In 1942, Bartlett et al. First prepared triptycene through a multi-step reaction, and then in 1956, Wittig et al. Reported a practical method for the one-pot synthesis of triptycene through the Diels-Alder addition reaction of benzyne and anthracene, which opened the prelude of pterylene chemistry^[1][2]. Triptycene is an aromatic compound in which three benzene rings are connected by two saturated bridgehead carbons (Fig. 1), with D_3h symmetry, and the dihedral angle between adjacent benzene rings is 120^°. Therefore, it has a unique rigid three-dimensional paddle-like structure and three open electron-rich cavities, which are widely used in host-guest chemistry, supramolecular chemistry and materials chemistry^[3].

Organic light-emitting diode (OLED) displays have the advantages of lightness, active emission, wide viewing angle, fast response, low energy consumption, excellent low temperature and shock resistance, and potential flexible design^[4]. In 1998, Forrest et al. Found that the electroluminescent internal quantum efficiency of phosphorescent materials can reach 100%, breaking through the limitation that the efficiency of traditional fluorescent materials can only reach 25%, thus accelerating the development of organic electroluminescent materials^[5]. In 2012, Adachi et al. Found that the electroluminescent internal quantum efficiency of pure organic molecules with thermally activated delayed fluorescence (TADF) could also reach 100%, which laid the foundation for the commercialization of low-cost OLED^[6]. The most difficult problem in the use of electroluminescent materials is the concentration quenching, which is often caused by the π-π stacking interaction between luminescent molecules. The unique three-dimensional structure of the triptycene skeleton can help to avoid this interaction, and this rigid structure is generally considered not to cause serious loss of excited state energy through non-radiative transitions.Moreover, the three benzene rings are connected by saturated carbon, and the conjugation between the benzene rings is mainly weak homoconjugation, which has little effect on the frontier orbital of the luminescent molecule, but does not have a serious adverse effect on the carrier transport. In addition, asymmetric polysubstituted triptycene derivatives have stable intrinsic chirality, which is convenient for the development of new chiral luminescent materials. In this context, triptycene group has been widely used in organic electroluminescent host materials, electron transport materials and luminescent materials, and has achieved good results.

In 2010, Zheng Jianhong et al. Introduced diphenyl phosphoryl into the three benzene rings of triptycene to synthesize the host material TPOTP (Fig. 2) by using the small conjugation of triptycene group, and its triplet energy level was E_T=3.15 eV.Compare with that mCP（E_T=2.70 eV） of the commercial conventional blue light host material, the invention is beneficial to the transfer of energy from the host material to the guest material, and meanwhile, the glass transition temperature is very high and （T_g=192℃）, which is far higher than the mCP（T_g=65℃）, and can effectively prolong the service life of the device^[7]. When it is used as the host material of FIrpic device, the maximum external quantum efficiency （EQE_max） of the device can reach 16.9%, which is excellent, but the lifetime of the device is not evaluated. In 2021, Chou et al. Prepared the electron transport material TPP by introducing a pyridine-phenyl group into the triptycene skeleton^[8]. The material has excellent thermal stability, the thermal weight loss temperature （T_d） of 5 wt% is as high as 443 ° C, while the T_g can reach 145 ° C, which exceeds the T_g（79℃） of the commercial electron transport material TmPyPB, and the electron fluidity （μ_e） is 1.5×10^-5, which is in the same order of magnitude as TmPyPB. Although the turn-on voltage (4. 2 V) of the device fabricated with TPP is lower than that of TmPyPB (3. 4 V), its efficiency is improved by 36%, and its high temperature stability is significantly improved. In these materials, the triptycene skeleton is excellent in improving the thermal stability of the material and inhibiting the concentration quenching, but the bridge carbon is an electron-donating group, which has a negative impact on increasing the energy gap and improving the electron mobility, and needs to be overcome by taking corresponding measures in molecular design, such as introducing more or stronger electron-deficient groups.

In 2015, Biegger et al. Combined a benzene ring of triptycene with a pyrazine derivative, and realized the adjustment from blue light to red light by changing the number of benzene rings fused with pyrazine^[9]. Among them, 2a (fig. 3) has the highest luminous efficiency, reaching 0.76 in dichloromethane solution, while the compound without triptycene parent structure does not emit light, and electroluminescence has been successfully achieved with this compound. Single crystal structure analysis shows that the introduction of triptycene structure effectively suppresses the intermolecular stacking and concentration quenching, thus significantly improving the luminous efficiency, so this strategy is universal for fluorescent materials.

In 2020, Amro et al. Introduced the triptycene group into tetraphenylsilole and synthesized the compound Tp-DMTPS (Fig. 4), which maintained the aggregation-induced luminescence characteristics of tetraphenylsilole and showed reversible force-induced fluorescence discoloration behavior^[10]. The mechanochromism is a very unique blue shift of the emission wavelength, which is analyzed in the literature to be caused by the enhancement of intermolecular C-H ⋯ π interaction by grinding. The compound shows a high luminous efficiency of 43% in a pure solid state, and the device efficiency is improved by 35 times compared with the parent compound when the compound is used for organic electroluminescence. It is pointed out that the introduction of triptycene reduces the charge transfer of the material from three dimensions to two dimensions, and increases the hole transport ability while having little effect on the electron transport, so this introduction is fruitful.

TADF materials emit light through charge transfer between donor and acceptor, which is usually prone to concentration quenching due to its long lifetime. The unique and stable three-dimensional structure of triptycene skeleton can endow TADF materials with rich designability, high thermal stability, low concentration quenching rate and good film-forming properties, so it is widely used in various TADF material molecules. In addition, triptycene can be designed as a stable chiral center, which also provides a new platform for the development of chiral TADF materials.

The three benzene rings of triptycene are connected by saturated carbon, which has been proved to have Homoconjugation effect and is an excellent model molecule for studying this effect^[11]. In 2015, Swager et al. Constructed donors and acceptors on different aromatic rings of the triptycene skeleton to synthesize compounds TPA-QNX(CN)₂（ Fig. 5)^[12]. Different from the common TADF materials which reduce the ΔE_ST by increasing the bond angle, the compound reduces the electron cloud overlap between the donor and acceptor through the homoconjugation effect, thereby reducing the ΔE_ST. The first triptycene TADF material is synthesized, and the EQE_max of the yellow light device is 9.4%, which provides a new way to realize TADF luminescence. In 2019, the group used a similar strategy to replace the receptor with thiadiazoline, and the synthesized compound T1 also had TADF properties, with a photoluminescence efficiency of 73.0% in solution, but its electroluminescent performance was not reported^[13]. In 2016, Gao et al. Proved by theoretical calculation that the ΔE_ST（ can be effectively reduced (from 0.076 eV to 0.033 eV) and the radiative transition rate can be enhanced (from 1.35×10⁵ to 9.94×10⁵）) by properly extending the conjugation length of the donor of these compounds^[14]. This strategy is difficult to prepare high-performance TADF materials because of the small overlap between the donor/acceptor and the difficulty of electron transfer.

In order to overcome the above problems, many research groups have considered using only the three-dimensional structural characteristics of triptycene to connect both the donor and the acceptor with the triptycene skeleton, but to construct TADF materials through charge transfer other than homoconjugation effect. In 2021, Zhang Xiaohong's group introduced donor and acceptor at positions 9 and 10 of triptycene, respectively, to synthesize the compound t-BuDMAC-TPE-TRZ^[15]. It is found that the molecule has intramolecular and intermolecular charge transfer, so the efficiency （EQE_max=10.0%） of the undoped device is much higher than that of the doped device （EQE_max=5.0%）, which provides a new idea for the design of electronic light-emitting devices with low concentration quenching. In 2020, the Kaji research team reported a new triptycene-based compound, TpAT-tFFO, by connecting the donor and acceptor units at positions 1 and 8 of triptycene, respectively, to achieve a "tilted" face-to-face arrangement of donor and acceptor segments at an optimized distance, resulting in a strong space charge transfer phenomenon^[16]. TpAT-tFFO shows a very fast inverse intersystem crossing （K_RISC=1.2×10⁷s^-1） as well as a very small ΔE_ST（0.019 eV） and negligible concentration quenching. The EQE_max of the device with 25 vol% doping is 19.2%, and the performance of the device is significantly improved by using optical coupling technology, the EQE_max reaches 29.0%, and the EQE still maintains 19.7% at a high brightness of 20000 cd·m^-2. This design idea has good extensibility, and can adjust the luminescent color and properties of materials by adjusting different donors and acceptors, which shows the unique advantages of triptycene skeleton in the design of luminescent materials. In 2022, Lu Canzhong's group connected phenazine and dimethylacridine donors and benzophenone acceptors at the ortho position of the same benzene ring of triptycene to synthesize TADF materials TP-BP-DMAC and TP-BP-PXZ through space and covalent bond charge transfer^[17]. The results show that the performance of TP-BP-DMAC is more excellent, the ΔE_ST is only 6. 7 meV, the luminescence quantum efficiency is as high as 0. 80, its doped device is filled blue emission, and the EQE_max reaches 20. 5%. Multi-resonance luminescent materials have the advantages of narrow-band luminescence and high luminous efficiency, but they are easy to cause concentration quenching due to their large planar structure. In 2023, Mubarok et al. Synthesized a deep blue emitting compound Tp-DABNA by integrating the triptycene structure into the multiple resonance molecular design, and compared it with the similar compounds substituted by tert-butyl^[18]. The results show that the thermal stability （T_d5=528℃） is obviously higher than that of the reference compound （T_d5=419℃）, and the half-wave width is narrower and the horizontal transition dipole ratio is higher, the efficiency （EQE_max=24.8%） of the doped device is also obviously higher than that of the reference compound （EQE_max=19.8%）, the EQE_max of the Superfluorescent device is as high as 28.7%, and the Dexter energy transfer is significantly reduced. These TADF materials with donor/acceptor connected with triptycene have excellent performance and make full use of the advantages of triptycene three-dimensional structure, but their synthesis is difficult and their expansibility is limited.

In order to reduce the difficulty of synthesis, there are also studies on linking the donor or acceptor in TADF molecule with triptycene alone to give full play to the advantages of triptycene group. In 2018, Huang et al. Fused a benzene ring of triptycene into the carbazole structure to form a donor moiety, and then combined with the triazine receptor to synthesize the compound TCZ-TRZ (Fig. 6)^[19]. The results show that compared with the parent compound, the single/triplet energy gap (Highest occupied molecular orbital) is significantly reduced due to the higher dispersion of the Highest occupied molecular orbital (HOMO) and the reduction of the overlap of the frontier molecular orbital, thus realizing the transition from ordinary fluorescent materials to TADF luminescent materials. Its electroluminescent device emits deep blue light with a EQE_max of 10.4%. In 2021, Yang Chuluo's research group fused triptycene and dimethylacridine to synthesize TADF molecule TDMAC-TRZ. The fused triptycene group not only ensures the rigidity of the molecule, but also provides the molecule with enhanced processing ability, morphological stability and inhibited molecular stacking performance^[20]. The EQE_max of the undoped device made of this molecule is as high as 23.0% and the efficiency roll-off is very small, which is the highest efficiency of the undoped TADF device reported at that time, fully demonstrating the advantages of triptycene structure in inhibiting concentration quenching. In 2021, Lu Canzhong's research group introduced the triptycene structure into benzophenone TADF materials to synthesize the compound TCO-DMAC. It was found that the introduction of triptycene effectively inhibited the intermolecular π-π stacking, and induced a large number of intermolecular C-H … … π interactions, resulting in excellent film-forming properties, high solid-state luminous efficiency (0.69) and reversible force-induced fluorescence discoloration behavior^[21]. The performance of the electroluminescent device is also excellent, the EQE_max of a doped device can reach 21.2%, and the EQE_max of an undoped device can reach 15.6%. Our group also studied the analogous compounds TPCOP and ATPCOP with phenoxazine as donor, and the results showed that these materials had obvious aggregation-induced luminescence characteristics, and the EQE_max of the undoped TPCOP device was 13.4%.However, the decrease of ATPCOP to 4.0% is due to the further alkyl modification on triptycene, which reduces the intermolecular interaction force and the carrier transport performance of the device, indicating that the intermolecular interaction has an important impact on the luminescence performance^[22]. In 2022, Lu Canzhong's group fused a benzene ring in the triptycene structure with pyrazine to synthesize a single-arm compound TBQ-DPXZ^[23]. The EQE_max of the material doped device is as high as 25.1%, which is one of the highest values of green TADF devices at that time. In the same year, Montanaro et al. Synthesized three similar compounds whose benzene rings were fused with pyrazine, and proved that the homo-conjugation effect of triptycene structure could enhance the electron transition rate of TADF material, and the EQE_max of its device was 11.9%.Compared with the parent compound without triptycene, the comprehensive performance of the parent compound without triptycene is obviously improved, but compared with the parent compound with a single arm, the effect of reducing the concentration quenching of the tripticene skeleton is reduced, and the channel of non-radiative transition is increased, so the efficiency is lower^[24]. These studies show that as long as the donor or acceptor is fused with the triptycene group alone, the electroluminescent performance of the material can be effectively improved, which is an effective strategy. At present, the luminescence color of reported triptycenyl TADF materials is mainly green or yellow, and photocolor regulation is rarely considered. Future research should focus on the design of efficient blue light materials required by the industry to break through the industry bottleneck.

Recently, the advantages of circularly polarized light in future displays and optoelectronic technologies, such as optical spintronics, optical quantum information and optical data storage, and three-dimensional displays, have attracted much attention^[25,26]. 50% of the light is lost by ordinary light filtering, while the light produced by circularly polarized OLED can be fully utilized, so it is particularly urgent to develop high-efficiency chiral luminescent materials in the context of the gradual maturity of OLED. The chiral center of common chiral luminescent materials is prone to racemization, resulting in a reduction in their service life. If the substituents of the three benzene rings of triptycene are different, the bridge carbon is two chiral carbon atoms. Because triptycene is a stable rigid structure, the inherent chiral tripticene structure is also stable and will not be racemized. For triptycene, a convenient way to achieve inherent chirality is to modify at positions 2 and 6 to obtain a pair of chiral enantiomers. In 2021, Chen Chuanfeng's research group synthesized TADF material TpAC-TRZ (Fig. 7) with chiral triptycene fused acridine as donor from 2,6-triptycene diamine enantiomer. The tripticene skeleton effectively avoids π-π stacking between molecules, so that the material has Aggregation-induced emission (AIE) properties, and the photoluminescence quantum efficiency of pure solid-state molecules is as high as 85.0%^[27]. The material exhibited significant TADF activity with a ΔE_ST of 0.03 eV and a delayed fluorescence lifetime of 1.1 μs. The absolute value of the asymmetry factor of photoluminescence is about 1.9×10^-3. The absolute asymmetry factor of the spin-coated non-doped device is between 1.5×10^-3~2.0×10^-3, and the EQE_max is as high as 25.5%. In the same year, the same triptycene donor was polymerized with diphenyl sulfone or diphenyl ketone monomer to synthesize polymeric TADF materials pTpAcDPS and pTpAcBP^[28]. The ΔE_ST of pTpAcBP is only 0. 01 eV, which shows that pTpAcBP has significant TADF activity.The photoluminescence quantum efficiency of the doped film of its enantiomer is as high as 0. 88/0.92, and the absolute value of the photoluminescence asymmetry factor is about 1.0×10^-3. The EQE_max of the doped spin-coating device can reach 22.1%, which is between the 1.0×10^-3~1.6×10^-3. This work is the first report of a circularly polarized OLED based on a chiral TADF polymer and provides useful and valuable guidance for the development of efficient circularly polarized electroluminescent polymers. In the same year, the same group also synthesized the imide TADF material CTRI-Cz starting from the 2,6-triptycene diamine enantiomer, with a ΔE_ST of 0.2 eV, a delayed fluorescence lifetime of 15.4 μs, a photoluminescence quantum efficiency of the doped film of 0.57 in vacuum, and an absolute value of the asymmetry factor of photoluminescence in air of about 0.9×10^-3[29]. The EQE_max of the doped device is about 15.0%, but the asymmetry factor has not been reported, which may be related to the distance between the chiral center and the luminescence center. In 2022, Zhang et al. Reported triptycenyl chiral TADF materials with carbazole as donor and micyboron as acceptor^[30]. The material exhibits efficient TADF luminescence via intramolecular charge transfer with a photoluminescence quantum efficiency of 0.86 in degassed solvent and a photoluminescence asymmetry factor of approximately 0.4×10^-3 in absolute value. The luminescence of the molecule in solution is very sensitive to temperature, and the luminescence color gradually changes from blue to yellow from low temperature to high temperature. At present, the electroluminescence efficiency of triptycenyl chiral TADF materials is excellent, but there is still a certain gap between the asymmetric factor and the （10^-2） of the highest value range reported at present, but the advantage is also very prominent, that is, the chiral stability is very high.It is almost impossible to racemize the molecule without decomposition, so how to improve the asymmetry factor of the material while maintaining its high device efficiency is the key goal of the follow-up research.

The heavy atom effect of metal ions in heavy metal complexes will make the complexes produce strong spin-orbit coupling, which is conducive to phosphorescence emission, so that the theoretical internal quantum efficiency of electroluminescence can reach 100%. However, the luminescence lifetime of complex phosphorescent materials is long, in the microsecond level, which is prone to concentration quenching and efficiency roll-off under high voltage. Therefore, reducing the intermolecular interaction of complex phosphorescent materials is one of the most important means to improve the electroluminescent properties of complex phosphorescent materials.

In 2019, our research group synthesized two cyclometalated ligands containing triptycene groups by connecting benzimidazole and benzothiazole at the No.1 position of triptycene, and prepared bicyclometalated iridium complexes Ir1 and Ir2 (Fig. 8)^[31,32]. The results show that these complexes with triptycene sterically hindered group have better thermal stability than the parent complex, and the photoluminescence and electroluminescence efficiencies are significantly increased by more than 47% and 31%, respectively, and the efficiency roll-off of the electroluminescent device is significantly reduced. It is worth pointing out that this study is the first report on the effect of triptycene group on the electroluminescent properties of iridium complexes.

Triptycene is an electron-rich group, and the heterocyclic fusion of triptycene and cyclometalated ligand can not only provide huge steric hindrance, but also improve the coordination ability of the ligand. Therefore, in 2023, our research group fused triptyciene with pyridazine and phthalazine to prepare tricyclic iridium complexes Ir3, Ir4 and Ir5^[33,34]. The results show that although the photoluminescence quantum efficiency of these complexes is above 80%, there is still a π-π stacking interaction between Ir3 and Ir4 molecules, which leads to severe concentration quenching, while there is no π-π stacking interaction between Ir5 molecules. Because of this feature, the EQE_max of the orange-red photodoped device based on Ir5 reaches 27.5%, and the EQE_max of the undoped device can also exceed 5.0%. This is the highest efficiency of the current phthalazine iridium complex device.

Compared with bidentate iridium complexes, ditridentate iridium complexes have more rigid ligand structure, so their stability and luminescence properties are stronger, but because of the larger conjugation of their ligands, the intermolecular force is stronger, and more effective steric groups are needed to suppress the intermolecular force. In 2023, our group introduced the triptycene group as a steric hindrance group into the bis-tridentate iridium complex to synthesize the yellow-green iridium complex Ir6^[35]. The EQE_max of the complex-doped device reaches 24.0%, which is 50% higher than that of the reference complex, while the efficiency of the undoped device is as high as 7.4%, which is 300% higher than that of the reference complex, indicating the great advantage of the triptycene group in suppressing concentration quenching.

At present, the research on triptycenyl electroluminescent complexes is still in its infancy, and the related research only focuses on the modification of iridium complexes. The synthesis of ligands is difficult, and the triptycene group has an impact on the coordination ability and coordination sites of ligands, so the related research is full of challenges, but because the triptycene group has a significant role in regulating the intermolecular forces and photoelectric properties of complexes, there is still a broad research space and great research potential in this direction.

At present, OLED has been well developed in the field of display, successfully commercialized, and is gradually becoming the mainstream technology. The main problems that have plagued OLED for a long time are short service life and high cost. With the development of pure organic light-emitting materials, the cost of materials has been reduced, but there is still much room for improvement in service life. After the triptycene group is fused into the organic electroluminescent functional material, the performance of the organic electroluminescent functional material can be improved in a plurality of aspects: in the aspects of host materials and electron transport materials, the thermal stability of the material can be obviously improved,In particular, increasing the glass transition temperature of the material, which is a crucial property for practical applications, can extend the lifetime of OLED devices. However, due to the contradiction between the electron-rich characteristics of triptycene and the highly electron-deficient characteristics of electron transport materials, the performance improvement effect is limited, and the host-guest potential of triptycene electron-rich cavity has not yet been applied to the design of host materials, so the design and application of related materials need to be further optimized; In the aspect of luminescent materials, the three-dimensional structure of triptycene can reduce the interaction between molecules, thus effectively reducing the concentration quenching and the influence of the concentration of luminescent molecules on the wavelength and efficiency of the device. Materials with high undoped device performance can even simplify the device structure and reduce the manufacturing cost. Chiral triptycenyl luminescent materials have super chirality retention ability, and racemization will not occur before the decomposition of molecules, thus endowing chiral devices with long-term stability. However, the asymmetry factor of triptycenyl chiral materials is relatively low at present, and how to improve the asymmetry factor is the main goal of future research. In general, triptycenyl electroluminescent materials have achieved good research results, and the triptycenyl group performs well in regulating intermolecular forces, which makes the materials have excellent performance, but there are still some problems. The main problem is that the synthesis and separation of triptycene derivatives are difficult, for example, a variety of isomers are formed when there are many substitutions, the triptycene skeleton is easily destroyed under strong acidic conditions, the variety of commercial raw materials is small, and the price is high.As a result, the preparation cost of materials remains high, which greatly hinders their industrialization. Therefore, the research on efficient, simple and low-cost synthesis methods is the biggest challenge faced by pteridine-based electroluminescent materials. In view of the excellent performance of triptycene group in electroluminescent materials, it is believed that triptycene group will play a more important role in OLED and has broad development prospects.