Achieving Efficient Violet-blue Organic Light Emitting Devices（CIEy=0.046）by Extending π-conjugation in D-π-A Structure

CUI Jiang-feng，SUN Jing，2*，WANG Guo-liang，WANG Zhi-heng，YANG Xiao-dong，XU Hui-xia，MIAO Yan-qin，2，WANG Long，YU Jun-sheng，WANG Hua*

（1. Key Laboratory of Interface Science and Engineering in Advanced Materials，Ministry of Education，Taiyuan University of Technology，Taiyuan 030024，China；2. Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering，Taiyuan 030024，China；3. State Key Laboratory of Electronic Thin Films and Integrated Devices，School of Optoelectronic Information，University of Electronic Science and Technology of China，Chengdu 610054，China）* Corresponding Authors，E-mail：sunjing@tyut.edu.cn；wanghua001@tyut.edu.cn

Abstract：Organic violet-blue luminescent materials have exhibited great potential in full-color displaying and lighting fields. However，it is a huge challenge to develop efficient violet-blue emitters because of the inherently wide energy gap. In this work，two violet-blue emitters with donor（D）-π-acceptor（A）type using 10H-phenothiazine 5，5-dioxide（2OPTZ）as a weak acceptor and N-（2-naphthyl）aniline（PNA）as the donor，namely 2OPTZ-PNA and 2OPTZ-BPPNA，were designed and synthesized. By extending π-conjugation length between donor and acceptor，local excited states and charge transfer excited states were adjusted，and absolute photoluminescence quantum yields（PLQYs）values in the evaporated film increased from 14% to 33%. Notably，non-doped device based 2OPTZ-BP-PNA exhibits violetblue light at 436 nm with a narrow full width at half-maximum（FWHM）of 54 nm，Commission Internationale de l’Eclairage（CIE）coordinates of（0.155，0.046）and external quantum efficiency（EQE）of 4.1%. Given this，our results provide an efficient design strategy for the high-quality violet-blue luminescent materials.

Key words：organic light-emitting diodes（OLED）；violet-blue；phenothiazine dioxide；D-π-A structure；π-conjugation length

1 Introduction

Organic light-emitting diodes（OLEDs）play an essential role in full-color flat-panel displaying and lighting fields due to high color contrast, low driving voltages, fast response and environmental friendliness[1-6]. High-efficiency violet-blue emitters are significant to OLEDs because they can not only be employed as energy donors to generate light covering the entire visible light region，but also reduce power consumption[7-10]. In addition, the violet-blue lightemitting materials are also widely used in phototherapy[11], biological and chemical sensing[12-13], highdensity information storage[14], sterilization and so on[15]. Therefore, developing high-performance violetblue light-emitting materials is an integral part of the OLEDs technology field[16-19].

In principle, excellent violet-blue emitters require wide optical band gaps, rigid planar structures,conjugated backbones, and the ability to balance electron and hole transport[20-23]. Recently, violetblue emitters with D-π-A structure were reported by using weak electron donors and acceptors[24-26]. On this basis, reasonable regulation of the intramolecular charge transfer（ICT）state is the key to achieve violet-blue emitter, which satisfies the color standards of violet-blue light with CIE coordinates（x,y）=（0.131, 0.046）, defined by the ITU-R Recommendation BT.2020[27-29].

It is known that phenothiazine has excellent electron donating ability and was widely used in the D-π-A system for high-performance electroluminescent materials[30-31]. Interestingly, 10H-phenothiazine 5,5-dioxide exhibits weaker electron-donating ability than phenothiazine and is widely used to construct room temperature phosphorescent materials[32-33]. In 2021, Prof. Routet al. reported the influence of the oxidation state of the sulfur atom in phenothiazine on the photophysical and electrochemical properties of the compound. The results show that the oxygenfunctionalized molecular structure is more rigid, the absorption and emission spectra are blue-shifted,and the highest occupied molecular orbital（HOMO）energy level is deeper[34]. In addition, it is also verified that the oxidized oxygen-functionalized molecules possess better thermal stabilities and significantly lower the lowest unoccupied molecular orbital（LUMO）energy levels, which endows better electroninjection and transport abilities[35]. In 2016, Prof.Ma and his co-workers designed and synthesized a series of triphenylamine-phenirimidazole derivatives.By gradually adding phenyls along with horizontal and vertical directions, respectively, the appropriate composition between the locally-excited（LE）state and the charge-transfer（CT）state components were adjusted and controlled, thereby effective electroluminescence（EL）efficiency was achieved[36-37].

In this work, we designed and synthesized two new D-π-A-typed violet-blue fluorescent emitters using 10H-phenothiazine 5,5-dioxide（2OPTZ）as the weaken acceptor and N-（2-naphthyl）aniline（PNA）as the donor, namely 2OPTZ-PNA and 2OPTZ-BPPNA. By extending the length of the π-bridge between the donor and acceptor, the local excited and charge transfer states were optimized, and their absolute PLQYs values increased from 14% to 33%. Nondoped devices of 2OPTZ-PNA and 2OPTZ-BP-PNA exhibited violet-blue emission with the CIE coordinates of（0.164,0.060）and（0.155,0.046）,respectively,maximum external quantum efficiency（EQE）of 1. 5% and 4. 1%, respectively, which indicated the excellent application prospect of 10H-phenothiazine 5,5-dioxide in the violet-blue OLED materials.

2 Experiment

2.1 Synthesis

The synthetic routes of 2OPTZ-PNA and 2OPTZBP-PNA are shown in Fig. 1, and the intermediates and target products were prepared by palladium-catalyzed Ullmann reaction and then purified by silica gel column chromatography[38-40]. Their precise structures were characterized by1H-NMR spectra,13CNMR spectra and high-resolution mass spectrometry（Fig.S1-S3）.

Fig.1 Chemical structures and synthetic routes of 2OPTZ-PNA and 2OPTZ-BP-PNA

2.1.1 2OPTZ-PNA

Synthesis of 1a. The materials of 10H-phenothiazine（1.99 g, 10 mmol）, 1-bromo-4-iodobenzene（5.66 g, 20 mmol）, tri-tert-butylphosphine tetrafluoroborate（0.17 g, 0.60 mmol）, sodium tert-butoxide（1.92 g, 20 mmol）, tris（dibenzylideneacetone）dipalladium（0.28 g, 0.30 mmol）were dissolved in toluene（50 mL）. The mixture solution was heated to 110 ℃and reacted for 8 h. When the reaction was completed, the reaction solution was extracted,dried and subjected to column chromatography to obtain a white solid（2.15 g, yield 61%）.1H NMR（600 MHz, chloroform-d）:δ=8.17（dt,J=8.0, 1.7 Hz, 2H）, 7.85-7.83（m, 2H）, 7.41（ddd,J=8.7,7.2, 1.6 Hz, 2H）, 7.30-7.26（m, 4H）, 6.62（dd,J=8.7,0.9 Hz,2H）.

Synthesis of 2a. The intermediate of 1a（1.20 g, 3.40 mmol）was dissolved in 40 mL dichloromethane and stirred. Glacial acetic acid（20 mL）and aqueous hydrogen peroxide solution（8 mL, 30%）were added to the reaction mixture,and then the mixture was heated to 70 ℃for 8 h. When the reaction was over, the reaction solution was extracted, dried and column chromatography to give a white solid（1.18 g, yield 90%）.1H NMR（600 MHz, chloroform-d）δ8.18（d,J=1.8 Hz, 1H）, 8.16（d,J=1.6 Hz, 1H）, 7.84（d,J=2.0 Hz, 1H）, 7.83（d,J=2.1 Hz, 1H）, 7.41（ddd,J=8.8, 7.2, 1.6 Hz, 2H）, 7.29（d,J=2.1 Hz, 1H）, 7.28-7.26（m, 3H）, 6.63（s,1H）,6.61（s,1H）.

Synthesis of 2OPTZ-PNA. The intermediate of 2a（1.16 g, 3.00 mmol）, N-phenylnaphthalen-2-amine（0.72 g, 3.30 mmol）, tri-tert-butylphosphine tetrafluoroborate（0.06 g, 0.2 mmol）, sodium tertbutoxide（0.63 g, 6.6 mmol）were dissolved in toluene（50 mL）and the catalyst of tris（dibenzylideneacetone）dipalladium（0.09 g, 0.1 mmol）was added.The mixture solution was reacted at 110 ℃for 8 h.When the reaction was over, the reaction solution was extracted, dried and column chromatography to give a white solid（1.02 g, yield 65%）.1H NMR（600 MHz, chloroform-d）:δ=8.18（d,J=1.6 Hz,1H）, 8.16（d,J=1.6 Hz, 1H）, 7.84（d,J=8.8 Hz,1H）, 7.82（d,J=7.9 Hz, 1H）, 7.70（d,J=8.0 Hz,1H）, 7.61（d,J= 2.3 Hz, 1H）, 7.49-7.45（m,3H）, 7.45-7.41（m, 2H）, 7.41-7.36（m, 3H）,7.34-7.31（m, 2H）, 7.29（dd,J=8.6, 1.1 Hz,2H）, 7.27-7.25（dd,J=15.0, 0.9 Hz, 1H）, 7.20-7.16（m, 3H）, 6.82（d,J=8.3 Hz, 2H）.13C NMR（101 MHz, chloroform-d）:δ=148.9, 147.0, 144.5,141.1, 134.4, 132.8, 131.5, 131.0, 130.8, 129.8,129.6, 127.7, 127.2, 126.6, 125.7, 125.3, 125.0,124.5,123.5,123.4,122.6,122.4,122.0,117.4.HRMS（FAB/magnetic sector）m/z: [M]+calcd for C34H24N2O2S 524.63;found 524.53.

2.1.2 2OPTZ-BP-PNA

Synthesis of 1b. It was prepared using a similar procedure to 1a. The obtained product was a white powder（2.41 g, yield 56%）.1H NMR（600 MHz,chloroform-d）:δ=7.76（s, 2H）, 7.63-7.59（m,2H）, 7.55-7.52（m, 2H）, 7.45（d,J=8.4 Hz, 2H）,7.05（dd,J=7.5, 1.6 Hz, 2H）, 6.95-6.75（m,4H）,6.32（d,J=8.2 Hz,2H）.

Synthesis of 2b. It was prepared using a similar procedure to 2a. The obtained product was a white powder（1.41 g, yield 89%）.1H NMR（600 MHz,chloroform-d）δ8.19（dd,J=1.6, 0.5 Hz, 1H）, 8.18（dd,J=1.7, 0.5 Hz, 1H）, 7.89-7.87（m, 1H）, 7.87（d,J=2.1 Hz, 1H）, 7.67-7.64（m, 2H）, 7.58-7.56（m, 2H）, 7.47（d,J=1.7 Hz, 1H）, 7.46（d,J=2.1 Hz,1H）,7.41（ddd,J=8.7,7.1,1.6 Hz,2H）,7.28-7.26（m,2H）,6.71（d,J=8.7 Hz,2H）.

Synthesis of 2OPTZ-BP-PNA. It was prepared using a similar procedure to 2OPTZ-PNA. The resulting product is a white powder（1.10 g, yield 61%）.1H NMR（600 MHz, DMSO-d6）:δ=8.10（d,J=1.6 Hz, 1H）, 8. 09（d,J=1. 6 Hz, 1H）, 8. 04（d,J=8.5 Hz,2H）,7.91（d,J=8.9 Hz,1H）,7.87（d,J=6.8 Hz, 1H）, 7.79（d,J=8.7 Hz, 2H）, 7.76（d,J=8.3 Hz, 1H）, 7.60（ddd,J=8.8, 7.2, 1.7 Hz, 2H）,7.57（d,J=8.5 Hz, 2H）, 7.53（d,J=2.3 Hz, 1H）,7.47-7.41（m, 2H）, 7.40-7.35（m, 4H）, 7.30（dd,J=8.8, 2.3 Hz, 1H）, 7.17（s, 1H）, 7.17-7.15（m, 3H）, 7.14（d,J=7.4 Hz, 1H）, 6.72（d,J=9.1 Hz, 2H）.13C NMR（101 MHz, Chloroform-d）:δ=146.5, 146.5, 144.1, 143.2, 139.3, 138.7, 133.4,133.0, 130.0, 129.2, 128.4, 128.0, 127.8, 126.8,126.6, 126.0, 125.8, 125.7, 125.3, 123.7, 123.7,123.6, 123.0, 122.3, 121.5, 119.8, 119.3, 115.2.HRMS（FAB/magnetic sector）m/z: [M]+calcd for C40H28N2O2S 600.18;found 600.10.

2.2 Fabrication of OLEDs

The non-doped and doped OLEDs were fabricated by vacuum deposition. The glass substrates were cleaned with deionized water, acetone and ethanol. The electroluminescence（EL）spectra and CIE coordinates were measured by PR-655 spectrophotometer. The current density-voltage-luminance（J-VL）characteristics were tested with the equipment of Keithley 2400 Source Meter and ST-900M Spot Brightness Meter. The characterization of devices was recorded at room temperature.

2.3 Characterizations

The Switzerland Bruker DR × 600 was used to measure the1H NMR and13C NMR spectra. Finnigan 4021C gas chromatography（GC）-mass spectrometry（MS）instrument was used to characterize mass spectra. Gaussian 09 program package with the B3LYP/6-31g（d）method was used to study theoretical calculations. The Lambda Bio 40, HORIBA FluoroMax-4 spectrophotometer and Edinburgh FLS 980 spectrometer were used to study the photophysical properties. Netzsch TG 209F3 and DSC Q2000 were used to investigate the TGA and DSC curves.CHI 660E electrochemical workstation was used to study cyclic voltammetry.

3 Results and Discussion

3.1 Theoretical Calculation

To further understand the relationship between the electronic structure and photophysical properties of 2OPTZ-PNA and 2OPTZ-BP-PNA, density functional theory（DFT） calculations were performed.The optimized molecular structures and the density state distributions of the LUMOs and the HOMOs were plotted in Fig. 2. From the optimized ground geometry, the dihedral angles of benzene ring and adjacent benzene ring, naphthalene group and benzene ring, phenothiazine oxide group and benzene ring of 2OPTZ-PNA were 68.13°, 66.82° and 86.25°.When the π-bridge length was extended to the biphenyl group, the corresponding dihedral angles became 68.94°, 67.25° and 87.16°, at the same time dihedral angle between the benzene rings on the biphenyl group was 35.97°. It indicated that the elongation of π-conjugation did not change the twisted molecular configuration.

The HOMO of 2OPTZ-PNA was mainly distributed on the N-（2-naphthyl）aniline and the adjacent benzene ring, and its LUMO was mainly localized over the naphthylamine and adjacent benzene ring.By contrast, the HOMO distribution of 2OPTZ-BPPNA was mainly on the N-（2-naphthyl）aniline and the biphenyl groups, and its LUMO was mainly localized over the naphthalene groups and adjacent biphenyl groups. In addition, it was found that both compounds had a significant degree of orbital overlap and separation between HOMO and LUMO,which may be led to a hybrid LE state and CT state.Furthermore, theoretically calculated band gaps of 2OPTZ-PNA and 2OPTZ-BP-PNA were 4.03 eV and 3.81 eV, respectively, guaranteeing emission in the violet-blue region. Fig. 2（c）showed that prominent CT states were found, and 2OPTZ-BP-PNA had stronger oscillator strength（f=0.036 1）which is favorable to achieve higher PLQY.

Fig.2 （a）The geometries and structural formulas of 2OPTZ-PNA and 2OPTZ-BP-PNA.（b）Spatial distribution of the HOMOs and LUMOs of 2OPTZ-PNA and 2OPTZ-BP-PNA.（c）The S0→S1 natural transition orbit（NTO）of 2OPTZ-PNA and 2OPTZ-BP-PNA.

3.2 Single-crystal Structure

Single crystal of 2OPTZ-PNA was obtained through slow diffusion of methyl alcohol into methylene dichloride. The crystal system of 2OPTZ-PNA was monoclinic, the space group wasP21/c, and there were four molecules in one unit cell（Tab.S1）. As shown in Fig. 3（a）, the twisted dihedral angle between the benzene ring and the central benzene ring, the naphthalene group and central benzene ring, the phenothiazine oxide group and central benzene ring of 2OPTZ-PNA are 76.17°, 57.65° and 80.83°,which is a small difference from the dihedral angle from theoretical calculations. There is strong intermolecular interaction of π-π stacking（0.336 6 nm and 0.338 0 nm） between phenothiazine oxide groups in adjacent molecules. Moreover, typical C—H···O hydrogen bonds（0.264 9 nm）between the oxygen atom and the central benzene groups were found. It indicated that more planar molecular structure in the single crystal resulted from the stronger π-π stacking and hydrogen bonds. Fig. 3（b）showed that the molecular stacking owned the headto-head arrangements.

Fig.3 Inter-molecular interactions（a）and molecular packing structure（b）of the 2OPTZ-PNA crystal

3.3 Photophysical Properties

Photophysical properties of 2OPTZ-PNA and 2OPTZ-BP-PNA were measured in 10-5mol/L toluene solution（Fig. 4（a））and vacuum-deposited neat films（Fig. 4（b））. From the absorption spectra in the toluene solution, absorption bands of both compounds between 300 nm and 340 nm were assigned as π-π*transition[41-42]and the long-wavelength absorption peaks between 370 nm and 400 nm, respectively, were attributed to the intramolecular charge transfer state. Photoluminescence（PL）emission peaks in toluene solution of 2OPTZ-PNA and 2OPTZ-BPPNA were located at 391/500 nm and 402 nm, respectively. In the neat films, PL emission peaks of 2OPTZ-PNA and 2OPTZ-BP-PNA were at 411 nm and 429 nm belong to the violet-blue light, respectively, which had a red-shift of about 20 nm and 27 nm owing to the aggregate state. By comparison,2OPTZ-BP-PNA had an obvious red shift than 2OPTZ-PNA because of the change of electronic structure by the extended π-bridge. Additionally,the absolute PLQY values（14% and 33%）and their lifetime（3.20 ns and 2.47 ns, Fig. S4）of 2OPZT-PNA and 2OPTZ-BP-PNA were obtained in the neat films.It indicated that the expansion of π-bridge could effectively enhance the PLQY values by adjusting the CT state that has stronger oscillator strength.

Fig.4 The UV-Vis absorption spectra and PL spectra of 2OPTZ-PNA and 2OPTZ-BP-PNA in toluene solution（10-5 mol/L）（a）and in the neat film（b）

In order to further investigate the photophysical properties,we measured the UV-Vis absorption spectra（Fig. S5）, PL spectra and time-dependent fluorescence spectroscopy（Fig. 5）of 2OPTZ-PNA and 2OPTZ-BP-PNA in different polar solvents. UV absorption bands of both compounds had no significant changes in the different solvents, indicating that their electron transition in the absorption process was similar. However, PL emission of 2OPTZ-PNA and 2OPTZ-BP-PNA exhibited a remarkable solvatochromic effect. As shown in Fig. 5（a）, emission peaks of 2OPTZ-PNA showed obvious red-shift from low-polarity n-hexane to high-polarity dimethyl sulfoxide（DMSO）and extra PL emission band presented at the long-wavelength region. It could be explained as that HOMO and LUMO of 2OPTZ-PNA were located at triarylamine groups leading to the hybrid emission from the locally excited states（LE）state and CT state. However, the emission peak of 2OPTZ-BP-PNA exhibited a red shift of 50 nm in the different solvents, which was attributed to the obvious CT characteristics. We used the Lippert-Mataga model[43]to estimate the excited state dipole moments with increasing the solvent polarities. Both compounds exhibited a linear relationship（Fig. S6）and their excited dipole moment（μe） values of 2OPTZ-PNA and 2OPTZ-BP-PNA were 18.79 D and 22.06 D, respectively. In addition, PL decay lifetime of 2OPTZ-PNA in different solvents ranged from 1.30 ns to 2.32 ns, while 2OPTZ-BP-PNA had a longer lifetime from 2.32 ns to 3.33 ns. The above results testified that the extension of π-bridge could optimize the intramolecular charge transfer（ICT）states.

Fig.5 Photophysical properties in different solvents. PL spectra of 2OPTZ-PNA（a）and 2OPTZ-BP-PNA（b）. Time-dependent fluorescence spectra of 2OPTZ-PNA（c）and 2OPTZ-BP-PNA（d）.Inset：time-dependent fluorescence spectra of 2OPTZPNA measured at 466，495，519，540 nm.

3.4 Thermal and Electrochemical Properties

Thermal properties of 2OPTZ-PNA and 2OPTZBP-PNA were investigated by thermalgravimetric analysis（TGA）and differential scanning calorimetry（DSC）under nitrogen atmosphere（Fig. 6（a）and Tab. 1）. It showed that 2OPTZ-PNA and 2OPTZBP-PNA possessed good thermal and morphological stabilities with the high decomposition temperatures（Td, corresponding to 5% weight loss）of 370/447 ℃and glass-transition temperatures（Tg）of 106/134 ℃,respectively. It indicated that both compounds were suitable to fabricate the stable OLEDs by vacuum thermal evaporation. It was worth mentioning that 2OPTZ-BP-PNA had more excellent thermal stability owing to the rigidly molecular structure by the extended π-bridge length.

Tab.1 The thermal properties and photophysical data of 2OPTZ-PNA and 2OPTZ-BP-PNA

Fig.6 TGA and DSC curves（inset）（a）and electrochemical CV curves（b）of 2OPTZ-PNA and 2OPTZ-BP-PNA

Electrochemical properties of 2OPTZ-PNA and 2OPTZ-BP-PNA were evaluated by cyclic voltammetry（CV）curves（Fig. 6（b））, which were used to calculate the HOMO and LUMO energy levels. According to the formula ofEHOMO=-4.8-e（Ecox-Efox）,the HOMO energy levels of 2OPTZ-PNA and 2OPTZBP-PNA were -5.40 eV and -5.32 eV, respectively, which were calculated from the oxidation potential with ferrocene as an internal reference. Correspondingly, the LUMO energy levels calculated from the formula ofELUMO=EHOMO+Egwere -1.93 eV and -2.12 eV,respectively.

3.5 Film Morphology Characteristic

As is well-known that the excellent film morphologies for the light-emitting layer are favor to reduce the interface problems and improve device efficiency. On this basis, film morphologies of both compounds in the thermally evaporated thin film were evaluated by atomic force microscopy（AFM）.As shown in Fig. 7, the root mean square surface roughness values of 2OPTZ-PNA and 2OPTZ-BP-PNA are 0.36 nm and 0.24 nm, respectively. It indicated that 2OPTZ-BP-PNA would have better contact with the adjacent functional layers owing to the smoother and more homogeneous surface morphology.

Fig.7 AFM images（5 µm × 5 µm）and the corresponding 3D height images of 2OPTZ-PNA and 2OPTZ-BP-PNA in the evaporated films

3.6 Electroluminescent Performance

In order to evaluate the EL performance of 2OPTZ-PNA and 2OPTZ-BP-PNA, non-doped devices were firstly fabricated. The device structures were as follows: ITO/MoO3（3 nm）/NPB（40 nm）/TCTA（10 nm）/2OPTZ-PNA or 2OPTZ-BP-PNA（20 nm）/TPBi（40 nm）/LiF（1 nm）/Al（100 nm）. The molecular structures of related functional layer materials, device structures and energy-level diagrams are shown in Fig. S8. Fig. 8 showed the EL properties of these non-doped devices,and the critical data were summarized in Tab. 2. Both 2OPTZ-PNA-based and 2OPTZBP-PNA-based non-doped devices achieved violetblue emission, and their emission peaks were located at 424 nm and 436 nm with the corresponding CIE coordinates at（0.164, 0.060）and（0.155, 0.046）,respectively. Here, apart from the slight red shift of EL emissions,EL spectra for two devices were similar to the PL spectra in the neat films prepared by vacuum-deposition, indicating that EL emissions for these devices were well originated from the intrinsic emission of two materials.

Tab.2 EL performance of devices based on 2OPTZ-PNA and 2OPTZ-BP-PNA

Fig.8 （a）EL performance of 2OPTZ-PNA and 2OPTZ-BP-PNA in the non-doped and doped devices. EL spectra at a driving voltage of 5 V.（b）J-V-L curves.（c）CE-L-PE curves.（d）EQE-L curves.

Moreover, EL spectra of 2OPTZ-PNA and 2OPTZ-BP-PNA at different driving voltages exhibited good color stability without noticeable changes（Fig. S9）. The lower turn-on voltages（3.0-3.2 V）were obtained, which could be attributed to the use of a suitable device structure and the balanced carrier transport properties characterized by the carrier-only devices（Fig. S10）. In particular, the non-doped 2OPTZ-BP-PNA-based device exhibited excellent EL performance with the maximum EQE and luminance（Lmax）of 4.1% and 2 602 cd/m2, respectively.Importantly, it also had the narrower FWHM of 54 nm, and CIE coordinates of（0.155, 0.046）at the driving voltage of 6 V was very close to the standard violet-blue light in the high-definition displays. The high EQE of 2OPTZ-BP-PNA was due to its better carrier transport performance, more suitable device energy levels and high PLQY.

Furthermore, we also tried to explore the EL properties of 2OPTZ-PNA and 2OPTZ-BP-PNA in the doped devices, where mCP was employed as the host material of 2OPTZ-PNA and 2OPTZ-BP-PNA.The device structure was ITO/MoO3（3 nm）/NPB（40 nm）/TCTA（10 nm）/mCP∶x% 2OPTZ-PNA or 2OPTZBP-PNA（20 nm）/TPBi（40 nm）/LiF（1 nm）/Al（100 nm）. Compared with their non-doped devices, the EL emission spectra of two doped devices had significantly blue-shifted emission peaks and narrower FWHMs（Fig. 8 and Tab. 2）. This was mainly attributed to the fact that the introduction of the host material（mCP）reduced the polarity of the emitting layer and weakened the intermolecular interaction. Moreover, we investigated the different doping concentrations of 70%, 50%, 30%, 20% and 10% in the doped 2OPTZ-BP-PNA-based devices（Fig. S11-S12）. As the doping concentration decreased（Tab. S2）, EL emission peaks had obvious blue-shift to a certain extent owing to increasing the distance between the chromophores. When the doping concentration was 70%, it showed high-quality violet-blue light emission at 428 nm with a CIE coordinate of（0.156,0. 036）. However, the maximum luminance and device efficiency were slightly lower than the non-doped device. It could be explained that the disordered molecular arrangement in the doped films decreased the light output. These results indicated that the new compounds of 2OPTZ-PNA and 2OPTZ-BP-PNA had great potential in lighting and full-color display.

4 Conclusion

In summary, we have designed and synthesized a kind of violet-blue fluorescent materials（2OPTZPNA and 2OPTZ-BP-PNA）with D-π-A-typed structures using 10H-phenothiazine 5, 5-dioxide as the weak acceptor and N-（2-naphthyl）aniline as the donor and the phenyl and biphenyl as the π-bridge.By prolonging the π-bridge length, the charge transfer state has been appropriately regulated and controlled. On this basis, PLQYs improved from 14% to 33%. Non-doped OLED devices based on 2OPTZPNA and 2OPTZ-BP-PNA exhibited purely violetblue light with the CIE coordinates of （0.164,0.060）and（0.155, 0.046）, respectively. Furthermore, the non-doped devices of 2OPTZ-BP-PNA exhibited the best EL performance with the narrow FWHM of 54 nm and maximum EQEs of 4.1%.This work provides a new strategy for high-performance violet-blue emitters.

Supplementary Information and Response Letter are available for this paper at：http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20220093.