NIR TADF emitters and OLEDs: challenges, progress, and perspectives

Near-infrared (NIR) light-emitting materials show excellent potential applications in the fields of military technology, bioimaging, optical communication, organic light-emitting diodes (OLEDs), etc. Recently, thermally activated delayed fluorescence (TADF) emitters have made historic developments in the field of OLEDs. These metal-free materials are more attractive because of efficient reverse intersystem crossing processes which result in promising high efficiencies in OLEDs. However, the development of NIR TADF emitters has progressed at a relatively slower pace which could be ascribed to the difficult promotion of external quantum efficiencies. Thus, increasing attention has been paid to NIR TADF emitters. In this review, the recent progress of NIR TADF emitters has been summarized along with their molecular design strategies and photophysical properties, as well as electroluminescence performance data of their OLEDs, respectively.


Introduction
Infrared light is dened as one kind of electromagnetic radiation whose wavelength is between those of visible light and microwaves. Within this range, the near-infrared (NIR) region typically spans from the longest wavelength of red light (680 nm) to 2500 nm. 1 In recent years, NIR materials have attracted great attention in organic light-emitting diodes (OLEDs), 2,3 solar cells, 4-6 bioimaging, 7-10 information storage, 11,12 and phototherapy devices. 13 As one of the most important applications of NIR luminescent materials, NIR OLEDs play important roles not only in the elds of night-vision devices, optical communication and information-secured displays, but also in the elds of at panel displays and lighting xtures. [14][15][16][17][18] Many types of emitters have been adopted in NIR OLEDs, such as metal complexes, conjugated polymers, phosphorescent metal complexes, etc. 19 These materials endow NIR OLEDs with excellent features such as light weight, low power consumption, fast response time, good processing performance, wide temperature range, low cost and so on. However, conventional NIR OLEDs usually suffer from inferior efficiencies due to the low exciton utilization. In recent years, researchers have successfully developed NIR thermally activated delayed uorescence (NIR TADF) materials, which are able to make full use of excitons and theoretically achieve 100% internal quantum efficiency (IQE). Therefore, the research on NIR TADF materials has become a hot topic nowadays.
TADF is not a new concept. It was rst reported in 1929 by Perrin et al. 20 Since then, TADF emitters have been made commercially available by many industrial companies, such as Kyulux and CYNORA. 21 Chihaya Adachi utilized the TADF mechanism to build an efficient OLED in 2012. 22 Unlike traditional uorescent and phosphorescent materials, TADF emitters oen have a narrow enough energy split (DE ST ) between the excited singlet (S 1 ) and triplet (T 1 ) states to allow reverse intersystem crossing (RISC) processes at ambient temperature ( Fig. 1). Thus, remarkable high external quantum efficiencies (EQEs) which are comparable to those of advanced phosphorescent emitters could also be realized. 23 However, designing NIR delayed uorescence materials is more difficult compared with designing other TADF emitters in the visible region due to the requirement of a relatively small energy gap.
In this review, we summarize the recent progress of NIR TADF emitters and their applications in OLEDs. The molecular design strategy of NIR TADF emitters, the relationship between molecular structures and photophysical properties, and device            performances are comprehensively discussed in this review. The photophysical properties of these TADF materials are summarized in Table 1 and the electroluminescence (EL) characteristics of the corresponding OLEDs are listed in Table  2. Moreover, the prospects of the TADF molecular design strategy are discussed to optimize the photophysical properties of TADF emitters and the performances of OLED devices. We believe that this review will have a benecial impact on the design of a variety of NIR TADF emitters and OLEDs, which will arouse more research interest in this eld.

Approaches to constructing NIR TADF emitters
According to spin quantum theory, the ratio of singlet and triplet excitons is 1 : 3 during the electrical excitation processes. As a result, traditional uorescent materials can only use the radiation energy from the S 1 state with 25% internal quantum efficiency. This also means that the remaining 75% of energy from triplet excitons is lost. Therefore, it is important to acquire more triplet energy to improve the OLED efficiencies. For this purpose, heavy-metal atoms are usually introduced to enhance spin-orbit coupling for promoting the RISC process to obtain higher exciton utilization. In this way, both triplet and singlet excitons can be harvested, which makes it possible for OLEDs to theoretically achieve 100% IQE. Nevertheless, heavy metals are rare and expensive. Researchers tried to develop new strategies to utilize triplet excitons, such as triplet-triplet annihilation, hybridized local and charge-transfer states, and TADF. In order to achieve TADF, researchers oen design molecules with twisted structures to reduce the overlapping between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In addition, bulky and twisted structures could restrict the conjugation which could result in an efficient intramolecular charge transfer (ICT) process. 24 In general, narrowing the bandgap, i.e., reducing the gap between the LUMO and HOMO energy levels, is one of the basic molecular design principles for donor-acceptor (D-A) type organic TADF materials. It has been proved to be an effective way to tune molecular energy levels because bandgaps could be easily adjusted by changing the electron density on the donor and acceptor units. 25 Considering that an appropriate D and A could minimize the singlet-triplet energy gap DE ST , it is essential to achieve efficient reverse ISC (RISC) processes. Meanwhile, the highly twisted structure required for small DE ST is not benecial for red-shiing the emission. Thus, it is important to rst explore acceptor moieties with strong electron-withdrawing ability. According to our review, rigid and planar fused heterocycles have been widely used for acceptors. Both DE ST and uorescence efficiencies should be taken into consideration. However, luminescence efficiency is oen low due to the small coupling between S 1 and S 0 from the separated HOMO and LUMO. But once the high probability of coupling of vibrational manifolds between the ground states and the excited states with the red-shied emission wavelengths exists, the non-radiative transition rates would be increased rapidly. 26 Therefore, the inherent contradiction between small DE ST and high uorescence efficiency results in a dilemma in designing NIR TADF molecules.
In addition to the general design rules of TADF emitters, employing rigid donor or acceptor structures is preferred for higher EQEs by suppressing non-radiative transition. Rigid groups can efficiently avoid molecular motions by freezing the molecular structures. Additionally, the rigid moieties could narrow the emission spectra, which also increases the EQE of the NIR TADF emitters. In the reported NIR TADF emitters, only a few D moieties are employed, such as triphenylamine, carbazole, phenoxazine and their derivatives, while there are a large variety of A groups that can be selected to manipulate the molecular properties, which has also become a hotspot nowadays. In general, acceptor groups for NIR TADF molecules have focused on pyrazine derivatives, boron-nitrogen compounds, diuoroboron compounds and so on. The development of new acceptor materials is benecial for improving device performance and broadening the application of NIR TADF materials. Thus, the development of new functionalized acceptor units with simple structures is still imperative.
Mostly, NIR OLEDs based on TADF emitters usually employ doping strategies to minimize possible annihilation problems of the emitters during the light-emitting process. However, doped devices are prone to phase separation and crystallization during the fabrication process, which affects the stability of the device and leads to a frustrating low energy utilization rate. Moreover, the multi-layer structure increases the difficulty in the fabrication process. 27,28 Thus, researchers are trying to develop high-efficiency light-emitting materials for non-doped OLED luminescent layers. For this purpose, light-emitting molecules must be well designed, which means that they should have a subtle conguration as well as the optimal optoelectronic properties. Firstly, the packing form of the molecules in the aggregation state can effectively inhibit emission quenching to achieve high-efficiency carrier transport. Secondly, the radiative transition rate should have an overwhelming advantage over non-radiation to achieve high photoluminescence quantum yield (PLQY). Moreover, rapid charge recombination and exciton radiation are benecial for avoiding exciton accumulation. 29 Therefore, we also introduced some non-doped OLED luminescent materials in this review.

NIR TADF emitters and their OLEDs
As mentioned above, it is important to get a small singlet energy level to realize NIR emission in D-A type TADF emitters. The degree of charge transfer (CT) character is decided by donor and acceptor moieties. Therefore, either strong electron-donating or electron-withdrawing groups are required to realize NIR emission. It was found that pyrazine derivatives can act as excellent acceptors for NIR TADF molecules according to Schemes 1 and 2. Based on the analysis of existing studies, it can be concluded that many of the electron donors of NIR TADF molecules based on pyrazine are triphenylamine and its derivatives. To facilitate the comparison of their photophysical properties, we divided them into derivatives with multiple donors (Scheme 1) and those with a single donor (Scheme 2).

Pyrazine derivatives with multiple donors
The rst NIR TADF molecule, TPA-DCPP (Scheme 1), was reported by Wang et al. in 2015. A pyrazine derivative with an extended conjugation structure, 2,3-dicyanopyrazino phenanthrene (DCPP), was chosen as the electron-withdrawing moiety. An effective HOMO-LUMO separation and partial orbital overlaps deriving from the D-p-A-p-D conguration ensure DE ST as small as 0.13 eV and a large k f value of 9.0 Â 10 À7 s À1 for TPA-DCPP. It showed a broad NIR emission with the maximum at 708 nm and a remarkable F PL value of 14% in the neat lm. The non-doped OLED device based on TPA-DCPP exhibited a maximum EQE of 2.1% and an EL peak at 710 nm with CIE coordinates of (0.70, 0.29) (as shown in Fig. 1(a) and (b)). 30 In addition to typical triphenylamine and diphenylamine, pyrazine-acceptor based NIR TADF materials could also employ carbazole (Cz) derivatives and phenoxazine (POZ) as donors. A new acceptor unit core, dibenzo-[a,j]phenazine (DBPHZ), was designed in 2016 and one of its derivatives, POZ-DBPHZ (Scheme 1), was constructed by an oxidative skeletal rearrangement of 1,1 0 -binaphthalene-2,2 0 -diamines. POZ-DBPOHZ showed a small energy gap of 0.02 eV as the effective HOMO/ LUMO separation. It was found that the exciplex formed with m-MTDATA (host material) and POZ-DBPHZ showed NIR emission with the emission wavelength at efficient TADF by triplet state coupling of acceptors. 31 To improve both the efficiency and color purity of NIR TADF OLEDs, an acenaphtho[1,2-b]pyrazine-8,9-dicarbonitrile acceptor (APDC) core was combined with two diphenylamine donor units to design the wedge-shaped D-p-A-p-D emitter APDC-DTPA (Scheme 1) in 2017. The new acceptor APDC exhibited strong electron-withdrawing ability due to the formation of the aromatic cyclopentadienide structure with six p-electrons in the central uoranthene core. The non-doped device based on APDC-DTPA exhibited NIR emission with a maximum of 777 nm and a high EQE of 2.19%. In addition, 10 wt% and 20 wt% doped devices showed excellent EQEs of 10.19% (emission peak at 693 nm) and 9.70% (emission peak at 696 nm), respectively. 32 Based on DCPP, Wang et al. developed DBPzDCN by extending the p-conjugated length and increasing the electronwithdrawing ability. Due to the relatively small twist angle, the orbital distribution of DPA-Ph-DBPzDCN (Scheme 1) has a partial orbital overlap of the HOMO/LUMO on the DBPzDCN acceptor, leading to a high oscillator strength of 0.0744, which would boost the radiative uorescence rate.
The reducing LUMO indicated that extending the p-conjugation led to an increase in electron-withdrawing ability. Thus, DPA-Ph-DBPzDCN was regulated to a redshi emission with the maximum of 765 nm in the non-doped thin lm. Notably, the doped OLED (Fig. 4(a)) device achieved an emission with the maximum at 698 nm and CIE coordinates of (0.68, 0.30), as well as a decent EQE of 7.68%. 33 Liao and coworkers developed a novel TADF emitter in which a new acceptor, dibenzo[a,c]phenazine-3,6-dicarbonitrile (PZCN), was used. The large and planar backbone of the emitter was designed for suppressing the nonradiative transition. TPA-PZCN (as shown in Scheme 2) showed a high F PL of 97% and an effective RISC process. The corresponding nondoped device exhibited an NIR emission peaking at 680 nm and a high EQE of 5.3%. 34 Yang et al. reported two typical TADF emitters by utilizing multiple Cz derivatives as electron donors in 2019, named HATNA-tCz and HATNA-tPCz, respectively (Scheme 1). 5,6,11,12,17,18-Hexaazatrinaphthylene (HATNA) was regarded as a very important core for TADF receptors among pyrazine derivatives too. It was constructed with a large and rigid pconjugated structure ensuring strong electron-withdrawing ability. The introduction of a peripheral tert-butyl unit provided great solubility with promising applications in solution-processed OLEDs. Doped devices based on HATNA-tCz and HATNA-tPCz ( Fig. 4(b)) displayed EL emissions with the maxima of 682 and 692 nm, respectively. 35 This work also provided an easy method with low cost to construct efficient solution-processable NIR TADF emitters.
To further enhance molecular rigidity and planarity, four new TADF molecules with a strong electron-withdrawing pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (DCPPr) core and electron-donating triarylamine (Ar 3 N) moieties, including N,N-diphenylnaphthalen-1-amine (a-NDPA), N,Ndiphenylnaphthalen-2-amine (b-NDPA), triphenylamine (TPA) and N,N-di([1,1 0 -biphenyl]-4-yl)phenylamine (DBPPA), were reported. The nitrogen atoms of DCPPr in pyridine rings were able to form hydrogen bonds with the adjacent phenyl rings of Ar 3 N, which largely improved the emission efficiency and horizontal orientation. All of the four new TADF molecules showed NIR emissions (692-710 nm) in neat lms. In addition, highperformance non-doped OLEDs with NIR light were attained based on these molecules. 36 Researchers also paid attention to the roll-off problems of EQE in NIR-OLEDs besides high EQE. More C^N groups were used to construct a new receptor named PZTCN, based on which TPA-PZTCN (Scheme 1), with a well suppressed roll-off behavior of EQE (EQE > 10% at 1 mA cm À2 ), exhibited intense NIR EL (EQE max : 13.4% AE 0.8%) at a peak wavelength of 734 nm by harvesting triplet excitons as delayed uorescence. It was outstanding compared with the previously reported NIR-TADF-OLEDs emitting beyond 700 nm (EQE < 6% at 1 mA cm À2 ), which could be attributed to the small RISC rate constant. TPA-PZTCN can achieve a deeper NIR uorophore to achieve a peak wavelength of approximately 900 nm, resulting in an EQE of over 1% in a TADF-sensitized NIR OLED with high operational device durability. 37 Nowadays, ultra-high-denition displays are treated as a key factor to drive the growth of the global 4 K technology market. TADF emitters also contribute to this eld. Very recently, an NIR TADF emitter, pCNQ-TPA with a quasi-planar structure (Scheme 1), showed great potential for applications in the eld of ultrahigh-denition displays. The pCNQ-TPA-based OLED showed desirable CIE coordinates of (0.69, 0.31) and the record maximum EQE of 30.3%, and is the best red TADF device with Rec. 2020 gamut for UHDTV (as shown in Fig. 2(c)). Besides, by tuning the doping concentration of pCNQ-TPA, the NIR device reached an emission peaking at 690 nm and a high PLQY of 90% ( Fig. 2(d)). The quasi-planar structure of pCNQ-TPA contributed to increasing the light out-coupling ratio to 0.34 for achieving high efficiency of devices. 38 CNPP was developed as a strong electron-withdrawing group by providing a lower LUMO. CNPP-TPA (Scheme 1) could form intramolecular hydrogen bonds between the introduced nitrogen atoms and adjacent phenyls. The conguration derived from a dihedral angle of 13 effectively enhances structural rigidity to improve the transition probability of the 1 CT state. According to the data in Tables 1 and 2, CNPP-TPA also became a promising NIR TADF emitter. 39 Very recently, Wang et al. developed a strategy to strengthen the electron-withdrawing capability and reduce the energy gap by attaching three AQ cyano groups. The resulting NIR emitter AQTC-DTPA (Scheme 1) exhibited an emission over 800 nm. The twisted conguration of D and A in AQTC-DTPA inhibited intramolecular electronic communication, but this was made up for by the strong intermolecular border orbital coupling, which was very successful in red-shiing the emission. TADF OLEDs based on AQTC-DTPA showed a record high EQE of 0.51%/0.41%/0.30%/0.23% with the EL peak wavelength at 810/ 828/852/894 nm, respectively. Increasing the thickness of the light-emitting layer apparently generated a red shi in emission and a higher efficiency of the EL device. For instance, a high EQE of 0.22% was achieved for 40-AQTC-DTPA (40 nm) with the EL peak at 910 nm and radiance of 961 mW Sr À1 m À2 , and an EQE of 0.17% was achieved at 908 nm for 30-AQTC-DTPA (30 nm). 40

Pyrazine derivatives with a single donor
Under the same conditions with the electron donor, powerful electron-withdrawing cyanide groups could efficiently minimize the energy gap of molecules and shi the emission wavelength to the NIR region to some extent. Besides, the non-radiative transition was suppressed by the strong intermolecular interactions among cyanide groups, which could induce a rigid microenvironment. The strategy of using intermolecular interactions to build a TADF channel was applied in some classic NIR TADF emitters with a single donor also exhibited great properties. Quinoxaline-6,7-dicarbonitrile (QCN) was also a typical acceptor for the NIR TADF emitter. By attaching the triphenylamine group to QCN, the TPA-QCN (Scheme 2) shows sufficient D-A separation and relatively high F PL with an emission maximum of 733 nm in the neat lm. 41 Phenanthro [4,5-abc]phenazine-11,12-dicarbonitrile (PPDCN) has been employed as an NIR TADF acceptor. The rigid planar framework of the PPDCN acceptor showed a benecial effect on reducing the non-radiative processes and inducing high photoluminescence (PL) efficiency F PL . In addition, C^N groups could endow the LUMO with a lower energy level which was advantageous for realizing long wavelength emission. TPA-PPDCN (Scheme 2) doped lms showed strong deep-red/NIR emission with F PL of 73-87%. TPA-PPDCN doped devices (Fig. 4(c)) achieved an EQE of 16.4% with NIR emission peaking at 692 nm. 42 By using the strong electron-donating triphenylamine (TPA), TPAAP (Scheme 2) formed J-aggregates due to its strong intermolecular charge-transfer properties ( Fig. 3(a) and (b)). It was conrmed that intermolecular charge transfer could stabilize excited states, reduce the non-radiative decay rate, and further induce highly efficient TADF even in the NIR region according to the experimental and theoretical investigations. In Jaggregates, the splitting of the original singlet exciton states could generate lower-energy and dipole-allowed exciton states with larger transition dipole moments. At the same time, pristine triplet exciton states are almost stable because of the vanishingly small transition dipole moments, thereby resulting in a signicant reduction of DE ST . 43 The structure based on AP was further developed. A new NIR TADF emitter, CAT-1 (Scheme 2), with an additional C^N group provided a low LUMO with a phenyl spacer-acceptor dihedral  angle of 61 (Fig. 3(c)), which ensured a narrow calculated DE ST of 0.11 eV which also greatly enhanced the electronwithdrawing ability. Therefore, the maximum emission wavelength of CAT-1 in the thin-lm state was 887 nm. Moreover, the PL emission was further red-shied with increasing doping ratio. The fabricated NIR device displayed impressive EL with the l max value beyond 900 nm (Fig. 3(d)). 44 To enlarge the conjugate of the TPAAP moiety, TPAAQ (Scheme 2) with stronger electron-withdrawing ability and high rigidity was investigated as a building block for NIR TADF materials. The neat lms of TPAAQ showed NIR emission with a maximum of 716 nm and F PL values of 16.3 AE 1.6%. For the 1 wt% dopant lm in TPBi, the transient PL spectra demonstrated double-exponential decays and the lifetime of delayed uorescence was 35.7 ms. Through a rational molecular design strategy, J-aggregates with strong ICT character were found in the solid state of TPAAQ. J-aggregates oen contain Frenkel excitons and CT excitons which could reduce DE ST and further enhance TADF by promoting radiative transition. In thin-lm states, TPAAQ (doped in TPBi) revealed a remarkable luminescence quantum efficiency of nearly 100%. 43 Apart from the zygomorphic structured pyrazine derivatives, some asymmetric structures with larger conjugated pyrazine derivatives, such as 11,12-di(pyridin-3-yl)dibenzo[a,c]phenazine-3-yl(m-DPBPZ), were also developed as NIR TADF emitters. Phenoxazine (PXZ) was used as the electron donor to optimize the balance between molecular rigidity and intermolecular stacking. Although m-DPBPZ contains rotatable pyridines, which could probably decrease the molecular rigidity, they can suppress molecular p-p packing and thus enhance the emission properties in the non-doped lms. As a result, m-DPBPZ TADF emitters (Scheme 1) have an extremely small DE ST of 0.04 eV. At the same time, mDPBPZ-PXZ (Scheme 2) showed slightly reduced efficiency with a F PL of 95 AE 1.3%, and an EQE of 21.7% in the doped OLED. Besides, the EL wavelength of nondoped OLEDs based on m-DPBPZ red-shied to the NIR region with peaks at 680 nm with noteworthy high efficiency with a maximum EQE of 5.2% at corresponding CIE coordinates of (0.68, 0.32). 45 According to the above studies, aromatic fused rings with an extended conjugation structure have been demonstrated to be a good choice to decrease the band gap into the NIR region. Based on this, DCPA was developed by replacing the naphthalene moiety with the classic anthracene chromophore as the acceptor core. In this way, DCPA could also keep the synergistic electron-withdrawing effect of the cyano group with the adjacent aceanthryleno [1,2-b]pyrazine. Asymmetry of the molecular structure was further increased by the introduction of an anthracene core, which contributed to the X-aggregate packing mode in the DCPA-TPA crystal. Moreover, the non-doped devices based on DCPA-TPA and DCPA-BBPA (Scheme 2) showed NIR emission with emission peaks at 838 and 916 nm and maximum EQEs of 0.58% and 0.07%. 46 Charge-transfer aggregation could signicantly stabilize exciton states and reduce the non-radiative decay process ratio; it also induced strong TADF for desirable NIR-II emission from a non-TADF molecule. So, it was regarded as a good strategy to design NIR TADF emitters. TPAAZ (Scheme 2) displayed a gradually red-shied emission (717 nm to 1010 nm for 1 wt% doped lms) in thin-lm with the rise of the doping concentration (Fig. 3(c)). The EL spectrum of a non-doped OLED (Fig. 4(d)) is shown in Fig. 3(d). 47 dpTPAAZ (Scheme 2) with extended conjugation also exhibited remarkable NIR TADF performance. By employing the electron receptor pyrazine on the receptor of dpTPAAP, the emitter not only enhances the ICT process, but also simultaneously makes NIR emission at the single-molecule level come true. 48 dpTPA, with additional phenyl groups compared to TPA, was considered to benecially increase the delocalization of the HOMO, which results in red-shied emission with a high PLQY. In dpTPA, an intermolecular charge-transfer aggregate (CTA) was employed to promote nonadiabatic coupling suppression, which seemed to be a feasible and innovative strategy to realize more high-efficiency NIR emission. The CTA is typically formed by intermolecular CT in the excited state, which ensured excellent photophysical performance and stabilized excited-state energy of the NIR TADF emitter (Fig. 2(c) and (d)). All these factors make the strong TADF performance in dpTPAAP achieve highly efficient NIR EL.
Liao et al. developed a spiro-type electron-donating moiety N,N-diphenyl-9,9 0 -spirobi[uorene]-2-amine (SDPA). Beneting from the strong electron-donating strength of SDPA, SDPA-APDC (Scheme 2) exhibited a narrower bandgap. The highly rigid structure endows SDPA-APDC (Scheme 2) with remarkable thermal stability. The PLQY of SDPA-APDC in the thin lm could reach 80.5%. The corresponding EL devices (Fig. 4(e)) exhibited an NIR emission peak at 782 nm with a maximum EQE of 2.55%. 49 Recently, designing novel NIR TADF molecules by theoretical calculation has become an effective strategy. Lin et al. reported several acceptors by changing the position of the cyano group or introducing the phenanthroline into CNBPz. Among them, 44 molecules were selected and studied theoretically. Moreover, the emission spectra of DA-oCNPPz, DA-pCNBPz, and DA-pCNPPz in toluene were simulated in the NIR region. Accordingly, DA-pCNPPz (Scheme 2) was demonstrated to exhibit the highest uorescence efficiency of 20% among reported NIR TADF molecules, with a deep red emission beyond 700 nm in toluene solution. 50 In 2021, introduction of the cyano group was once again proved to be an effective strategy to rationally regulate the emission wavelengths of TADF emitters to the NIR region (such as CN-TPA, Scheme 2). Devices based on CN-TPA exhibited a leading EQE max of 18.41% at 688 nm and 15.05% at 698 nm. 51 This work provided new insights on the design of efficient NIR TADF emitters.
By investigating the intrinsic inuence of the isotope effect on the luminescence efficiency of TADF emitters, deuterated donors TPA, TPAAP-D and TPAAQ-D (Scheme 2) were developed. As expected, deuteration greatly reduced the non-radiation internal conversion rate, resulting in a signicant increase in the PLQY of TADF molecules. Moreover, TPAAP-D achieved an outstanding record with the EL peak at 760 nm (FWHM ¼ 45 nm) and EQE of 2.8%, and the corresponding device structure is shown in Fig. 4(f). 52 By employing the new electron donor based on tetraphenylethylene (TPE) triphenylamine, CAT-TPE (Scheme 2) realized good solubility and aggregation induced emission behavior. As the rst solution-processable NIR TADF molecule based on a fused polycyclic aromatic electron acceptor, TPE-CAT provided a new horizon for NIR TADF emitters. 53 The combined effects of solvatochromism and aggregation increased the concentration of CAT-TPE in TPBi and the PL could be red-shied. The PL of a pristine drop-cast lm was even further red-shied close to 1 mm (l max ¼ 961 nm).
Researchers adopt some typical congurations as we mentioned above. Aside from those emitters, a novel D-A 1 -A 2 -A 3 conguration has also been developed recently. To enhance the electron-withdrawing ability, the acceptor was incorporated by three types of sub-acceptor units. Moreover, according to the experiment results, TPA-CN-N4-2PY (Scheme 2) provided an extended p-backbone, which inuenced the EL emission and improved the horizontal ratio of the emitting dipole orientation at the same time. 54

Others
There are some other excellent electron acceptors without pyrazine also worth discussing due to their outstanding properties as listed in Scheme 3.
Poly(dendrimer)s have provided a new strategy for the development of efficient solution-processed OLEDs in recent years. 57 Sun and coworkers designed and synthesized a self-host TADF dendrimer for a solution-processed non-doped NIR OLED. In the D-A type dendrimer, the electron donating groups and electron-withdrawing groups combined well with exible alkyl chains, which can effectively reduce the intermolecular interaction between emitting centers and ensure stable performances in solution-processed OLED devices. It was shown that the spin-coated organic EL device with MPPA-MCBP (Scheme 3) as the NIR emitter featured the highest EQE of 0.62% (emission maximum at 698 nm). This work showed that the design of an autonomous TADF dendrimer with a bipolar tree structure might be a promising strategic solution to improve EL performance. 55 Data and coworkers developed TPA-cNDI (Scheme 3) as an efficient NIR TADF emitter. Due to the linking phenylene ring being in a non-planar conguration in the solution state, charge transfer processes between TPA and cNDI are promisingly enhanced in a more planar, conjugated geometry in the aggregation state. NIR OLED devices were fabricated based on TPA-cNDI (10 wt% in CBP) with the emission maximum at ca. 730 nm and EQE of 2.4%. 56 Boron diuoride curcuminoid (BDC) derivatives BDC-1 and BDC-2 (as shown in Scheme 3) were composed of one or two triphenylamine (TPA) moieties as the donor group, while the acetylacetonate boron diuoride groups acted as the strong acceptors. Adachi and coworkers reported BDC-1 as an NIR TADF emitter. OLEDs based on BDC-1 showed a maximum EQE of nearly 10%. In addition, NIR amplied spontaneous emission was also detected due to the strong spatial overlap between the hole and electron wavefunctions. The maximum EL emission wavelength could be adjusted from 700 to 780 nm by controlling the polarity of the active medium. With decreasing doping concentration, the PLQY values of the CBP blends increase rapidly, which could reach the highest value of ca. 70% with a dopant concentration of 6 wt%. 57 Ye and coworkers designed and synthesized a solutionprocessable NIR TADF emitter, BDC-2 (ref. 58) (Scheme 3). NIR OLEDs based on BDC-2 showed a maximum EQE of 5.1% with an emission wavelength of 758 nm. The ASE emission bands were also observed and the maxima could be gradually shied from 801 to 860 nm at various doping concentrations. BDC-2 in blend lms displayed an amplied spontaneous emission band above 800 nm with a threshold as low as 7.5 mJ cm À2 . These reports illustrated that TADF boron diuoride curcuminoid derivatives were promising candidates for highperformance NIR OLEDs and NIR organic semiconductor lasers. In 2019, Fu et al. adopted a TADF organic solid-state laser based on a borondiuoride curcuminoid derivative, CAZ-A (Scheme 3), by employing a Cz derivative as a donor. 59 Researchers also developed TPAM-BF 2 by replacing the phenyl groups of BDC-1 with methylphenyl which exhibited better OLED performances. 60 An acceptor named 2-dicyanomethylene-3-cyano-4,5,5trimethyl-2,5-dihydrofurance (TCF) with linearly distributed cyano groups was exploited by Xu and coworkers. Based on this acceptor, two NIR TADF materials, named CzTCF and tBCzTCF, were constructed (Scheme 3). The ICT effects and HOMO-LUMO overlaps of these two molecules were signicantly improved by the linear distribution of cyano groups in TCF and the styryl p-bridge. All the decay processes of CzTCF and tBCzTCF lms exhibited biexponential properties indicating the presence of TADF processes. Moreover, the non-doped solution-Scheme 3 Other types of NIR molecules mentioned in the review.

Review
Chemical Science processed OLED based on tBCzTCF was successfully fabricated with an emission maximum of 715 nm. 61 Phenanthro[9,10-d]imidazole (PI) has a rigid planar p conjugation and two different unique bonding types of nitrogen atoms which results in high F PL and thermal stability and ambipolar carrier transport properties. As PI is oen used as a weak donor, choosing suitable electron-withdrawing groups could adjust the emission wavelength from the deep-blue to the green range according to previous results. 62 Lu and coworkers developed three TADF emitters PIBz-10-PTZ, PIBz-10P-PTZ, and PIBz-3-PTZ (Scheme 3) by utilizing PI and PTZ as donors and benzothiadiazole (Bz) as an acceptor. These compounds exhibit different photophysical properties and device performances by changing the linking positions between PTZ and Bz. Among these molecules, PIBz-3-PTZ exhibits a strong DR/NIR emission and aggregation-induced emission properties with a high F PL of 35% in neat thin lms in particular. The non-doped OLEDs achieved a maximum EQE of 2.02% with an emission peak at 672 nm and brightness of up to 3403 cd m À2 . In addition, the device was able to maintain an EQE of 1.69% at a high luminance of 100 cd m À2 , with a low roll-off of 16%, suggesting that the non-doped device can keep a relatively high efficiency at very high brightness. These results demonstrate that PI could be a useful donor to construct highly efficient NIR-emission emitters. 63 A simple and fairly highly electron-decient receptor, 5,6dicyano[2,1,3]benzothiadiazole (CNBz), was presented. A typical D-A-D type system was constructed by end-capping with the electron-donating triphenylamine (TPA) unit. TPACNBz (as shown in Scheme 3) exhibited an efficient NIR TADF emission (l em ¼ 750 nm) with a very small DE ST of 0.06 eV. The device based on TPACNBz exhibited an excellent performance i.e., a high maximum radiance of 10 020 mW Sr À1 m À2 , an impressive maximum EQE of 6.57%, and a peak wavelength of 712 nm. 64 Recently, TADF materials with polycyclic heteroaromatics embedded in multiple boron (B)-and nitrogen (N)-atoms have also been developed. [65][66][67] Introduction of multiple B and N atoms into polycyclic heteroaromatics resulted in the successful formation of restricted p-bonds on the phenyl-core for delocalized excited states, thus narrowing the energy gap. As the rst NIR multi-resonance TADF emitter developed in 2021, the R-TNB (Scheme 3) based OLED exhibited an impressive high maximum EQE of 27.6%; the value is the highest recorded among all reported results of NIR TADF devices as summarized in Table 2. 68 This work provides an effective strategy of multiple B and N atoms in polycyclic heteroaromatics, showing viable potential to generate DR/NIR emitters with bright and efficient emission without nonradiative transitions.
Very recently, two pairs of TADF enantiomers (R/S-DOBP and R/S-HDOBP) (Scheme 3) with tetracoordinate boron geometries were developed by Yang et al. R/S-DOBP and R/S-HDOBP exhibited high PLQY and efficient reverse intersystem crossing in neat lms. The R-DOBP based non-doped solution-processed OLEDs revealed an NIR emission (peaking at 716 nm) with a maximum external quantum efficiency of 1.9% and high exciton utilization efficiency of 86%, and are some of the best solution-processed non-doped NIR-OLEDs. 69 In this review, besides Schemes 1-3 as we mentioned above, we also listed the host materials mentioned in the review in Scheme 4. Moreover, the photophysical properties of TADF emitters we have mentioned before can be found in Table 1, and the corresponding device structures and performances are further listed in Table 2. The chemical structures mentioned in Table 2 are also shown in Scheme 4 to make them clearer. Besides, we also summarize the PLQYs of the representative NIR TADF emitters in Fig. 5 and EQEs of the representative NIR TADF OLEDs in Fig. 6. In Fig. 5, an asterisk (*) indicates that the data for that emitter were acquired from the neat lm, while for the other emitters not marked by an asterisk they were acquired from the doped lm. They are also shown in the patterns, where triangles represent data from neat lms and squares represent data from doped lms. In Fig. 6, the same emitter-based OLEDs are shown in the same pattern but different colors to make it easier to read. With the supplement of Fig. 5 and 6, we believe that our review can further provide inspiration and ideas for Scheme 4 Host materials mentioned in the review. receptors will be prosperously developed in the future. Moreover, for scholarly study and real-world use, more structural receptor types ought to be developed. Therefore, more receptor developments deserve attention especially multiple boron (B)and nitrogen (N)-atom embedded polycyclic heteroaromatic molecules and so on with extended conjugation and small DE ST . As a result, more useful molecular design strategies would be adopted. Considering that polymer dots could be used as powerful probes for bioimaging, 70 biosensing, 71 and photodynamic therapy, 72 NIR TADF polymers with higher efficiencies and stronger penetration will be more worthy of exploration with more excellent practical applications.
In terms of the future development of OLEDs, we believe more solution-processed NIR OLEDs would be designed and developed based on the TADF NIR emitters, thus signicantly reducing production cost. Production of commercially available exible OLEDs is now based on low temperature polysilicon. With the development of technology, the excellent properties of TADF materials will play a role since they can not only be exposed to high temperature but also emit better emission intensity at high temperature. At the same time, the luminescence efficiency and lifespan will have greater development potential in the future. More applications like all-organic optical upconversion devices would prosper based on the development of NIR TADF emitters. The investigation of NIR TADF emitters and related OLED devices will witness rapid growth in coming years.

Author contributions
Yuxin Xiao, Hailan Wang, and Zongliang Xie made equal contributions to this review. All the authors were involved in the revision of this manuscript.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.