminance. d) External quantum efficiency (EQE)–
luminance–power efficiency.
Table 1. Photophysics, electrochemical and thermal properties of 2PXZDPS and 2DMACDPS Td/Tga
λabsb
λonsetc
λemtd
λphose
PLQYsf
(℃)
(nm)
(nm)
(nm)
(nm)
(%)
2PXZDPS
408/-
238
429
590
448
53.5
2DMACDPS
388/-
228
411
531
425
77.6
Cpd.
EHOMO/LUMOg
E gh
Es i
ET i
∆ESTj
(eV)
(eV)
(eV)
(eV)
-5.31/-2.42
2.89
2.79
2.77
0.02
-5.42/-2.41
3.01
3.00
2.92
0.08
(eV)
a.Td obtained from TGA measurements and Tg obtained from DSC measurements;b.The maximum absorption peak of UV absorption peak; c. The cut-off wavelength of UV absorption spectrum; d. The highest emission peak of fluorescence spectrum at room temperature; e. The first emission peak of low-temperature phosphorescence spectrum; f. photoluminescence quantum yields ;g. Highest occupied molecular orbital energy level EHOMO= -[Eonset+(4.8-EFc/Fc+)] eV and the lowest unoccupied molecular orbital energy level ELUMO = Eg + EHOMO; h. Optical energy gap Eg = 1240 / λonset; i. Lowest excited singlet (ES) and triplet (ET) energies estimated from onset wavelengths of the time-resolved fluorescence and phosphorescence spectra, respectively; j. Singlet−triplet energy splitting determined experimentally using ∆EST = ES-ET.
Table 2. Summarized device performances of the 2PXZDPS and 2DMACDPS devices EQEc ELpeak
Von
a
τPFb
τDFb
(nm)
(V)
(ns)
(µs)
Cpd.
PEcmax,100,1000
CEcmax,100,1000
Luminanced
CIEe
(lm W-1)
(cd A-1)
(cd m-2)
(x,y)
max,100,1000
(%) 2PXZDPS
550
2.2
88
0.40
20.7/10.7/5.6
20.7/15.1/8.8
20.9/18.7/16.5
12670
(0.32,057)
2DMACDPS
490
3.2
51
0.21
19.2/7.0/3.8
27.9/13.0/3.8
21.8/16.9/8.2
2398
(0.22,038)
a. Turn-on voltage at a luminance of 1 cd/m-2. b. The prompt and delayed fluorescence lifetimes of the investigated molecules in toluene solution; c. Maximum external quantum efficiency (EQE), power efficiency (PE) and current efficiency (CE) value, values at 100 cd/m-2 and 1000 cd /m-2; d. Maximum luminance; f. Commission Internationale de L’Eclairage (CIE) coordinates at 100 cd/m-2.
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Highlights Two molecules were constructed with diphenylsulfone as acceptor, phenoxazine or 9,9-dimethyl-9,10-dihydroacridine as donor. The molecule with Ipsilateral Double Substituted Diphenylsulfone structures can increase the singlet exciton yield of fluorescent. With increased thermally activated delayed fluorescence, The OLEDs based on 2PXZDPS and 2DMACDPS achieve the maximum external quantum efficiency (EQEmax) of 20.7% and 19.2%, respectively.
Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
S0143-7208(19)31752-8
DOI:
https://doi.org/10.1016/j.dyepig.2019.108028
Reference:
DYPI 108028
To appear in:
Dyes and Pigments
Received Date: 27 July 2019 Revised Date:
10 October 2019
Accepted Date: 5 November 2019
Please cite this article as: Zhang D, Wei H, Wang Y, Dai G, Zhao X, Synthesis and properties of ipsilateral double substituted diphenylsulfone thermally activated delayed fluorescent materials, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.108028. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Two novel donor-acceptor (D-A) type materials containing ipsilateral double substituted diphenylsulfone were designed and synthesized for application in thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs).
Synthesis and Properties of Ipsilateral Double Substituted Diphenylsulfone Thermally Activated Delayed Fluorescent Materials
Di Zhang1, Huaixin Wei1, 2*, Yan Wang1, Guoliang Dai1, Xin Zhao1* 1. College of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, Suzhou, Jiangsu, 215009, China. 2. Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft
Materials (FUNSOM), Soochow University, Suzhou，Jiangsu, 215123, China.
Abstract Two novel donor-acceptor (D-A) type thermally activated delayed fluorescent (TADF) materials 2PXZDPS and 2DMACDPS are synthesized by attaching two donors phenoxazine (PXZ) or 9,9-dimethyl-9,10-dihydroacridine(DMAC) to the ipsilateral side of the acceptor diphenylsulfone group. The two materials show high yield, good thermal properties, suitable photophysical properties, high triplet energy and small energy difference between singlet and triplet states, leading to feasible thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). Both 2PXZDPS and 2DMACDPS show excellent performance in OLED. 2PXZDPS based OLED give the maximum external quantum efficiency (EQEmax) of 20.7% and low turn-on voltage of 2.2V. Keywords：organic light-emitting diodes, diphenylsulfone, thermally activated delayed fluorescent, photoelectric performance Corresponding author: Huaixin Wei, Address: School of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, 215009 Suzhou, China, Email: [email protected]
Xin Zhao, Address: School of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, 215009 Suzhou, China, Email: [email protected]
Introduction In recent years, organic light emitting diodes (OLEDs) have attracted more and more attention due to their potential application in flat-panel displays and large-area solid state lighting
[1-3]
. Although dramatic breakthroughs have been achieved through many
researchers’ efforts, there still exist some difficulties hampering the future development of OLED.
It is well known that only singlet exciton can decay radiatively in traditional
fluorescent materials according to the spin statistics. Approximately 75% of the triplet excitons are wasted in nonradiative processes, leading to an upper limit of the internal quantum efficiency (IQE) of 25%, which is not suitable for the application of Industrial production [4]. In contrast, phosphorescent materials can approach 100% IQE than traditional fluorescent materials due to spin-orbit coupling of the heave atom effect. However, the choice of precious metals used in phosphorescent materials will increase high cost, in addition, the triplet-triple annihilation under the condition of high concentration in phosphorescent materials are remain to be solved [5-9]. how to make pure organic fluorescent materials to achieve a comparable high EQE with those of high efficiency PHOLEDs have attracted much interest by researchers. In recent years, thermally activated delayed fluorescence (TADF) materials obtained more and more attention due to its ability to increase the singlet exciton yield of fluorescent OLEDs [10-11]
. The common method at present is to design bipolar molecules containing a
pre-twisted donor-acceptor (D-A) type structure with intramolecular charge transfer (ICT) effect. In the pre-twisted D-A molecule, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are located on the electron donor group (D)
and the electron acceptor group (A), respectively. HOMO and LUMO of the molecule are easily separated, resulting in a smaller energy difference (∆EST) between the S1 and T1 states [12-13]
. Small ∆EST is conducive to the reverse intersystem crossing (RISC) and TADF
performance. Meanwhile, the limited overlap of HOMO and LUMO of the D-A molecule is favorable for obtaining high PLQYs by choosing suitable donor and acceptor. It is well known that Diphenyl sulfone (DPS) units are the more typical electron-transporting
groups
in
TADF
materials
because
of
their
excellent
electron-withdrawing ability. Phenoxazine (PXZ) and 9,9-dimethyl-9,10-dihydroacridine (DMAC) were selected as the electron donor to connect the ipsilateral side of the diphenylsulfone (DPS) phenyl. According to previous studies, steric hindrance of the space occurs when two donors are connected on the ipsilateral side. The twist angle between the donor and the acceptor will increase under the steric hindrance effect which can effectively separating HOMO and LUMO. Finally, a smaller energy difference (∆EST) was obtained between the S1 and T1 states. In 2014, Zhang[15]et.al reported two novel TADF materials by connecting two PXZ donors or DMAC donors on the opposite side of DPS. The two compounds exhibited superior TADF properties and the performance of corresponding OLED devices achieved the maximum EQE of 19.5% and 17.5% with the turn-on voltage values of 2.7 and 3.7 V, respectively. In the same year, Michael P. Gaj[14] et al. designed and synthesized the TADF material mCPSOB which with two carbazole units at the two meta positions of the benzene ring on the diphenylsulfone side, the corresponding devices show the maximum external quantum efficiency EQE as high as 26.5%, which surpasses all OLEDs at that time. It is maybe the increased steric hindrance effect by connecting the two
electron donors on the same side, resulting in raised dihedral angle between donor (D) and acceptor (A), reduced small ∆EST and improved device efficiency. According to previous reports, it is clearly that diphenyl sulfone modified by a strong electron-withdrawing group with a twist angle will show enhanced fluorescent. However, the molecules reported are mostly symmetrical structure, asymmetric structure with donors in one side are still scarce, and it is interest to clarify the properties of asymmetric molecule for the understanding and optimization of TADF materials. In this paper, we report two novel efficient diphenylsulfone TADF materials, 2PXZDPS and 2DMACDPS (as shown in Fig.1).
The widely used hole-transporting moiety,
phenoxazine (PXZ) and 9,9-dimethyl-9,10-dihydroacridine (DMAC) were selected as the electron donor to connect the same side of the diphenylsulfone (DPS) phenyl for increasing the distortion of the molecular structure as a design center. 2PXZDPS and 2DMACDPS exhibit higher photoluminescence quantum yields (PLQYs) and obvious TADF phenomena. The∆EST values are 0.02eV and 0.08eV, respectively. Besides, the maximum external quantum efficiency values of the corresponding OLEDs based on 2PXZDPS and 2DMACDPS were as high as 20.7% and 19.2%, respectively. In addition, the turn-on voltage values of the two TADF OLEDs based on 2PXZDPS and 2DMACDPS were as low as 2.2V and 3.2V. To the best of our knowledge, performance of the devices are among the best of OLEDs with diphenylsulfone as emitters. Results and discussion The TADF molecules 2PXZDPS and 2DMACDPS were synthesized by connecting two phenoxazine (PXZ) donors or 9,9-dimethyl-9,10-dihydroacridine(DMAC) donors to the
same side of the diphenylsulfone (DPS). The detailed synthetic routes and chemical structure of 2PXZDPS and 2DMACDPS are shown in Fig.1 and supporting information. The melting point of 2PXZDPS is around 169 ℃ and the decomposition temperature (mass loss 5%) Td is 408 ℃ (Fig.S4). The melting point of 2DMACDPS is around 215℃ and the decomposition temperature (mass loss 5%) Td is 388 ℃ (Fig.S4). Both molecules exhibit higher thermal stability because of the introduction of two donor units, which increased spatial distortion of the compound. In addition, it is clearly that the molecular weight of the compound was increased by adding the sulfone unit result in prominent thermal stability. Theoretical calculation of 2PXZDPS and 2DMACDPS was carried out by using the Gaussian-03 program. The structures of two compounds were determined and optimized by density functional theory (DFT) B3LPY/6-31G(d) [17-18]. Optimized structures were used for the calculation of HOMO and LUMO energy levels. As shown in Fig. 2, the two compounds possessed larger dihedral angles between the donor and the acceptor unit and larger spatial distortion in the optimized geometries. Moreover, the HOMOs and LUMOs of the two compounds are almost completely separated, in which the HOMOs are mainly localized at the electron-donating PXZ or DMAC units with a slight extent on the adjacent benzene ring but the LUMOs are mainly localized at DPS moiety. The almost completely separation and limited overlap of the frontier molecular orbitals (FMOs) will endow the two compounds small ∆ESTs and high PLQYs [19]. The experimental HOMO and LUMO energy levels of the two molecules were measured by cyclic voltammetry (CV). As shown in Fig.S5, The initial positions of oxidation peaks of 2PXZDPS and 2DMACDPS are 0.95 and 1.05 V, respectively. The oxidation peak starting position of the internal standard ferrocene is
located at 0.44 and 0.43 V, respectively. The HOMO and LUMO energy levels of the compounds 2PXZDPS and 2DMACDPS were -5.31, -5.42 eV and -2.42, -2.41 eV, respectively. In order to verify the theoretical calculation, cyclic voltammetry (CV) measurement were carried out to analyze the value of HOMO and LUMO energy levels. As shown in Fig.S5, it is clearly that the first oxidation peak of 2PXZDPS and 2DMACDPS are at 0.96 eV and 0.97 eV. The starting position of the internal standard ferrocene oxidation peak is located at 0.34 eV and 0.34 eV, respectively. According to the photophysics properties shown in Fig. 3, The HOMO and LUMO of compound 2PXZDPS were -5.42 eV and -2.43 eV, respectively and 2DMACDPS were -5.43 eV and -2.42 eV, respectively. Fig.3 shows the normalized spectrum of UV-Vis, PL and Low Tem Phos of 2PXZDPS and 2DMACDPS. It is clearly to see that the UV absorption of 2PXZDPS is at 238 and 317 nm in dilute solution of 10-5 mol/L-1 in CH2Cl2, which attribute to the π-π * charge transition in phenoxazine and the π-π * electronic transition. The optical bandgap (Eg) was calculated to be 2.89 eV based on the absorption band edge of the 2PXZDPS (429 nm). 2DMACDPS shows three absorption peaks at 228 nm, 280 nm and 364 nm, respectively. The three peaks corresponding to the π-π * transitions of benzene ring and sulfone group, respectively. The optical band gap (Eg) was calculated to be 3.01 eV based on the absorption edge (411 nm) of 2DMACDPS. The fluorescence emission peaks of 2PXZDPS and 2DMACDPS in the dilute solution of 10-5 mol/L-1 CH2Cl2 are located at 590 nm and 531 nm at room temperature, respectively. The low temperature phosphorescence emission curves of 2PXZDPS and 2DMACDPS
were also measured at 77K by using 2-methyltetrahydrofuran as the solvent. As shown in Fig.3, the peak position of the fluorescence emission peak of 2PXZDPS was at 444 nm. The calculated singlet energy level (ES) was 2.79 eV, and the peak of the phosphorescence emission peak was at 448 nm. The calculated triplet level (ET) is 2.77 eV, and its singlet-triplet energy level difference (∆EST) is 0.02 eV. The peak position of the fluorescence emission peak of 2DMACDPS was at 414 nm. The calculated singlet energy level (ES) was 3.00 eV, and the peak position of the phosphorescence emission peak was at 425 nm. The calculated triplet level (ET) is 2.92 eV, and the singlet-triplet energy level difference (∆EST) is 0.08 eV. It is apparently to see that both 2PXZDPS and 2DMACDPS have a small ∆EST, which is favorable for reverse intersystem crossing (RISC) of triplet excitons and improvement of delayed fluorescence properties. The detailed performance data of 2PXZDPS and 2DMACDPS are listed in Table 1. In order to investigate the fluorescence properties of 2PXZDPS and 2DMACDPS, the fluorescence profiles of the two materials were studied in the presence of oxygen and without oxygen (Fig.S6). It is clearly that the fluorescence peaks of the two materials are obvious higher than that in oxygen. The fluorescence quantum yields of 2PXZDPS and 2DMACDPS are 60.1% and 85.8% after deoxygenation with nitrogen bubbling. It is well known that oxygen can quench the triplet exciton and reduce the fluorescence quantum yield of TADF molecules, thus the fluorescence quantum yields of the two compounds are increased by 6.6% and 8.2% after deoxygenation, respectively. The transient PL decay curves of 2PXZDPS and 2DMACDPS in toluene solution were also measured to certify the delayed fluorescence performance. As shown in Fig.4, The two
compounds both show a double exponential decay, indicating that a normal fluorescence decay process and a delayed fluorescence emission process, which is the most important property of the delayed fluorescent material. Therefore, 2PXZDPS and 2DMACDPS are well worth considering as delayed fluorescent materials. The instantaneous fluorescence lifetime of 2PXZDPS is 88 ns and the delayed fluorescence lifetime is 0.40 µs; 2DMACDPS has a transient fluorescence lifetime of 51 ns and a delayed fluorescence lifetime of 0.21 µs. Therefore, both 2PXZDPS and 2DMACDPS have explicit delayed fluorescence performance. In order to assess the device performances of 2PXZDPS and 2DMACDPS, multilayer OLEDs devices were fabricated by doping the two TADF emitters into the mCBP host as the emissive layer (EML). As shown in Fig.5, the device structure is ITO/MoO3(5 nm)/TAPC(40 nm)/TCTA(10 nm)/mCBP: TADF(15%, 20 nm)/TmPYPB(35 nm)/LiF(1 nm)/Al(100 nm)[20]. In the device, TCTA was selected as the electron blocking layer (EBL)[21], MoO3 and LiF acted as hole injection layer (HIL) and electron injection layer (EIL), respectively. TAPC and TmPYPB served as hole transport layer (HTL) and electron transport layer (ETL)[22], respectively. The energy-level diagram, EL spectra, current density-voltage-luminance
(J-V-L),
current
efficiency-luminance
(CE-L),
EQE-luminance-power efficiency (EQE-L-PE) characteristics of these devices are shown in Fig.6. it is clearly from the spectra that the devices based on 2PXZDPS and 2DMACDPS exhibited green and bright sky-blue EL emission with peaks at 550 nm and 490 nm, respectively. The green TADF OLED based on 2PXZDPS presented the higher maximum external quantum efficiency (EQEmax) value of 20.7% with lower turn-on voltages (Von) of
2.2V. In contrast, The sky-blue TADF OLED based on 2DMACDPS exhibited the maximum external quantum efficiency (EQEmax) value of 19.2% with low turn-on voltages (Von) of 3.2V. Among the two TADF devices, the 2PXZDPS device achieved the maximum power efficiency value (PEmax) and the maximum current efficiency value (CEmax) were 20.7 lm/W-1 and 20.9 cd/A-1, respectively. The maximum luminance of the 2PXZDPS device was 12670 cd m-2. For the 2DMACDPS device, the PEmax and CEmax were 27.9 lm/W-1 and 21.8 cd/A-1, respectively. It should be noted that the maximum luminance of the 2DMACDPS device was 2398 cd m-2 (Table 2). When the luminance increased in the range of 100/cd m-2 and 1000/cd m-2, the EQE, PE and CE values of both TADF devices showed a negligible reduction. The device performances of 2PXZDPS exhibited a lower EQE value of 10.7 %, PE of 15.1 cd A-1 and CE of 18.7 lm W-1 at 100 cd m-2, EQE of 5.6 %, PE of 8.8 cd A-1 and CE of 16.5 lm W-1 at 1000 cd m-2. Similarly, for the 2DMACDPS device, the EQE, PE and CE were 7.0 %, 13.0 cd A-1 and 16.9 lm W-1 at 100 cd m-2 compared to those of 3.8 %, 3.8 cd A-1 and 8.1 m W-1 at 1000 cd m-2, respectively. Commission Internationale de L’Eclairage (CIE) coordinates of 2PXZDPS and 2DMACDPS were (0.32, 0.57) and (0.22, 0.38) at 100 cd m-2, respectively. Conclusion In summary, two novel TADF materials 2PXZDPS and 2DMACDPS were designed and synthesized by connecting two donors PXZ or DMAC to the ipsilateral side of the acceptor diphenylsulfone. Quantum computational simulations suggest that both compounds have a distorted D-A structure, which allows the material to have good orbital separation
while allows partial orbital overlap. So 2PXZDPS and 2DMACDPS have not only smaller ∆EST (0.02 and 0.08 eV), but also higher photoluminescence quantum yields (PLQYs). The OLEDs based on the two emitters show well electroluminescence performance. The TADF-OLEDs based on 2PXZDPS and 2DMACDPS achieve the maximum external quantum efficiency (EQEmax) of 20.7% and 19.2% with the turn-on voltage of 2.2 and 3.2 V, respectively. Acknowledgements The work described in this paper was supported by National Natural Science Foundation of China (No. 61705154); The Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 17KJB140022); China Postdoctoral Science Foundation (No. 2018M632358); Suzhou University of Science and Technology, China (No. 331512301).
Fig. 1 Synthetic routes and chemical structure of 2PXZDPS and 2DMACDPS
Fig.2 a) Chemical structures, b) Molecular structure models of 2PXZDPS and 2DMACDPS, and c) HOMO and LUMO distributions of 2PXZDPS and 2DMACDPS
Fig.3 UV-Vis absorption spectra, fluorescence spectra (both measured at room temperature) and phosphorescence spectra (measured at 77 K) of the two emitters in CH2Cl2 solution and 2-methyltetrahydrofuran solution
Fig.4 Transient PL decay of of 2PXZDPS and 2DMACDPS in toluene
Fig.5 (a) The energy level diagrams, (b) Commission Internationale de L’Eclairage (CIE) coordinates of 2PXZDPS and 2DMACDPS, and (c) the chemical structure of the materials used in the TADF devices
Fig. 6 (a) EL spectra of the TADF OLEDs at 10 V. (b) Current density and luminance versus the voltage (J–V–L) characteristics.(c) Current efficiency–luminance. (d) External quantum efficiency (EQE)–luminance–power efficiency
Fig. 1 Synthetic routes and chemical structure of 2PXZDPS and 2DMACDPS
Fig.2 Molecular structure, optimized structure, HOMO and LUMO distributions of a) 2PXZDPS and b) 2DMACDPS, respectively.
Fig.3 UV-Vis absorption spectra, fluorescence spectra (both measured at room temperature) and phosphorescence spectra (measured at 77 K) of the two emitters in CH2Cl2 solution and 2-methyltetrahydrofuran solution.
Fig.4 Transient PL decay of of 2PXZDPS and 2DMACDPS in toluene
Fig.5 a) The energy level diagrams, b) Commission Internationale de L’Eclairage (CIE) coordinates of 2PXZDPS and 2DMACDPS, and c) the chemical structure of the materials used in the TADF devices
Fig. 6 a) EL spectra of the TADF OLEDs at 10 V. b) Current density and luminance versus the voltage (J– V–L) characteristics.
c) Current efficiency–luminance. d) External quantum efficiency (EQE)–
luminance–power efficiency.
Table 1. Photophysics, electrochemical and thermal properties of 2PXZDPS and 2DMACDPS Td/Tga
λabsb
λonsetc
λemtd
λphose
PLQYsf
(℃)
(nm)
(nm)
(nm)
(nm)
(%)
2PXZDPS
408/-
238
429
590
448
53.5
2DMACDPS
388/-
228
411
531
425
77.6
Cpd.
EHOMO/LUMOg
E gh
Es i
ET i
∆ESTj
(eV)
(eV)
(eV)
(eV)
-5.31/-2.42
2.89
2.79
2.77
0.02
-5.42/-2.41
3.01
3.00
2.92
0.08
(eV)
a.Td obtained from TGA measurements and Tg obtained from DSC measurements;b.The maximum absorption peak of UV absorption peak; c. The cut-off wavelength of UV absorption spectrum; d. The highest emission peak of fluorescence spectrum at room temperature; e. The first emission peak of low-temperature phosphorescence spectrum; f. photoluminescence quantum yields ;g. Highest occupied molecular orbital energy level EHOMO= -[Eonset+(4.8-EFc/Fc+)] eV and the lowest unoccupied molecular orbital energy level ELUMO = Eg + EHOMO; h. Optical energy gap Eg = 1240 / λonset; i. Lowest excited singlet (ES) and triplet (ET) energies estimated from onset wavelengths of the time-resolved fluorescence and phosphorescence spectra, respectively; j. Singlet−triplet energy splitting determined experimentally using ∆EST = ES-ET.
Table 2. Summarized device performances of the 2PXZDPS and 2DMACDPS devices EQEc ELpeak
Von
a
τPFb
τDFb
(nm)
(V)
(ns)
(µs)
Cpd.
PEcmax,100,1000
CEcmax,100,1000
Luminanced
CIEe
(lm W-1)
(cd A-1)
(cd m-2)
(x,y)
max,100,1000
(%) 2PXZDPS
550
2.2
88
0.40
20.7/10.7/5.6
20.7/15.1/8.8
20.9/18.7/16.5
12670
(0.32,057)
2DMACDPS
490
3.2
51
0.21
19.2/7.0/3.8
27.9/13.0/3.8
21.8/16.9/8.2
2398
(0.22,038)
a. Turn-on voltage at a luminance of 1 cd/m-2. b. The prompt and delayed fluorescence lifetimes of the investigated molecules in toluene solution; c. Maximum external quantum efficiency (EQE), power efficiency (PE) and current efficiency (CE) value, values at 100 cd/m-2 and 1000 cd /m-2; d. Maximum luminance; f. Commission Internationale de L’Eclairage (CIE) coordinates at 100 cd/m-2.
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Highlights Two molecules were constructed with diphenylsulfone as acceptor, phenoxazine or 9,9-dimethyl-9,10-dihydroacridine as donor. The molecule with Ipsilateral Double Substituted Diphenylsulfone structures can increase the singlet exciton yield of fluorescent. With increased thermally activated delayed fluorescence, The OLEDs based on 2PXZDPS and 2DMACDPS achieve the maximum external quantum efficiency (EQEmax) of 20.7% and 19.2%, respectively.
Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.