CN110078656B - Intramolecular twist charge transfer triplet electroluminescent material and device - Google Patents

Abstract

An electroluminescent material and an electroluminescent device using the electroluminescent material as a luminescent layer, wherein the electroluminescent material comprises a compound shown as a formula I:

wherein R is₁、R₂、R₃、R₄、R₅Each independently selected from H, C1-C4 alkyl groups or halogen atoms; r₆、R₇、R₈、R₉、R₁₀、R₁₁、R₁₂、R₁₃Each independently selected from the group consisting of H, C1-C4 alkyl groups, halogen atoms, or carbazolyl groups.

Description

Intramolecular twist charge transfer triplet electroluminescent material and device

Technical Field

The invention belongs to the technical field of organic electroluminescence. In particular to an intramolecular twist charge transfer triplet electroluminescent material and an electroluminescent device manufactured by adopting the electroluminescent material.

Background

Organic Light Emitting Diodes (OLEDs), which are used as light sources or display panels, have been developed vigorously in academic or industrial fields, showing great application prospects over conventional technologies. The currently popular fluorescent OLEDs have an inherent problem: three-quarters of the electrical energy is dissipated by triplet excitons. Thermally Activated Delayed Fluorescence (TADF) is one method of efficiently utilizing triplet exciton energy. In contrast, a dual radiative channel through singlet and triplet states is a direct route to 100% exciton utilization. However, since the triplet radiation decay rate of organic Room Temperature Phosphorescent (RTP) compounds is very slow, they cannot be applied to organic electroluminescent devices without effective spin-orbit coupling enhanced by heavy metals.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention aims to provide an electroluminescent material capable of effectively utilizing energy of a singlet excited state and a triplet excited state simultaneously and an electroluminescent device manufactured by adopting the electroluminescent material, and the invention provides the following technical scheme:

embodiments of the present invention provide an electroluminescent material, which includes a compound represented by formula I:

wherein R is₁、R₂、R₃、R₄、R₅Each independently selected from H, C1-C4 alkyl groups or halogen atoms; r₆、R₇、R₈、R₉、R₁₀、R₁₁、R₁₂、R₁₃Each independently selected from the group consisting of H, C1-C4 alkyl groups, halogen atoms, or carbazolyl groups.

According to one embodiment of the invention, for example, the electroluminescent material comprises at least one of 9-benzoyl-9H-carbazole (BCz), 9- (4-bromobenzoyl) -9H-carbazole (BrBCz), 9H- [3, 9' -biscarbazole ] -9-yl (phenyl) methanone (BDCz), said BCz, BrBCz, BDCz having the respective formulae:

embodiments of the present invention provide an electroluminescent device comprising a cathode, an anode, and a light-emitting layer between the cathode and the anode, the light-emitting layer comprising a compound according to formula I:

wherein R is₁、R₂、R₃、R₄、R₅Each independently selected from H, C1-C4 alkyl groups or halogen atoms; r₆、R₇、R₈、R₉、R₁₀、R₁₁、R₁₂、R₁₃Each independently selected from the group consisting of H, C1-C4 alkyl groups, halogen atoms, or carbazolyl groups.

According to one embodiment of the present invention, for example, the light emitting layer includes at least one of 9-benzoyl-9H-carbazole (BCz), 9- (4-bromobenzoyl) -9H-carbazole (BrBCz), 9H- [3, 9' -biscarbazole ] -9-yl (phenyl) methanone (BDCz), the chemical formulas of BCz, BrBCz and BDCz being respectively:

according to an embodiment of the present invention, for example, the electroluminescent device further includes a hole transport layer between the anode and the light-emitting layer, a hole blocking layer between the cathode and the light-emitting layer, and an electron transport layer between the cathode and the hole blocking layer.

According to one embodiment of the present invention, for example, the light emitting layer has a thickness of 10nm to 30nm, the hole transport layer has a thickness of 30nm to 50nm, the hole blocking layer has a thickness of 5nm to 15nm, and the electron transport layer has a thickness of 40nm to 80 nm.

According to one embodiment of the present invention, for example, LUMO levels of the electron transport layer, the hole blocking layer, the light emitting layer, and the hole transport layer are sequentially increased, and HOMO levels of the hole transport layer, the light emitting layer, and the hole blocking layer are sequentially decreased.

According to an embodiment of the present invention, for example, the hole transport layer includes 4,4 ', 4 ″ -tris (N-carbazolyl) triphenylamine (4, 4', 4 ″ -tris (N-carbazolyl) Triphenylamine) (TAPC), the hole blocking layer includes bis (2- (diphenylphosphino) phenyl) ether oxide (bis (2- (diphenylphosphino) phenyl) ether oxide) (DPEPO), and the electron transport layer includes 1,3,5-tris (2-N-benzimidazole) benzene (1,3,5-tris (2-N-phenylbenzazolyl) benzene) (TPBi).

According to one embodiment of the present invention, the light-emitting layer has a thickness of 20nm and the hole transport layer has a thickness of 20nm, for exampleThe thickness of the hole blocking layer is 10nm, and the thickness of the electron transport layer is 60 nm. The electroluminescent device designed in this way can obtain the highest maximum External Quantum Efficiency (EQE)_max)。

According to one embodiment of the present invention, for example, the light emitting layer has a thickness of 20nm, the hole transport layer has a thickness of 40nm, the hole blocking layer has a thickness of 10nm, and the electron transport layer has a thickness of 50 nm. The highest light-emitting brightness can be obtained by adopting the electroluminescent device designed in such a way.

According to one embodiment of the invention, for example, the anode is ITO/MoO₃The cathode is LiF/Al.

According to one embodiment of the invention, for example, the MoO₃The ITO is evaporated and plated in vacuum, and the thickness is 2 nm; the LiF is evaporated on the Al in vacuum, and the thickness of the LiF is 1 nm.

According to one embodiment of the invention, for example, the light emitting layer is composed of 9H- [3, 9' -biscarbazol ] -9-yl (phenyl) methanone (BDCz).

The invention has the beneficial effects that:

by using the electroluminescent material of the invention as a light-emitting layer material for the manufacture of OLED devices, an internal quantum efficiency of theoretically up to 100% can be achieved, since in such OLED devices an internal quantum efficiency of the order of magnitude of 100% is achieved¹TICT and³the TICT is in transition luminescence, and singlet excitons and triplet excitons can be effectively harvested, so that the luminescence brightness and the external quantum efficiency of the electroluminescent device are effectively improved, and the electroluminescent device with excellent performance is obtained.

Drawings

Fig. 1 is a spectrum of BCz and BrBCz in a dilute solution state, in which 1 is an ultraviolet-visible absorption spectrum, 2 is an excitation spectrum, and 3 is an emission spectrum.

FIG. 2 is a comparison of the lifetime decay curves of BCz and BrBCz in n-hexane solution, under excitation at a wavelength of 340nm, in an air atmosphere and in an argon atmosphere.

FIG. 3 is a graph on the left showing the stable PL spectrum (top left) for 5 wt% BCz in a PMMA film and the decay-related spectrum (bottom left) for BCz in a 5 wt% PMMA film; the right panel shows the stable PL spectrum (top right) for 5 wt% BrBCz in PMMA film and the decay-related spectrum (bottom right) for 5 wt% BrBCz in PMMA film.

FIG. 4 is a graphical illustration of the difference in TICT behavior in polymer films and in solution.

FIG. 5 is a left panel showing the chemical structures of two model compounds pBCz and BCz-DMe; the right panel shows the PL spectrum of 1 wt% pBCz and BCz-DME in a PMMA film after air removal at room temperature under 330nm excitation.

FIG. 6 shows BCz, BrBCz and BCz-DME on CD₂Cl₂In (1)¹H-NMR spectrum in which the intensities of the resonance peaks in the chemical shift region of 9.2 to 5.8ppm are normalized.

FIG. 7 is a steady state PL spectrum of 100 μ M BCz in 2-MeTHF solvent with the test temperature decreasing from 295K to 77K.

FIG. 8 is a graph of the luminescence lifetime of 100 μ M BCz in 2-MeTHF, the temperature drop from 295K to 160K, and plots¹TICT fluorescence and³the ratio of the TICT phosphorescence intensity versus temperature curve.

FIG. 9 is the spin relaxation times (. tau.) of BCz and BrBCz in 2-MeTHF solvent_rot) Plotting the temperature ratio (. eta./T) of the viscosity.

FIG. 10 is a graph of BCz and BrBCz in CHCl₃Transient absorption spectrum in (5 mM).

FIG. 11 is BCz-DMe in CHCl₃Transient absorption spectrum in (5 mM).

FIG. 12 is a graph of lifetime decay for BCz and BrBCz at 420nm,720nm emission and 550nm transient absorption before and after nitrogen gassing.

Fig. 13 shows the structural formula of BDCz (left) and the electroluminescent device structure (right).

FIG. 14 is the ultraviolet-visible absorption spectrum (UV-Vis), normalized excitation spectrum, and photoluminescence spectrum (PL) of a pure BDCz film; wherein the electroluminescence spectrum (EL) is at 100cd m^-2Measuring; the phosphorescence spectra (PL) were measured in a 1 wt% PMMA film at a low temperature of 77K.

Fig. 15 is a current density (J) -voltage (V) -light emission luminance (L) curve for an optimized OLED device.

Fig. 16 is a Power Efficiency (PE) -light emission luminance (L) -External Quantum Efficiency (EQE) curve for an optimized OLED device.

Fig. 17 is a chemical structure of a portion of the materials in the OLED device structure of example 5.

Fig. 18 is an energy level diagram of a material in the structure of the OLED device of example 5, with the HOMO level at the lower side and the LUMO level at the upper side.

Detailed Description

The electroluminescent materials and devices according to the invention will be further described with reference to specific examples. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the technologies realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.

Unless otherwise indicated, the raw materials and reagents used in the following examples are all commercially available products or can be prepared by known methods.

Apparatus and device

The elemental analysis in the following examples was performed using a VARIO elemental analyzer manufactured by Elementar Analyzteme GmbH.

Thermogravimetric analysis in the following examples was performed on a thermogravimetric analyzer model Q600SDT, manufactured by t.a. company, with a temperature rise rate of 10 ℃/min, tested under a nitrogen flow of 100 mL/min.

The UV-visible absorption spectra in the examples described below were performed on a spectrometer model UV3600PLUS from Shimadzu.

Photoexcitation, photoemission spectroscopy was performed on a spectrometer model FLS980, manufactured by edinburg instruments, inc.

The photoluminescence spectra at different temperatures (room temperature to 77K) in the following examples were performed on an FLS980 spectrometer using a liquid nitrogen cryovial with ITC controller manufactured by oxford instruments.

The fluorescence/phosphorescence lifetimes in the examples below were measured by single photon counting on a 340nm or 320nm excitation, Edinburgh instruments model FLS980 spectrometerAnd (4) a spectrometer. The lifetime data were fitted using software provided by the FLS980 spectrometer. The fitting result adopts a parameter chi²Evaluation was carried out (in all cases<1.3)。

The absolute quantum efficiency in the following examples was performed using a photoluminescence quantum absolute efficiency test system model C9920-02 manufactured by hamamatsu.

The transient uv-vis absorption spectra in the following examples were performed using a nanosecond resolution transient absorption spectrometer model LP920, manufactured by edinburg instruments, inc; the exciting light comes from 355nm laser pulse (1Hz,10 mJ/pulse, fwhm ≈ 7ns) generated by a Nd: YAG laser with the model number of Surelite II produced by Continum company; the analysis light was from a pulsed Xe lamp at 450W.

The photoelectron spectroscopy in the following examples was carried out on an AC-2 photoelectron spectrometer manufactured by Nikkiso K.K.K.

The electroluminescence spectra, current density-voltage-luminescence (J-V-L) characteristic curves, External Quantum Efficiency (EQE) characteristic curves in the following examples were carried out on a computer-controlled Keithley 2400 meter and absolute EQE measurement system (model C9920-12, manufactured by Hamamatsu photonics), and a photon multichannel analyzer (model PMA-12, manufactured by Hamamatsu photonics).

EXAMPLE 1 preparation method

Sodium hydride (0.60g of a 60% dispersion in mineral oil, 15mmol) was placed in a 50mL three-necked round bottom flask connected to a dropping funnel. A solution of carbazole (10mmol) in 20 ml of anhydrous Tetrahydrofuran (THF) was added to the above dropping funnel. The flask was evacuated and purged three times with nitrogen. Another 5mL of anhydrous THF was charged to the flask and stirred for 5 minutes. The carbazole solution was then added dropwise over 10 minutes. The reaction mixture was heated to 60 ℃ and stirred for 30 minutes, then cooled to 0 ℃. Benzoyl chloride (or its derivative) (12mmol) was added dropwise to the reaction mixture while stirring. The resulting mixture was further heated at 60 ℃ and stirred overnight. After cooling, the product was collected by removing THF in vacuo, dissolved with Dichloromethane (DCM), washed three times with waterOver anhydrous Na₂SO₄Dried and then purified by flash chromatography.

9-benzoyl-9H-carbazole (BCz) white solid. The total yield is 72 percent. Nuclear magnetic characterization data¹H NMR(500MHz,DMSO-d₆)8.25-8.17(m,2H),7.78-7.69(m,3H),7.65-7.55(m,2H),7.44-7.32(m,6H).¹³C NMR(500MHz,DMSO-d₆)169.54,139.01,135.93,132.97,129.56,129.06,127.36,125.92,124.00,120.82,115.78. Mass Spectrometry ESI-MS (m/z) calculated value C₁₉H₁₃NO 271.10, test value 272.1(M + H)⁺). Calculated value of elemental analysis (test value) C₁₉H₁₃NO:C 84.30(83.96),H 4.90(4.87),N 5.04(5.19)。

9- (4-bromobenzoyl) -9H-carbazole (BrBCz) white crystals. The total yield is 70 percent. Nuclear magnetic characterization data¹H NMR(400MHz,CD₂Cl₂)8.12-8.00(m,2H),7.75-7.67(m,2H),7.65-7.58(m,2H),7.56-7.49(m,2H),7.37(tt,J＝7.4,5.6Hz,4H).¹³C NMR(400MHz,CDCl₃)168.51,138.99,134.48,132.26,130.77,127.31,126.91,126.87,126.10,123.64,119.96,115.71. Mass Spectrometry ESI-MS (m/z) calculated value C₁₉H₁₂BrNO 349.01, test value 350.0(M + H)⁺) Calculated value of elemental analysis (test value) C₁₉H₁₂BrNO:C 65.16(65.13),H 3.45(3.55),N 4.00(4.01)。

(9H-carbazol-9-yl) (2, 6-dimethylphenyl) methanone (BCz-DME). The total yield is 27%. 2, 6-Dimethylbenzoyl chloride is prepared according to the known methods (Du, X.J.; Bian, Q.; Wang, H.X.; Yu, S.J.; Kou, J.J.; Wang, Z.P.; Li, Z.M).(ii) a Zhao, w.g.org.biomol.chem.2014,12,5427.). Nuclear magnetic characterization data¹H NMR(400MHz,CD₂Cl₂)8.89(d,J＝8.4Hz,1H),8.03(dd,J＝19.2,7.5Hz,2H),7.59(t,J＝7.7Hz,1H),7.48(t,J＝7.6Hz,1H),7.41(t,J＝7.7Hz,1H),7.29(t,J＝7.5Hz,1H),7.22(d,J＝7.7Hz,2H),7.07(t,J＝8.2Hz,1H),6.06(d,J＝8.4Hz,1H),2.17(s,6H).¹³C NMR(400MHz,DMSO-d₆)169.59,138.66,137.53,137.19,134.20,130.71,128.75,128.47,127.76,126.66,126.30,125.21,124.43,121.14,120.61,117.67,113.29,18.89. Mass Spectrometry ESI-MS (m/z) calculated value C₂₁H₁₇NO 299.13, test value 300.1(M + H)⁺) Calculated value of elemental analysis (test value) C₂₁H₁₇NO:C 84.25(84.37),H 5.72(5.73),N 4.68(4.68)。

9H-indole [3,2,1-de]Phenanthridin-9-one (pBCz). pBCz was prepared according to the known methods (Markgraf rf, J.H.; Dowst, A.A.; Hensley, L.A.; Jakobsche, C.E.; Kaltner, C.J.; Webb, P.J.; Zimmerman, P.W.tetrahedron 2005,61, 9102.). A pale yellow solid. The total yield is 28 percent. Nuclear magnetic characterization data¹H NMR(400MHz,CD₂Cl₂)8.79(dt, J ═ 8.2,0.9Hz,1H),8.67-8.60(m,1H),8.38-8.30(m,1H),8.19(dd, J ═ 7.8,1.0Hz,1H),8.14-8.06(m,2H),7.85(ddd, J ═ 8.0,7.2,1.5Hz,1H),7.67(ddd, J ═ 8.2,7.2,1.1Hz,1H),7.64-7.61(m,1H),7.61-7.55(m,1H),7.51(td, J ═ 7.5,1.1Hz, 1H). Mass Spectrometry ESI-MS (m/z) calculated value C₁₉H₁₁NO 269.08, test value 270.1(M + H)⁺) Calculated value of elemental analysis (test value) C₁₉H₁₁NO:C 84.74(84.70),H 4.12(4.17),N 5.20(5.16)。

9H- [3, 9' -bis-carbazole]Reference is made to the known methods for 9-yl (phenyl) methanone (BDCz).9H-3,9' -biscarbazole (Kim, S.J.; Kim, Y.J.; Son, Y.H.; Hur, J.A.; Um, H.A.; Shin, J.; Lee, T.W1; cho, m.j.; kim, j.k.; joo, S.; yang, j.h.; chae, g.s.; choi, k.; kwon, j.h.; choi, d.h.chem.commun.2013,49,6788.). A white solid. Total yield 17%. Nuclear magnetic characterization data¹H NMR(400MHz,DMSO-d₆)8.56(d,J＝2.3Hz,1H),8.31(dd,J＝20.7,7.2Hz,3H),7.90-7.74(m,4H),7.71-7.57(m,3H),7.51-7.36(m,6H),7.37-7.24(m,3H).¹³C NMR(400MHz,DMSO-d₆)169.57,141.18,139.61,138.03,135.75,133.21,132.96,129.68,129.22,127.96,127.52,126.75,126.29,125.52,124.14,123.09,121.61,120.99,120.45,119.63,117.25,115.83,110.15. Mass Spectrometry ESI-MS (m/z) calculated value C₃₁H₂₀N₂O436.16, test value 437.2(M + H)⁺) Calculated value of elemental analysis (test value) C₃₁H₂₀N₂O:C 85.30(85.01),H 4.62(4.63),N 6.42(6.33)。

Example 2 photophysical Properties of BCz and BrBCz

Several important photophysical properties of BCZ and BRBCZ were measured in dilute solution by uv-visible absorption and Photoluminescence (PL) and the results are shown in table 1 and fig. 1. FIG. 1 is the room temperature CH of BCZ and BRBCZ₂Cl₂Spectrum in a dilute solution state, where 1 is an ultraviolet-visible absorption spectrum, 2 is an excitation spectrum, and 3 is an emission spectrum, the excitation spectrum being measured at a long-wavelength emission position, the excitation wavelength of the emission spectrum being 300 nm. BCz showed dual emission of the benzoylimine under argon atmosphere: a 544nm long wavelength band with a very large stokes shift (229nm) and a short wavelength band around 350 nm. Unexpectedly, the PL decay curve monitored at 544nm shows that two components, rather than one, coexist in the broadband (on fig. 2). The fast decay component has a fitted lifetime of 18.4ns and the slow decay component has a fitted lifetime of 137ns (table 1). For BrBCz, the long wavelength band (554nm) has a 10nm red shift, which can be explained by the weak electron withdrawing effect of bromide substitution. Similar dual dynamical decay was observed in BrBCz (fig. 2, below, table 1). The observed lifetime of 355nm and the fast decay component of the long wavelength band are typical lifetimes for unimodal excited states. Slow-decay components with lifetimes of hundreds of nanoseconds may originate from Room Temperature Phosphorescence (RTP) or delayed fluorescence.

TABLE 1 BCz and BrBCz in CH₂Cl₂Photophysical Properties in solution (50. mu.M)

Due to intramolecular charge transfer properties, the long wavelength bands of BCz and BrBCz undergo a pronounced red shift as the polarity of the solvent increases. At the same time, the short-wavelength emission is hardly changed and can be assigned to carbazole moieties¹LE state emission. According to the Lippert-Mataga model, the excited-state dipole moments of the Charge Transfer (CT) states of BCz and BrBCz were estimated to be 29D and 35D, respectively. It can be concluded that significant charge transfer between the carbazole moiety and the benzoyl moiety occurred.

In addition, the slow attenuation components of the long wavelength band in BCz and BrBCz can be quenched by oxygen molecules (fig. 2). Their life in air is greatly reduced. Molecular oxygen (³O₂) The energy transfer between the ground state of (a) and the triplet excited state of the solute suppresses the emission of the triplet excited state. CDCl at BCz and BrBCz at ambient conditions₃Singlet oxygen at 1270nm was detected in solution (¹O₂) Weak emission of (2). This further verifies that the triplet state is involved in long wave emission.

In Polymethylmethacrylate (PMMA) films, the rotational freedom is limited to a limited range along the relaxation path, the PL spectra of BCz and BrBCz show an emission peak around 500nm, whereas the 350nm emission in solution becomes a shoulder in BCz-doped PMMA films and disappears in BrBCz-doped PMMA films (fig. 3). In two PMMA samples, the PL decay curves monitored at the maximum emission wavelength show two components with different decay indices, which is the same decay behavior as experimentally observed in long wavelength emission in solution. The applicable PL lifetimes at-500 nm for BCz and BrBCz PMMA samples were-2 ns (fast attenuation component) and-1.3 μ s (slow attenuation component). Compared to the data measured in solution, the lifetime is significantly increased by more than 1 microsecond.

If a change in PL characteristics in an environment of increased stiffness is interpreted as being affectedThe effect of the micro-viscosity dependent excited state formation and non-radiative decay processes, both of which are related to the internal spin rate constant (k)_rot) This is understood in relation to (fig. 4). To gain insight into the relationship between rotation around amide bonds and electronic properties, the ground state (E) was calculated by varying the dihedral angle (φ) between the carbazole plane and the benzene ring_GS)、¹LE(E¹ _LE) And¹CT(E¹ _CT) Potential energy curve of (2). For BCz and BrBCz, these calculations strongly support that the fluorescence emission from the planar (0 °) and distorted conformation (90 °) should be derived from¹LE and¹the CT status. The transition from the planar to the twisted coherency state is facilitated, which best explains the dual emission bands assigned to the LE and CT states, respectively, in fig. 1. Therefore, the fast attenuation component in the CT emission band is due to¹TICT fluorescence. Since there may be rotational freedom around the central bond, the mechanism of TICT is understood as the excited state equilibrium between the LE state in the planar conformation and the TICT state in the perpendicular conformation. When flexible molecules are doped in a polymer matrix, however, due to the restriction of free rotation in a physically constrained environment,¹TICT→¹the rotational hindrance of the LE process is greatly increased. Therefore, in the PMMA film, the balance is in the direction of¹TICT state transition, resulting in¹The relative intensity of the LE emission decreases.

Furthermore, the inventors have also attempted to find the source of slow attenuation components hidden in the CT emission band. From the lifetime measurements in the PMMA film and in the solution, it is speculated that the decay process of the slow decay component is sensitive to the micro-viscosity of the dye environment. This suggests that the formation of the corresponding excited state is related to the internal rotation of the amide bond in the BCz and BrBCz molecules, which is related to¹The TICT states are similar. The inventors performed quantum calculations using Density Functional Theory (DFT) and time dependent DFT (TD-DFT) at the CAM-B3LYP/6-311G (d, p) level (using the LANL2DZ base set for Br atoms). T is₁The hole and electron distributions of the states have planar (phi-30 deg.) and twisted (phi-90 deg.) geometries. T is₁The hole and electron distribution in both BCz and BrBCz planar conformations is concentrated in the carbazole moiety. In contrast, the lumen in both distorted conformationsApparently located in the benzoyl moiety. These results demonstrate that phosphorescence emission in planar and distorted conformations should be derived from respectively³LE and³the CT state is characterized by a rotationally driven change in the phosphorescent emission wavelength. The inventors have tentatively assigned the slow attenuation components hidden in the CT emission band as³TICT RTP. PL spectra of BCz and BrBCz³LE RTP disappears because intramolecular spin activity leads to an ultra-long lifetime³Quenching of LE state.

Example 3 time-resolved emission spectra of pBCz, BCz-DMe, BCz, BrBCz

Time Resolved Emission Spectroscopy (TRES) can readily identify overlapping broad bands around 500nm in PMMA films, given the enhanced PL in PMMA films. Pre-exponential values (weighted by lifetime values) are plotted against wavelength for each attenuation component to generate an attenuation-related spectrum. Both BCz and BrBCz have a wavelength around 465nm (2.67eV)¹TICT fluorescence, at 505nm (2.46eV)³TICT RTP, as shown in FIG. 3. From¹TICT and³BCz emission wavelengths calculated in the TICT state are 459nm and 569nm respectively; for BrBCz, 470nm, 510nm, respectively. Except for BCz³Besides the TICT-RTP wavelength, other values are better consistent with the experimental result.

For a deeper understanding of the above³RTP mechanism of TICT State, the inventors have specifically validated stereochemical evidence to support internal rotation pairs³Effect of TICT luminescence. The inventors synthesized two model compounds in which the amide bond (N-C ═ O) could not be rotated. The first compound (9H-carbazol-9-yl) (2, 6-dimethylphenyl) methanone (BCz-DMe, fig. 5) loses the rotational freedom around the amide bond due to steric hindrance caused by the two methyl groups adjacent to the amide bond. Of BCz-DMe¹The H-NMR spectrum confirms this because only one set of resonance signals is resolved into clusters of peaks (FIG. 6), and therefore BCz-DMe does not rotate at room temperature. As shown in FIG. 5, BCz-DME does not emit light at room temperature in a PMMA matrix. The other compound 9H-indole [3,2,1-de]Phenanthridin-9-one (pBCz, fig. 5) is a ring-fixed planar analogue of BCZ, and also does not exhibit long wavelength emission (fig. 5). Both compounds demonstrated that the distortion of the amide bond is a productRaw material¹TICT and³the requirement of the TICT state.

Of BCz and BrBCz obtained at different temperatures¹The H-NMR spectrum (FIG. 6) reveals the effect of temperature on the rotational movement around the amide bond. For BCz and BrBCz, the rotational motion speed is so fast that it is indistinguishable within the nmr response time. However, at 223K, the rotation is slow enough, so multiple bodies of revolution cause a chemical shift (H assigned to the benzene ring) of 7.47ppm_aAnd H_b) The peak of (a) becomes broad. In 2-MeTHF solution, when temperature and viscosity changes are studied, intramolecular torsional motion pairs¹TICT and³the change in PL behavior of the TICT emission gives a reasonable explanation. The gradual blue shift of the long-wave emission when melting above 160K can be explained by the continuous decrease in the dielectric constant of 2-MTHF, which destabilizes the geometrically relaxed TICT state (FIG. 7). As the temperature is lowered, the temperature of the reaction solution is lowered,¹TICT and³the lifetime of the TICT emission continues to increase (fig. 8). In a 77K high viscosity glass solution, it is expected that rotation is greatly impeded. The temperature and viscosity dependence of BCz and BrBCz luminescence indicates that³The behavior of TICT depends on the degree to which the triplet excited state is geometrically relaxed by molecular twisting. In addition, simple hydrodynamics give low viscosity (. eta.)<2cP) of BCz and BrBCz. Rotational relaxation time (. tau./T) as a function of viscosity with temperature (eta./T) over the range of experimental data (FIG. 9)_rot) Shows that_rotHas good linear relation with eta/T. With increasing size of the rotating phenyl group, τ_rotThe slope to η/T increases, consistent with the Debye-Stokes-Einstein (DES) hydrodynamic model.

Example 4 time-resolved transient absorption Spectroscopy

Through the research of time-resolved transient absorption spectrum, the method reveals³Photophysical processes in the generation of the TICT state. In the case of BCz and BrBCz (fig. 10), the transient absorption spectrum can be described by the sum of the spectra of the subsystem components: radical anions and radical cations corresponding to the CT state. In the early stage (t ═ 30ns), excitation was carried out with 355nm excitation pulsesPhotolytic CHCl₃BCz in (iii) yields two broad absorption bands and an emission peak at 550 nm. During the time evolution (Δ t 60ns), the two absorption peaks at 420nm and 720nm decay synchronously, and no new absorption peak appears until 270 ns. The lifetime of the transient species can be analyzed from the transient spectrum (Δ a) as a function of time. A transmission with a lifetime of 156ns may be attributed to³And (4) carrying out TICT emission. Transient absorption in the 600nm-850nm region is due to radical cations of the carbazole (Cz) chromophore, and another absorption band in the 380nm-520nm region may be due to Acetophenone (AP) radical anions. In contrast to the literature, the transient absorption maxima of the carbazole radical cation and the acetophenone radical anion are slightly changed, mainly due to the interaction between them. Furthermore, the decay kinetics of the carbazole radical cation and the acetophenone radical anion are identical and both sensitive to oxygen. After the solution is lightly purged with nitrogen, the lives of the two transient absorption substances are increased from 160ns to 280ns, which fully indicates that the two transient absorption substances are³Counterpart of CT structure (fig. 12). However, where rotation of BCz-DMe around N-C ═ O is sterically hindered, no significant signal for any transient species is shown in fig. 11. These results support the following conclusions: RTP in BCz and BrBCz is formed by electron transfer from the carbazole triplet to the benzoyl acceptor triplet³The TICT state (and consequent rotation into a distorted conformation).

TADF and triplet-triplet annihilation (TTA) are two processes that can produce Delayed Fluorescence (DF) with long emission lifetime. To distinguish the two processes from³TICT RTP, the inventors of the present invention, conducted experiments. Heating can enhance the TADF fluorescence emission, and thus increasing the temperature can result in an increase in delayed fluorescence intensity. As shown in fig. 8, the intensity ratio of the slow and fast fading components fluctuated around the average value with the increase in temperature, indicating that the occurrence probability of TADF was low. Delayed fluorescence due to TTA may be represented by the formula F ═ K_TTA[T]²To simply express, where K_TTAIs the rate parameter of TTA, [ T]Is the probability density of the triplet. Thus, as the triplet density increases, the TTA-induced delayed fluorescence will be largeAnd (4) greatly enhancing. The test shows that CH of 1mMBrBCz₂Cl₂The PL intensity of the solution is linear with excitation intensity. The inventors confirmed that the broad peak emission of BrBCz was free of TTA processes.

EXAMPLE 5 electroluminescent device

By passing¹TICT and³the dual emission channel of the two excited states of the TICT is a feasible way to obtain the energy of the singlet excited state and the triplet excited state in the OLED, so that higher exciton utilization efficiency (and ultimately high power efficiency, light emission efficiency) can be obtained. The inventors of the present invention have designed 9H- [3, 9' -biscarbazole]-9-yl (phenyl) methanone (BDCz), which has better thermal stability than BCz and BrBCz, is used in this example as a light emitting layer material for OLEDs by³The TICT mode emits light. PL spectra of BDCz in different solvents also show LE and CT emissions with very large stokes shifts; the attenuation curve of the PL spectrum measured at the CT emission band indicates the presence of two different emissions. With unchanged emission center, BDCz exhibits the same as BCz and BrBCz¹TICT fluorescence and³TICT RTP. The inventors have further investigated the photophysical properties of BDCz in the pure film state to better understand the OLED performance. Fig. 13 shows the structure of the undoped optimized OLED device in this embodiment, which is: ITO/MoO₃(2nm)/4,4 ', 4 "-tris (N-carbazolyl) triphenylamine (4, 4', 4" -tri (N-carbazolyl) triphenylamine) (TAPC,40nm)/BDCz (20 nm)/bis (2- (diphenylphosphino) phenyl) ether oxide (bis (2- (diphenylphosphino) phenyl) ether oxide) (DPEPO,10nm)/1,3, 5-tris (2-N-benzimidazole) benzene (1,3,5-tris (2-N-phenylbenzimidazole) benzene) (TPBi,60nm)/LiF (1nm)/Al (100nm), wherein BDCz is a luminescent layer material, TAPC is a hole transport layer material, DPEPO is a hole barrier layer material, TPBi is an electron transport layer material, MoO is an electron transport layer material, and LiF (1nm)/Al (100nm), wherein BDCz is a luminescent layer material, TAPC is a hole transport layer material, DPEPO is a hole barrier layer material, and TPBi is a polymer material₃And LiF is used to change the work function of the electrode to improve charge injection. Fig. 17 is a chemical structure of a portion of the materials in the OLED device structure of example 5. Fig. 18 is an energy level diagram of a material in the structure of the OLED device of example 5, with the HOMO level at the lower side and the LUMO level at the upper side. CIE of the OLED device structure_(x,y)(Commission Internationole de L' Eclairage) color coordinates (0.28, 0.46). The device is in 100cd m^-2The Electroluminescence (EL) spectrum (fig. 14) under the condition is almost the same as the PL spectrum of BDCz in the thin film (fig. 14). Fig. 15 shows the current density (J) -voltage (V) -light emission luminance (L) curves of the OLED device. The turn-on voltage (turn-on voltage) is 4.1V, which is quite high in green OLEDs. By passing³The mechanism of TICT can be understood as the triplet excitons being first excited³LE state (3.0eV) capture, then transfer to³TICT state (2.4 eV). The maximum External Quantum Efficiency (EQE) can reach 4.0% (fig. 16), while the photoluminescence quantum efficiency (PLQY) of BDCz pure films is only 16%. The OLED device achieves 100% potential triplet utilization efficiency, which means that in the optimized device, by¹TICT and³in the TICT state, singlet excitons and triplet excitons can be effectively utilized.

In addition, the thicknesses of the Hole Transport Layer (HTL) and the Electron Transport Layer (ETL) were optimized in this example, and the results are shown in the following table.

TABLE 2 thickness optimization of Hole Transport Layer (HTL) and Electron Transport Layer (ETL) in OLED device structures

From the results in the above table, it is understood that the maximum external quantum efficiency of 4.01% was obtained at 40nm for TAPC and 60nm for TPBi.

The embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiment. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (8)

1. An electroluminescent device comprising a cathode, an anode and a light-emitting layer between the cathode and the anode, wherein the light-emitting layer comprises a compound of formula I:

wherein R is₁、R₂、R₃、R₄、R₅Each independently selected from C1-C4 alkyl; r₆、R₇、R₈、R₉、R₁₀、R₁₁、R₁₂、R₁₃Each independently selected from C1-C4 alkyl groups, or carbazolyl groups;

the electroluminescent device further comprises a hole transport layer positioned between the anode and the light-emitting layer, a hole blocking layer positioned between the cathode and the light-emitting layer, and an electron transport layer positioned between the cathode and the hole blocking layer;

the thickness of the light-emitting layer is 10nm-30nm, the thickness of the hole transport layer is 30nm-50nm, the thickness of the hole blocking layer is 5nm-15nm, and the thickness of the electron transport layer is 40nm-80 nm.

2. The electroluminescent device of claim 1, wherein the LUMO levels of the electron transport layer, the hole blocking layer, the light emitting layer, and the hole transport layer increase in order, and the HOMO levels of the hole transport layer, the light emitting layer, and the hole blocking layer decrease in order.

3. An electroluminescent device according to claim 1 or 2, characterized in that the hole transport layer comprises 4, 4', 4 "-tris (N-carbazolyl) Triphenylamine (TAPC), the hole blocking layer comprises bis (2- (diphenylphosphino) phenyl) ether oxide (DPEPO), and the electron transport layer comprises 1,3,5-tris (2-N-benzimidazole) benzene (TPBi).

4. An electroluminescent device as claimed in claim 1 or 2, characterized in that the light-emitting layer has a thickness of 20nm, the hole transport layer has a thickness of 40nm, the hole blocking layer has a thickness of 10nm and the electron transport layer has a thickness of 60 nm.

5. An electroluminescent device as claimed in claim 1 or 2, characterized in that the light-emitting layer has a thickness of 20nm, the hole transport layer has a thickness of 40nm, the hole blocking layer has a thickness of 10nm and the electron transport layer has a thickness of 50 nm.

6. An electroluminescent device as claimed in claim 1 or 2, characterized in that the anode is ITO/MoO₃The cathode is LiF/Al.

7. An electroluminescent device as claimed in claim 1 or 2, characterized in that the light-emitting layer consists of 9H- [3, 9' -biscarbazol ] -9-yl (phenyl) methanone (BDCz).

8. The electroluminescent device of claim 6, wherein the MoO is₃Vacuum evaporating on the ITO, the MoO₃The thickness of (2 nm); and the LiF is evaporated on the Al in vacuum, and the thickness of the LiF is 1 nm.

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