State Key Laboratory of Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications,Nanjing 210023, China
Thermally activated delayed fluorescence materials with circularly polarized luminescence characteristics have received extensive attention due to their application prospects in data storage, bio⁃imaging, and 3D display. The circularly polarized thermally activated delayed fluorescence (CP⁃TADF) devices based on these materials exhibit excellent circularly polarized electroluminescence performance. In this paper, starting from the molecular design strategies and luminescence mechanism of CP⁃TADF molecules, we comprehensively summarize their design strategies according to the different construction methods, and the chemical structures, photoelectric properties, as well as applications in electroluminescent devices are systematically reviewed. Finally, we discuss the current problems of CP⁃TADF materials, and our outlook on their future development prospects and challenges is also given.
2 Basic luminescence parameters of circularly polarized light
3 Molecular design of CP⁃TADF materials and their applications in electroluminescent devices
3.1 Molecular design strategies
3.2 Intrinsically chiral CP⁃TADF materials
3.3 Chiral perturbation
4 Conclusion and outlook
1 Introduction
Thermally activated delayed fluorescence (TADF) materials are a new type of organic electroluminescent materials with low cost and high efficiency. Through reasonable and ingenious molecular design, the energy difference (Singlet⁃triplet energy splitting,ΔEST) between singlet state (Thermally activated delayed fluorescence) and triplet state (Reversed intersystem crossing) is small, which can drive triplet exciton through Reversed intersystem crossing under the action of external heat.The RISC process is converted into a singlet exciton, which produces Delayed fluorescence (DF) emission and achieves 100% internal quantum efficiency. It has been widely used in Organic light emitting diodes (OLEDs) and other fields, and is one of the research hotspots in the field of Organic light emitting devices[1,2][3,4][5,6]. In the aspect of molecular design strategies, high Photoluminescence quantum yield (PLQY) and good color purity of the material were achieved through the multiple resonance effect of heteroatoms, space charge transfer, intramolecular charge transfer and other strategies[7][8][3]. In the aspect of electroluminescent devices, the devices based on TADF materials have achieved excellent device performance: the TADF device prepared by vacuum evaporation with strong donor-acceptor structure as the light-emitting layer achieves red emission at 622 nm, the turn-on voltage is as low as 2.9 V, and the maximum CE is 33.7 cd/A, PE is 37.8 lm/W, and EQE is 24.7%[9]; A TADF material synthesized with octahydrobinaphthol as the acceptor and carbazole as the donor was used as the light-emitting layer to prepare a TADF device, which achieved green emission with a turn-on voltage as low as 3. 5 V, a maximum CE of 93. 7 cd/a, a PE of 59. 6 lm/W, and a maximum EQE of 32. 6%[10]. A TADF device with benzotriazine as the acceptor and helical group as the donor in the blue light field has a turn-on voltage of 2.7 V, a maximum CE of 90.0 cd/a, and a maximum EQE of 38.4%[11]. A white TADF device with a turn-on voltage of 2. 6 V, a maximum CE of 95. 4 cd/A, a PE of 99. 9 lm/W, and a maximum EQE of 32. 8% was realized by doping the high-performance orange-red TADF material into the blue TADF host[12]. TADF materials can achieve efficient and full-color OLED luminescence, which has great application prospects in information storage, three-dimensional display and so on[13,14].
Circularly polarized luminescence (CPL) refers to the phenomenon that chiral luminescent substances exhibit Left-handed (L) or Right-handed (R) circular polarization when excited[15,16]. Circularly polarized light-emitting materials have become a hot research topic in the field of chemical materials in recent years due to their broad application prospects in the fields of chemical sensing, biological probes, three-dimensional display and optical storage[17⇓⇓~20]. Conventional circularly polarized light emission is mainly produced by using polarizers and anti-glare filters, but the use of polarizers may bring problems such as increased manufacturing cost of light emitting devices, decreased brightness and efficiency of devices. If CPL material is directly used as the light-emitting layer of the device, the utilization rate of the light source can reach nearly 100%, and the brightness and clarity of the display can be greatly improved, but the efficiency of the electroluminescent device will be greatly limited[21⇓~23]. In 1997, Peeters et al. Of Eindhoven University of Technology reported Circularly polarized organic light emitting diodes (CPOLED) for the first time.Since CP _ OLEDs), people have begun to design and study various circularly polarized organic electroluminescent materials, such as conjugated polymers modified by chiral side chains or co-doped with chiral emitters, chiral metal complexes, chiral small molecules, etc., in the hope of obtaining more efficient electroluminescent device performance[24][25][26][27].
In view of the broad application prospects of TADF materials in flat panel display, 3D display and other aspects, it is of great significance to develop Circularly polarized thermally activated delayed fluorescence (CP TADF) materials of chiral molecules[28]. CP TADF materials can effectively combine the characteristics of TADF and CPL, and can achieve circularly polarized light emission on the premise of achieving low ΔEST and high PLQY, thus greatly improving the performance of devices and further prolonging the service life of devices (Figure 1). At present, CP-TADF-based materials can achieve the performance of circularly polarized thermally activated delayed fluorescence electroluminescent devices with EQE up to 30%, and achieve full-color display, which greatly improves and enriches the application range of materials. In this paper, based on the luminescence principle of CP TADF materials, the molecular structure design strategy of CP TADF materials, and the basic characteristics of chiral molecules, the research work on the design, performance and device application of CP TADFs in recent years is summarized.Finally, the development and main application prospects of chiral TADF materials are prospected, and the future development direction is expected, in order to provide a reference for later researchers to design more excellent CP TADF molecules.
2 Fundamental parameters of circularly polarized light
Absorption of asymmetry factor of circularly polarized light is a measure of the degree of left-handed (Absorption of asymmetry factor) and right-handed (εR) differential Absorption of circularly polarized light by chiral molecules in the ground state, which is quantified by gabs and is independent of solution concentration[29,30]. Electronic Circular dichroism (CD) spectroscopy has been successfully applied to the determination of absolute configuration, the prediction of enantiomeric excess of asymmetric reaction, the analysis of molecular and supramolecular conformation, and the study of chiral molecular and supramolecular polymer self-assembly. In the measurement process, two beams of left/right circularly polarized light are alternately generated by a xenon lamp, then pass through the chiral material, and finally the CD spectrum can be obtained by comparing the original light intensity and the residual light intensity[31,32].
In most CD spectra, the first to be obtained is the molar circular dichroism (Δε) versus wavelength1.
In Equation 1, εL and εR are the molar extinction coefficients of left-handed circularly polarized light and right-handed circularly polarized light, respectively. gabs is the ratio of the molar extinction coefficient of CD to the total molar extinction coefficient, see Equation 2.
Where ε represents the average molar extinction coefficient, see Equation 3.
Therefore, the gabs is finally defined as Equation 4.
The degree of polarization of circularly polarized light is measured by the Luminescent dissymmetry factor, denoted by Luminescent dissymmetry factor, which is determined by the electric and magnetic dipole transition moments[33,34]. In organic CPL materials, the general electric dipole transition moment is much larger than the magnetic dipole transition moment. Therefore, small organic molecules with large electric dipole transition moment tend to have high luminescence efficiency, but small glum. In the CPL spectrum, the luminous intensity difference (.DELTA.I) between the left and right circularly polarized light is defined as Equation 5.
Where IL, IR represent the intensity of left and right circularly polarized light, respectively[35]. However, the determination of the absolute values of I and ΔI is difficult. Therefore, the degree of asymmetry of CPL is generally quantified by the luminous asymmetry factor (glum or gem), which is expressed as the ratio of the difference of light intensity divided by the average total luminous intensity, see Equation 6[36].
Obviously, Equation 4 and Equation 6 have similar mathematical expressions. Therefore, the values of gabs and glum are between + 2 to − 2. A maximum of 2 indicates ideal left polarized light, a minimum of − 2 indicates ideal right polarized light, and glum=0 indicates no circularly polarized luminescence.
Electroluminescence dissymmetry factor for circular polarization is measured from the Electroluminescence spectrum and is expressed as a Electroluminescence dissymmetry factor, similar to the measurement of the CPL spectrum, which is the ratio of twice the intensity difference of the left and right circular electroluminescence spectra divided by the total electroluminescence spectrum intensity, as shown in Equation 7.
3 Molecular Design and Electroluminescent Device Performance of CP TADF Material
3.1 Molecular design strategy
CP TADF materials mainly include small molecules and polymers, and the design principles mainly include two aspects: one is to construct CP TADF materials based on inherent chiral chromophores, and the other is to generate CPL signals by chiral unit perturbation. The former has a higher glum value, but it is expensive to resolve the enantiomers of TADF materials by chiral high-performance liquid chromatography or diastereoisomer separation. In contrast, the latter strategy is based on the perspective of synthesis, using commercial chiral enantiomeric products as the initial raw materials, which greatly promotes the large-scale synthesis and later separation of CP TADF materials, and is conducive to the purification of products and the application of CP TADF materials in electroluminescent devices. The design strategies commonly used to construct CP TADF materials are shown in Figure 2: (a) CP TADF materials with inherent chirality, mainly including central chirality, planar chirality and helical chirality[37]; (B) Chirally perturbed CP TADF materials, mainly including binaphthol, octahydrobinaphthol, biphenyl, 1.2diaminocyclohexane, alkyl chain polymers, etc.
3.2 Inherent chirality
The design strategy of intrinsically chiral CP TADF materials is to embed chiral elements into active fluorophores, which can be divided into three types according to the different chiral elements: central chirality, planar chirality and helical chirality.
3.2.1 Central chirality
Central chiral molecules hybridized with sp3 are the most common type of chiral molecules. CP TADF materials can be effectively constructed by introducing central chiral molecules into known TADF chromophores, which is also the earliest CP TADF material reported. In 2015, Hirata et al. Of Tokyo University of Agriculture and Technology first reported DPHN, a compound with CP TADF properties, using triphenylamine as donor and naphthoquinone as acceptor (Fig. 3)[38]. The HOMO (Highest occupied molecular orbital) and LUMO (Lowest unoccupied molecular orbital) of the compound were assigned to the triphenylamine and naphthoquinone units, respectively, and the calculated Highest occupied molecular orbital was 0. 07 eV. In toluene solution, the PLQY of DPHN is 4%, and the fluorescence lifetime is 13.9 ns. The film prepared by doping 9% DPHN with 9,9 '- (1,3) phenyl) diphenylcarbazole (mCP) showed green emission with a PLQY of 26%. In toluene solution, the absorption asymmetry factor |gabs| of DPHN is 1.2×10-3, and the luminescence asymmetry factor |glum| is 1.1×10-3. Although the compound has not been characterized for device performance, this work is the first CP TADF material reported in the literature.
In 2020, Zhang Xiaohong et al. Of Soochow University reported a dual-conformation CP TADF material, TTR PMAc[39]. A pair of chiral isomers were synthesized by introducing a central chiral carbon atom into the compound, and the R/S TTR PMAc compound was synthesized by using the chiral isomers as donors and 5,5,10,10 tetroxide as acceptors (Figure 3). Both chiral molecules exhibit nearly planar (dihedral angle of 173.4 °) and orthorhombic (dihedral angle of 85.57 °) bistable conformations, resulting in high thermal stability of the molecules with thermal decomposition temperatures (Thermal decomposition temperature,Td) of 403 ° C and 405 ° C. The ΔEST of R/S, TTR and PMAc were measured to be 0.02 and 0.05 eV, respectively. This work confirms that the molecular conformation has an important influence on the circularly polarized luminescence properties.
In 2021, Yang Chuluo et al. Of Wuhan University reported CP TADF materials with sky blue and orange luminescence properties, TRZ MeIAc and NID MeIAc (Fig. 3)[40]. In molecular design, acridine derivative MeIAc was used as chiral donor, and triphenyltriazine and naphthalimide derivatives were used as acceptors to synthesize TRZ and NID compounds, respectively. Quantum chemical calculations show that the HOMO is mainly distributed in the acridine derivative unit, and the LUMO is mainly distributed in the triphenyltriazine (TRZ) and naphthalimide (NID) units. The ΔEST of TRZ/MeIAc and NID/MeIAc are 0. 19 eV and 0. 22 eV, respectively, the emission peaks of thin film are 473 and 565 nm, PLQY are 89% and 86%, the fluorescence lifetimes are 18. 7 and 26. 3 ns, and the delayed fluorescence lifetimes are 82. 3 and 235. 4 μs. The rigid structure of the molecule results in good thermal and configurational stability, with Td of 409 and 379 ° C, respectively. In toluene solution, the luminescence asymmetry factors glum of TRZ ∥ MeIAc and NID ∥ MeIAc are 5.9×10-4 and 2.0×10-3, respectively. TRZ and NID are used as a luminescent layer,
The maximum EQE of the CP OLED device prepared by vacuum evaporation (Figure 4) is 20.3% and 23.7%, respectively, and the asymmetry factor of circular polarization electroluminescence reaches 2.4×10-3 |gEL|. This work not only expands the design of central chiral molecules, but also provides theoretical guidance for improving the performance of CP TADF devices.
3.2.2 Planar chirality
Planar chirality refers to the construction of CP TADF materials through symmetrical planar chiral units. According to the R/S naming rules, chiral molecules are named Rp and Sp. Among them, the face-to-face benzene rings in the [2.2] -paracyclophane unit make it have the characteristics of planar chiral structure, which is an important chiral unit for the construction of circularly polarized luminescent materials. In 2018, Zhao Cuihua et al. Of Shandong University constructed two CP TADF materials ——g⁃BNMe2⁃Cp and m⁃BNMe2⁃Cp (Fig. 5) with [2.2] paracyclophane as the chiral unit, in which dimethylamine was used as the donor on one aromatic ring and dimethylboron was used as the acceptor on the other aromatic ring[41]. Quantum chemical calculations show that the HOMO and LUMO are mainly distributed in the donor and acceptor units, and the experimentally measured ΔEST values are 0.17 eV(g⁃BNMe2⁃Cp) and 0.12 eV(m⁃BNMe2⁃Cp), respectively. In toluene solution, the emission peak of g⁃BNMe2⁃Cp is at 529 nm, and the glum is 46% 4.24×10-3,PLQY, while in cyclohexane, the PLQY of the two compounds is 72% and 39%, respectively, and the PLQY of the powder is 53% and 33%, respectively. This work introduces the planar chiral structure into the field of circularly polarized luminescence, which provides a new idea for the later design of materials.
In 2019, Zysman Colman et al., University of St. Andrews, UK, reported two new TADF materials, R/S CzpPhTrz (Fig. 5), using [2.2] -paracyclophane unit as donor and diphenyltriazine as acceptor[42].
With the increase of the steric volume of the donor unit, the torsional force between the carbazole and the aryl bridge increases, resulting in a decrease of the ΔEST to 0. 16 eV, a highly rigid structure, and a Td of 360 ℃. In the doped film, the emission peak is at 482 nm, and the PLQY is 69%. In toluene solution, R/S CzpPhTrz exhibits an absorption asymmetry factor gabs of 5.0×10-3 and -7.0×10-3,The luminescence asymmetry factors glum are -1.2×10-3 and 1.3×10-3, respectively (Fig. 6). An electroluminescent device was fabricated by vacuum evaporation of 10% DPEPO doped with the planar chiral molecule. The device had a turn-on voltage of 3.2 V, CIE coordinates of (0.17,0.25), maximum CE of 34.8 cd/A, PE of 32.5 lm/W, and EQE of 17%.
In 2021, Ye Shanghui et al. Of Nanjing University of Posts and Telecommunications reported a CP TADF material with planar chiral phenoxazine (PXZp) as the donor unit, 4,6 diphenyl 1,3,5 triazine (TRZ) as the acceptor unit, and [2.2] paracyclophane as the planar chiral unit — Rp/Sp PXZp Ph TRZ (Fig. 5)[43]. The two compounds have efficient luminescence properties and excellent CPL properties with a ΔEST of only 0.03 eV. The compound has a Td of 388 ° C and a glass transition temperature (Glass transition temperature,Tg) of 179 ° C. These results indicate that the compound has good thermal and morphological stability. The emission peaks are 548 nm and 527 nm in solution and doped film, respectively, and the PLQY is increased from 47% to 60%. In toluene solution, the molecules exhibited good CPL signals with glum values of -2.4×10-3, 3.3×10-3, respectively. The CP OLED device fabricated by the solution spin-coating method with Rp/Sp ∥ PXZp ∥ Ph ∥ TRZ as the light-emitting layer realized yellow emission with a maximum brightness of up to 34 293 cd/m2,EQE and up to 7.8%,gEL as 4.6×10-3.
3.2.3 Helical chirality
Helical chirality is a kind of chiral molecule with helical structure, which is obtained by ortho-condensation of aromatic and non-aromatic compounds. This kind of molecule has neither chiral center nor other symmetry factors.Because of the special helical structure, the molecular skeleton is twisted to a certain extent, so that all parts of the molecule are not in the same plane, resulting in helical chirality. Helical chirality not only has a high rigid structure, but also has the characteristic of keeping the configuration stable at high temperature. In 2020, Jiang Zuoquan et al. Of Soochow University constructed two CP-TADF molecules, SFST and SFOT (Fig. 7), using the space charge transfer strategy and asymmetric helical structure. These two compounds use 2,4,6-phenyl-1, 3,5-triazine (TPZ) as the acceptor to connect to the C1 position of fluorene, and 10-phenyl-1, 10-phenyl-10H-phenothiazine (PTZ) or 10-phenyl-10H-phenoxazine (PXZ) as the donor to connect to the C9 position of fluorene[44]. Both compounds have good thermal stability with Td of 382 and 393℃,Tg of 171 and 157 ° C, respectively. The experimentally measured ΔEST is 0.052 and 0.053 eV, and the theoretically calculated ΔEST is 0.0256 and 0.0118 eV, and the emission peaks are 512 nm,kRISC 9.93×10-4 and 11.7×10-4S-1, respectively. The large intersystem crossing rate makes the maximum PLQY of the film reach 53.1 and 89.0%, respectively, when doped with mCBP. The SFST and SFOT fluorescence lifetimes are 141 and 10 ns, respectively, corresponding to the delayed fluorescence lifetimes of 6.78 and 7.98 μs, respectively. In toluene solution, the corresponding glum of (S) SFOT and (R) SFOT reach 2.2×10-3 and -2.0×10-3, respectively.While the corresponding glum of (S) ∥ SFST and (R) ∥ SFST are 3.4×10-3 and 4.0×10-3, respectively. At the same time, the maximum CE, PE and EQE of the electroluminescent device with SFST and SFOT as the light-emitting layer are 37. 0, 68.0 cd/A, 31.8, 53.6 lm/W and 12. 5%, 23.1%, respectively. In addition, (S) -SFST and (S) -SFOT with obvious circularly polarized EL signals were tested, and the gEL were 1.30×10-3 and 1.0×10-3, respectively. This work verifies that the face-to-face connection of donor-acceptor molecules is the key to achieve effective space charge transfer, and the combination of space charge transfer and asymmetric helical carbon can obtain chiral properties, which provides a new idea for the design of CP TADF molecules.
In 2020, Chen Chuanfeng et al. Of the University of Chinese Academy of Sciences designed and synthesized a pair of helicene enantiomers with rigid skeletons, (P) HAI and (M) HAI (Fig. 7, 8)[45]. Both helicene enantiomers have high thermal stability, chiral configurational stability and good photophysical properties. The Td is at a 408℃,Tg of 316 ℃ with an emission peak at 547 nm. In toluene solution, the PLQY reaches 24% with a fluorescence lifetime of 5.1 ns. Both (P) HAI and (M) HAI have strong CPL properties, and their luminescence asymmetry factors glum are -5.9×10-3 and 6.2×10-3, respectively. Using (P) HAI and (M) HAI as the chiral luminescent guest of the device, the CP OLED device was prepared by thermally activating the helicene enantiomer with TADF sensitizer, the turn-on voltage was 2. 6 and 2. 8 V, respectively, the efficiency roll-off was 1. 9% at the brightness 1000 cd/m2, and the maximum external quantum efficiency (EQE, 5.3%) was increased by 4 times. The gEL values for (P) HAI and (M) HAI are -2.3×10-3 and 3.0×10-3, respectively. This is the first example of CP OLED based on thermally activated delayed fluorescence material sensitized circularly polarized luminescence, which provides an effective strategy for improving the circularly polarized luminescence efficiency of chiral fluorescent molecules.
In 2021, Zheng Youxuan et al. Of Nanjing University reported two helical CP TADF materials, R/S OSFSO, using phenoxazine as a donor and sulfur dioxide anthracene derivatives as an acceptor (Fig. 7)[46]. The measured ΔEST is only 0.022 eV, the emission peak is at 470 nm, and the maximum PLQY is as high as 285 ℃ 81.2%,Tg. In toluene solution, R/S ￠ OSFSO exhibited luminescence asymmetry factors glum of -1.6×10-3 and 1.4×10-3, respectively. The blue CP TADF devices fabricated with R and S OSFSO as the light-emitting layer have a turn-on voltage of 3. 3 V, a maximum CE of 37. 1 and 38. 1 cd/A, a maximum PE of 21. 6 and 28. 1 lm/W, and a maximum EQE of 19. 5% and 20. 0%, respectively. The electroluminescence asymmetry factor gEL of the device is -3.0×10-3, 3.1×10-3, respectively, corresponding to an EQE of 19.3%, 18.0% at a device luminance of 1000, 2000cd/m2, and an efficiency roll-off of only 3.5%.
3.3 CP TADF materials realized by chiral perturbation
In terms of synthesis, CP TADF materials based on chiral perturbation units can directly obtain enantiomerically pure products from the market without resolution process, which greatly promotes large-scale synthesis and application in organic light-emitting devices. Chiral units such as binaphthol, octahydrobinaphthol, biphenyl, 1.2 cyclohexanediamine and chiral alkyl chain were introduced into the TADF luminophore, and the CP TADF properties were induced by chiral perturbation.
3.3.1 Binaphthol
Binaphthol is an important chiral perturbation unit, which has both chiral and fluorescent properties. Binaphthol can be introduced into TADF materials to show excellent circularly polarized luminescence properties. In 2016, Pieters et al. Of Paris-Thackeray University first reported a TADF material with carbazole as the donor, terephthalonitrile as the acceptor, and binaphthol as the chiral unit — R/S fBN fDCz (Fig. 9), which has a PLQY of 74% and a luminescence asymmetry factor |glum| of 1.3×10-3[47]. The maximum CE, PE and EQE of the CP TADF device prepared by doping S, BN and DCz into mCP are 34. 7 cd/A, 16.3 lm/W and 9. 1%, respectively, when the doping mass ratio is 20%.
In 2018, following the same design principle, Tang Benzhong et al. Of the Hong Kong University of Science and Technology reported four pairs of CP TADF materials, R/S BN CF, R/S BN CCB, R/s BN 〕 DCB and R/s BN 〕 AF (Fig. 9), using carbazole, 3,6 tert-butylcarbazole and 9,10 dihydro9, 9 dimethylacridine as donor units, terephthalonitrile unit as acceptor unit and binaphthol as chiral unit, respectively[17]. The Td of the four chiral molecules are higher than 360 ℃, and the corresponding ΔEST are 0. 15,0.11,0.02 and 0. 11 eV, respectively. By changing different donor units, the emission peaks of the doped film and the pure film are 493 ~ 571 nm and 537 ~ 597 nm, respectively, and the multicolor luminescence from green to orange is realized. The PLQYs of S-BN CF, S-BN CCB, S-BN DCB and S-BN AF were 37.6%, 44.4%, 52.2% and 7.8% respectively in the solid state, and 0.5%, 0.8%, 1.2% and 2.0% respectively in the solution state. The average values of the |glum| of R/S chars BN CF, BN CCB, BN DCB and BN AF under the film are 2.0×10-2~4.2×10-2, which are larger than the average value of the |glum| of toluene solution 5.0×10-4~1.2×10-3, confirming that this chiral molecule has an aggregation-induced effect, which leads to a significant increase in the fluorescence and CPL signals in the aggregated state. A series of electroluminescent devices were fabricated with these four molecules with S configuration. The maximum CE of the doped and undoped CP TADF devices with S/BN/CF as the light-emitting layer were 26.4 and 10.3 cd/A, respectively. The PE were 19.6 and 6.7 lm/W, respectively. The maximum EQE were 9.3% and 3.5%, respectively.
In 2019, our research group reported two pairs of CP TADF materials, R/S CPDCz and R/S CPDCB, with carbazole and 3,6-di-tert-butylcarbazole as donors, cyanopyridine as acceptor, and binaphthol as chiral unit (Fig. 9)[48]. The two pairs of molecules show excellent thermal stability, the Td is 420 ℃, the 439℃,Tg is 329 ℃/326 ℃ and 248 ℃/244 ℃, and the corresponding ΔEST is 0. 08 eV and 0. 03 eV, respectively. When the two molecules were doped with mCP at a mass ratio of 10%, the maximum PLQY was as high as 55%, which was significantly higher than that of 37% PLQY in solution. In tetrahydrofuran solution, R/S ￠ CPDCz and R/S ￠ CPDCB exhibited luminescence asymmetry factors glum of 3.4×10-4, -3.0×10-4 and 3.2×10-4, -2.1×10-4, respectively (Fig. 10).
The devices fabricated by vacuum evaporation and solution spin coating using R/S ∥ CPDCB as the light-emitting layer have the maximum CE of 39.5 and 33.3 cd/A, PE of 30.3 and 26.1 lm/W, EQE of 12.4% and 10.6%, and EL asymmetry factors gEL of 6.0×10-4, -8.6×10-4, 3.5×10-3, and -3.9×10-3, respectively. This work provides a guiding direction for the preparation of CP TADF devices by solution spin coating.
In 2019, Cheng Yixiang et al. Of Nanjing University reported the TADF materials with phenoxazine as donor and binaphthol as chiral unit, R/S μ1 and R/S μ2 (Fig. 9),Td is 494 ℃, 364℃,Tg is 212 ℃, 217℃,ΔEST is 0.059 and 0.076 eV, emission peak under film is 568 and 530 nm, PLQY is 18.5% and 15.7%, delayed fluorescence lifetime is 1.03 and 0.97 μs, respectively)[49]. The emission peak of R/S arrowhead 1 in toluene solution is at 581 nm, and the asymmetry factor glum is -1.2×10-3, 1.6×10-3, while the CPL signal of R/S arrowhead 2 can not be detected because the acceptor structure inhibits the effective chirality transfer. The CPL exhibited similar signals in the doped and pure film States, with luminescence asymmetry factors glum of -7.1×10-4, 9.2×10-4 and -7.2×10-4, 8.2×10-4, respectively. The turn-on voltage of the fabricated circularly polarized light-emitting device is 3.4 V, the maximum brightness is 40 470 cd/m2, the maximum EQE is 4.1%, and the electroluminescence asymmetry factor gEL is -0.9×10-3 and 1.0×10-3 with R/S ∥ 1 as the light-emitting layer.
At the same time, Pieters et al. Reported chiral TADF materials with 10 pairs of binaphthol units — B1, B2, B3, B4, C ′ 1, C ′ 2, C ′ 3, C1, C2 and C3 (Fig. 9). They systematically studied the structure-performance relationship of TADF molecules with binaphthol as the chiral unit, and proposed a new strategy to improve the CPL performance by shortening the distance between the TADF luminophore and the chiral unit[50]. To this end, they synthesized and studied three series of molecules: B, C ', C. Molecule B has phthalonitrile as acceptor, binaphthol linked at positions 4 and 5, and different carbazole derivatives as electron donors at positions 3 and 6. On the other hand, compounds C ′ and C were obtained from phthalonitrile with the binaphthol unit at positions 3 and 4. For compound C ', only one carbazole derivative is attached to the donor at position 5, while two donors of compound C are attached at positions 5 and 6, respectively. Quantum chemical calculation shows that the orbital distribution of HOMO and LUMO is similar, HOMO is mainly distributed in the donor unit, LUMO is mainly distributed in the acceptor unit, and binaphthol chiral unit is almost not involved in the frontier orbital distribution. The fluorescence colors of all these compounds range from blue (B1,λem=469 nm) to green (C3,λem=519 nm), in toluene solution, exhibit quantum efficiencies ranging from 2% to 30%, and have AIEE behavior. Bi-exponential luminescence decay was observed upon doping into PMMA films, confirming the TADF nature of these molecules. Except for C1 and C2, the other compounds show significant absorption asymmetry factors, indicating that the chiral perturbation strategy plays an effective role in the ground state of molecules. DFT calculations were carried out for each molecule to study the effect of carbazole and its carbazole derivative substitution on the efficiency of ground state chiral perturbation, and the results showed that compound B had a good orientation and the carbazole substitution had a great effect on these key parameters. Notably, compound B1 showed the highest luminescence asymmetry factor glum (was 2.1×10-3). In addition, in view of the complexity of bottom-emitting CP OLED device fabrication, the top-emitting device structure is considered to be the best choice for future display applications. In this work, the first top-emitting CP OLED based on CP TADF material was prepared, which is a key step for the practical application of CP OLED.
In 2021, Zheng Youxuan et al. Of Nanjing University and Zhou Liang et al. Of Changchun Yinghua Institute cooperated to prepare a new type of chiral donor structure by combining the chiral binaphthol unit with the electron donor structure, and directly constructed chiral TADF materials — R/S p BAMCN (rod-like) and R/S o BAMCN (helical) with D*⁃A⁃D* as the skeleton (Fig. 9)[51]. Due to the rigid molecular structure and special intramolecular arrangement, the Tm of the chiral TADF material is 485 ℃ and 465 ℃, respectively, and the ΔEST is 0. 18 eV and 0. 12 eV, respectively.Meanwhile, the chiral TADF material has a high PLQY of 86% in toluene solution, a half-peak width of 38 nm in cyclohexane solution, and a 51 nm,|gPL| of 5.3×10-3 in toluene solution. In particular, the rod-shaped R/S-p-BAMCN light-emitting layer shows higher device performance, and the maximum external quantum efficiency is close to 28%. At the same time, the semi-transparent CP matrix OLED based on R/S matrix BAMCN shows obvious circular polarization electroluminescence characteristics, and the |gEL| is about 4.6×10-3. The D*⁃A⁃D*structure with special chiral arrangement provides an effective way to obtain efficient CP TADF materials with narrow-band emission spectra and better CPL properties.
3.3.2 Octahydrobinaphthol
In 2019, Zheng Youxuan et al. Of Nanjing University reported a CP TADF material with octahydrobinaphthol as the chiral unit, diphenylamine as the donor, and dicyanobenzene as the acceptor — R/S OBN DPA (Fig. 11)[52]. The emission peaks of the two materials are located at the 538 nm,ΔEST of 0. 09 eV, and the delayed fluorescence lifetimes of the doped and pure films are 9. 8,10.8 ns and 8. 3,13.5 μs, respectively, and the maximum PLQY are 84. 67 and 78. 59%, respectively. At 560 nm, R/S OBN DPA exhibits a luminescence asymmetry factor |glum| of 2.0×10-3. The maximum EL asymmetry factors gEL, CE, PE and EQE of the undoped and doped EL devices fabricated by vacuum evaporation with R/S/OBN/DPA as the emitting layer are 2.9×10-3, -2.2×10-3 and 2.3×10-3, -1.8×10-3, 23.0 and 45.3 cd/A, 16.8 and 30.7 lm/W, 6.6% and 12.3%, respectively.
In 2019, Zheng Youxuan et al. Of Nanjing University replaced the donor group in the above chiral molecule with carbazole, and designed and synthesized a new CP TADF material, R/S OBN Cz (Fig. 12), whose ΔEST was reduced to 0.037 eV, the maximum emission peak was 503 nm, and the maximum PLQY of doped film and pure film were 92% and 81%, respectively[10]. The luminescence asymmetry factors glum of R/S ￠ OBN ￠ Cz at 548 nm and 541 nm are -2.0×10-3 and 2.14×10-3, respectively. The turn-on voltage of the TADF device with R/S/OBN/Cz as the light-emitting layer is as low as 3.5V, the maximum CE is 93.7 cd/A, the PE is 59.6 lm/W, the maximum EQE is 32.6%, and the corresponding efficiency roll-off of the device is 2.5%, 1.0% and 4.4% at the brightness of 1000, 3000 and 5000 cd/m2, respectively (Fig. 12).
In 2021, Tang Jianxin et al. Of Soochow University designed and synthesized two pairs of axial CP TADF materials, R/S ODQPXZ and R/S-ODPPXZ, with octahydrobinaphthol as the chiral unit, diphenylquinoxaline as the acceptor unit and dibenzophenazine as the donor unit (Fig. 11)[53]. The rigid structure of the molecule makes it have good thermal stability, the Td are 470 ℃ and 469 ℃ respectively, the Tg of R/S ■ ODQPXZ is 214 ℃, and R/S ■ ODPPXZ does not show obvious glass transition temperature. The corresponding experimental ΔEST values of R/S ∥ ODQPXZ and R/S ∥ ODPPXZ are 0.16 and 0.07 eV, respectively, and the orange and red emissions are achieved at 589 and 630 nm, respectively, with PLQY as high as 92% and 89%, respectively.The circularly polarized photoluminescence asymmetry factor gPL of R/S ≠ ODPPXZ is -1.4×10-3, 1.9×10-3, which is nearly an order of magnitude larger than that of R/s ≠ ODQPXZ. The EL devices based on these two TADF materials have a turn-on voltage of 2.7 V, maximum CE, PE, and EQE of 81.0 and 41.7 cd/A, 89.7 and 43.9 lm/W, 28.3%, and 20.3%, respectively, and the EL asymmetry factors gEL are 6.0×10-4 and 2.4×10-3, respectively.
In 2021, Wang Yue et al. Of Jilin University reported two CP TADF materials with efficient green emission, R/S OBN 2CN BN and R/S OBN 4CN BN (Fig. 12)[54]. They were synthesized by a simple two-step reaction using octahydrobinaphthol as the chiral unit, benzonitrile as the acceptor unit and DtBuCzB as the donor unit. R/S OBN o2CN OBN and R/S oBN o4CN oBN have good thermal stability with Td of 482 and 483 ℃, respectively. The HOMO and LUMO levels of R/S/OBN/2CN/BN and R/S/OBN/4CN/BN are effectively separated by quantum chemical calculation, and the ΔEST are 0. 12 and 0. 13 eV, respectively. The green emission was achieved at 493 and 500 nm, and the PLQY reached 99% and 96% in toluene solution, respectively. However, the PLQY decreased to 95% and 90% in solid state, and there was fluorescence quenching in the aggregated state. The fluorescence lifetimes of R/S OBN 2CN BN and R/S OBN 4CN BN are 9.8, 10.0 ns and 95.3, 97.4 μs, respectively. Also under doped films, the corresponding luminescence asymmetry factors of R/S ￠ OBN ￠ 2CN ￠ BN and R/S ￠ OBN ￠ 4CN ￠ BN are 9.0×10-4, -9.1×10-4 and 8.0×10-4, -10.4×10-4, respectively. With these two molecules as the light-emitting layer, the fabricated EL devices have half-peak widths of 30 and 33 nm, maximum EQE of 29.4% and 24.5%, and EL asymmetry factors gEL of 1.43×10-3, -1.27×10-3 and 4.6×10-4, 4.76×10-4, respectively.
3.3.3 Biphenyl
In 2020, Chen Chuanfeng et al. Of the University of Chinese Academy of Sciences reported a CP TADF material with biphenyl as the chiral unit and carbazole molecule as the donor, R/S Cz Ax CN (Fig. 13)[55]. Molecules with this structure also have good solubility and thermal stability, and the Td is 260 ℃ 388℃,Tg. The minimum ΔEST is 0. 029 eV due to the π-conjugated charge transfer in the molecule, the emission peak is 458 nm, and the maximum PLQY is 68. 2%. The CD and CPL spectra of R/S/Cz/Ax/CN show a mirror image relationship due to the intramolecular charge transfer, in which the fluorescence asymmetry factors glum are 4.5×10-3 and -4.8×10-3, respectively. High efficiency blue CP TADF devices were fabricated by using R/S Cz Ax CN as the light-emitting layer, and the gEL of the devices were 12.5% for 1.4×10-2 and 12.7% for -1.2×10-2,EQE.
Subsequently, they designed a new axisymmetric CP TADF material, R/S 4 tBuCzPN, by changing the donor unit and replacing the carbazole group with 3,6-tert-butylcarbazole (Fig. 13)[56]. Benefiting from the effective separation of frontier orbitals, the compound has a ΔEST of only 0.05 eV, an emission peak at 497 nm, and a PL QY up to 74%,Td of 498 ° C. The glum of R/S/4 tBuCzPN in toluene solution and film state are 5.4×10-3, 5.2×10-3 and -5.0×10-3, -5.2×10-3, respectively.
In addition, the electroluminescent device with 4 t BuCzPN as the light-emitting layer achieves bright green luminescence, and the maximum CE, PE, and EQE are 34. 7 cd/A, 16.3 lm/W, and 9. 1%, respectively.
In 2020, Zheng Youxuan et al. Of Nanjing University used the same design concept to report a CP TADF material with phenoxazine and carbazole as donors, R/S BPPOACz (Fig. 14)[57]. The compound has excellent thermal stability, the Td is only 0. 04 eV 338℃,ΔEST, the emission peak is 537 nm, and the PLQY is up to 86. 10%. The glum of R/S/BPPOACz in toluene solution and doped film are -8.7×10-3, 9.7×10-3 and -1.72×10-2, 1.85×10-2, respectively. The bright green emission at 537 nm with CIE coordinates of (0.36, 0.57), maximum luminance, CE and EQE of 35 711, 38 128, 37 899 cd/m2,56.9, 61.0, 51.9 cd/A and 16.6%, 15.1%, 17.8% were achieved with rac/BPPOACz and R/S/BPPOACz as the emitting layers, respectively. This work greatly broadens the luminescence range of CP TADF and enriches the device performance.
3.3.4 1,2-diaminocyclohexane
In 2018, Chen Chuanfeng et al. of the University of Chinese Academy of Sciences reported a CP TADF material based on 1,2-diaminocyclohexane as a chiral unit, R/S CAI Cz (Fig. 15)[58]. The ΔEST of these materials is only 0. 06 eV, the lower emission peak of the film is 533 nm, and the 98%,Td of PLQY in vacuum is 349 ℃.The excellent thermal stability makes TADF materials have high morphological stability in device fabrication. The |glum| of R/S in the doped film is 1.1×10-3. The TADF devices fabricated by doping S/CAI/Cz and R/CAI/Cz with mCBP as the light-emitting layer have the gEL of -1.7×10-3, 2.3×10-3, the turn-on voltage of 3.4 V, and the maximum CE, PE, EQE of 59.0, 59.4 cd/A, 53.0, 52.9 lm/W and 19.7%, 19.8%, respectively. These devices also have low efficiency roll-off, with EQE of 14.2% and 6.3% at 100 and 1000 cd/m2, respectively.
Subsequently, they prepared a CP TADF material, R/S CAI DMAC, with 1.2diaminocyclohexane as the chiral unit by introducing a 1,8naphthalimide molecule (Figure 15)[59]. The ΔEST is only 0. 07 eV, the emission peak is 583 nm in solution, and the 26%,Td of PLQY is 405 ℃. Compared with CAI ⑥ Cz, CAI ⑥ DMAC showed a |glum| of 9.2×10-4. The TADF devices were fabricated with R-CAI-DMAC and S-CAI-DMAC doped with CBP as the light-emitting layers, respectively. The corresponding CP-OLED had a turn-on voltage of 3.4 V and a broad orange-red emission at 592 nm. The maximum CE, PE, and EQE were 28.5 and 28.8 cd/A, 28.8 and 26.6 lm/W, and 12.4% and 12.3%, respectively.
3.3.5 CP TADF polymer materials with alkyl chain as chiral unit
From the perspective of molecular design strategy, TADF polymers use the design idea of the electron donor-acceptor (D-A) structure of TADF organic small molecules. The main design strategy is based on an indirect way to introduce small molecules with known TADF activity behavior into different positions of the polymer structure as side groups, backbone components or cores. In addition to these, there are self-luminescent TADF polymers. In this case, the small molecule originally without TADF properties reduces the energy splitting between the corresponding singlet and triplet States after polymerization due to the expansion of the conjugated structure, so that the ΔEST is relatively small enough to be overcome by heat, and thus the polymer shows excellent TADF properties. Among them, chiral alkyl chains are widely used in the field of chiral luminescence because of their diversity and availability. In 2019, our group realized chiral ultra-long room temperature phosphorescent materials with photoactivation properties by combining the chiral alkyl chain of alanine methyl ester with carbazole chromophore[60]. Similarly, CP TADF materials with circularly polarized luminescence properties can also be prepared by introducing chiral units into thermally activated delayed fluorescence polymer molecules. In 2019, Tang Benzhong et al. Of Hong Kong University of Science and Technology used poly (carbazole) dimethyl acridine as donor, dibenzothiophene 2 phenylmethanone as acceptor, and alanine as chiral alkyl chain to modify conjugated macromolecules — P5 and P10 (Fig. 16). The chiral polymer has good stability, with Td of 285 ℃ and 293℃,Tg of 130 ℃, respectively[61]. Because the conjugated polymer has small electronic coupling and effective orbital separation, the ΔEST is relatively small, which is beneficial to the RISC process. The corresponding kRISC of P5 and P10 are 2.07×106S-1 and 2.50×106S-1, respectively. They also show strong green emission under the film, with PLQY of 6.7% and 10.3%, and delayed fluorescence lifetime of 1.358 and 1.366 μs, respectively. Under the solid film, the fluorescence asymmetry factors glum of P5 and P10 were -2.01×10-3 and -1.39×10-3, respectively. Undoped and doped electroluminescent devices were fabricated by using the polymer as the light-emitting layer, and the maximum brightness was 2. 52 cd/A in 1477 cd/m2,CE and 0. 87% in EQE.
4 Conclusion and prospect
TADF materials have been widely studied in the field of organic electroluminescence due to their high exciton utilization, good thermal stability and high PLQY. Circularly polarized luminescence can effectively reduce the energy loss in the process of electroluminescence, and achieve higher brightness with lower energy consumption. In this paper, the latest research progress of CP TADF materials in recent years is reviewed. Although CPL materials have been widely studied, CP TADF materials are still in the initial stage of development. CP TADF materials have made great progress in recent years by using two molecular design strategies, inherent chirality and chirality perturbation. Electroluminescent devices based on CP TADF materials greatly simplify the device structure, while the EQE and glum can reach 30% and 10-2, respectively. However, there are still many problems to be solved in the synthesis and device application of CP TADF materials.
(1) At present, CP TADF materials mainly emit green or yellow. Due to the relative difficulty of molecular design, effective blue (below 480 nm) and red and even near-infrared (above 600 nm) emissions are less, and high PLQY and high glum values are also very rare. The innovative synergistic combination of chiral units with traditional efficient blue and red TADF chromophores may provide an effective way for molecular design strategies, such as trying a series of chiral units with TADF molecules with red and blue emission, or improving PLQY and glum by molecular crystallization, chiral units and chromophores host-guest doping, etc.
(2) There is still no systematic study on the measurement of the asymmetry factor of CP TADF materials and its influencing factors. In recent years, the development of theoretical chemistry has led to a deeper understanding of the relative orientation and magnitude of the electric and magnetic transition dipole moments of excited States. Due to the complexity of its measurement in the solid or thin film state, a detailed and systematic comparison between the glum value in solution or in thin film and the gEL value of the OLED is still necessary, which may be affected by the measuring instrument. Therefore, in order to achieve a higher asymmetry factor, the strategy of amplifying the CPL signal without affecting the photophysical properties of the molecule seems to provide a more optimized scheme for practical applications.
(3) From the perspective of electroluminescent devices, high-performance light-emitting devices need to have the advantages of low efficiency roll-off, good color stability and long device life, which will determine the commercial prospects of TADF materials in the future. However, achieving high-performance solution-processable undoped devices, especially CP TADF devices, remains a challenge. In this regard, designing TADF polymers with blue, red, and white luminescent colors is one of the ideal molecular design strategies. Energy transfer between CP TADF assisted dopants and traditional fluorescent materials (in small molecule and polymer devices) is an effective way to improve the performance of TADF devices.
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