Thermally activated delayed fluorescent materials with dual charge transfer channels and aggregation-induced emission characteristics and their applications

Abstract

The present invention relates to a thermally activated delayed fluorescent material, and more specifically to a thermally activated delayed fluorescent material having both dual charge transfer channels and aggregation-induced emission properties, and its application. By using rigid indolequinoxaline as an electron acceptor unit, introducing electron-donating triphenylamine at positions 6 and 8, and electron-withdrawing cyano at positions 2 and 3, a near-infrared TADF material with both dual CT channels and AIE properties was constructed. The material has three distinct characteristics: aggregation-induced emission, a redshift in the emission wavelength, and an accelerated anti-gap crossing rate and weakened non-radiative transitions, which are beneficial for improving the material's luminescence performance. Therefore, this type of dual-CT channel TADF material exhibits a quantum yield of up to 83.8% in a 10wt% CBP-doped thin film; and 716nm near-infrared emission was obtained in a solution-processed pure film device, with an external quantum efficiency of 0.408%.

Description

Technical Field

[0001] The present invention relates to a thermally activated delayed fluorescent material, in particular to a thermally activated delayed fluorescent material having dual charge transfer channels and aggregation-induced luminescence characteristics and applications thereof. Background Art

[0002] In order to improve the external quantum efficiency (EQE) of near-infrared luminescent materials, it is crucial to maximize the utilization of triplet excitons for luminescence. Thermally activated delayed fluorescent materials, due to their small singlet and triplet energy gaps, can 100% utilize triplet excitons through anti-gap crossing (RISC) and are considered the development direction of near-infrared luminescent materials. However, according to the energy gap law, the luminous efficiency of luminescent materials generally decreases as the band gap of the material decreases. Therefore, compared with visible light luminescent materials such as red, green, and blue with larger band gaps, near-infrared luminescent materials with smaller band gaps are generally more difficult to achieve high-efficiency luminescence.

[0003] Improving the utilization rate of triplet excitons is an effective way to improve the luminous efficiency of materials. To this end, increasing the RISC rate constant (k RISC ) is crucial. In theory, k RISC and E ST is proportional to is the spin-orbit coupling (SOC) matrix element; ΔE ST is the energy gap between the singlet (S) and triplet (T) excited states. Obviously, increasing the SOC matrix component or reducing ΔE ST will help improve k RISC Early studies have shown that ΔE can be effectively reduced by using highly twisted electron donor (D)-electron acceptor (A) type molecular designs. ST However, the singlet state of this single CT channel ( 1 CT) and triplet states ( 3 CT) between the SOC matrix components, sometimes close to zero, but not conducive to speeding up k RISC Therefore, exploring new paths and improving the SOC matrix component are the key to improving the luminous efficiency of near-infrared luminescent materials. Summary of the Invention

[0004] In response to the technical problems pointed out in the background technology section, the present invention provides a near-infrared luminescent material with dual CT channels, aiming to fundamentally solve the problem of reducing the SOC matrix component within the molecules of single CT channel near-infrared luminescent materials, and to maximize the luminous efficiency of near-infrared luminescent materials.

[0005] Using indoloquinoxaline as the electron-deficient rigid skeleton unit and triphenylamine (TPA) as the electron donor unit, positioned at the 6 and 8 positions of indoloquinoxaline; using cyano as the electron acceptor unit, positioned at the 2 and 3 positions of indoloquinoxaline; a near-infrared thermally activated delayed fluorescence (TADF) material with dual CT channels and aggregation-induced emission (AIE) properties was constructed.

[0006] The molecular structure of this type of luminescent material is shown in Formula 1 below:

[0007]

[0008] The fluorescent material is used as a luminescent material to prepare an organic light-emitting device through a solution processing method.

[0009] The structural characteristics of the indoloquinoxaline luminescent material of the present invention are:

[0010] 1. Utilizing the electron-deficient rigid skeleton of indole-quinoxaline, a dual CT channel was obtained by introducing electron-donating triphenylamine (TPA) at the 6 and 8 positions.

[0011] 2. Enhance the molecular AIE properties through the steric effect of the bistriphenylamine unit;

[0012] 3. By introducing a cyano group (-CN) with strong electron-withdrawing properties at the 2 and 3 positions of indolequinoxaline, the CT properties of the molecule are enhanced and the SOC matrix component of the molecule is increased.

[0013] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0014] 1. By simultaneously introducing electron-donating triphenylamine at the 6- and 8-positions of the electron-deficient rigid unit of indole-quinoxaline, a near-infrared thermally activated delayed fluorescent material TPA2-IQD with dual CT channels and AIE properties was prepared;

[0015] 2. This type of indole-quinoxaline derivative material effectively enhances SOC through dual CT channels, thereby accelerating k RISC , and its rigid skeleton structure of indole and quinoxaline helps to increase the oscillator strength (f) and improve the luminescence efficiency of the material.

[0016] 3. The spatial effect of the ditriphenylamine unit in the molecule makes the TPA2-IQD luminescent material have AIE characteristics, which can eliminate the aggregation-induced quenching effect (ACQ) and further improve the luminescence efficiency.

[0017] 4. The molecular structure is simple, easy to synthesize, and the preparation cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 11 is the hydrogen spectrum of the intermediate compound SM 1 of Example 1.

[0019] Figure 2 This is the hydrogen spectrum of the TPA2-IQD luminescent material of Example 1.

[0020] Figure 3 These are the relevant theoretical calculations of TPA2-IQD in Example 1, where a is a schematic diagram of the CT channel; b: torsion angle under the optimized structure; c: ESP diagram of TPA2-IQD; df: DFT analysis of the molecule; gj: molecular excited state energy levels and intergap crossing mechanism diagram under SOC matrix elements.

[0021] Figure 4 Figure 1 shows the absorption spectra and temperature-dependent PL spectra of TPA2-IQD in different solvents in Example 1, where a is the absorption spectra of TPA2-IQD in different solvents; bd is the PL spectra of the material in tetrahydrofuran at different temperatures, the normalized PL spectra, and the relationship between temperature and PL intensity.

[0022] Figure 5 PL intensity correlation spectrum of TPA2-IQD in THF / H2O in Example 1, wherein a is the PL intensity correlation spectrum in THF / H2O with different concentrations; b is the PL intensity at different volume fractions of THF / H2O.

[0023] Figure 6 This is the CV diagram of TPA2-IQD in Example 1.

[0024] Figure 7 This is the low-temperature fluorescence phosphorescence spectrum of TPA2-IQD in Example 1 at 77K.

[0025] Figure 8 This is the temperature-dependent lifetime spectrum of TPA2-IQD in Example 1.

[0026] Figure 9 This is the instantaneous lifetime fluorescence spectrum of TPA2-IQD in Example 1, where af is the instantaneous lifetime at different ratios under CBP doping; and f is the integration of all results, i.e., the corresponding relationship between doping concentration and lifetime.

[0027] Figure 10 This is the extended lifetime fluorescence spectrum of TPA2-IQD in Example 1, where af is the extended lifetime at different ratios under CBP doping; and f is the integration of all results, i.e., the corresponding relationship between doping concentration and lifetime.

[0028] Figure 11 This is the PLQY data diagram of TPA2-IQD in Example 1.

[0029] Figure 12 This is the EL spectrum of the TPA2-IQD in Example 1 in a 10% doped CBP device.

[0030] Figure 13 This is the EQE curve of the TPA2-IQD in Example 1 in a 10% doped CBP device.

[0031] Figure 14 This is the EL spectrum of TPA2-IQD in pure film device in Example 1.

[0032] Figure 15 This is the EQE curve of TPA2-IQD in pure film device in Example 1. DETAILED DESCRIPTION

[0033] The following are specific examples of near-infrared AIE-TADF materials with dual CT channels to further illustrate the present invention, but these specific embodiments do not limit the scope of protection of the present invention in any way.

[0034] Example 1

[0035] Synthesis of AIE-TADF Material TPA2-IQD with Dual CT Channels

[0036]

[0037] Synthetic route of TPA2-IQD

[0038] Synthesis of compound SM 1

[0039] Under nitrogen protection, 6-bromoisatin (1.15 g, 5 mmol), triphenylamine borate (2.16 g, 7.5 mmol), copper acetate (1.0 g, 5 mmol), and triethylamine (0.6 g, 6 mmol) were added to a round-bottom flask in sequence, followed by the addition of 1,2-dichloroethane. The reaction was stirred magnetically at room temperature for 4 h, and then ice water was added to quench the reaction. The copper acetate catalyst was removed by filtration through diatomaceous earth, and the remaining liquid was washed three times with brine. The resulting solution was added with anhydrous magnesium sulfate and dried for 1 h. The solution was filtered and concentrated by rotary evaporation. The resulting solid mixture was washed three times with methanol and then separated by silica gel column to obtain yellow needle-like crystals SM 1 (896 mg, yield 78%). 1 HNMR (400MHz, Chloroform-d) δ7.53 (d, J = 8.0Hz, 1H), 7.38–7.28 (m, 5H), 7.22–7.05 (m, 11H). See the NMR spectrum. Figure 1 .

[0040] Synthesis of compound SM 2

[0041] SM1 and an equivalent amount of 4,5-diaminophthalonitrile (0.20 g, 1.28 mmol) were added to a round-bottom flask, ethanol was added, 20 ml of acetic acid was slowly added at low temperature, the temperature was raised to 115 ° C and stirred for 16 h, then the resulting mixture was poured into ice water and extracted with dichloromethane. The separated extract was concentrated to obtain a red solid, which was recrystallized from methanol and dichloromethane to obtain an orange solid SM 2 (766 mg, yield 80%).

[0042] Synthesis of compound TPA2-IQD

[0043] SM2 and 1.5 equivalents of triphenylamine borate (0.5 g, 1.8 mmol) were added to a round-bottom flask, followed by tetrakis(triphenylphosphine)palladium (0.1 g, 0.0915 mmol), 1.7 g of potassium carbonate, and a mixture of toluene, ethanol and water (toluene: ethanol: water = 8:1:1). The mixture was stirred magnetically and reacted at 80°C under a nitrogen atmosphere for 24 h. The mixture was cooled to room temperature, extracted with dichloromethane, washed with water, and the resulting dichloromethane solution was dried over anhydrous magnesium sulfate for 1 h. The solution was filtered and the filtrate was concentrated by rotary evaporation to obtain a red solid, which was separated by silica gel column to obtain an orange-yellow solid TPA2-IQD (656 mg, 78% yield). 1 H NMR (400 MHz, CH3Cl-d) δ8.65 (s, 1H), 8.46 (d, J = 8.2 Hz, 2H), 7.69–7.59 (m, 2H), 7.49 (d, J = 8.7 Hz, 2H), 7.45–7.37 (m, 2H), 7.33–7.21 (m, 10H), 7.18 (s, 4H), 7.13–6.98 (m, 10H). Figure 2 .

[0044] Example 2 Theoretical Calculation of TPA2-IQD

[0045] Density functional theory (DFT) calculations were performed on TPA2-IQD at the B3LYP / 6-31G level to explore their optimal geometric structures and electronic distributions. Figure 3 As shown in the optimized TPA2-IQD structure, the dihedral angles θ between the triphenylamine unit and the indoloquinoxaline rigid unit are 34° and 55°, respectively. The LUMO is primarily distributed on the indoloquinoxaline unit, the HOMO is primarily distributed on the triphenylamine attached to the 8-carbon atom, and the HOMO-1 is distributed on the triphenylamine attached to the 6-nitrogen atom. Clearly, these HOMOs and HOMO-1 promote the internal conversion (IC) of S1 and S2 during TADF.

[0046] Since the symmetry of the excited state of the molecule is broken, the electron transition is distributed on a double charge transfer state inside the molecule, that is, two triphenylamines with different distributions, forming two quasi-degenerate excited states CT I and CT II This dual degenerate charge transfer excited state can enhance SOC and accelerate the k RISC The conversion between triplet excited state and singlet excited state involves T1 and T2 states. The RISC process is improved by fast spin coupling conversion under multiple promotion.

[0047] Example 3 UV test and PL test of TPA2-IQD

[0048] The UV-visible absorption and photoluminescence spectra of TPA2-IQDs were tested in carbon tetrachloride (CCl4), toluene (Tol) and dichloromethane (DCM) solvents, respectively. Figure 4 As shown in a), with the increase of solvent polarity, the width of the emission peak gradually increases, and there is an obvious red shift, indicating that its emission is mainly characterized by ICT, and the positive solvation effect is particularly obvious. The photoluminescence wavelength in dichloromethane solution is 748nm; TPA2-IQD still has relatively obvious characteristic absorption bands in solvents of different polarities, but the absorption band changes little in solutions of different polarities. The strong absorption band before 400nm is attributed to the electronic transition of π-π* and n-π* within the molecule, and the weak absorption band between 400-600nm is attributed to the intramolecular charge transfer (ICT) from triphenylamine to quinoxaline structure. From b)-d), it can be seen that in tetrahydrofuran, the emission spectrum of TPA2-IQD will gradually increase in luminescence intensity as the temperature decreases. This is because the molecules themselves aggregate at low temperatures, that is, AIE characteristics may exist. In order to detect the AIE phenomenon, the PL spectrum of TPA2-IQD in tetrahydrofuran / water was measured. The water molecule content of each group ranged from 0% to 80%. Figure 5 It can be seen that when the water molecule content reaches 80%, the luminescence intensity suddenly increases, indicating that the molecule has obvious AIE characteristics.

[0049] Example 4 CV test of TPA2-IQD

[0050] like Figure 6 As shown, cyclic voltammetry (CV) was used to test the voltammetric analysis using a CHI620 voltammetric analyzer. A platinum disk working electrode, a platinum wire counter electrode, and an Ag / AgCl reference electrode were used. Tetrabutylammonium hexafluorophosphate (Bu4NPF6, 0.1M) acetonitrile solution was used as the electrolyte, and ferrocene / ferrocene cations (Fc / Fc +) redox couple as reference, the scan rate was 50 mV / s, and the test was carried out in anhydrous DMF solution in a nitrogen environment. The test results showed that the oxidation potential of TPA2-IQD was 1.09 eV, and the ferrocene / ferrocene cation (Fc / Fc + ) has a potential of 0.62 eV. Calculated by the formula, the HOMO energy level of TPA2-IQD is -5.27 eV, and the LUMO energy level is -2.88 eV calculated from the optical band gap.

[0051] Example 5 Low-temperature fluorescence and phosphorescence test of TPA2-IQD

[0052] At 77K, the low-temperature fluorescence-phosphorescence spectrum of TPA2-IQD in toluene solution was tested. Figure 7 As shown. Based on the low-temperature fluorescence phosphorescence spectrum, the energy level difference between the singlet and triplet states of TPA2-IQD is calculated to be ΔE ST =0.16eV.

[0053] Example 6 Temperature-dependent lifespan test of TPA2-IQD pure film

[0054] Test the temperature-varying lifetime of TPA2-IQD in pure film state, such as Figure 8 It can be clearly seen that as the temperature increases, the luminescence lifetime gradually becomes longer, which is enough to prove that it has TADF characteristics.

[0055] Example 9 Transient Fluorescence Lifetime Test of TPA2-IQD Doped CBP Film

[0056] Using TPA2-IQD as the guest material and CBP as the host material, TPA2-IQD-doped CBP films with different doping concentrations were prepared. The instantaneous lifetime and delayed lifetime of the TPA2-IQD-doped CBP films were tested. Figure 9 and 10 It is found that with the increase of doping concentration, its instantaneous lifetime gradually shortens and its delayed lifetime increases.

[0057] Example 10 PLQY measurement of TPA2-IQD doped CBP film

[0058] The luminescence quantum yield (PLQY) of TPA2-IQD doped CBP films at different doping concentrations was tested. Figure 11 As shown in Figure 2, PLQY of the TPA2-IQD-doped CBP film reaches a high PLQY of 83.8% at a 10% doping concentration, as the host-guest energy transfer is incomplete below 10% doping.

[0059] Example 11 Fabrication of a light-emitting device based on TPA2-IQD material and testing of its luminescent properties

[0060] The organic electroluminescent device structure is: ITO / PEDOT:PSS (30nm) / PVK (35nm) / CBP:TPA2-IQD (10wt% or 100wt%, 35nm) / TmPyPB (45nm) / LiF (1nm) / Al (100nm). The hole injection layer is polyethylene dioxythiophene / poly (p-styrene sulfonic acid) (PEDOT:PSS) (Bayer Batron P4083); the hole transport layer is polyvinyl carbazole (PVK); the electron transport layer is 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB); the emissive layer is a blend coating of the guest material (TPA2-IQD) and the host material 4,4'-bis(9-carbazole)biphenyl (CBP); the electron injection layer is lithium fluoride (LiF); and the cathode layer is aluminum (Al).

[0061] The device fabrication procedure was as follows: the ITO glass was cleaned with isopropyl alcohol, acetone, and deionized water, dried in a drying oven, and treated with UV-ozone at 100°C for 15 minutes. Then, 40 nm of PEDOT:PSS, 35 nm of PVK, and 35 nm of the luminescent layer were spin-coated. Then, 45 nm of TmPyPB, 1 nm of LiF, and 100 nm of Al were evaporated in sequence. The device had a luminescent area of ​​0.04 cm 2 .

[0062] The thickness of the hole injection layer, hole transport layer and light-emitting layer was measured by a probe surface profiler (Dektak-XT from Bruker). The thickness and deposition rate of TmPyPB, LiF and Al were measured by a thin film coating controller (SQC-310C from Inficon). The deposition rates of TmPyPB, LiF and Al were and All operations were performed in a nitrogen glove box.

[0063] The electroluminescence (EL) spectrum was measured using a PR-735 spectroradiometer from Photo Research; the current-voltage (IV) curve and the luminescence intensity-voltage (LV) curve were measured using a Keithley 2400 source measurement unit from Tektronix. The external quantum efficiency was calculated based on the electroluminescence spectrum, current density, and luminance assuming Lambert emission.

[0064] The electroluminescence spectrum and EQE-L curve of the light-emitting device based on TPA2-IQD material are shown in Figure 2. Figure 12-15 As shown. Figure 12 and Figure 14 It can be seen that the emission wavelengths of the doped / pure film devices based on TPA2-IQD are 626nm (EQE is 2.337%) / 716nm (EQE is 0.408%) respectively.

[0065] Table 1

[0066]

[0067] where τp and τd are the lifetimes of the instantaneous and delayed components, θp and θd are the instantaneous and delayed luminescence quantum efficiencies, respectively. kp and kd are the rate constants of instantaneous and delayed fluorescence, respectively. kISC is the interslit crossing rate, and kRISC is the anti-interslit crossing rate between S1 and T1. kr and knr are the radiative and nonradiative decay rates from S1 to S0, respectively.

[0068] Although the present invention has been described in conjunction with the preferred embodiments, the present invention is not limited to the above embodiments, and it should be understood that the appended claims summarize the scope of the present invention. Under the guidance of the inventive concept, those skilled in the art should appreciate that certain changes made to the various embodiments of the present invention will be covered by the spirit and scope of the claims of the present invention.

Claims

1. A thermally activated delayed fluorescent material having both dual charge transfer channels and aggregation-induced emission characteristics, characterized in that: The thermally activated delayed fluorescent material has indole and quinoxaline as an electron-deficient rigid skeleton unit, triphenylamine as an electron donor unit, which is located at the 6 and 8 positions of indole and quinoxaline; and cyano as an electron acceptor unit, which is located at the 2 and 3 positions of indole and quinoxaline. The chemical structure of the thermally activated delayed fluorescent material is shown in Formula 1: ， Formula 1.

2. The use of the thermally activated delayed fluorescent material having both dual charge transfer channels and aggregation-induced emission characteristics according to claim 1, characterized in that: The fluorescent material is used as a light-emitting material to prepare an organic light-emitting device.

3. The use of the thermally activated delayed fluorescent material having both dual charge transfer channels and aggregation-induced emission characteristics according to claim 2, characterized in that: The organic light-emitting device is prepared by a solution processing method.

4. The use of the thermally activated delayed fluorescent material having both dual charge transfer channels and aggregation-induced emission characteristics according to claim 2, characterized in that: The non-doped organic light-emitting device has a light emission wavelength of 716 nm, a light emission quantum yield of 15.0%, and an external quantum efficiency of 0.408%. The 10% doped device has a light emission wavelength of 626 nm, a light emission quantum yield of 83.8%, and an external quantum efficiency of 2.337%.

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