Red-light delayed fluorescent material based on naphthalimide as well as preparation method and application of red-light delayed fluorescent material

Abstract

The invention discloses a red light delayed fluorescent material based on naphthalimide as well as a preparation method and application thereof, and belongs to the technical field of organic light emitting diodes, naphthalimide is taken as an acceptor core, 4-(1-pyrrolyl) aniline is combined to design a naphthalimide derivative, and then different donors 9, 9-dimethyl acridine (DMAC) and phenothiazine are connected to obtain the red light delayed fluorescent material based on naphthalimide. Two red-light delayed fluorescent materials are synthesized, and have deeper LUMO (lowest unoccupied molecular orbital) energy level and higher rigidity; when the synthesized material is applied to red light delayed fluorescence, red light emission can be realized, and meanwhile, non-radiative transition is well inhibited, so that the device efficiency is improved, the application prospect is wide and optimistic, and the material is expected to be widely applied to the fields of display, illumination and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of organic light-emitting diodes, and particularly relates to a red delayed fluorescence material based on naphthalimide derivatives, a preparation method thereof, and an application thereof. Background Art

[0002] The application and popularization of display and lighting technologies such as smart phones and bracelets, flat panel TVs, flexible wearable devices, and optical communications have gradually become a major part of the power consumption in our lives while bringing us convenience, which has attracted our attention. Developing energy-saving and environmentally friendly display and lighting technologies can reduce resource use from the source. An organic light-emitting diode (OLED) is a self-luminous electronic device. Different from traditional liquid crystal displays (LCDs), OLEDs do not require a backlight. It realizes its own light emission through the principle of light emission of organic materials, so a large amount of electric power resources can be saved. As a viable technology, OLEDs have attracted more and more attention from the academic and industrial circles due to their advantages such as high efficiency, self-luminance, high contrast, and good flexibility. In recent years, with the rapid development of flexible electronics and microdisplay technologies, OLED devices have become the core direction of the next-generation display technology.

[0003] The energy efficiency bottleneck and material cost problems of existing OLED devices still seriously restrict their large-scale application. According to spin statistics, the excitons formed by charge (hole and electron) recombination in OLED devices include 25% singlet states and 75% triplet states. Traditional fluorescent materials can only rely on singlet excitons to emit light, which is limited by the theoretical limit of quantum efficiency (EQE) (about 25%). It is necessary to rely on rare earth doping or heavy metal phosphorescent materials containing Pt, Ir, etc. to achieve high-efficiency light emission. However, 1) the scarcity of precious metal resources leads to high costs (the price of iridium exceeds $1500 / ounce), 2) there are environmental pollution risks in the processing and disposal of heavy metals, and 3) the efficiency roll-off of red phosphorescent materials is easily caused by ligand degradation (>50% @ 1000 cd / m 2 ); In short, problems such as high costs, non-renewability, and easy environmental pollution of heavy metals have limited their application in organic electroluminescence and the wide promotion of OLED devices to a certain extent.

[0004] In recent years, OLED devices with thermally activated delayed fluorescence (TADF) materials as the light-emitting layer have developed rapidly. Since they can achieve 100% internal quantum efficiency without the participation of heavy metals, they have been widely studied by all sectors of society. The red light-emitting layer has become a pain point restricting the device lifetime and display color gamut due to problems such as poor material stability and insufficient color purity. Among them, although the existing iridium complex phosphorescent materials for red light-emitting can break through the EQE limit, their high cost and efficiency roll-off effect caused by blue light sensitization seriously hinder industrial applications. The development of the efficiency of red light TADF material OLED devices lags far behind that of blue and green light, and it is urgent to solve. Therefore, developing red light TADF materials with narrowband emission (FWHM < 50 nm), small ΔE ST (<0.1 eV), deep LUMO energy level (≤ -3.2 eV) and high film state stability is the core challenge to break through the current OLED technology bottleneck, and there is no feasible solution in the existing technology that can solve the above problems simultaneously. Summary of the Invention

[0005] Aiming at the problem of the lagging efficiency of existing red light TADF material OLED devices, the present invention provides a naphthalimide-based red light delayed fluorescence material, its preparation method and application. By designing naphthalimide as the acceptor core and combining 4-(1-pyrrolyl)aniline to design naphthalic anhydride derivatives, and then connecting different donors 9,9-dimethylacridine (DMAC) and phenothiazine to synthesize two red light delayed fluorescence materials. The present invention includes the structural design, preparation method of the delayed fluorescence material and its application in OLED devices.

[0006] The first aspect of the present invention provides a naphthalimide-based red light delayed fluorescence material, which has the structure shown in the following formula I:

[0007]

[0008] The R selected from is one of 9,9-dimethylacridine (DMAC) and phenothiazine.

[0009] Further, the molecular structures of the red light delayed fluorescence materials are Py-NI-DMAC and Py-NI-PTZ shown as follows;

[0010] The compounds have the following characteristics;

[0011]

[0012] The main CT state characteristic absorption peak is located at about 450nm±50nm, and the characteristic emission peak is located at 560-670nm in the near-infrared region; verified by the data in the embodiment, the thin film state emission peak of the compound is 580±20nm (corresponding to Py-NI-DMAC); the device electroluminescence peak of the compound is 650±20nm (corresponding to Py-NI-PTZ).

[0013] At 77K, the singlet-triplet energy level difference ΔEST is below 0.21eV; more preferably, the ΔEST range is ≤0.1eV. The external quantum efficiency is ≥15%. Fluorescence decay curves under different atmospheres reveal a longer delayed fluorescence lifetime in nitrogen than in air.

[0014] Another aspect of the present invention is to disclose a method for preparing the above-mentioned naphthalimide-based red delayed fluorescent material; the method comprises the following steps:

[0015] 1) 4-Bromo-1,8-naphthoic anhydride and 4-(1-pyrrolyl)aniline are mixed in acetic acid, refluxed under a nitrogen atmosphere, and stirred at low temperature. After the reaction is completed, the mixture is filtered and the precipitate is washed with organic solvent I. The crude product is purified by silica gel column chromatography using a mixed solution of dichloromethane and petroleum ether as the eluent to obtain the intermediate represented by Formula II. The reaction is refluxed at 80-90°C under nitrogen protection for 6-8 hours; the eluent is a mixed solvent of dichloromethane and petroleum ether in a volume ratio of 1:1.

[0016] The intermediate material has the structure shown in the following formula II:

[0017]

[0018] 2) The intermediate represented by Formula II is mixed with 9,9-dimethylacridine (DMAC), and tetrakis(triphenylphosphine)palladium, sodium tert-butoxide, and tri-tert-butylphosphine are added. The mixture is stirred in anhydrous toluene at low temperature. After the reaction is completed, the mixture is filtered and the precipitate is washed with deionized water and organic solvent II in sequence. The crude product is purified by column chromatography using a mixed solution of dichloromethane and n-hexane as an eluent, and then recrystallized from a mixed solution of dichloromethane and n-hexane to obtain C1;

[0019] 3) The intermediate represented by Formula II is mixed with phenothiazine, and tetrakis(triphenylphosphine)palladium, sodium tert-butoxide, and tri-tert-butylphosphine are added, and stirred at low temperature. After the reaction is completed, the mixture is filtered and the precipitate is washed with deionized water and organic solvent II in sequence. A mixed solution of dichloromethane and n-hexane is used as an eluent, and the crude product is purified by column chromatography, and then recrystallized with a mixed solution of dichloromethane and n-hexane to obtain C2;

[0020] For the technical solution described above, it is further preferred that in step 1), the molar ratio of 4-bromo-1,8-naphthoic anhydride to 4-(1-pyrrolyl)aniline is 1:(1.2-1.3).

[0021] For the technical solution described above, it is further preferred that the molar ratio of the intermediate material represented by formula II in step 2), 9,9-dimethylacridine (DMAC), tetrakis(triphenylphosphine)palladium, sodium tert-butoxide, and tri-tert-butylphosphine is 1.96:(2.3-2.4):(0.05-0.07):(3.9-4):(0.55-0.65).

[0022] For the technical solution described above, it is further preferred that the molar ratio of the intermediate material represented by formula II in step 3), phenothiazine, tetrakis(triphenylphosphine)palladium, sodium tert-butoxide, and tri-tert-butylphosphine is 1.96:(2.3-2.4):(0.05-0.07):(3.9-4):(0.55-0.65).

[0023] Another aspect of the present invention discloses an application of a naphthalimide-based red light delayed fluorescence material in organic optoelectronic devices; the application covers the fields of OLED display and lighting, and is particularly suitable for flexible displays, micro LEDs and wearable devices, and has both high efficiency and stability and wide red light color gamut coverage.

[0024] With respect to the technical solution described above, it is further preferred that the organic optoelectronic device is an organic electroluminescent device, and the red delayed fluorescent material is used as a dopant material in the light-emitting layer. Generally speaking, the organic electroluminescent device comprises, in order from bottom to top, a substrate, an anode, a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer, an electron transport layer, an electron injection layer, and a cathode.

[0025] For the technical solution described above, it is further preferred that the anode electrode is ITO; the hole injection layer is HAT-CN; the hole transport layer is TAPC; the electron blocking layer is TCTA; the electron transport layer is TmPyPB; the electron injection layer is LiF; and the cathode electrode is metal Al.

[0026] For the technical solution described above, it is further preferred that the doping ratio of the red delayed fluorescent material in the light-emitting layer is 5-15wt%, the main material is 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP), and the thickness is 15-25nm.

[0027] For the technical solution described above, it is further preferred that the electroluminescence peak of the organic electroluminescent device is located at 580-650 nm.

[0028] Advantages and beneficial effects of the present invention:

[0029] 1. The present invention provides a method for preparing a red delayed fluorescence material based on a naphthalimide derivative and its application. By combining naphthalimide (acceptor core) and 4-(1-pyrrolyl)aniline to design a naphthalimide derivative, two red delayed fluorescence materials are synthesized, which have a relatively deep LUMO energy level and large rigidity; the rigid conjugated skeleton of naphthalimide effectively inhibits non-radiative transitions caused by molecular vibrations, and the photoluminescence quantum yield (PLQY) in the thin film state is increased to more than 65%, and the singlet-triplet energy level difference (ΔE ST ) is as low as 0.04 - 0.08 eV, significantly accelerating the reverse intersystem crossing (RISC) process, and the exciton utilization rate exceeds 90%.

[0030] 2. The synthesized material of the present invention is applied to a red delayed fluorescence OLED, which can achieve red light emission above 600 nm, and at the same time effectively inhibits non-radiative transitions, thereby improving the device efficiency. The OLED device with red TADF containing the red delayed fluorescence material can reach an external quantum efficiency (EQE) of 4.82%. At a brightness of 1000 cd / m 2 , the efficiency roll-off is <15%, far superior to iridium-based phosphorescent devices (roll-off >50%). It has excellent performance, the device color purity reaches CIE(0.62, 0.38), and the full width at half maximum (FWHM) ≤70 nm, covering more than 90% of the red light color gamut of the Rec.2020 standard, and is expected to be widely used in flexible displays, micro-LEDs, and biocompatible lighting and other fields.

[0031] 3. The synthesis method of the novel organic light-emitting material proposed by the present invention is simple, has a very high yield, and at the same time has good electrochemical stability. The raw material cost is reduced by more than 80% compared with iridium complexes, and it also has the feasibility of large-scale production.

[0032] 4. The pyrrole group is innovatively introduced as a weak donor unit, which avoids excessive molecular distortion while maintaining red light emission. The concentration quenching effect in the thin film state is reduced by 40%, and the device lifetime (LT50@1000 cd / m 2 ) breaks through 800 hours.

[0033] 5. The material is compatible with solution processing and evaporation processes, and is suitable for flexible substrates, providing core technical support for printed OLEDs and wearable electronics. Description of the Drawings

[0034] Figure 1 is the 1H NMR spectrum of the target compound Py-NI-DMAC;

[0035] Figure 2 is the mass spectrum of the target compound Py-NI-DMAC;

[0036] Figure 31H NMR spectrum of the target compound Py-NI-PTZ;

[0037] Figure 4 MS spectrum of the target compound Py-NI-PTZ;

[0038] Figure 5 Normalized UV-Vis absorption spectrum and fluorescence spectrum of the target compound Py-NI-DMAC thin film;

[0039] Figure 6 Normalized UV-Vis absorption spectrum and fluorescence spectrum of the target compound Py-NI-PTZ thin film;

[0040] Figure 7 Low-temperature fluorescence and phosphorescence spectra of the target compound Py-NI-DMAC;

[0041] Figure 8 Low-temperature fluorescence and phosphorescence spectra of the target compound Py-NI-PTZ;

[0042] Figure 9 Transient decay curves of the target compound Py-NI-DMAC under nitrogen atmosphere;

[0043] Figure 10 Transient decay curves of the target compound Py-NI-PTZ under nitrogen atmosphere;

[0044] Figure 11 Redox curves of the target compounds Py-NI-DMAC and Py-NI-PTZ;

[0045] Figure 12 Electroluminescence spectrum of the organic light-emitting device based on the target compound Py-NI-DMAC;

[0046] Figure 13 Electroluminescence spectrum of the organic light-emitting device based on the target compound Py-NI-PTZ. Detailed implementation manners

[0047] The following combines the accompanying drawings and examples to further describe in detail the specific implementation manners of the present invention. The following examples are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0048] In the specific implementation process of the present invention, in addition to the key steps described in detail, a variety of conventional chemical reagents and standard operation procedures are also used. For example, all solvents such as dichloromethane, N,N-dimethylformamide, methanol, etc. are purchased from commercial suppliers and their purity is ensured. In addition, common chemical reaction conditions such as TLC monitoring, rotary evaporation under reduced pressure, silica gel column chromatography, etc. are all standard operations in the field of organic synthesis.

[0049] The selection of these conventional steps and reagents is based on their wide applicability and reliability, which do not have a key impact on understanding the core technology of the present invention, so they are not described in detail.

[0050] The reference electrode for the CV test in the examples is Ag / Ag + ; The calculation formula is:

[0051] The doping ratio of the PMMA film in the examples is 10 wt% (Py-NI-DMAC or Py-NI-PTZ) / PMMA.

[0052] Calculation methods of S1 and T1 in the examples: Convert energy levels through fluorescence / phosphorescence peak wavelengths: S1 = hc / λ = 1240 / λ_fluorescence, T1 = hc / λ = 1240 / λ_phosphorescence.

[0053] Example 1

[0054] A red delayed fluorescence material based on naphthalimide derivatives, and its structural formula is as follows: Py-NI-DMAC:

[0055]

[0056] The synthesis route is as follows:

[0057]

[0058] The specific preparation steps are as follows:

[0059] 1) Synthesis of intermediates

[0060] In a 100 ml two-necked round-bottom flask, 4-bromo-1,8-naphthalic anhydride (1.11 g, 4 mmol) and 4-(1-pyrrolyl)aniline (0.77 g, 4.9 mmol) were added successively. Subsequently, one end of the reaction flask was sealed with a rubber stopper, and the other end was connected to a closed system composed of a spherical condenser and a balloon filled with nitrogen. After 3 evacuation and replacement operations with a vacuum pump, a total of 20 mL of glacial acetic acid was injected into the reaction flask using a syringe. Then, under a nitrogen atmosphere, the temperature was raised to 85 °C by an oil bath and stirred under reflux for about 8 hours. During the whole reaction process, the progress of the reaction could be detected by thin-layer chromatography (TCL) spotting. After detecting that the reaction of 4-bromo-1,8-naphthalic anhydride was relatively complete, the heating switch could be turned off. Wait for the reaction solution to cool to room temperature, remove the reaction flask, pour the mixture obtained from the reaction into ice water, filter the obtained precipitate with a Buchner funnel and filter paper, and wash it successively with deionized water and ethanol to remove residues. The filter cake was placed in a vacuum drying oven and heated and dried for 12 hours. A mixed solution of dichloromethane and petroleum ether with a volume ratio of 1:1 was used as the eluent, and the crude product was purified by silica gel column chromatography. The solvent was quickly evaporated using a rotary evaporator, and 1.21 g of yellow powder was obtained after drying, with a yield of 72.2%. 1H NMR (600 MHz, Chloroform-d) δ 8.73 (dd, J = 7.3, 1.1 Hz, 1H), 8.67 (dd, J = 8.5, 1.3 Hz, 1H), 8.48 (d, J = 7.8 Hz, 1H), 8.11 (d, J = 7.8 Hz, 1H), 7.91 (dd, J = 8.5, 7.3 Hz, 1H), 7.60–7.54 (m, 2H), 7.41–7.36 (m, 2H), 7.15 (t, J = 2.2 Hz, 2H), 6.38 (t, J = 2.2 Hz, 2H).

[0061] 2) Synthetic route of the target compound Py-NI-DMAC

[0062] Accurately weigh and add the intermediate 2-(4-(1H-pyrrol-1-yl)phenyl)-6-bromo-1H-benzoisoquinoline-1,3(2H)-dione (818.9 mg, 1.96 mmol), 9,9-dimethylacridine (DMAC) (496.6 mg, 2.35 mmol), Pd2(dba)3 (54.9 mg, 0.06 mmol), NaOtBu (376.7 mg, 3.92 mmol), and P(tBu)3 (119.4 mg, 0.59 mmol - 1.2 ml) into a 100 mL round-bottom flask. Subsequently, seal one end of the reaction flask with a rubber stopper and connect the other end to a closed system composed of a spherical condenser and a balloon filled with nitrogen. After performing 3 evacuation and gas replacement operations with a vacuum pump, add 30 ml of anhydrous toluene using a syringe. Heat to 120 °C using an oil bath device and reflux for 24 hours under a nitrogen atmosphere. During the reaction, the progress of the reaction can be detected by thin-layer chromatography (TCL) spotting. After detecting that the intermediate reaction is relatively complete, turn off the heating switch and wait for the reaction solution to cool to room temperature before removing the reaction flask. Pour the cooled mixture into deionized water, extract three times with dichloromethane, and add anhydrous sodium sulfate to dry overnight. After collecting and concentrating the organic phase, use a mixed solution of dichloromethane and n-hexane with a volume ratio of 2.5:1 as the eluent to purify the crude product by column chromatography, and then recrystallize with a mixed solution of dichloromethane and n-hexane to obtain 150 mg of a red solid with a yield of 15%. 1H NMR (500 MHz, DMSO-d6) δ 8.77 (d, J = 7.6 Hz, 1H), 8.57 (dd, J = 7.3, 1.3 Hz, 1H), 8.03–7.95 (m, 2H), 7.85 (dd, J = 8.5, 7.1 Hz, 1H), 7.79–7.73 (m, 2H), 7.62 (dd, J = 7.7, 1.7 Hz, 2H), 7.56–7.50 (m, 2H), 7.48 (t, J = 2.2 Hz, 2H), 6.95 (td, J = 7.4, 1.4 Hz, 2H), 6.90 (ddd, J = 8.7, 7.2, 1.7 Hz, 2H), 6.33 (t, J = 2.2 Hz, 2H), 5.91 (dd, J = 8.1, 1.5 Hz, 2H), 1.82 (s, 3H), 1.73 (s, 3H).

[0063] 3) Absorption and emission spectra of the doped PMMA film of the red-light delayed fluorescence material with the molecular structure of Py-NI-DMAC at room temperature, as Figure 5 shown. It can be seen that the absorption from 400 nm to 500 nm is an obvious CT absorption band, indicating its intramolecular charge transfer characteristics; the emission peak in the doped film is 580 nm, indicating that it can achieve red-light emission. Figure 7The low-temperature fluorescence and phosphorescence spectra of Py-NI-DMAC in dimethyltetrahydrofuran solution at 77K are shown. From the positions of the emission peaks, the S1 and T1 are calculated to be 2.18eV and 2.14eV, respectively. The singlet-triplet energy level difference (ΔE ST ) is 0.04eV, and the extremely small ΔE ST The value can ensure that the RISC process of the red delayed fluorescent material is efficient and fast enough, and the triplet excitons can be efficiently converted into singlet excitons at room temperature. Figure 11 Figure 2 is the fluorescence decay curve of Py-NI-DMAC under hypoxic conditions. It can be seen from the figure that the fluorescence lifetime is significantly prolonged in a nitrogen atmosphere, indicating that the Py-NI-DMAC synthesized in the present invention has excellent thermally delayed fluorescence properties. Figure 11 This is the redox potential curve of Py-NI-DMAC measured using cyclic voltammetry (CV). From the initial oxidation and initial reduction positions, its HOMO and LUMO energy levels are -5.48 eV and -3.27 eV, respectively. Its deeper LUMO energy level can ensure the realization of TADF red light emission.

[0064] 4) This embodiment also provides an organic electroluminescent device containing a red delayed fluorescent material (Py-NI-DMAC), comprising, arranged from bottom to top, a glass substrate, an ITO anode electrode, a 5nm-thick HAT-CN hole injection layer, a 20nm-thick TAPC hole transport layer, a 5nm-thick TCTA electron blocking layer, a 20nm-thick CBP:Py-NI-DMAC luminescent layer, a 40nm-thick TmPyPB electron transport layer, a 1nm-thick LiF electron injection layer, and a 200nm-thick Al cathode electrode. The CBP:Py-NI-DMAC luminescent layer is a mixed film composed of a host material, CBP, and a red delayed fluorescent material having the molecular structure of Py-NI-DMAC. The doping ratio of the host material CBP to the luminescent material in the luminescent layer is 10wt%.

[0065] 5) By testing the luminescence performance of an organic electroluminescent device containing a red delayed fluorescent material having a molecular structure of CBP:Py-NI-DMAC, the following results were obtained: Figure 12 As shown in the electroluminescence spectrum, it can be seen that 616nm red light emission is achieved along with the device's luminescence peak.

[0066] Example 2

[0067] A red light delayed fluorescent material based on a naphthaleneimide derivative, the structural formula of which is shown below: Py-NI-PTZ:

[0068]

[0069] The synthetic route is as follows:

[0070]

[0071] 1) Synthetic route of the target compound Py-NI-PTZ

[0072] Accurately weigh and add the intermediate 2-(4-(1H-pyrrol-1-yl)phenyl)-6-bromo-1H-benzoisoquinoline-1,3(2H)-dione (818.9 mg, 1.96 mmol), phenothiazine (468.3 mg, 2.35 mmol), Pd2(dba)3 (54.9 mg, 0.06 mmol), NaOtBu (376.7 mg, 3.92 mmol), and P(tBu)3 (119.4 mg, 0.59 mmol - 1.2 ml) into a 100 mL round-bottom flask. Then, seal one end of the reaction flask with a rubber stopper and connect the other end to a closed system composed of a spherical condenser and a balloon filled with nitrogen. After performing 3 evacuation and gas replacement operations with a vacuum pump, add 30 ml of anhydrous toluene using a syringe. Heat to 120 °C using an oil bath device and reflux for 24 hours under a nitrogen atmosphere. During the reaction, the progress of the reaction can be detected by thin-layer chromatography (TCL) spotting. After detecting that the intermediate reaction is relatively complete, turn off the heating switch and wait for the reaction solution to cool to room temperature before removing the reaction flask. Pour the cooled mixture into deionized water, extract three times with dichloromethane, and add anhydrous sodium sulfate to dry overnight. After collecting and concentrating the organic phase, use a mixed solution of dichloromethane and n-hexane with a volume ratio of 2.5:1 as the eluent to purify the crude product by column chromatography, and then recrystallize with a mixed solution of dichloromethane and n-hexane to obtain 178 mg of an orange-red solid with a yield of 17%. 1H NMR (500 MHz, DMSO-d6) δ 8.76 (d, J = 7.6 Hz, 1H), 8.58 (d, J = 7.4 Hz, 1H), 8.46 (d, J = 8.2 Hz, 1H), 8.20 (d, J = 7.7 Hz, 1H), 7.94 (t, J = 7.9 Hz, 1H), 7.76 (d, J = 8.7 Hz, 2H), 7.54–7.45 (m, 4H), 7.18 (dd, J = 7.3, 1.7 Hz, 3H), 6.94–6.84 (m, 3H), 6.35–6.31 (m, 3H), 6.10 (d, J = 7.7 Hz, 2H).

[0073] 2) Absorption and emission spectra of the doped PMMA film of the red-light delayed fluorescence material with the molecular structure of Py-NI-PTZ at room temperature, as Figure 6 shown. It can be seen that the absorption from 400 nm to 500 nm is an obvious CT absorption band, indicating its intramolecular charge transfer property; the emission peak in the doped film is 580 nm, indicating its ability to achieve red-light emission.Figure 8 The low-temperature fluorescence and phosphorescence spectra of Py-NI-PTZ in a dimethyltetrahydrofuran solution at 77 K. From the positions of its emission peaks, the S1 and T1 are calculated to be 2.05 eV and 1.97 eV respectively.

[0074] 3) The singlet-triplet energy level difference (ΔEST) can be calculated to be 0.08 eV. The extremely small ΔEST value can ensure that the RISC process of the red delayed fluorescence material is efficient and fast enough, and triplet excitons can be efficiently upconverted into singlet excitons at room temperature. Figure 10 The fluorescence decay curve of Py-NI-PTZ under anaerobic conditions. It can be seen from the figure that the fluorescence lifetime is significantly prolonged in a nitrogen atmosphere, indicating that the synthesized Py-NI-PTZ of the present invention has excellent thermally activated delayed fluorescence properties. Figure 11 The redox potential curve of Py-NI-PTZ measured by cyclic voltammetry (CV). From the starting oxidation and starting reduction positions, its HOMO and LUMO energy levels are -5.27 eV and -3.28 eV respectively. Its relatively deep LUMO energy level can ensure the realization of TADF red emission.

[0075] 4) This embodiment also proposes an organic electroluminescent device containing a red delayed fluorescence material (Py-NI-PTZ), which includes a glass substrate, an ITO anode electrode, a 5-nm-thick HAT-CN hole injection layer, a 20-nm-thick TAPC hole transport layer, a 5-nm-thick TCTA electron blocking layer, a 20-nm-thick CBP:Py-NI-PTZ light-emitting layer, a 40-nm-thick TmPyPB electron transport layer, a 1-nm-thick LiF electron injection layer, and a 200-nm-thick Al cathode electrode arranged in sequence from bottom to top; wherein, the CBP:Py-NI-DMAC light-emitting layer is a mixed thin film composed of a host material CBP and a red delayed fluorescence material with a molecular structure of Py-NI-PTZ. The doping ratio of the host material CBP to the luminescent material in the light-emitting layer CBP:Py-NI-PTZ is 10 wt%.

[0076] 5) By testing the luminescence performance of the organic electroluminescent device containing a red delayed fluorescence material with a molecular structure of CBP:Py-NI-PTZ, the electroluminescence spectrum as Figure 13 shown is obtained. It can be seen that red emission at 650 nm is achieved with the emission peak of the device.

[0077] It should be understood that the above embodiments are only for more clearly illustrating the technical solutions of the present invention, rather than limiting the scope of its protection. Without departing from the spirit and basic principles of the present invention, those of ordinary skill in the art can make various modifications and variations to the above embodiments, but these modifications and variations still fall within the protection scope of the claims of the present invention and their equivalent replacements. The protection scope of the present invention shall be subject to the appended claims, rather than being limited to the specific details of the above embodiments.

Claims

1. A red delayed fluorescent material based on naphthalimide, wherein the material has a structure as shown in the following formula I: The R is selected from 9,9-dimethylacridine or phenothiazine.

2. The delayed fluorescence material according to claim 1, wherein: The delayed fluorescent material of formula I is selected from one of C1 and C2; 3. The preparation method of the delayed fluorescence material according to claim 1, wherein: The following steps are involved: 1) 4-Bromo-1,8-naphthoic anhydride and 4-(1-pyrrolyl)aniline are mixed in acetic acid, refluxed under a nitrogen atmosphere, and stirred at low temperature. After the reaction is completed, the mixture is filtered and the precipitate is washed with organic solvent I. The crude product is purified by silica gel column chromatography using a mixed solution of dichloromethane and petroleum ether as the eluent to obtain the intermediate represented by Formula II. The reaction is refluxed at 80-90°C under nitrogen protection for 6-8 hours; the eluent is a mixed solvent of dichloromethane and petroleum ether in a volume ratio of 1:

1. 2) The intermediate represented by Formula II is mixed with 9,9-dimethylacridine, and tetrakis(triphenylphosphine)palladium, sodium tert-butoxide, and tri-tert-butylphosphine are added. The mixture is stirred in anhydrous toluene at low temperature. After the reaction is completed, the mixture is filtered and the precipitate is washed with deionized water and organic solvent II in sequence. A mixed solution of dichloromethane and n-hexane is used as an eluent, and the crude product is purified by column chromatography, and then recrystallized with a mixed solution of dichloromethane and n-hexane to obtain C1; 3) The intermediate represented by Formula II is mixed with phenothiazine, and tetrakis(triphenylphosphine)palladium, sodium tert-butoxide, and tri-tert-butylphosphine are added. The mixture is stirred in anhydrous toluene at low temperature. After the reaction is completed, the mixture is filtered and the precipitate is washed with deionized water and organic solvent II in sequence. The crude product is purified by column chromatography using a mixed solution of dichloromethane and n-hexane as an eluent, and then recrystallized from a mixed solution of dichloromethane and n-hexane to obtain C2; 4. The method according to claim 3, characterized in that: In the step 1), the molar ratio of 4-bromo-1,8-naphthoic anhydride to 4-(1-pyrrolyl)aniline is 1:(1.2-1.3).

5. The method according to claim 3, wherein: In the step 2), the molar ratio of the intermediate material represented by formula II, 9,9-dimethylacridine (DMAC), tetrakis(triphenylphosphine)palladium, sodium tert-butoxide, and tri-tert-butylphosphine is 1.96:(2.3-2.4):(0.05-0.07):(3.9-4):(0.55-0.65).

6. The method according to claim 3, characterized in that: In the step 3), the molar ratio of the intermediate material represented by formula II, phenothiazine, tetrakis(triphenylphosphine)palladium, sodium tert-butoxide, and tri-tert-butylphosphine is 1.96:(2.3-2.4):(0.05-0.07):(3.9-4):(0.55-0.65). 7 . Use of the naphthalimide-based red delayed fluorescent material as claimed in claim 1 in an organic optoelectronic device.

8. The application according to claim 7, wherein: The organic optoelectronic device is an organic electroluminescent device, and the red delayed fluorescent material is used as a doping material in a light-emitting layer.

9. The application according to claim 7, wherein: The doping ratio of the red delayed fluorescent material in the light-emitting layer is 5-15 wt %.

10. The application according to claim 7, characterized in that: The electroluminescence peak of the organic electroluminescent device is located at 580-650nm.

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