A thermally activated delayed fluorescent material with a rigid π-bridge construction strategy and its application

Abstract

The present invention relates to the technical field of organic electroluminescent materials, and specifically discloses a thermally activated delayed fluorescent material with a rigid π-bridge construction strategy and an application thereof. The material comprises: using pyrenequinoxaline as a conjugated π bridge, connecting a cyanobenzene acceptor (A) unit and a triphenylamine (TPA) donor (D) unit, and constructing a novel TADF material TPA-PyQl-BzCN with a D-A molecular structure through Suzuki coupling. The present invention uses pyrenequinoxaline as a conjugated π bridge, connecting a cyanobenzene acceptor (A) unit and a triphenylamine (TPA) donor (D) unit, and constructing a novel TADF material TPA-PyQl-BzCN with a D-A molecular structure through Suzuki coupling. The material is a completely new TADF material constructed with large conjugated π bridge units, is used in the molecular construction of the thermally activated delayed fluorescent material, provides a new construction strategy for the thermally activated delayed fluorescent material, and expands the types of organic near-infrared luminescent materials.

Description

Technical Field

[0001] The invention belongs to the technical field of organic electroluminescent materials, and in particular relates to a thermally activated delayed fluorescent material with a rigid π-bridge construction strategy and applications thereof. Background Art

[0002] Thermally activated delayed fluorescence emitters, also known as TADF emitters, are highly desirable for use in organic light-emitting diodes (OLEDs) because they have the potential to achieve 100% internal quantum efficiency (IQE) compared to pure organic fluorophores. The transition from the triplet excited state to the singlet state, requiring anti-intersegment crossing (RISC), is essential.

[0003] This is because RISC is an important factor in determining the use of triplet excitons, and therefore, the rate constant (k RISC ) must be increased appropriately. Theoretically, (k RISC ) is proportional to ΔEst, where and ΔEst are the energy gaps between the spin-orbit coupling (soc) matrix elements and the singlet (S) and triplet (T) excited states, respectively. or reducing ΔEst may help improve the final k RISC Many early studies have shown that ΔEst can be easily reduced by using highly twisted electron donor (D)-electron acceptor (A) type molecular designs. This has been demonstrated by many different research groups. This is because such a geometric structure can limit the overlap between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs), thereby reducing ΔEst during the charge transfer (CT) transition;

[0004] On the other hand, in this pure CT singlet state ( 1 CT) and triplet states ( 3 CT) may become closer and closer to zero, which completely violates the principle of accelerating k RISC Fortunately, there is a straightforward answer to this puzzle, which is to tune the energy level arrangement and guide the local excited triplet state ( 3 CT) state to promote the RISC process. This has been proven to be successful in solving the problem;

[0005] So far, several TADF molecules have been reported to have 6 s -1 K RISC value. Since these emitters 3 The energy levels of the LE state are very close to 1 CT / 3The energy level of the CT state is determined, thus making a significant contribution to the improvement of the RISC process. 3 LE states are usually much higher than their CT levels ( 1 CT and 3 CT), making them unable to affect RISC. Although it has only recently been discovered that the triplet energy levels of some large fused aromatic A segments are in the range required for red TADF emitters, not all of these large fused aromatic A segments are like this. When they are used to construct orange / red TADF emitters, their 3 LE states may contribute to the improvement of kRISC; however, the A segment with similar electron-sucking ability may be 3 Significant differences are shown in the LE energy levels; however, the reasons that determine these differences have not been elucidated, and therefore, in-depth analyses are urgently needed to reveal them and further develop standards for designing efficient TADF emitters. Summary of the Invention

[0006] The purpose of the present invention is to provide a thermally activated delayed fluorescent material with a rigid π-bridge construction strategy and its application, so as to solve the problems raised in the above background technology.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A thermally activated delayed fluorescent material with a rigid π-bridge construction strategy includes: using pyrenequinoxaline as a conjugated π bridge, connecting a cyanobenzene acceptor (A) unit and a triphenylamine (TPA) donor (D) unit, and constructing a new TADF material TPA-PyQl-BzCN with a DA molecular structure through Suzuki coupling. The specific molecular structure is shown below:

[0009]

[0010] Preferably, TADF luminescent material, TPA-PyQl-BzCN as the luminescent layer dopant, and CBP as the main material are used to prepare the luminescent layer through spin coating and heating annealing process.

[0011] Preferably, a doped TADF organic electroluminescent device is prepared to obtain a TADF organic electroluminescent device with an emission peak of 564 nm and an EQE of 7.2%.

[0012] Preferably, the structure of the TADF organic electroluminescent device is:

[0013] ITO / PEDOT:PSS(35nm) / PVK(30nm) / EML(35nm) / TmPyPB(45nm) / LiF(0.5nm) / Al(120nm);

[0014] Among them, PVK is the hole transport layer, TmPyPB is the electron transport layer, PEDOT:PSS is the hole injection layer, EML is the light-emitting layer, which is a mixture of guest material TPA-PyQl-BzCN and host material (4,4'-di(9-carbazole)biphenyl), and the cathode layer is composed of lithium fluoride and aluminum.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) In the present invention, a new TADF material TPA-PyQl-BzCN with a DA molecular structure is constructed by Suzuki coupling using pyrenequinoxaline as a conjugated π bridge to connect a cyanobenzene acceptor (A) unit and a triphenylamine (TPA) donor (D) unit. This is a new TADF material constructed with a large conjugated π bridge unit and is used in the molecular construction of thermally activated delayed fluorescent materials. It provides a new construction strategy for thermally activated delayed fluorescent materials and expands the types of organic near-infrared luminescent materials.

[0017] (2) Compared with the common TADF materials constructed with phenanthroquinoxaline conjugated π-bridge units, it has a stronger spin coupling constant (SOC), a smaller root mean square deviation (RMSD) and a higher quantum yield (PLQY).

[0018] (3) Through simple and reasonable molecular construction, it has a high-position anti-gap crossing channel, and its quantum yield PLQY is as high as 90%, which has the ability to be solution-processed electroluminescent devices.

[0019] (4) The molecular structure is simple, easy to synthesize, and the preparation cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The UV-visible absorption spectra of the compounds TPA-PyQl-BzCNT and TPA-PhTh-BzCN prepared in Example 1 of the present invention in toluene are shown;

[0021] Figure 2 The photoluminescence spectra of the compounds TPA-PyQl-BzCNT and TPA-PhTh-BzCN prepared in Example 1 of the present invention in toluene solution;

[0022] Figure 3 The photoluminescence spectra of the compounds TPA-PyQl-BzCNT and TPA-PhTh-BzCN prepared in Example 1 of the present invention in thin films;

[0023] Figure 4 CV spectra of the compounds TPA-PyQl-BzCNT and TPA-PhTh-BzCN prepared in Example 1 of the present invention;

[0024] Figure 5 The theoretical calculation spectra of the compounds TPA-PyQl-BzCNT and TPA-PhTh-BzCN prepared in Example 1 of the present invention;

[0025] Figure 6 The low-temperature fluorescence and lifetime spectra of the compounds TPA-PyQl-BzCNT and TPA-PhTh-BzCN prepared in Example 1 of the present invention are shown;

[0026] Figure 7 Device correlation diagrams of the compounds TPA-PyQl-BzCNT and TPA-PhTh-BzCN prepared in Example 1 of the present invention;

[0027] Figure 8 This is the synthetic route of the target compounds TPA-PyQl-BzCNT and TPA-PhTh-BzCN of the present invention. DETAILED DESCRIPTION

[0028] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0029] Example 1: Synthesis of TADF luminescent materials TPA-PyQl-BzCNT and TPA-PhTh-BzCN Figure 8 As shown;

[0030] Synthesis of compound M1

[0031] In a round-bottom flask, dissolve pyrene (10.00 g, 49.50 mmol, 1.0 eq.) and RuCl3·xH2O (1.23 g, 4.74 mmol, 0.096 eq.) in 200 mL of acetonitrile. Dissolve NaIO4 (42.75 g, 198 mmol, 4.0 eq.) in 100 mL of hot water and carefully add to the pyrene solution. Add 100 mL of DCM and stir vigorously at room temperature.

[0032] The reaction mixture was stirred for 18 h. The suspension was filtered through Celite, and the filtrate was extracted with DCM (3 × 100 mL). The combined organic phases were washed with saturated aqueous Na2S2O3 and water, and then dried over Na2SO4. The solvent was removed under reduced pressure. The crude product was a dark orange solid (7.56 g, 66%). The crude product was purified by silica gel column chromatography. 3.25 g of bright red pyrrole-4,5-dione (1) crystals were obtained. The yield was 27%.

[0033] 1 H NMR (300MHz, Chloroform-d): δ / ppm 8.42 (dd, J=7.4, 1.3Hz, 2H), 8.12 (dd, J=8.0, 1.3Hz, 2H), 7.79 (s, 2H), 7.71 (t, J=7.7Hz, 2H). 13 CNMR(75MHz,Chloroform-d):δ / ppm135.75,132.00,130.08,128.33,127.98,127.26.MODI-TOF-MS:m / z calculated forC 16 H8O2[M]+:232.05; found:232.09.Elemental analysis(%)for C 16 H8O2:C82.75,H 3.47,O 13.78; found:C 82.73,H 3.48,O 13.47.

[0034] Synthesis of compound M2

[0035] In a round-bottom flask, pyrene-4,5-dione (1) (2.00 g, 8.62 mmol, 1.0 eq.) was dissolved in 20 ml of concentrated sulfuric acid. n-Bromosuccinimide (2.75 g, 15.50 mmol, 1.8 eq.) was added and stirred at 0°C for 15 minutes. Stirred at room temperature for 1 hour. The reaction mixture was precipitated in 2 liters of water / ice, collected by filtration, and washed with water and methanol. It was then dried under high vacuum to obtain a bright yellow solid with a quantitative yield of 9,10-dibromopyrene-4,5-dione (2).

[0036] 1 H NMR (500MHz, DMSO-d6, 120℃): δ / ppm 8.72(d,J=8.3Hz,2H),8.47(d,J=7.4Hz,2H),7.98(t,J=7.9Hz,2H).Mp=360℃(decomposition).Elemental analysis(%)calculated C 16 H6Br2O2: C 49.27, H 1.55, Br 40.97, O 8.20; Found: C 49.41, H 1.73, Br41.03, O 8.21.

[0037] Synthesis of compound M3

[0038] 4,5-Dibromobenzene-1,2-diamine (1.00 g, 3.76 mmol, 1 eq.), (4-(diphenylamino)phenyl)boronic acid (2.72 g, 9.40 mmol, 2.5 eq.), K2CO3 (2.08 g, 15.04 mmol, 4 eq.), and Pd(PPh3)4 (87 mg, 0.075 mmol, 0.02 eq.) were added to a mixture of 1,4-dioxane and water (10 / 1, v / v) in 100 mL of argon. The mixture was stirred at 90°C for 24 h, cooled to room temperature, poured into 100 mL of water, and extracted with dichloromethane (DCM). The resulting layer was evaporated under reduced pressure and purified by column chromatography using petroleum ether and ethyl acetate (2 / 1, v / v) as eluent to obtain 1.79 g of a waxy solid with a yield of 78%.

[0039] 1 H NMR(400MHz,Chloroform-d)δ / ppm 7.21(t,J=7.7Hz,8H),7.07(d,J=7.8Hz,8H),6.99(d,J=8.0Hz,8H),6.92(d,J=8.3Hz,4H),6.79(s,2H).13C NMR(101MHz,DMSO-d6)δ147.27,144.47,137.34,134.44,130.55,129.35,128.59,123.42,123.24,122.51,115.98.MODI-TOF-MS:m / z calculated for C 42 H 34 N4[M] + :594.76; found:594.15.Elemental analysis(%)for C 42 H 34 N4:C 84.82,H 5.76,N 9.42;found:C 84.79,H5.63,N 9.41.

[0040] Synthesis of compound M4

[0041] 9,10-Dibromopyridine-4,5-dione (2) (700 mg, 0.74 mmol, 1 eq.) and N4,N4,N4",N4"-tetraphenyl-[1,1':2',1"-terphenyl]-4,4',4",5'-tetramine (3) (443 mg, 0.74 mmol, 1 eq.) were added to 70 mL of AcOH and 5 mL of EtOH under argon. The mixture was stirred at 85°C for 16 h, cooled to room temperature, and mixed with ice water. The resulting mixture was filtered under reduced pressure and recrystallized to obtain a yellow solid containing 530 mg of the product in a yield of 75%.

[0042] 1 H NMR(400MHz,Chloroform-d)δ9.57(d,J=24.9Hz,2H),8.69(d,J=27.4Hz,2H),8.42–8.32(m,2H),8.09(d, J=34.8Hz,2H),7.29(d,J=7.7Hz,10H),7.23(s,4H),7.15(d,J=7.9Hz,8H),7.06(s,6H).MODI-TOF-MS:m / z calculated for C 58 H 36 Br2N4[M]+:948.13; found:948.11.Elemental analysis(%)for C 58 H 36 Br2N4: C 73.43, H 3.82, Br16.84, N 5.91; found: C 73.33, H 3.85, Br 16.90, N 5.93.

[0043] Synthesis of compound M5

[0044] N4,N4,N4",N4"-tetraphenyl-[1,1':2',1"-terphenyl]-4,4',4",5'-tetraamine (3) (522 mg, 0.88 mmol, 1 eq.) and 3,6-dibromophenanthracene-9,10-dione (820 mg, 0.88 mmol, 1 eq.) were added to 70 mL of AcOH and 5 mL of EtOH under argon. The mixture was stirred at 85°C for 16 h, cooled to room temperature, and mixed with ice water. The resulting mixture was filtered under reduced pressure and recrystallized to obtain 653 mg of a yellow solid with a yield of 80%.

[0045] 1H NMR(400MHz,Chloroform-d)δ / ppm 9.15(d,J=8.5Hz,2H),8.51(s,2H),8.27(s,2H),7.81(d,J=8.5Hz,2H),7.32–7.26(m,8H ),7.20(d,J=8.1Hz,4H),7.14(d,J=7.7Hz,8H),7.04(d,J=8.0Hz,8H).MODI-TOF-MS:m / z calculated for C 56 H 36 Br2N4[M]+:924.74; found:924.43.Elemental analysis(%)for C 56 H 36 Br2N4: C 72.74, H 3.92, Br17.28, N 6.06; found: C 72.69, H 4.01, Br 17.30, N 6.00.

[0046] Synthesis of compound TPA-PyQl-BzCNT

[0047] 4,4'-(4,5-Dibromophenylthio[4,5-abc]phenazine-11,12-diyl)(N,N-diphenylaniline)(4) (0.50 g, 0.53 mmol, 1 eq.), (4-cyanophenyl)boronic acid (0.17 g, 1.16 mmol, 2.2 eq.) and K2CO3 (0.58 g, 4.42 mmol, 8 eq.) were added to 80 mL of NMP. Pd(PPh3)4 (36 mg, 0.032 mmol, 0.06 eq.) was then added under nitrogen atmosphere. After the solution was heated at 130°C for 24 hours, the reaction mixture was cooled to room temperature. The product was poured into 100 mL of water and extracted with dichloromethane (DCM). After evaporation of the organic layer, the product was purified by column chromatography using dichloromethane / n-hexane as the eluent to obtain 0.28 g of an orange solid. The yield was 53%.

[0048] 1 H NMR(400MHz,Chloroform-d)δ9.68(d,J=8.2Hz,2H),8.43(s,2H),8.01(t,J=7.9Hz,2H),7.77(d,J=8.5Hz,2H),7.66(d,J= 7.9Hz,4H),7.39(d,J=8.0Hz,4H),7.29(t,J=7.8Hz,8H),7.23(d,J=8.5Hz,4H),7.15(d,J=7.9Hz,8H),7.11–7.01(m,8H).13 C NMR (101MHz, CDCl3) δ147.56,147.26,143.77,141.91,136.01,132.06,131.80 ,130.89,129.39,128.15,127.38,124.93,124.70,123.21,122.64,111.59.The carbon peak is not complete due to solubi l ity relation.MODI-TOF-MS:m / z calculated forC 72 H 44 N6[M]+:992.36; found:993.08.Elemental analysis(%)for C 72 H 44 N6:C 87.07,H4.47,N 8.46;found:C 87.01,H 4.51,N 8.44.

[0049] Synthesis of compound TPA-PhTh-BzCN

[0050] 4,4′-(3,6-Dibromodibenzo[a,c]phenazine-11,12-diyl)(N,N-diphenylaniline)(5) (0.45 g, 0.49 mmol, 1 eq.), (4-cyanophenyl)boronic acid (0.16 g, 1.09 mmol, 2.2 eq.) and K2CO3 (0.20 g, 1.45 mmol, 3 eq.) were added to 80 mL of a mixture of toluene, ether and water (8 / 1 / 1, v / v). Pd(PPh3)4 (17 mg, 0.015 mmol, 0.03 eq.) was then added under nitrogen atmosphere. After the solution was heated at 98°C for 24 h, the reaction mixture was cooled to room temperature. The product was poured into 100 mL of water and extracted with dichloromethane (DCM). After evaporation of the organic layer, the product was purified by column chromatography using dichloromethane / n-hexane as eluent to obtain 0.37 g of an orange solid. The yield is 78%.

[0051] 1H NMR(400MHz,Chloroform-d)δ9.52(d,J=8.4Hz,2H),8.77(d,J=1.7Hz,2H),8.38(s,2H),7.99(d,J=8.3Hz,2H),7.93(d,J=8 .1Hz,4H),7.86(d,J=8.1Hz,4H),7.30(d,J=7.9Hz,8H),7.22(d,J=8.4Hz,4H),7.15(d,J=7.9Hz,8H),7.06(t,J=7.7Hz,8H). 13 C NMR (101MHz, CDCl3) δ146.52,131.83,130.98,129.80,128.34,127.22,123.63,122.15.The carbon peak is not complete due to solubi lity relation.MODI-TOF-MS:m / zcalculated for C 70 H 44 N6[M]+:968.36; found:969.79.Elemental analysis(%)forC 70 H 44 N6: C 86.75, H 4.58, N 8.67; found: C 86.77, H 4.61, N 8.62.

[0052] Example 2: UV and PL testing of TADF-based luminescent materials TPA-PyQl-BzCNT and TPA-PhTh-BzCN

[0053] The ultraviolet-visible (UV / Vis) absorption and photoluminescence (PL) spectra of these emitters in toluene solutions at room temperature are shown in Figure 2. Figure 1 and 2As shown. Strong, high-energy absorption bands in the lower wavelength region before 400 nm are attributable to n-π* and π-π* transitions. Weaker, broad absorption bands in the 400-550 nm range are attributed to the transition of ICT from donor to acceptor. In toluene solution, these emitters exhibit distinct ICT emissions and structure-less PL spectra upon photoexcitation, with maxima ranging from 557 nm to 545 nm. Compared to TPA-PhTh-BzCN and TPA-PyQl-BzCN, the PL spectrum of TPA-PyQl-BzCN exhibits a significant blue shift, indicating weaker ICT properties. Compared to TPA-PyQl-BzCN, TPA-PhTh-BzCN also exhibits significantly red-shifted emission, which is attributed to the incorporation of a strong electron-accepting unit and more delocalized π orbitals, which enhance their ICT states. This is a common phenomenon in most organic DA molecules.

[0054] Example 3: CV test of TADF-based luminescent materials TPA-PyQl-BzCNT and TPA-PhTh-BzCN

[0055] Figure 4 The cyclic voltammetry curves of TPA-PhTh-BzCN and TPA-PyQl-BzCN in CH2Cl2 and CH3CN solutions are shown with ferrocene as the reference. In the anodic scan, the oxidation potential peak of TPA-PhTh-BzCN is 1.079V, and the oxidation potential peak of TPA-PyQl-BzCN is 1.075V. Among them, the two TADF compounds exhibit very similar oxidation potentials. The HOMO energy level (EHOMO) and LUMO energy level (ELUMO) of TPA-PhTh-BzCN are calculated to be -5.29 / -3.04eV and -5.28 / -2.91eV, respectively. It is observed that TPA-PhTh-BzCN and TPA-PyQl-BzCN have similar energy gaps, which is mainly due to the similar DA electron cloud distribution.

[0056] Example 4: Theoretical calculation based on TADF luminescent materials TPA-PyQl-BzCNT and TPA-PhTh-BzCN

[0057] Density functional theory (DFT) calculations were performed on these compounds at the B3LYP / 6-31G basis level to explore their geometry and electronic distribution. Figure 5As shown, the dihedral angles θ1 between the TPA moiety and the rigid conjugated π-bridge in the optimized TPA-PhTh-BzCN and TPA-PyQl-BzCN are 48.23° and 45.41°, respectively. Generally speaking, typical TADF molecules with a dihedral angle close to 90° between the donor and acceptor exhibit nearly complete separation of the HOMO and LUMO distributions, resulting in smaller oscillation intensities (f). TPA-PhTh-BzCN and TPA-PyQl-BzCN exhibit relatively small dihedral angles (approximately 45°). This facilitates moderate overlap between the HOMO and LUMO, thereby increasing f. Here, we observe that the LUMO of the acceptor triphenyl group in TPA-PhTh-BzCN, after attachment to the conjugated π-bridge, is not distributed over the strong acceptor unit. This explains the slight blue shift in TPA-PyQl-BzCN compared to TPA-PhTh-BzCN, primarily due to the large torsion angle (θ2) between the conjugated π-bridge and the A unit. The twist angle of TPA-PyQl-BzCN increases from 37.74° to 66.88° relative to TPA-PhTh-BzCN, nearly doubling the twist angle to achieve a complete separation of the electron cloud distribution. This complete HOMO-LUMO separation drives a decrease in ΔEST (0.17 eV and 0.24 eV for TPA-PhTh-BzCN and TPA-PyQl-BzCN, respectively), which facilitates an efficient RISC process.

[0058] In further experiments, the root mean square deviation (RMSD) of the conformational changes of the ground state (S0) and excited state (S1) were calculated based on the optimized molecular geometry. Significantly higher than TPA-PyQl-BzCN The higher the RMSD, the greater the conformational change between S0 and S1, which leads to more non-radiative transition channels for excited state molecules;

[0059] Using time-dependent density functional theory, natural transition orbitals (NTO) and spin-orbit coupling matrices (SOC) were studied to further evaluate the excited-state properties. The ground-state S0 / S1 transition represents a charge transfer (CT) transition from the TPA donor to the conjugated aromatic ring acceptor. Interestingly, the lower triplet states (T1 and T2) of both compounds exhibit mixed charge transfer with charge transfer and localized excitation (LE) characteristics, which is crucial for enhancing the RISC process through non-adiabatic coupling. These results indicate that the two molecules composed of the same unit cell exhibit potential as TADF materials.

[0060] In addition, Figure 5We found that after TPA-PyQl-BzCN increased the conjugated bridge area of ​​TPA-PhTh-BzCN, the SOC between S0, T1 and T2 increased significantly, which also led to the k RISC Faster than TPA-PhTh-BzCN. Surprisingly, TPA-PyQl-BzCN involves the hRISC process, which is not observed in TPA-PhTh-BzCN, and the increase in surface conjugation stiffness allows the diversification of RISC process channels. Not only that, the oscillator strength of TPA-PyQl-BzCN (0.3272) is also improved compared with TPA-PhTh-BzCN (0.3153). At the same time, the reorganization energy (λ) is an indicator reflecting the geometric changes of the S0 and S1 states, and also reflects the contribution of intramolecular motion to non-radiative decay. However, we found that although the rigid skeletons are different, due to the similar DA structure, the two TADF materials have the same reorganization energy (λ = 0.426 eV), which also indicates that the introduction of the rigid skeleton does not improve the non-radiative transition behavior caused by intramolecular transitions.

[0061] Example 5: Low-temperature fluorescence phosphorescence test and delayed fluorescence lifetime test based on TADF luminescent materials TPA-PyQl-BzCNT and TPA-PhTh-BzCN

[0062] The singlet and triplet energies of both emitters were calculated from their fluorescence and phosphorescence, which were measured in toluene at 77 K ( Figure 6 a and d). The singlet-triplet energy gaps (ΔEST) of TPA-PhTh-BzCN and TPA-PyQl-BzCN are 0.18 and 0.20 eV, respectively; these values ​​are sufficiently small for an efficient RISC process. The broad fluorescence and phosphorescence spectra of both emitters indicate ICT-dominated singlet and triplet excited states.

[0063] In order to further verify the emission properties of these two emitters, the temperature-dependent transient PL decay ( Figure 6 b, c, e, and f). As expected, the TPA-PhTh-BzCN and TPA-PyQl-BzCN doped films exhibit biexponential decay curves with transient (τp) and delayed (τd) lifetimes of 23.72 ns / 4.77 μs and 22.13 ns / 2.31 μs, respectively. The proportion of the delayed component increases with increasing temperature from 100 K to 300 K, which can be attributed to the TADF process via RISC.

[0064] Example 6: Device fabrication scheme and luminescence performance testing based on TADF luminescent materials TPA-PyQl-BzCNT and TPA-PhTh-BzCN

[0065] To further verify our theory, solution-processed OLEDs were fabricated using an ITO / PEDOT device structure: PSS (35 nm) / PVK (30 nm) / EML (35 nm) / TmPyPB (45 nm) / liff (0.5 nm) / Al (120 nm). Here, the EML is composed of 4,4'-bis(n-carbazolyl)-1,1'-biphenyl (CBP), TPA-PhTh-BzCN, and TPA-PyQl-BzCN. Each OLED was doped with an optimal concentration of 10 wt%, as shown in Figure 2. Figure 7 As shown;

[0066] The current efficiency of TPA-PhTh-BzCN doped with 10% CBP is 11.52 cdA at 584 nm. -1 , power efficiency is 4.76lmW -1 , EQE is 4.76%, while the current efficiency of TPA-PyQl-BzCN at 564nm is 12.01cdA -1 ,5.23lmW -1 and 7.18%. The increase in the EQE value of TPA-PyQl-BzCN is attributed to the effect of the rigid backbone strategy, in which the rigid π-bridge suppresses the nonradiative transition.

[0067] As can be seen from the above, the present invention takes the problems raised in the background technology as the starting point, and starts from the aspects of material design and device preparation. We have developed two new TADF materials. Since they use the same D-segment triphenylamine (TPA) and their A segment contains the same main electron-deficient substructure as benzonitrile, their HOMO and LUMO energy levels are very similar. Under these conditions, phenanthrene and pyrene are selected as connecting bridges. By changing the conjugated area of ​​the rigid π-bridge, we can better understand the effect of the rigid conjugated bridge on the luminescent material. In photophysical measurements, they show very similar charge transfer (CT) characteristics in the emitted light, as expected. On the other hand, the TADF performance of TPA-PyQl-BzCN is significantly better than that of TPA-PhTh-BzCN. This can be attributed to the different aromatic hydrocarbons of the entire A and conjugated area framework. Although both materials have the same excellent 3 LE state, but the increase in rigid conjugated area provides a relatively strong SOC and a smaller RMSD, indicating that k RISCThe luminescence efficiency of TADF is faster, with fewer nonradiative transition channels. We also observed a high-positioned anti-interstitial crossing process after increasing conjugation. Most importantly, increasing the rigid conjugation area increased the PLQY from 75% to 90%. This result provides insight into the relationship between the conjugation area of ​​the rigid π-bridge and molecular properties. Furthermore, the rigid backbone strategy enriches the diversity and stability of TADF molecular structures. Therefore, this study further explores the luminescence mechanism of rigid π-bridges in TADF materials, providing an effective guiding strategy for the development of efficient TADF materials.

[0068] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. Application of a thermally activated delayed fluorescent material with a rigid π-bridge construction strategy, characterized in that: include: Using TADF luminescent material, TPA-PyQl-BzCN as the luminescent layer dopant, and CBP as the main material, a luminescent layer was prepared by spin coating and heating annealing process; The TADF material is a novel TADF material TPA-PyQl-BzCN with a DA molecular structure, using pyrenequinoxaline as a conjugated π bridge to connect a cyanobenzene acceptor (A) unit and a triphenylamine (TPA) donor (D) unit. The specific molecular structure is shown below: The structure of the TADF organic electroluminescent device is: ITO / PEDOT:PSS(35nm) / PVK(30nm) / EML(35nm) / TmPyPB(45nm) / LiF(0.5nm) / Al(120nm); Among them, PVK is the hole transport layer, TmPyPB is the electron transport layer, PEDOT:PSS is the hole injection layer, EML is the light-emitting layer, which is a mixture of guest material TPA-PyQl-BzCN and host material (4,4'-di(9-carbazole)biphenyl), and the cathode layer is composed of lithium fluoride and aluminum.

2. The use of a thermally activated delayed fluorescent material with a rigid π-bridge construction strategy according to claim 1, characterized in that: A doped TADF organic electroluminescent device was prepared, and a TADF organic electroluminescent device with an emission peak of 564 nm and an EQE of 7.2% was obtained.

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