1,3,5‐Triazine‐Functionalized Thermally Activated Delayed Fluorescence Emitters for Organic Light‐Emitting Diodes

The 1,3,5‐triazine electron acceptor has become one of the most popular building blocks for the design of thermally activated delayed fluorescence (TADF) materials. Many TADF design strategies are first applied in compounds that contain triazines, and there are numerous examples of organic light‐emitting diodes (OLEDs) with triazine‐containing emitters that show high efficiencies and long operating lifetimes. A comprehensive review of triazine‐containing TADF emitters is provided. This review is organized according to the triazine‐derived structural motifs, such as number and position of electron‐donor groups in donor–acceptor‐type emitters, the π‐bridging linkers employed, orientation control of the transition dipole moment, and the design of chiral and through‐space charge‐transfer emitters. The structure of the compounds with their optoelectronic properties and the corresponding performance of the OLED devices is correlated.


Introduction
Since 1987, when the first viable electroluminescent device was reported by Tang and Van Slyke, [1] significant progress has been made in terms of materials development such that organic lightemitting diodes (OLEDs) have now been commercialized in both the solid-state lighting and display markets. The current mature OLED products heavily rely on the phosphorescent emitters that contain noble metals such as iridium because these materials permit 100% utilization of the electrogenerated singlet and triplet excitons to produce light. The breakthrough of TADF materials in 2012 by Adachi and co-workers [2] provides an alternative route to reach equally efficient OLEDs and without the use of these scarce metals. To date, thousands of TADF materials have been reported, and their use in red, green, and blue OLEDs has been demonstrated. Importantly, there are now many examples of devices that show high external quantum efficiency (EQE), device stability, and color purity, these reports demonstrate that TADF OLEDs are attractive alternatives to phosphorescent devices. Indeed, the first commercialized TADF OLED devices are now on the market. [3] For instance, one of the best red TADF OLED, which uses TPA-PZCN as the emitter, has realized a record high maximum EQE (EQE max ) of 28.1% with an electroluminescence maximum, λ EL , of 648 nm. [4] For green OLEDs, the use of CzDBA as the emitter resulted in a device with an EQE max of 37.8% with the λ EL at 528 nm. [5] Blue OLED employing TDBA-DI as the emitter showed a comparable high EQE max of 38.2% with CIE coordinates of (0.15, 0.28) [6] and an OLED using a multiple resonance TADF (MR-TADF) emitter, ν-DABNA, achieved an EQE max of 34.4% with a λ EL at 469 nm and a full-width at half-maximum (FWHM) of only 18 nm. [7] All these achievements in terms of device performance are inextricably linked to the development of high-performance highly twisted donor-acceptor emitter architectures. The vast majority of electron-donor groups used within TADF emitters are hole-transporting N-heterocycles and are typically chosen from carbazole and its derivatives (e.g., bicarbazole, benzofurocarbazole, thienocarbaozle, and indolocarbazole), arylamine, acridan, phenoxazine, phenothiazine, and phenazine, and their derivatives. There is more structural diversity in the electronacceptor moiety, with popular motifs containing borane, sulfone, ketone, pyrimidine, benzonitrile, phthalonitrile, triazole, oxadiazole, thiadiazole, benzothiazole, benzooxazole, quinoxaline, anthraquinone, heptazine, and, of course, triazine. In particular, 1,3,5-triazine has been one of the most popular electron-deficient heteroaromatic acceptors used in green and blue TADF emitters and is also a popular component used in host materials design due to its moderate electron affinity with the LUMO values in the range of À2.7 to 3.1 eV. [8] Most commonly, the 1,3,5-triazine is decorated by three phenyl groups attached to the 2,4,6-position of triazine in a triphenyltriazine structure (TRZ). There are also examples of triazine-containing materials where the electron donor is directly attached to the triazine. However, in many of these materials, as there are negligible steric interactions between the triazine and the donor, thus leading to a predominantly coplanar conformation, there is significant overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), thereby resulting in a correspondingly large singlet and triplet energy splitting (ΔE ST ) and no TADF. For example, DPhCzT, [9] a compound DOI: 10.1002/adpr.202200203 The 1,3,5-triazine electron acceptor has become one of the most popular building blocks for the design of thermally activated delayed fluorescence (TADF) materials. Many TADF design strategies are first applied in compounds that contain triazines, and there are numerous examples of organic light-emitting diodes (OLEDs) with triazine-containing emitters that show high efficiencies and long operating lifetimes. A comprehensive review of triazine-containing TADF emitters is provided. This review is organized according to the triazine-derived structural motifs, such as number and position of electron-donor groups in donor-acceptor-type emitters, the π-bridging linkers employed, orientation control of the transition dipole moment, and the design of chiral and throughspace charge-transfer emitters. The structure of the compounds with their optoelectronic properties and the corresponding performance of the OLED devices is correlated.
containing a carbazole directly coupled to a diphenyltriazine, was reported as a host material, but did not show any TADF properties due to its large ΔE ST of 0.3 eV (Figure 1).
The genesis of organic TADF materials as emitters for OLEDs was in 2011 and relied on a triazine-based compound ( Figure 2). A sufficient separation between HOMO and LUMO was induced by strengthening the electron donor by extending the conjugation length as was done in PIC-TRZ (ΔE ST ¼ 0.11 eV), the first example of a purely organic TADF emitter. The design of PIC-TRZ also paved the way for the adoption of one of the most successful design strategies for TADF emitters, which is based on a strongly twisted donor-π-acceptor motif. Exciplex systems are another route to small ΔE ST as the weak intermolecular interactions between electron-donor and electron-acceptor compounds ensure suitable physical separation of the HOMO and LUMO. Triazine-containing electron acceptors appear prominently in exciplex systems due to their largely planar conformation since the first exciplex system was reported in 2012. [10] PO-T2T, for instance, has   been widely used as the acceptor for exciplex since it was first reported in 2014. [11] Triazine electron acceptors were incorporated into some of the first TADF dendrimer emitters in 2015, such as G3TAZ, this due in part to the D 3 -symmetric structure of the TRZ and the ease with which chemical modification of this core can occur. High-efficiency nondoped OLEDs (EQE max ¼ 20%) using the small molecule emitter DMAC-TRZ showed comparable performance to the doped device, indicating that it is possible to design a compound that shows significantly suppressed concentration quenching in neat films. DMAC-TRZ is still one of the most widely studied donor-acceptor-type TADF emitters. Disposing the electrondonor adjacent to the TRZ results in improved performance in the OLED, as exemplified by TRZ-oCz (EQE max ¼ 9.3%). This is partially due to the more strongly twisted conformation that must be adopted for this compound and the additional through space charge transfer (TSCT) state that is induced.
Since the first example in 2017, TRZ-containing TSCT TADF emitters have developed rapidly. Presently, the highest performing TRZ-based TADF emitter is SpiroAC-TRZ.
Compared to the OLED with DMAC-TRZ, the EQE max of the OLED with SpiroAC-TRZ is improved to 36.7%. Here, by employing a related electron donor in 10 H-spiro [acridine-9,9 0 -fluorene] (SpiroAC), a highly horizontally orientated transition dipole moment (TDM) was achieved, leading to enhanced light outcoupling. [12] Recently, a triazinecontainning TADF emitter 5Cz-TRZ with five carbazoles closely packed onto a central phenylene bridge afforded a material with one of the fastest reverse intersystem crossing (RISC) rate constants of k RISC ¼ 1.5 Â 10 7 s À1 . There are several advantages for the use of triazine within TADF materials. These include (1) triazine and its derivatives are easy to synthesize from inexpensive starting materials; (2) triazine is easily functionalized at the 2,4,6 positions and can be done both symmetrically or asymmetrically, thereby increasing structural diversity; (3) the linkage between the electron donor and the triazine core can be rationally adjusted; and (4) triazine is a stable aromatic structure that contributes to improved operational lifetimes of the devices. Documenting the popularity of this moiety, in this review, we summarize the recent progress of triazine-containing TADF materials. First, the TADF emitters based on carbazole-triazine structures are introduced and classified according to 1) the number and substitution position of the carbazole; 2) the influence of the bridge between the donor and the triazine acceptor; 3) the influence of the structure on the orientation of the emitter within the emitter layer, which will affect the orientation of the TDM and hence the light outcoupling efficiency in the device; 4) trisubstitution about the triazine core; 5) dimerization strategies where there are at least two triazines within the material; 6) examples where the linker group goes beyond a phenylene; 7) adjustment of the triazine electron-withdrawing strength through peripheral decoration; and 8) examples where there is TSCT in addition to through-bond charge transfer (TBCT). Finally, moving beyond carbazole-based electron donors, examples of emitters using other donors will be summarized ( Figure 3).

Influence of Carbazole (Number and Position) on the Properties of TRZ-Based TADF Emitters
Carbazole (Cz) has been widely used as an electron-donating group in optoelectronic materials because of its moderate donor strength and the ease with which chemical derivatization can occur. [13] Carbazole is also a fully aromatic structure, with no bonds to a sp 3 -centre as exists in acridine, phenoxazine, and phenothiazine, bonds that are prone to scission and routes to degradation in the device.
First, we compare the optoelectronic properties of Cz-TRZ derivatives where the Cz is either ortho, meta or para disposed with respect to the diphenyltriazine, as shown in Figure 4. The para-analog CzTRZ (here, it is renamed as p-CzTRZ), first reported by Lee et al., [14] is not a TADF emitter due to its too strong conjugation between Cz and TRZ, leading to a large ΔE ST of 0.36 eV; the compound is a blue emitter with a λ PL of 449 nm and a photoluminescence quantum yield, Φ PL , of 71% (10 wt% DPEPO). A blue OLED with CIE coordinates of (0.17, 0.11) showed only an EQE max of 4.2%. A slightly improved OLED efficiency (EQE max of 5.8%) was later reported by our group, [15] which provided further confirmation that in the device p-CzTRZ acts as a fluorescent emitter. Liao et al. [16] designed two analogs, SFCCN and SFCCNO ( Figure 5), based on the strategy of introducing a secondary donor group onto the carbazole moiety of p-CzTRZ. The solution-state properties remained unchanged because the spiro-carbon linkage between the secondary donor with carbazole is not conjugated. However, in the neat films, the aggregation of these molecules triggered the TADF as a result of the intermolecular TSCT. The planar molecular conformation is believed to facilitate face-to-face intermolecular interactions required for the TSCT to occur. The nondoped device based on SFCCNO exhibited an EQE of 12.9% at 100 cd m À2 , which is much improved compared to the device based on the non-TADF emitter p-CzTRZ (1.5% at 100 cd m À2 ).
The meta-and ortho-analogs, by contrast, are efficient TADF emitters. CzTRZ (renamed as o-CzTRZ; λ PL ¼ 455 nm, τ d ¼ 3.90 μs in 5 wt% mCP film) possesses a much smaller overlap between the HOMO and LUMO as a result of the more strongly twisted conformation (87.3°for the dihedral angle between carbazole and phenylene bridge determined from the crystal structure [17] ), which translates into a compound with a ΔE ST of 0.1 eV in 5 wt% mCP film. By doping 5 wt% o-CzTRZ in mCP host as the emitting layer, the OLED exhibited a blue emission with a λ EL at 470 nm and an EQE max of 9.3% despite the Φ PL of o-CzTRZ being only 16.7% (5 wt% mCP), which likely implies that the Φ PL has been underestimated, considering the www.advancedsciencenews.com www.adpr-journal.com device performance. Interestingly, the full-width at halfmaximum (FWHM) of the blue TADF device is only 66 nm, which we attribute to a much reduced geometric reorganization in the excited state to the small degree of conformational freedom present in the ortho-linked system. Indeed, many other compounds containing ortho-linked donors and acceptors also show narrow emission spectra (in section 9 related to TSCT). Normally, meta-disposed electron donors [18] and electron acceptors in D-A-type emitters show the weakest conjugation compared to their ortho-and para-analogs, which should lead to a correspondingly smaller ΔE ST . However, it was observed by Wu et al. [19] that the ΔE ST of m-CzTRZ (0.22 eV in 10 wt% in mCP film) is indeed larger than 0.03 eV for o-CzTRZ (determined again by Wu et al. [19] although these values are extracted from the energy difference between the PL at room temperature and the phosphorescence spectra at 77 K in 2-MeTHF and so differ from those of Gong et al. [17] ). The authors contended that the smaller observed ΔE ST for o-CzTRZ resulted from the near orthogonal conformation of the donor, leading to near complete suppression of the conjugation between Cz and TRZ. The DFT calculations ( Figure 6) from their work suggest that the 3 LE state of m-CzTRZ is much more stabilized than the 3  In comparison to the three aforementioned triazine-based TADF emitters, each possessing only one carbazole donor, emitters with more than one carbazole have also been investigated. Lee et al. [20] reported a stable deep blue emitter DCzTRZ (λ PL : 459 nm; Φ PL : 43% in toluene) in which the two carbazole donor groups are disposed mutally meta to each other and to the triazine acceptor. The ΔE ST of DCzTRZ is 0.25 eV, which is similar to that of m-CzTRZ (0.22 eV in 10 wt% mCP). The blue OLED has an EQE max of 17.8% at CIE coordinates of (0.15, 0.15), which is slight less efficient than that of m-CzTRZ (19.2%). It is surprising to observe a much bluer emission spectrum in comparison with the device based on m-CzTRZ, although the solution PL spectrum in toluene is eactly the same. The blud-shift of the EL may be caused by the different device configurations, choice of host, and doping concentration between the two devices.  Reproduced with permission. [19] Copyright 2019, Royal Society of Chemistry.
A number of other analogs containing two carbazole donors were also prepared by Lee et al. [21] The different substitution patterns of the two carbazoles for six emitters 23CT, 24CT, 25CT, 26CT, 34CT, and 35CT (here DCzTRZ was renamed as 35CT) are correlated with their photophysical properties. Ortho-substitution to the triazine acceptor induces a large dihedral angle between the carbazole and triazine moieties, thus leading to a stronger charge transfer state and smaller ΔE ST , which is evident by comparing 23CT (ΔE ST : À0.02 eV, 1 wt% in Zeonex), 24CT (ΔE ST : 0.11 eV, 1 wt% in Zeonex), 25CT (ΔE ST  There are also a number of analog emitters possessing three carbazole donors. Compared with 35CT (ΔE ST : 0.23 eV; Φ PL : 43% in toluene), TCzTRZ [22] (ΔE ST : 0.16 eV; Φ PL : 100% in toluene), as shown in Figure 7, possesses a smaller ΔE ST coupled with a significantly improved Φ PL . The structural difference between these two compounds is the addition of a third carbazole para to the triazine, which results in a HOMO that is distributed over the three donor carbazoles. The even distribution of the electron density of the HOMO across multiple donors also is in operation in TmCzTRZ (ΔE ST : 0.07 eV; Φ PL : 99% in toluene), which also possesses a smaller ΔE ST and a higher Φ PL than DCzmCzTRZ (ΔE ST : 0.20 eV; Φ PL : 84% in toluene), which has two different carbazole-based donors. OLEDs with these three emitters achieved comparably high efficiencies, with EQE max of 25% and CIE coordinates of (0.18, 0.33) for the device with TCzTRZ and an EQE max of 25.5% and CIE coordinates of (0.25, 0.50) for the device with TmCzTRZ; the red-shift in the EL of the latter is due to the use of a stronger dimethylcarbazole donor. The EQE max decreased to 21.3%, with CIE coordinates of (0.23, 0.46) for the device with DCzmCzTRZ, reflecting to the lower Φ PL for the emitter.
Lee et al. [23] later probed the effect of the regiochemistry of the three carbazole donors on the photophysical properties of the emitters. A cross-comparison of the photophysical properties of compounds 234CzTRZ (ΔE ST : 0.07 eV; Φ PL : 90% in DPEPO; τ d : 4.1 μs), 235CzTRZ (ΔE ST : 0.14 eV; Φ PL : 100% in DPEPO; τ d : 8.4 μs), and 245CzTRZ (ΔE ST : 0.17 eV; Φ PL : 98.4% in DPEPO; τ d : 9.7 μs) revealed that the substitution pattern of the three carbazole donors affects both ΔE ST and τ d ; all three compounds possess a shorter delayed fluorescence lifetime than that of TCzTRZ (τ d : 13.5 μs). As shown in Figure 8, the ortho-substituted carbazoles to the diphenyltriazine acceptor adopt a large dihedral angle, which leads to a shortened delayed fluorescence lifetime, compound 234CzTRZ, with three   (Figure 9c) that contains five carbazole donors. [24] Compound 5Cz-TRZ possesses a very high k RISC of 1.5 Â 10 7 s À1 , which is in part due to the large density of excited states and the small ΔE ST . The authors contended that the fast RISC in 5Cz-TRZ would contribute to a smaller efficiency roll-off in the device as efficiency rolloff is typically caused by the accumulation of long-lived triplet excitons. The resulting device showed an outstanding performance with an EQE max of 29.3%, which decreased a mere 2.3% at 1000 cd m À2 and a device lifetime, T 90 , of %600 h from an initial brightness of 1,000 cd m À2 . All the photophysics and electrochemical characteristics of the aforementioned materials are summarized in Table 1. Representative device performance is summarized in Table 2. 3. Importance of Conjugated Bridge between TRZ and Carbazole/Carbazole Derivatives PIC-TRZ [25] (λ PL : 466 nm; τ d : 120 μs; ΔE ST : 0.11 eV; Φ PL : 39%; 6 wt% in mCP) was the first reported purely organic TADF emitter for use in OLEDs ( Figure 2). Unlike the most commonly used design strategy for TADF emitters, which is to separate electrondonor and electron-acceptor groups by a conjugated phenylene bridge, PIC-TRZ contains two carbazole derivatives that are directly connected to a triazine core. A relatively small ΔE ST (0.11 eV) was obtained due to the large twist angle between the carbazole-based donors and the triazine acceptor. Even though the reported EQE max could only reach 5.3% for the sky-blue OLED, this nevertheless indicated that upconversion of triplet excitons to singlet excitons was occurring as the theoretical EQE max of the device was limited to 2% based on its relatively low Φ PL of 39% and assuming no triplet exciton harvesting ( Figure 10). . a) EQE versus luminance characteristics. b) Normalized EL spectra. c) Transient EL decay characteristics of devices A, B, and C at 10, 1000, and 5000 cd m À2 . a.u., arbitrary units. d) Operational lifetimes of the TADF and hyperfluorescent OLEDs. Reproduced with permission. [24] Copyright 2020, Nature Portfolio.   Figure 10. Comparison of TADF emitters with and without a phenylene bridge between triazine and carbazole derivatives.
Adachi et al. [28] also performed a study to elucidate the importance of the bridging phenylene by comparing CzT (λ PL : 502 nm; ΔE ST : 0.09 eV; τ d : 43 μs; Φ PL : 40%; 3 wt% in DPEPO) with BCzT (λ PL : 483 nm; ΔE ST : 0.3 eV; τ d : 33 μs; Φ PL : 96%; 6 wt% in DPEPO) and demonstrated that the bridging phenylene contributes to increasing the overlap density between the electronic wave functions of the ground state and the lowest excited singlet state, leading to both a larger ΔE ST and higher Φ PL , yet surprisingly a blue-shifted λ PL . CzT, first reported in 2013 by Adachi et al., [29] shows a small ΔE ST of 0.09 eV. The reported OLED shows an EQE max of only 6% due to the relatively low Φ PL of 40%. The S 1 ! S 0 radiative decay of BCzT is significantly enhanced, leading to a much higher Φ PL of 96% (6 wt% in DPEPO), while the emission blue-shifted by 19 nm with a higher doping concentration in DPEPO matrix. Despite BCzT obtaining a near unity Φ PL , the observed ΔE ST was dramatically raised to 0.3 eV. Despite the larger ΔE ST , the delayed fluorescence lifetimes are of a similar magnitude, which can only be explained by the implication of intermediate triplet states in the RISC process of BCzT. The BCzT-based sky-blue OLED exhibits an EQE max of 21.7% with λ EL at 492 nm, which is significantly improved compared to the 6% EQE max of the CzT-based OLED. The photophysical and electrochemical data of the aforementioned materials are summarized in Table 3. Representative device performance is summarized in Table 4.
The more extended structure of IndCzpTr-2 leads to a preferential horizontal orientation of the TDM in the neat film with an orientation order parameter, S, of À0.264 (estimated by VASE). The S value of IndCzpTr-1 in the neat film is only À0.1, which is far away from the theoretical value of S ¼ À0.5 for a perfectly horizontal orientation of the TDM. From these measurements in neat films, the orientation factor θ was calculated as 0.  www.advancedsciencenews.com www.adpr-journal.com The emitter DACT-II (ΔE ST : 0.009 eV; Φ PL : 100% as 9 wt% doped film in CBP)) [32] is a derivative of Cz-TRZ but contains peripheral diphenylamino groups decorating the Cz donor. Their addition significantly strengthens the donor and turns on TADF compared to Cz-TRZ. DACT-II exhibits a near zero ΔE ST of 0.009 eV yet a large oscillator strength, f, reflected in the unity Φ PL (9 wt% in CBP film). These desirable traits translate into a green OLED that shows an impressive EQE max of 29.6% with λ EL at %520 nm. The high efficiency of the device is also due to the preferential horizontal orientation of the TDM of DACT-II as determined by VASE measurements (S ¼ À0.32) in Figure 12a and angular-dependent PL measurements (S ¼ À0.29) in Figure 12b. Kim et al. [33] later showed that efficiency roll-off can be suppressed in the OLED if the CBP host is replaced by a TCTA:B3YPMPM exciplex-forming host, as the authors found a 1.5-fold increased RISC rate in this system (k risc : CBP 1.37 Â 10 5 s À1 ; TCTA:B3PYMPM 2.06 Â 10 5 s À1 ). As a result, the device gives a higher efficiency with an EQE max of 34.2%, which only slightly decreased to 31% at a luminescence of 1000 cd m À2 .
However, the close structural analog, 3CzTRZ [34] (λ PL : 468 nm; τ d : 415 μs; ΔE ST : not reported; Φ PL 72%; 6 wt% in PPT), seems to be oriented randomly in the solid state. 3CzTRZ presents a blue-shifted emission at 468 nm and a lower Φ PL of 72% as a 6 wt% doped PPT film compared with DACT-II (9 wt% in CBP). The OLED with 3CzTRZ thus shows much poorer performance with an EQE max of 10.5% [λ EL : 480 nm; CIE ¼ (0.17, 0.26)], which implies that there is not even a 100% IQE, assuming an outcoupling efficiency of 0.2-0.25 based on an isotropic orientation of the TDM. However, a structural analog of 3CzTRZ, TCzTRZ (ΔE ST : 0.01 eV; Φ PL : 99%; 10 wt% in DPEPO), developed by Lee et al., [14] who replaced the C-N bonded tercarbazole dendron of 3CzTRZ with a C-C bonded tercarbazole dendron, does show 95% horizontal orientation of the TDM; a similar analog, BCzTRZ, also shows a horizontal dipole orientation of its TDM as high as 89%, indicating the preferential in-plane alignment is formed both in tercarbazole and bicarbazolecontaining compounds. Thus, subtle changes in molecular design can have profound effects on both the propensity for horizontal orientation of the TDM and the efficiency of the device. The Φ PL values of the 10 wt% doped DPEPO film are 99% and 96% for TCzTRZ and BCzTRZ, respectively, both of which are significantly higher than that of 3CzTRZ (72% in DPEPO matrix). Thanks to the near unity Φ PL and near perfect horizontal Figure 12. a) Results of VASE measurements; extinction coefficient k (black) and refractive index n (red). The solid and dashed lines represent spectra for the ordinary and extraordinary optical constants, respectively. b) Results of angular-dependent PL experiments. c) Optical simulations for the DACT-II-9 device. The numbers in parentheses for respective modes are obtained by the integration with respect to emission wavelengths with weighting of the PL intensity. d) Dependence of the out-coupling mode on the thicknesses of hole and electron-transport layers. [32] .
www.advancedsciencenews.com www.adpr-journal.com orientation, the blue OLED shows an outstanding EQE max of 31.8% at a λ EL of 496 nm and CIE coordinates of (0.20, 0.44). TRZ-DI (ΔE ST : 0.023 eV), designed by Kwon et al., [35] incorporates a D 3 -symmetric triazatruxene donor. Compared with Cz in Cz-TRZ, the extended conjugation within triazatruxene effectively localizes the HOMO only on the donor group, leading to a very small ΔE ST of 0.02 eV. A small dihedral angle (24.5°) between the plane of triazatruxene and the TRZ acceptor, along with the rigid molecular structure, contributes to a Φ PL of 87% (25 wt% in TCTA: Bepp2 exciplex host). A green OLED using TRZ-DI [λ EL : 526 nm; CIE ¼ (0.31, 0.57)] as the emitter exhibits an outstanding EQE max of 31.4%. The orientation of the TDM of TRZ-DI was not investigated, but it is not unreasonable to hypothesize that TRZ-DI possesses a highly horizontally orientated TDM based on the analysis of the Φ PL and the EQE max .
TRZ-SBA-NAI (ΔE ST : 0.16 eV; Φ PL : 87%; 3 wt% in mCPCN), designed by Yang et al., [36] contains an A-D-A 0 motif, where a spirobiacridine (SBA) donor is used to link the acceptors NAI and TRZ, as shown in Figure 13. Because of the weak electronic communication through the central sp3 carbon on the SBA donor, two distinct CT emission bands at λ PL of around 474 and 586 nm were observed in toluene, with the former assigned to TRZ-SBA and the later to SBA-NAI. By contrast, the solid-state emission 3.0 wt% doped in mCPCN matrix only showed a band at λ PL of 577 nm from the SBA-NAI moiety, suggesting efficient energy conversion from the high-lying excited states of TRZ-SBA to SBA-NAI. The different PL character in toluene and film was ascribed to the higher intramolecular charge transfer rate via FRET to the radiative decay rate of the TRZ-SBA counterpart.
The TDM vector aligns with the molecular long axis, which contributes to the high horizontal dipole ratios (Θ ║ ) of 88% in the mCPCN host matrix ( Figure 14). Finally, the orange-red OLED based on TRZ-SBA-NAI showed an EQE max of 31.7% at λ EL of 593 nm and CIE coordinates of (0.55, 0.45).
A similar design using SBA as the donor and triazine as the acceptor has also been explored by Kido et al. [37] to enable the horizontal orientation of the TDM of emitters in OLEDs. Three molecules, tBuTZ-SBA-Ph, TZ-SBA-Ph, and tBuPhTZ-SBA-Ph, were prepared with the difference in structure associated with the choice of distal groups on the triazine acceptor unit. The Φ PL values of tBuTZ-SBA-Ph, TZ-SBA-Ph, and tBuPhTZ-SBA-Ph in toluene are modest at 34%, 55%, and 43%, respectively. However, the Φ PL values in neat films increased to 41% for tBuTZ-SBA-Ph, 90% for TZ-SBA-Ph, and 65% for tBuPhTZ-SBA-Ph. By comparing with the Φ PL values of the 10 wt% doped films in DPEPO matrix (87% for tBuTZ-SBA-Ph, 100% for TZ-SBA-Ph, and 97% for tBuPhTZ-SBA-Ph), the similarly high Φ PL values for TZ-SBA-Ph in both neat and doped films indicate a strong suppression of intermolecular concentration quenching. The orientation ratio (Θ) in evaporated doped films was quantified to be 56%, 82%, and 82% for tBuTZ-SBA-Ph, TZ-SBA-Ph, and tBuPhTZ-SBA-Ph, respectively, as shown in Figure 15. The larger π-plane in TZ-SBA-Ph and tBuPhTZ-SBA-Ph, as asserted by the authors, would be more likely to interact with the deposition surface, thus enhancing the horizontal orientation. The OLEDs achieved EQE max of 24.1%, 31.2%, and 28.3% for the devices with tBuTZ-SBA-Ph, TZ-SBA-Ph, and tBuPhTZ-SBA-Ph, respectively. The  Table 5, and representative device performance metrics are summarized in Table 6.

Symmetric Substitution of Carbazole/Carbazole Dendrons on Triphenyltriazine Core
In this section, we focus on trisubstituted triazine-based dendrimer emitters. The very first TADF dendrimers, GnTAZ Reproduced with permission. [36] Copyright 2021, Royal Society of Chemistry. (n refers to the generation of the dendrons) ( Figure 16), were developed by Yamamoto et al. [38] These compounds contain three symmetrical carbazole donor dendrons (from second generation to fourth generation; 1 st generation with only Cz as the donor was not studied due to the insolubility of this compound) attached to a central TRZ acceptor. All three dendrimers showed near unity Φ PL and very small ΔE ST in toluene solution: G2TAZ (ΔE ST : 0.03 eV; Φ PL : 94% in toluene), G3TAZ (ΔE ST : 0.06 eV; Φ PL : 100% in toluene), and G4TAZ (ΔE ST : 0.06 eV; Φ PL : 94% in toluene). However, the Φ PL decreased significantly for the neat films to 52%, 31%, and 8.5%, respectively, which was rationalized in terms of increasing concentration quenching in the neat films along the series. OLEDs with neat films of GnTAZ dendrimers as the emitting layer performed poorly, with the highest EQE max of 3.4% at CIE coordinates of (0.27, 0.49) for the device with G3TAZ.
Although the EQE max of the OLED based on tBuG2TAZ improved to 9.5%, this performance is still not comparable to the efficiencies reported for nondoped vacuum-deposited devices based on small-molecule TADF emitters. This is due to the low Φ PL (<50%) in the neat film that results from concentration quenching. To solve the problem and further improve the device performance, Yamamoto et al. [40] developed two carbazole-based dendrimers (G3Ph and G4Ph as shown in Figure 17) as host materials to suppress the observed concentration quenching of tBuG2TAZ (λ PL : 500 nm; Φ PL : 76%; ΔE ST : 0.07 eV; 15 wt% in G3Ph). The doping concentration of 15% was chosen to optimize the Φ PL in the blended film. The emission spectra remain unchanged at 500 nm between the neat film and the films doped in G3Ph and G4Ph. However, the Φ PL increased from 44% in neat film to 76% and 70% in G3Ph and G4Ph (15 wt%), respectively. The EQE max of the green-emitting OLED was significantly improved to 16.1% at CIE coordinates of (0.28, 0.49).
Recently, our group [41] has developed a series of green TADF dendrimers that contain carbazole dendron donors surrounding a TRZ core acceptor unit, linked via either a para-/metaphenylene (tBuCz2pTRZ/ tBuCz2mTRZ) or combination of both connections (tBuCz2m2pTRZ), as shown in Figure 18. The design strategy of tBuCz2m2pTRZ (λ PL : 520 nm; Φ PL : 86%; ΔE ST : 0.04 eV; neat film) demonstrates that tBuCz2m2pTRZ     with the excited-state behavior being modulated as a result of the interactions between the two adjacent donor dendrons. The paraconnection of the donor dendrons in tBuCz2m2pTRZ leads to strong electronic coupling between donor and acceptor, as evidenced by the strong molar absorption for the ICT transition, while the additional meta-connection in tBuCz2m2pTRZ results in a small ΔE ST as the meta-disposed donor and acceptor groups are electronically decoupled. The λ PL of tBuCz2pTRZ is similar to that of tBuCz2mTRZ, indicating similar energies of their 1 CT states regardless of the electronic coupling between donor and acceptor. The spectral red-shift of tBuCz2m2pTRZ indicates the stabilized 1 CT states with more dendritic moieties than tBuCz2pTRZ and tBuCz2mTRZ. Nondoped solution-processed OLEDs using a simple device configuration ( Figure 19) without exciton barrier layers and containing only the dendrimers in the emissive layer exhibited the EQE max of 18.5%, 19.9%, and 28.7% for tBuCz2pTRZ, tBuCz2mTRZ, and tBuCz2m2pTRZ, Figure 18. Molecular structures and properties of TADF dendrimer emitters with symmetric disubstitution of carbazole dendrons. Reproduced with permission. [41] Copyright 2022, Wiley-VCH. Figure 19. Electroluminescence characteristics of host-free OLEDs using tBuCz2pTRZ, tBuCz2mTRZ, and tBuCz2m2pTRZ as emitters. a) Device configuration. b) Normalized electroluminescence spectra. c) Current density and luminance versus driving voltage characteristics. d) EQE versus brightness for tBuCz2pTRZ (black), tBuCz2mTRZ (red), and tBuCz2m2pTRZ (blue) based devices. (The photos shown from bottom left to right for tBuCz2pTRZ, tBuCz2mTRZ, and tBuCz2m2pTRZ-based devices, respectively). e) Statistical histogram of EQE max for tBuCz2m2pTRZ-based OLEDs. f ) The EQE max of all reported solution-processed host-free TADF OLEDs as a function of wavelength.
www.advancedsciencenews.com www.adpr-journal.com respectively. The performance of the device with tBuCz2m2pTRZ represents a step change in the efficiency of the nondoped solution-processed OLEDs. Importantly, the efficiency roll-off of the OLED based on tBuCz2m2pTRZ is significantly improved by doping 30 wt% OXD-7, an electrontransporting material, into the emissive layer. As a result of the improved charge balance, the EQE of the optimized device not only reached a similar EQE max of 28.4% but also maintained its efficiency of 22.7% at a luminance of 500 cd m À2 . A related work [42] documents a detailed photophysical investigation to rationalize the structure-property relationship of the TADF dendrimers tBuCz3pTRZ and tBuCz3mTRZ (the DFT optimized structures without peripheral tertbutyl groups are shown in Figure 19). Both dendrimers exhibit high Φ PL values (89% for tBuCz3pTRZ and 81% for tBuCz3mTRZ, 10 wt% doped in mCP). While tBuCz3mTRZ (2.92 eV) possesses a slightly higher S 1 state than that of tBuCz3pTRZ (2.84 eV) due to the weak electron coupling of meta-connection, both have a similarly small ΔE ST (0.1 eV for tBuCz3pTRZ and 0.08 eV for tBuCz3mTRZ). The activation energy barriers (E act ) for RISC are approximately half those of the corresponding ΔE ST values. However, the contribution of the delayed emission to the total emission (19% for tBuCz3pTRZ and 63% for tBuCz3mTRZ) and the k RISC (0.5 Â 10 5 s À1 for tBuCz3pTRZ and 3.7 Â 10 5 s À1 for tBuCz3mTRZ), both reflecting the efficiency of the RISC process, are much greater/faster in the case of the meta-connected dendrimer tBuCz3mTRZ.
The comparison of the DFT calculation for the model dendrimers without tert-butyl groups is shown in Figure 20. Unlike Cz3pTRZ, Cz3mTRZ has negligible overlap between the hole and electron NTO distribution in the T 1 states. This leads to a greater CT character in the T 1 state of Cz3mTRZ (ω CT ¼ 0.54) as compared to Cz3pTRZ (ω CT ¼ 0.38). The calculated spin-orbital coupling (SOC) matrix element was found to be higher for Cz3mTRZ (0.53 cm À1 ) as compared to Cz3pTRZ (0.31 cm À1 ). The intramolecular (λ intra ) reorganization energy for Cz3pTRZ (275 meV) was also calculated to be higher than that of Cz3mTRZ (155 meV). Most TADF molecule design strategies focus on the minimization the ΔE ST ; however, it is observed that the enhancement of the SOC for the T 1 ! S 1 transition and the reduced reorganization energy also significantly contribute to the faster k RISC in tBuCz3mTRZ compared to tBuCz3pTRZ, despite their similar ΔE ST values. This work demonstrates the importance of the regiochemistry of the donor dendrons on the control of the SOC and reorganization energies, which is a heretofore unexploited strategy that is distinct from the involvement of intermediate triplet states through a nonadiabatic (vibronic) coupling with the lowest singlet charge transfer state.
The introduction of tert-butyl groups to carbazole donor dendrons in G1TAZ was reported by Ulanski et al. [43] Dendrimers TR1 and TR2 ( Figure 21) contain one and two tert-butyl groups on each carbazole unit, respectively, their inclusion was designed to improve the solubility of the dendrimers. Similar to Cz-TRZ, both TR1 (λ PL : 398 nm; Φ PL : 83% in hexane) and TR2 (λ PL : 407 nm; Φ PL : 74% in hexane) are deep blue emitters, with only very short lifetimes of a few nanoseconds, suggesting that these dendrimers do not show TADF. A subsequent study [44] compared the para-substituted compound TR2 (renamed as TpCz) with a newly designed meta-linked donor dendron dendrimer TmCz. The photophysical properties of TmCz (λ PL : %450 nm; Φ PL : 25%; τ d : 80 μs (1 wt% in PMMA); ΔE ST : 0.125 eV) and TpCz (λ PL : %440 nm; Φ PL : 35% τ d : 500 μs (1 wt% in PMMA); ΔE ST : 0.249 eV) revealed that TpCz possesses a larger oscillator strength leading to a higher Φ PL , but at the expense of a lower triplet energy, stronger charge transfer character, and larger ΔE ST . A blue vacuum-deposited OLED using TmCz as the emitter showed an EQE max of 9.5% at CIE coordinates of (0.16, 0.23) and λ EL at 475 nm, demonstrating that TmCz emits via TADF in the device.
Another design strategy, developed by Sun et al., [45] for TADF dendrimers incorporates nonconjugated aliphatic chains linked to distal carbazole moieties that act as host units. Compounds TZ-Cz and TZ-3Cz ( Figure 21) are based on a para-connected  , which the authors assert is due to the change in local environment that results from the encapsulation. The best solution-processed OLED was achieved using TZ-3Cz (λ EL : 520 nm; CIE ¼ (0.24, 0.51)) in a host-free configuration and showed a much-improved EQE max of 10.1% compared to the OLED based on TZ (1.09%). The photophysics and electrochemical characteristics of the aforementioned materials are summarized in Table 7, and representative device performance metrics are summarized in Table 8.

TADF Emitters Containing Two TRZ and Carbazole/Carbazole Derivative Donors
There are a small number of examples of emitters containing two triazine acceptors. Many of these examples are based on a dimerization strategy. As shown in Figure 22, Lee et al. [46] reported two related green TADF emitters mCBPTRZ-1 (λ  . Dimerization is expected to enhance the overlap between frontier molecular orbitals (FMOs), which leads to an increase of the oscillator strength, thus resulting in higher Φ PL , but at the expense of a red-shifted emission along with a larger ΔE ST . The OLED using mCBPTRZ-1 (λ EL : 503 nm; CIE ¼ (0.23, 0.52) emits in the green with an EQE max of 20.8%, while the device using mCBPTRZ-2 (λ EL : 521 nm; CIE ¼ (0.31, 0.59)) that contains the more electron-rich 3,6-di-tert-butylcarbazole shows a red-shifted emission and a significantly lower EQE max of 9.3% despite of the high Φ PL of the emitter. The low device efficiency was rationalized by the authors as due to the weak exciton conversion efficiency of 41% in the mCBPTRZ-2-based device compared to 90% in the mCBPTRZ-1-based device. However, this explanation does not seem plausible given their similar molecular structures and photophysics. Lee et al. [47] also reported two triazine analog emitters to 2CzTPN and 2CzIPN, p2Cz2TRZ and m2Cz2TRZ (Figure 22), which contain ortho-disposed carbazole units to the bis(diphenyltriazine) moiety. According to the computed FMOs in Figure 23, the HOMOs of p2Cz2TRZ and m2Cz2TRZ are evenly distributed over two carbazole donors whereas the LUMOs are evenly  Kwon et al. [35] used a diindolocarbazole as a central donor core to connect two diphenyltriazine moieties ( Figure 22). Like the diindolocarbazole analog TRZ-DI (λ PL : 521 nm; Φ PL : 87%; τ d : 1.32 μs; ΔE ST : 0.02 eV; 25 wt% in TCTA:Bepp2) shown in Figure 9, DTRZ-DI (λ PL : 521 nm; Φ PL : 83%; τ d : 1.47 μs; ΔE ST : 0.03 eV; 25 wt% in TCTA:Bepp2) showed the same green emission and similar photophysical properties. The introduction of a second TRZ acceptor did not improve the Φ PL as other studies [46,48] had documented. The performance of the OLED based on DTRZ-DI is poorer with an EQE max of 26.2% [λ EL : 526 nm; CIE ¼ (0.32, 0.58)] in contrast to that for the TRZ-DI device where the EQE max is 31.4%.  www.advancedsciencenews.com www.adpr-journal.com The dimerization strategy ( Figure 24) could also be used to convert a non-TADF emitter into a TADF emitter, as Lee et al. [48] demonstrated by transforming the fluorescent emitter Cz-TRZ into a TADF emitters (TRZoCz and TRZotCz) by increasing the separation of the FMOs in the dimer structures. The highly twisted geometries in TRZoCz and TRZotCz lead to markedly smaller ΔE ST (0.20 eV for TRZoCz, 0.06 eV for TRZotCz) compared to Cz-TRZ (0.36 eV). The Φ PL of TRZoCz and TRZotCz are 96% and 98%, respectively, for the 10 wt% films in DPEPO. The PL spectra are red-shifted compared to that of Cz-TRZ due to the increased conjugation within their structures, the photophysics of the two compounds are also less sensitive to the doping concentration. The EQE max of the TRZoCz and TRZotCz-based OLEDs are 27.8% and 26.6%, respectively, at CIE coordinates of (0.15, 0.32) and (0.20, 0.51), respectively. It is noteworthy that the emission profiles of these two emitters are not as broad than mostly common TADF emitters (FWHM ¼ 90-110 nm), with FWHM for TRZoCz (67 nm) and TRZotCz (74 nm). This is due to the restricted conformational motion available in these compounds. The photophysics and electrochemical characteristics of the aforementioned materials are summarized in Table 9. The representative device performance is summarized in Table 10.

Modification to the Phenylene Bridge
The degree of orbital overlap between the HOMO and LUMO is controlled not only by the choice of donor and acceptor but also by the bridging unit that mediates the conjugation between the two. For example, the introduction of a methyl group or phenyl group onto the phenylene bridge affects one of the torsion angles, leading to a more twisted conformation and thus a smaller ΔE ST , while the inclusion of more strongly electron-withdrawing groups such as nitrile or trifluoromethyl substituents also contributes to strengthening the electron-acceptor and localizing the electron density of the LUMO.
Buchwald et al. [49] reported a series of TADF emitters ( Figure 25) that incorporate a triptycene-fused carbazole donor para-linked to a diphenyltriazine acceptor via a arylene bridge.  . Design strategy for the TADF molecule TRZoCz. Reproduced with permission. [48] Copyright 2020, Wiley-VCH.    Liao et al. [50] reported two blue TADF materials TRZ-CF (λ PL : However, the increased oscillator strength caused by the enhanced conjugation of the donors results in a higher Φ PL in these two compounds compared to TCZTRZ(Me), but at a cost of larger ΔE ST , indicating that the methyl group is not sufficiently bulky to induce a highly twisted structure that can result in the required separation of the FMOs. Despite the red-shifted PL spectra, the OLEDs still exhibited blue emission with λ EL at 476 and 460 nm and EQE max of 20% and 13.3% for the devices with TRZ-CF and TRZ-CzF, respectively. Strengthening the donor by incorporating a distal carbazole served to red-shift the emission in TRZ-DCF (Φ PL : 84%; τ d : 5.43 μs; 20 wt% in DPEPO) while the other photophysical properties remain unchanged. [51] Compared to the OLED with TRZ-CF [λ EL : 476 nm; CIE ¼ (0.17, 0.27)], a slightly lower EQE max of 18.7% was obtained for the sky-blue device with TRZ-DCF [λ EL : 484 nm; CIE ¼ (0.20, 0.33)].
The use of a phenyl substituent on the arylene bridge was explored by Lee et al. [52] to modulate the conformation of the emitter. 10 wt% in DPEPO) reveals how ortho-disubstitution leads to a more twisted conformation (dihedral angle for 1PCTRZ of 60.2°is smaller than that of 2PCTRZ of 68.2°), which translates into a smaller ΔE ST and a shorter τ d , but without negatively impacting Φ PL . However, the donor group adopts a less twisted conformation that those of TCZTRZ(Me), TCZTRZ(Me2p) and TCZTRZ(Me2o), all of which contain methyl substituents. This suggests that the phenyl group is effectively less bulky than the methyl substituent. Liao et al. have also explored the use of fluorine atom decoration of the phenylene bridge [53] in order to explore their influence on the photophysical properties of the emitters. They found that fluoro-substitution can reduce the ΔE ST as a result of an increased dihedral angle between the bridge and the donor by cross-comparing TCTZ (Φ PL : 84%; τ d : 10.09 μs; ΔE ST : 0.13 eV; 6 wt% in DPEPO), TCTZ-F (Φ PL : 92%; τ d : 9.20 μs; ΔE ST : 0.10 eV; 6 wt% in DPEPO) and TCTZ-2 F (Φ PL : 88%; τ d : 6.41 μs; ΔE ST : 0.08 eV; 6 wt% in DPEPO), as shown in Figure 26. The calculated dihedral angles of the ground-state optimized structures between the tercarbazole donor with the phenylene bridge increase slightly from 53.2°, 56.7°to 57.3°with increasing number of fluorine atoms. The minor changes to the dihedral angle are consistent with the small volume of the fluorine atoms. The strongly inductively withdrawing character of fluorine is responsible for the progressive stabilization of the S 1 state from 3.06, 2.96, to 2.90 eV for TCTZ, TCTZ-F and TCTZ-2 F, respectively, while the triplet energies remain almost the same at 2.87, 2.86 to 2.85 eV, respectively. It is interesting to observe that TCTZ-F has the best balance between a small ΔE ST and reasonably high oscillator strength, which is manifested in the highest Φ PL of the three compounds. The best EQE max was  Isosteric trifluoromethyl groups have also been introduced onto the arylene bridge in a series of blue TADF emitters: TRZCz-Me, TRZCz-Me-1, TRZCz-DMe, TRZCz-CF3, TRZCz-CF3-1, TRZBuCz-CF3, and TRZBuCz-CF3-1, as shown in Figure 27. Another strong electron-withdrawing group, CN, was used in lieu of the trifluoromethyl groups in a series of emitters reported by Hong et al. (Figure 27). [55] Compared to TRZCz-CF3, CzCNTRZ (λ PL : 458 nm; Φ PL : 46%; τ d : 47.9 μs; ΔE ST : 0.21 eV; 10 wt% in DPEPO) shows a similar emission maximum and improved Φ PL but at the cost of a larger ΔE ST of 0.21 eV. The similar emission was attributed to the identical electron-withdrawing ability of the two electron-withdrawing groups; however, the larger ΔE ST is indicative of a smaller steric bulk from the CN substituent. Thanks to the higher Φ PL , the EQE max of the CzCNTRZ-based device reached 13.9%. Similar to the PL spectrum, the CzCNTRZ device showed blue (λ EL ¼ 461 nm) emission with CIE coordinates of (0.15, 0.16), which resembles the emission profile of the TRZCz-CF3 device (λ EL ¼ 464 nm).
Hong et al. [56] also examined the effect of incorporation of a CN unit acting as a secondary electron acceptor on the TADF properties ( Figure 27). The addition of the nitrile intensified the CT character of the emitters due to the enhanced acceptor strength, evidenced by the red-shifted emission spectrum.   Lee et al. [57] reported dBFCzTRZ (λ PL : 420 nm; Φ PL : 90%; τ d : 30 There is a small sacrifice in Φ PL of 81% for dBFCzCNTRZ compared to 90% for dBFCzTRZ. The OLEDs based on dBFCzCNTRZ and dBFCzTRZ demonstrated high EQE max of 27.5% and 22.6%, respectively. dBFCzCNTRZ with the lower Φ PL , however, produced a higher efficiency device which may be attributed to the better electron mobility resulting from the presence of the nitrile group. The operational lifetime (LT 80 Table 11, and representative device performance is summarized in Table 12. www.advancedsciencenews.com www.adpr-journal.com www.advancedsciencenews.com www.adpr-journal.com

Functionalization of the Triazine
Lee et al. [58] developed a blue TADF emitter, mtCzTRZ (λ PL : 469 nm; Φ PL : 65%; τ d : 118.6 μs in 1 wt% in PS; ΔE ST : 0.06 eV), by attaching a para-methoxy group to one of the phenyl rings of the TRZ acceptor, which was designed to blue shift the emission compared to tCzTRZ by weakening its electron-withdrawing capacity ( Figure 28). The control compound, tCzTRZ (λ PL : 469 nm; Φ PL : 67%; τ d : 89.4 μs in 1 wt% in PS; ΔE ST : 0.05 eV), showed identical PL and similar photophysical properties to that of mtCzTRZ, indicating that there is negligible influence of the photophysical properties due to the presence of the methoxy group on the TRZ acceptor. However, the modification resulted in an improvement in the device performance. The solution-processed OLED with mtCzTRZ dispersed into the (5-(tert-butyl)-2-(4-(tert-butyl)phenoxy)phenyl)diphenylphosphine oxide (POBBPE) [59] host as the emitting layer, a more soluble analog of DPEPO, showed an EQE max of 16.1% at λ EL of 461 nm and CIE coordinates of (0. 16 [60] is an emitter containing a triazine acceptor with an inductively electron-withdrawing para-fluoro group onto one of the phenyl rings of the TRZ ( Figure 28). 3,6-Di-tert-butylcarbazole groups connected at the ortho positions of the other two phenyl rings complete the donor-acceptor structure. Their placement was to prevent intermolecular interactions, a similar strategy to that used in o-CzTRZ ( Figure 4). FTRZTCz showed a PL spectrum in toluene with the maximum at 450 nm, which though not directly comparable is similar to the PL maximum of o-CzTRZ (λ PL 455 nm, 5 wt% in mCP) in mCP matrix. The missing information related to the compound without the fluoro group makes it difficult to conclude how much the fluoro substituent contributes to the shift of the PL given the similar donor-ortho-acceptor skeleton. However, it is still possible to infer the influence of the fluoro group by comparing the properties of the compound with the previously reported molecule o-CzTRZ, which only has one ortho-connected carbazole. Xu et al. [61] employed diphenylphosphine oxide (PO) as secondary acceptors within a series of D-A-A-type TADF emitters with the collective name of xtBCznPO3-nTPTZ (x ¼ o, m, and p, corresponding to placement of the diphenylphosphine oxide at the ortho-, meta-, and para-positions; and n ¼ 1 and 2, corresponding to the number of diphenylphosphine oxide acceptors), in which tBCz is 3,6-di-tert-butylcarbazole, and TPTZ is triphenyltriazine (aka TRZ) as shown in Figure 29. The incorporation of the electron-withdrawing PO group serves to stabilize the excited states and to enhance the CT character, leading to a greater localization of the FMO distributions. By virtue of this design strategy, ptBCzPO2TPTZ (λ PL : 494 nm; Φ PL : 96%; in 10 wt% in DPEPO; ΔE ST : 0.01 eV) as the best example among this series, has almost unity Φ PL (96%) and very efficient RISC efficiency, Φ RISC , of 98% and k RISC of 5.42 Â 10 4 s. These properties established the basis for the high-performance OLED, which showed an EQE max of 28.9% at λ EL of 492 nm, corresponding to CIE coordinates of (0.18, 0.42). It is worth noting that the analog p-CzTRZ (Figure 4) without the PO groups does not show any TADF properties due to its too large ΔE ST of 0.36 eV in 10 wt% in DPEPO. The other derivatives in this work showed moderate Φ PL s ranging from 38% to 74%. The photophysics and electrochemical data of the aforementioned materials are summarized in Table 13, and representative device performance is summarized in Table 14.

Intramolecular Through Space Charge Transfer (TSCT) of Triazine Based TADF Emitters
The prior examples have implicated an S 1 state that has CT character and that the charge transfer process is mediated by www.advancedsciencenews.com www.adpr-journal.com a π-conjugated bridging moiety. This type of CT is termed TBCT. [62] There exists a second CT process that is mediated by π-stacking of the donor and acceptor units, termed through-space charge transfer (TSCT). Distinct from most of the TBCT emitters that feature strong electron coupling through covalent bonds, TSCT emitters possess weaker electronic interaction between donors and acceptors due to the relatively smaller overlap of the FMOs in these systems. The degree of electronic coupling is controlled by the distance and the relative angles between the donor and acceptor groups. For example, an  www.advancedsciencenews.com www.adpr-journal.com ortho-substitution pattern between the donor and acceptor moieties leads to sterically congested, co-facially aligned donor and acceptor groups in the molecule, inducing either a π···π* type (as in the B-oTC emitter) or an n···π* type (as in TRZ-oCz) interaction. The conformationally restricted structures result in limited change in the geometry of the emitter in the excited states and small reorganization energies, reflected in the narrower emission profiles of these compounds. Swager et al. [63] reported a series of TADF emitters possessing a TSCT state. As shown in Figure 29, the design of XPT, XCT, and XtBuCT is based on a xanthene scaffold with a donor and an acceptor co-facially aligned at a well-defined distance. These compounds possess very small ΔE ST values as evidenced by DFT calculation (Figure 30), and the close alignment between the donor and acceptor groups restricts motion in the solid state, resulting in aggregation induced delayed fluorescence (AIDF) as exemplified by the increase in Φ PL from dilute toluene to 10 wt% DPEPO films of XPT and XtBuCT [XPT (Φ PL : 7.7% in toluene, Φ PL : 66% in 10 wt% in DPEPO), and XtBuCT (Φ PL : 6.0% in toluene, Φ PL : 35% in 10 wt% in DPEPO). The strength of the electron donor directly impacted the corresponding emission color of the emitters as evidenced by the PL maximum shifting from 419 nm in XCT to 451 nm in XPT and 562 nm in XtBuCT. The OLED with XPT showed a 10% EQE max with λ EL at 586 nm while XtBuCTbased device exhibited a 4% EQE max (limited by it lower Φ PL ) with λ EL at 488 nm.
Lee et al. [64] found that ortho-linked donor and acceptor compounds showed superior TADF properties to those where the donor was either meta-or para-disposed. The authors compared the optoelectronic properties of oBFCzTRZ, mBFCzTRZ, and pBFCzTRZ, which are based on ortho-, meta-, and para-linked diphenyltriazine and benzofurocarbazole groups ( Figure 31 Expanding on the ortho-disposed donor-acceptor platform, Lee et al. [65] reported green TADF emitters DPA-o-TRZ and MPA-o-TRZ (Figure 31), which incorporated a diphenylamine group whose electron donating ability is stronger than benzoflurocarbazole as in oBFCzTRZ. Using DFT calculations, Bredas et al. [66] investigated the origins of the TSCT in a number of TADF emitters ( Figure 32). Two pathways for TSCT were identified: www.advancedsciencenews.com www.adpr-journal.com intramolecular π···π and n···π noncovalent interactions. The mechanism of the π···π interaction is the one that is operational for XPT, XCT, and XtBuCT. [63] However, in the cases of TRZ-oCz [17] and TRZ-oBFCz, the computations indicated a very close packing between one of the nitrogen atoms of the triazine ring with the carbazole plane, which the authors asserted triggered noncovalent interactions between the lone-pair electrons of this triazine nitrogen atom and the carbazole π electrons. The n ! π* interactions were identified as critical for enhancing the single-tÀtriplet spinÀorbit coupling, and as a result, greatly facilitating the RISC process. Further examples based on an ortho-connection strategy include the incorporation of a dibenzofuran as the bridge in BCzTRZDBF, TCzTRZDBF, and IDCzTRZDBF ( Figure 33) Figure 30. HOMO and LUMO orbital distributions and calculated bandgaps, singlet (S 1 ), triplet (T 1 ) energy levels, and oscillator strengths ( f ) for XPT, XCT, and XtBuCT based on TD-DFT at the B3LYP functional and 6-31 G* basis set. Reproduced with permission. [63] Copyright 2017, American Chemical Society.
www.advancedsciencenews.com www.adpr-journal.com where substitution next to the oxygen of the dibenzofuran induced a large conformational distortion. [67]  TADF molecules based on ortho-disposed donors and acceptors were shown by Bredas and co-workers to contain emissive excited states that comprise both TBCT and TSCT character. Yang et al. [68] reported a series of highly twisted emitters, SF34oTz, SF23oTz and SF12oTz, containing indolin-fused spirobifluorene donors (Figure 33). According to the DFT computations, the ratios of TBCT/TSCT were initially calculated by integrating the transition density localized on/not on the phenylene bridge. Among them, SF34oTz (Φ PL : 65% in 10 wt% DPEPO) showed a dominant TSCT (96.8% as suggested by TD-DFT calculations) and a relative larger ΔE ST of 0.29 eV, whereas SF23oTz (Φ PL : 86% in 10 wt% in DPEPO; ΔE ST : 0.08 eV) and SF12oTz (Φ PL : 92% in 10 wt% in DPEPO; ΔE ST : 0.05 eV) exhibit higher TBCT contributions, with the ratio increasing to 21% and 32%, respectively. The higher TBCT contribution in SF12oTz leads to much higher ratio of delayed fluorescence (79.1% vs 39.2%) compared with SF23oTz, which the authors asserted was due to the more effective channel of TBCT over TSCT to realize charge transfer from donors to the acceptor. However, the largest ΔE ST of 0. 29    Many OLEDs that contain TADF compounds that emit from an excited state containing some TSCT character still suffer from low device performance. [63] Recently, Kaji et al. [69] and Liao et al. [70] elucidated the importance of the relative orientation of donor and acceptor groups on the electronic coupling between the two in TSCT compounds that influences also the magnitude of the nonradiative decay. Similar to the other TSCT molecules  www.advancedsciencenews.com www.adpr-journal.com proportional to the Φ PL of these emitters. In contrast, the devices based on the more flexible and less electronically coupled DM-X (20 wt% in DPEPO) and DM-Z (30 wt% in DPEPO) exhibit considerably lower maximum efficiencies of 4.3% and 3.2%, respectively. Kaji et al. [69] reported a TSCT compound TpAT-tFFO ( Figure 35) based on the design strategy of controlling the distance and relative orientation between the adjacent tilted donor and acceptor moieties attached via the triptycene (Tp) scaffold. According to the DFT calculations, there is an increase in the energy levels of 1 CT and 3 CT states with increasing distance between the donor and acceptor (Figure 36b top). By contrast, the 3 LE state only shows a weak dependence on the donoracceptor distance. RISC is mediated by the intervening 3 LE state between the 1 CT and 3 CT states, and the high k RISC of 1.2 Â 10 7 s À1 is ensured by the energy level matching of the three states. Importantly, the authors showed by DFT calculations that tilted face-to-face orientation between the donor and acceptor greatly enhances the SOC between the 1 CT and 3 LE states, which is not the case for the coplanar conformation. TpAT-tFFO exhibited strong sky-blue emission with a λ PL of 485 nm. The Φ PL in toluene markedly increased from 1.8% to 84% after 30 min of Ar bubbling, while the Φ PL of the film 25 wt% doped into the mCBP host and for the neat film were determined to be 76% and 71%, respectively. A device using an optimized doping concentration of 25 wt% showed an EQE max of 19.2%. A gentle efficiency rolloff was obtained with EQEs of 19.1% and 18.1% at 100 cd m À2 and 1,000 cd m À2 , respectively; moreover, EQE of 14.4% and 11.6% were retained even at high luminance of 10 000 cd m À2 and 20 000 cd m À2 .
Monkman et al. [72] developed two unsymmetric donordonor 0 -acceptor (D-D'-A) type emitters, Ph 3 TRZCzTPA and Ph 2 TRZCzTPA (Figure 35), where the co-facial overlap between D and A is controlled by the introduction of a weak (rigid) carbazole donor bridge (D'). According to the crystallographic data, the pendant (spacer) aryl rings are highly twisted from www.advancedsciencenews.com www.adpr-journal.com the carbazole bridge (D 0 ) due to congested steric interactions. The short distances between the donor and acceptor enable the through space interaction. By comparing with the emission at 492 nm from the exciplex between TPA and TRZ in toluene, the identical emission observed from Ph 3 TRZCzTPA and Ph 2 TRZCzTPA indicates that the CT emission in both comes from a through-space intramolecular TRZ-TPA CT excited state.
No contribution from a through-bond, TRZ-weak D' CT pair was observed. Both Ph 3 TRZCzTPA and Ph 2 TRZCzTPA have the same small ΔE ST of 20 meV in Zeonex matrix. However, Ph 2 TRZCzTPA has a faster k RISC than Ph 3 TRZCzTPA, which is ascribed to the more optimal tilted co-facial orientation of D and A forming the through-space CT state, which the authors contend is critical in controlling SOC and thus the RISC rate, as proposed by Kaji et al. [69] The EQE max values of the devices based on Ph 3 TRZCzTPA (20 wt% in 26DCzPPy) and Ph 2 TRZCzTPA (12 wt% in CBP) are 13.3% and 16.3%, respectively, which correlate with the relative RISC rates of the materials.
The π-stacked scafold has also been expanded to spatially confine donor/acceptor/donor (D/A/D) motifs. [73] Similar to the monomer analogs of DM-B and DM-Bm, [70] DM-BD1 and DM-BD2 also rely on spatially confined donor-acceptor interaction but with two donors within one molecule in a sandwich-like structure ( Figure 35). The single-crystal structures of DM-BD1 and DM-BD2 are shown in Figure 37. The additional donor is asserted to be helpful in realizing a more parallel D/A alignment, which results in a small ΔE ST (0.00 eV for DM-BD1 and À0.07 eV for DM-BD2) and fast k RISC (2.9 Â 10 5 s À1 for DM-BD1 and 3.1 Â 10 5 s À1 for DM-BD2). The torsion angles between the donor and acceptor moieties were determined to be 30°for DM-BD1 and 25°for DM-BD2 from the crystal structures, where the not quite co-facial orientation has been demonstrated by Kaji et al. [69] to be critical to turn on spin-orbit coupling and facilitate RISC. Interestingly, both compounds showed broad, structureless CT emission that are almost identical to those in solution, with a λ PL of 495 nm. The Φ PL of DM-BD1 (10 wt% doped in DPEPO matrix) and DM-BD2 (10 wt% doped in DPEPO matrix) www.advancedsciencenews.com www.adpr-journal.com were determined to be 94.2% and 92.8%, respectively. The near unity Φ PL provides evidence of the effectiveness of this strategy to suppress nonradiative loss mechanisms. The best OLEDs contained a 30 wt% emitter and showed EQE max of 28.0% and 26.6%, respectively, for devices with DM-BD1 and DM-BD2, corresponding to CIE coordinates of (0.21, 0.47) and (0.20, 0.46), respectively. To enhance the D-A electronic interactions by adjusting the D-A distance, Zhang et al. [74] employed a xanthene bridge in two TSCT TADF emitters, mCz-Xo-TRZ and dCz-Xo-TRZ  www.advancedsciencenews.com www.adpr-journal.com ( Figure 38), that possess space-confined face-to-face D-A alignment and minimized D-A distance down to 2.7-2.8 Å, which is shorter than twice the van der Waals radius of a carbon atom (1.7 Å). As a result, the greatly strengthened electronic interaction between D and A leads to fast k r of 9.9 and 8.7 Â 10 6 s À1 for mCz-Xo-TRZ and dCz-Xo-TRZ, respectively. The stronger donor in dCz-Xo-TRZ versus mCz-Xo-TRZ leads to a red-shift of the PL spectrum in toluene (from 454 to 461 nm) and the 30 wt% doped film in bis(diphenylphosphinyl)-dibenzofuran (PPF) matrix (from 469 to 482 nm,). The Φ PL values of 90% and 92% for mCz-Xo-TRZ and dCz-Xo-TRZ, respectively, are high; however, the k RISC values are 3.0 Â 10 5 s À1 and 3.3 Â 10 5 s À1 , respectively, which are significanlty slower than that of TpAT-tFFO (1.2 Â 10 7 s À1 ). The blue OLEDs showed broad emission with λ EL of 477 and 464 nm for devices with dCz-Xo-TRZ and mCz-Xo-TRZ, corresponding to the CIE coordinates of (0.16, 0.29) and (0.15, 0.20), respectively. The EQE max of 27.8% for the device with dCz-Xo-TRZ and 21.0% for the device with mCz-Xo-TRZ showed only modest efficiency roll-off where the EQE values remained at 24.0% and 17.1% at a luminance of 1000 cd m À2 . Zysman-Colman and co-workers recently reported a systematic study that documented explicitly in the PL spectrum emission from a TSCT in the compound TPA-ace-TRZ [75] (Figure 39). This work provides a direct evidence that the TSCT plays a major role in the communication between the donor and acceptor. The photophysical studies in toluene of TPA-ace-TRZ show two characteristic lifetimes corresponding to the fast-decaying throughbond CT (TBCT) state (τ PL ¼ 9.6 ns) and longer lived TSCT state (τ PL ¼ 51 ns). The existance of the two different CT states was ascribed to rapid decay from the initially populated TBCT state (with moderate D-A dihedral angles of 48°between the TPA and ace units and 57°between the ace and TRZ units) to the more stable TSCT state. The Φ PL of TPA-ace-TRZ was measured to be only 17%, which is due to the weak electronic communication between the donor and aceeptor through the ace bridge (the ace unit is orthogonal and electronically decoupled from both D and A groups). It was found the the lowest-energy triplet state resides on the ace bridge, which also leads to a large ΔE ST of 0.48 eV (determined in 1 wt% ZEONEX film). The weak electronic communication makes it difficult for the triplet harvesting through the spin-vibronic coupling mechanism because the potentially mediating local triplet state resides on the ace bridge, which is orthogonal to both D and A and thus cannot efficiently couple to the TSCT state. Therefore, even though TPA-ace-TRZ possesses a strong TSCT state, it can hardly produce TADF because of the lack of coupling to a mediating LE triplet state. This study reveals the intimate interplay that the bridging ace group has on mediating both the TBCT state and the TSCT state.
Wang et al. [76] incorporated TSCT states in the design of two star-shaped TADF emitters, containing DMAC (Ac3TRZ3) or a dendritic teracridan (TAc3TRZ3) as donors and TRZ as acceptors about a hexaphenylbenzene scaffold ( Figure 40). Because of the Figure 38. TSCT-TADF molecules with different types of donor-acceptor alignments and chemical structures of mCz-Xo-TRZ and dCz-Xo-TRZ with their HOMO/LUMO distribution. Reproduced with permission. [74] Copyright 2022, Wiley-VCH. Figure 39. Molecular strucutre of TPA-ace-TRZ and schematic diagram of the potential energy involved in the S 0 and S 1 states. Reproduced with permission. [75] Copyright 2021, American Chemical Society.
www.advancedsciencenews.com www.adpr-journal.com steric hindrance inherent in the structure both molecules adopt a propeller-shaped conformation in which the peripheral aromatic units are almost perpendicular to the central phenyl ring. As a result of the strong TSCT character coupled with the weak TBCT character due to the physical separation of donors and acceptors, both molecules show small ΔE ST values of 0.04 eV for TAc3TRZ3 and À0.08 eV for Ac3TRZ3. The negative ΔE ST is likely due to different molecular geometries in the fully relaxed singlet and triplet states as also identified in DM-Bm and DM-G. Compared to Ac3TRZ3 [Φ PL : 54% 10 wt% in Ac6 (control compound contains six acridan donors], TAc3TRZ3, containing the stronger dendritic donor, shows a higher Φ PL of 63% (10 wt% in Ac6). The authors ascribed this higher photoluminescence quantum yield to the more efficient charge transfer in TAc3TRZ3 mediated by the spatial π-π interactions. The use of the stronger dendritic donors also leads to a red-shift of the emission spectrum from λ PL of 486 nm for Ac3TRZ3 to λ PL of 508 nm for TAc3TRZ3.  [77] also prepared a series of blue TADF polymers that exploit only TSCT to electronically couple donor and acceptor moieties (Figure 41). The polymers use a nonconjugated polyethylene backbone, 9,9-dimethyl-10-phenyl-acridan (Ac) or 9,9-bis(1,3-di-tert-butylphenyl)-10-phenyl-acridan (TBAc) as the donor pendant and TRZ as the acceptor pendant. In this design, D and A units are not directly electronically decoupled, but are spatially close to each other, allowing through-space, rather than the TBCT processes to occur. The TSCT character observed from the Ac-based polymer results in both a small ΔE ST of 0.019 eV and moderate Φ PL of 60% in the neat film. In comparison, the TBAc-based polymers only exhibited fast prompt fluorescence emission without TSCT contribution because the steric 1,3-dibutylphenyl groups separate the electron-donating acridan unit from the electron-accepting triazine unit, which weakens the TSCT transition. The device with a polymer consisting of 95 mol% Ac and 5 mol% TRZ content demonstrated the best device performance with CIE coordinates of (0.18, 0.27) and an EQE max of 12.1%. The optoelectronic properties of the aforementioned materials are summarized in Table 15, and the device performance metrics are summarized in Table 16.

Chiral TADF Emitters Containing Triazine
Circularly polarized luminescence (CPL) materials have attracted great attention due to their potential applications in optical data storage, [78] chirality sensing, [79] organic electronic devices, [80] and bio imaging. [81] In 1997, Meijer et al. [82] developed the first example of a circularly polarized OLED (CP-OLED). Since then, many chiral emitters have been explored, including chiral polymers, [83]   In 2019, Zysman-Colman and co-workers [15] introduced the carbazolophane (Czp) donor unit (indolo [2.2]paracyclophane) for the design of chiral TADF emitters, R p -CzpPhTrz and S p -CzpPhTrz ( Figure 42). The bulky carbazolophane donor unit increased the torsion between the donor and the phenylene bridge compared to the control compound CzPhTrz (same structure as CzTRZ discussed in Section 2). As discussed above, CzPhTrz was known to be a non-TADF compound; however, the larger torsion between the donor and the bridge in CzpPhTrz leads to a decreased ΔE ST of 0.16 eV, which contributes to the turn on of the TADF. rac-CzpPhTrz is a sky-blue emitter with λ PL of 482 nm, and high Φ PL of 69% in 10 wt% doped DPEPO films. The chiroptical properties of the enantiomers R p -CzpPhTrz and S p -CzpPhTrz reveal mirror image circular dichroism (CD) and CPL, with g lum values of 5 Â 10 À3 / À7 Â 10 À3 , respectively. Sky blue-emitting OLEDs were fabricated with an EQE max of 17% and associated CIE coordinates of (0.17, 0.25).

TADF Emitters Based on Triazine and Other Donors
In addition to carbazole, many alternative N-heterocycle donors have also been investigated as building blocks in triazine-based TADF emitters. 9,9-dimethyl-9,10-dihydroacridine (DMAC), 10 H-phenoxazine (PXZ), and 10 H-phenothiazine (PTZ) are the most popular six-membered N-heterocyclic donors used in the design of TADF emitters. The sequence of electron-donating strength typically follows PTZ > PXZ > DMAC>carbazole, with carbazole being the weakest donor. Unlike carbazole, the use of these larger N-heterocyclic donors generally produces a highly twisted conformation between the donor and the bridging units when they are N-bound because of the increased steric hindrance. As a result, the highly twisted conformation leads to a greater separation of the FMOs, which generally results in a small ΔE ST . Wu et al. [91] reported what is now considered one of the most widely studied TADF emitters, DMAC-TRZ (λ PL : 495 nm; Φ PL : 90%; τ d : 1.9 μs; ΔE ST : 0.05 eV; in 8 wt% mCPCN) as shown in Figure 49. DMAC-TRZ shows high Φ PL (90%) in an 8 wt% mCPCN doped film, which is not much reduced in the neat film (83%). The EQE max of doped and non-doped device based on DMAC-TRZ were reported to be 26.5% and 20%, respectively, reflecting in part the differences in Φ PL . Due to its good solubility, a solution-processed non-doped OLED showed only a slightly lower EQE max of 17.6%. Hu et al. [92] observed that photoexciting CT states can lead to a magneto-PL signal in the SOC regime, but not found when photoexciting LE states. This is the first experimental evidence that SOC is produced in CT states. Furthermore, they found that the DMAC-TRZ-based OLEDs demonstrated magneto-EL in the high field regime (>10 mT). [92] This high-field magneto-EL signal provides direct evidence to indicate that the SOC is indeed enhanced, in the absence of heavy elements, towards developing spin-dependent TADF in OLEDs. Compared with DMAC-TRZ (λ PL : 495 nm; Φ PL : 90%; τ d : 1.    . Representative CP-TADF molecular structures. Reproduced with permission. [89] Copyright 2020, American Chemical Society. may be ascribed to the energy gap law. [91] An OLED containing 6 wt% PXZ-TRZ doped in CBP as the emitter layer exhibited an EQE max of 12.5% (λ EL of 529 nm). [93] The authors also demonstrated that the orientation of the TDM of PXZ-TRZ can be modulated in mCBP by varying the temperature on the ITO glass holder during the deposition of the emitting layer. The horizontal orientation of the TDM of PXZ-TRZ can be enhanced by lowering the temperature of the ITO glass from 300 K to 200 K. As a result, the EQE max is improved from 9.6% at 300 K to 11.9% at 200 K. [94] The use of PTZ as a donor introduces additional conformational dynamics due to the existence of two ground-state conformers resulting from puckering of the PTZ ring, each with their own associated ΔE ST . [95] PTZ-TRZ shows a slightly red shifted emission maximum (λ PL : 562 nm; in toluene) and similar Φ PL (65.8%; in 2 wt% mCBP) to those of its analog PXZ-TRZ (545 nm in toluene, Φ PL 65.7%; 6 wt% in CBP), [93] while the ΔE ST (0.07 eV) of PTZ-TRZ is the largest amongst the three compounds: DMAC-TRZ, PXZ-TRZ, and PTZ-TRZ. The relatively similar electron-donating strength of the PXZ and PTZ groups in PXZ-TRZ and PTZ-TRZ translates to similar emission spectra and Φ PL . The device containing PTZ-TRZ exhibited an EQE max of 10.8%, which is also of similar performance to that of PTZ-TRZ-based OLED although the two reports [93,95] used different hosts within the emissive layer. The EL spectrum shows two emission bands, one high-energy band at around 393 nm and a stronger broad band at 532 nm, resulting from the simultaneous emission from the two conformers. Reproduced with permission. [89] Copyright 2020, American Chemical Society. www.advancedsciencenews.com www.adpr-journal.com 3ACR-TRZ, an analog of DMAC-TRZ containing 3 donor groups, reported by Kaji et al. [96] shows somewhat similar photophysical properties to DMAC-TRZ, with near unity Φ PL (98%; in 16 wt% CBP) and a slightly red-shifted emission (λ PL : 504 nm; in 16 wt% CBP). The small red-shift implies that there is only a weak influence on the stabilization of the singlet state by increasing the number of DMAC units. However, the multiple donors could further delocalize the distribution of HOMO, thus leading to a smaller ΔE ST (0.015 eV). The solution-processed device showed an EQE max of 18.6% at an emission of λ EL % 520 nm. Analog compounds containing either trisubstituted PXZ or PTZ donor groups have also been reported. [97] Similar evolution of the photophysical properties from the linear D-A compounds to the D 3 -symmetric analogs was observed for tri-PXZ-TRZ (λ PL : 568 nm; Φ PL : 58% in 6 wt% mCP; τ d : 1.10 μs in toluene) and TRZ3(Ph-PTZ) (λ PL : 575 nm in toluene; τ d : 7.2 μs in 2 wt% mCP). [98] The OLEDs based on TRZ3(Ph-PTZ) shows yellowish-green electroluminescence with a λ EL % 550 nm and exhibits a much higher EQE max of 17.4% than those of the devices of the tri-PXZ-TRZ-based OLEDs (EQE max : 13.3%, λ EL : 553 nm). The Zysman-Colman group reported yellow-emitting OLEDs based on tri-PXZ-TRZ by doping the emitter into a bespoke host 4-mCBPy, thus demonstrating an improved device performance with an EQE max of 19.4% and a dramatically reduced efficiency roll-off (EQE ¼ 16.0% at a luminance of 10 000 cd m À2 ). [99] i-DMAc-TRZ (λ PL : 452 nm; Φ PL : 55%; τ d : 1840 μs; ΔE ST : 0.35 eV; in 3 wt% DPEPO) is a constitutional isomer of DMAC-TRZ but where the phenylene bridge is C-bound para to the nitrogen atom of the DMAC donor ( Figure 50). [100] Distinct from the highly twisted conformation of DMAC-TRZ, such a structural change results in a flattened conformation, leading to enhanced conjugation between the DMAC and TRZ, and hence a much larger ΔE ST (0.35 eV) than that found for DMAC-TRZ (0.06 eV). Despite the emission spectrum being blue-shifted from DMAC-TRZ (λ PL : 495 nm) to i-DMAc-TRZ (λ PL : 452 nm), the Φ PL is reduced by almost half. The device-based i-DMAc-TRZ (10 wt% in mCBP) presents a deep-blue emission with λ EL at 450 nm and CIE coordinates of (0.15, 0.11) and an EQE max of 10.9%.
Replacement of the methyl groups on the acridan with a spiro adamantyl unit, as in a-DMAc-TRZ (λ PL : 479 nm; Φ PL : 86.1%; τ d : 4.09 μs in 20 wt% DPEPO; ΔE ST : 0.20 eV) results in a www.advancedsciencenews.com www.adpr-journal.com blue-shifted emission, but with otherwise comparable photophysical properties to that of DMAC-TRZ [91] (λ PL : 495 nm; Φ PL : 90%; τ d : 1.9 μs; ΔE ST : 0.05 eV; in 8 wt% mCPCN). The introduction of the rigid and bulky adamantanyl moiety not only suppresses the geometry relaxation in the excited state but also induced the formation of quasi-axial conformer (QAC) and quasi-equatorial conformer (QEC) geometries corresponding to a shoulder emission peak at around 419 nm and a main emission peak at around 479 nm in the PL spectrum, respectively. The ΔE ST values of two conformers were confirmed separately by different excitation wavelength. With an excitation wavelength of 360 nm, the S 1 and T 1 energies of QAC were calculated to be 2.97 and 2.66 eV, demonstrating a large ΔE ST of 0.31 eV. At an excitation wavelength of 420 nm, the S 1 and T 1 energy levels of a-DMAC-TRZ for QEC were calculated to be 2.79 and 2.59 eV, thus translating to a smaller ΔE ST of www.advancedsciencenews.com www.adpr-journal.com 0.20 eV. Owing to the effect of the rigid molecular backbone and the degenerate alignment of 3 LE of QAC and 3 CT of QEC for efficient dual fluorescence emission, the resulting OLEDs achieved a high EQE max of 28.9% at CIE coordinates of (0.18, 0.35). [104] Kaji et al. [105] demonstrated that by replacing the distal phenyl groups attached to the triazine with adamantyl substituents, the acceptor becomes weaker, leading to a blue-shifted emission compared to DMAC-TRZ, [91] DPAC-TRZ, [102] and SpiroAc-TRZ. [102] Solution-processed devices employing the emitters FA-TA (λ PL : 452 nm in toluene; Φ PL : 76%; τ d : 44.4 μs in 10 wt% CzSi; ΔE ST : 0.16 eV), MA-TA (λ PL : 469 nm in toluene; Φ PL : 83%; τ d : 18.3 μs in 10 wt% CzSi; ΔE ST : 0.14 eV), and PA-TA (λ PL : 450 nm in toluene; Φ PL : 70%; τ d : 69.5 μs in 10 wt% CzSi; ΔE ST : 0.17 eV) exhibited EQE max of 11.2% with CIE coordinates of (0.15, 0.13), 22.1% with CIE coordinates of (0.15, 0.19) and 6.7% with CIE coordinates of (0.15, 0.10), respectively, as shown in Figure 50. MA-TA represents one of the most efficient blue emitters for solution-processed OLEDs reported to date. However, the much poorer performance of the devices with PA-TA and FA-TA is likely in part due to the random orientation of the TDMs in the spin-coated films when compared to the highly horizontally oriented TDMs of DPAC-TRZ and SpiroAc-TRZ in evaporated 12 wt% mCPCN, although the emitters are similar in structure. The higher energy excited states of PA-TA and FA-TA also make it difficult to find suitable host materials or blocking layers in devices, which might be the reason for this poor performance.
The addition of a second phenylene ring within the bridge in DTPDDA resulted in an increased spatial separation between the donor and acceptor, as well as an improved horizontal dipole ratio of the TDM. DTPPDDA (λ PL : 450 nm in toluene; Φ PL : 38%; τ d : 20 ns in 8 wt% mCP/TSPO1; ΔE ST : 0.04 eV) [107] showed a TDM θ value of 0.73 compared to that of DTPDDA (0.66: isotropic). However, the extremely short "delayed lifetime" of 20 ns clearly indicates that DTPPDDA is not a TADF emitter. The biphenylene ring will adversely result in a lower local-excited triplet state energy than the energy of its lowest charge transfer triplet state ( 3 LE < 3 CT) because the former is sensitive to the enhanced conjugation while the latter is predominantly determined by the charge transfer strength. As a result, the upconversion of the triplet excitons become impossible due to its large barrier from the 3 LE to the S 1 state. Although the ΔE ST was determined to be as small as 0.04 eV, it does not align with this analysis. The triplet energy was determined according to the delayed fluorescence that, however, did not provide the delayed time for the measurement of phosphorescence. The OLED employing DTPPDDA showed an EQE max of 4.7% and deep blue emission at CIE www.advancedsciencenews.com www.adpr-journal.com coordinates of (0.15, 0.09). [107] The optoelectronic characterization of the aforementioned materials is summarized in Table 19, and device performance metrics are summarized in Table 20.

Conclusions
The objective of this review is to provide a detailed overview of triazine-based TADF materials, to compare and contrast their optoelectronic properties and to report their performance as emitters in OLEDs. The emission properties of these TADF emitters were mainly modulated via the tuning of the donor strength, modifying the substituents about the triazine, varying the nature of the D-A bridge and switching CT channels (intramolecular CT vs. through-space CT). The plethora of examples contained within this review reveals the versatility of triazine as an acceptor in the design of TADF emitters. By simply varying the strength and number of donors along with the structure of the bridging aryl groups, the emission spectrum can be easily tuned from deep blue through to yellow. The planar conformation adopted by the TRZ moiety can facilitate to the formation of intermolecular exciplexes or through space charge transfer interactions, which enriches the photophysical behavior of these compounds and can contribute to enhancing k RISC . PXZ-TRZ ITO/αÀNPD/6 wt% PXZÀTRZ: CBP/TPBi/LiF/Al 529 -3.5 12.5/À/ À/À [91] PTZÀTRZ ITO/αÀNPD/2 wt% PTZÀ TRZ:mCBP/TPBi/LiF/Al %532 --10.8/À/À À /À [95] Despite the progress that has been made, the potential of triazine as a moiety in TADF emitter design still has not been fully realized. First, one of the major factors affecting the device lifetime of OLEDs is the stability of the emitter. TADF compounds containing a TRZ acceptor have already demonstrated some potential for improved device lifetimes over their analogs containing acceptors like benzophenone or diphenyl sulfone. TRZ has long been recognized as a chemically stable moiety in the design of epoxy resin, [108] polymers, [109] and covalent TRZ frameworks (CTFs); [110] however, OLED stability studies based on TRZ-functionalized TADF emitters remain limited. Studies that probed the influence of intramolecular hydrogen bonding, glass transition temperatures, and charge mobility on the stability of TRZ-functionalized molecules and the impact on the device lifetime would be welcome.
The orientation of the TDM of the emitters, which impacts the light out-coupling efficiency of the device, is correlated with the EQE of the OLED. Due to the rigid and planar structure of TRZ linked to extended donors, a number of TRZ-based compounds have been documented to show preferential horizontal orientation of their TDM, leading to enhanced light outcoupling efficiency in the device, with EQE max > 30%. The parameters controlling the orientation of the TDM during thermal evaporation remain unclear, [111] and this is clearly a design feature that can be exploited further.
Solution-processing techniques such as ink-jet printing are promising for producing large-area OLEDs, which remains challenging and expensive for thermal evaporation. More and more attention has been paid to the development of TRZ-containing TADF dendrimers and polymers as attractive classes of emitters suitable for solution-processed OLEDs. However, in contrast to the advances reported for small molecular weight TADF emitters that show high PLQY, horizontal orientation of their TDM, fast RISC rates, and examples of chiral analogs that show CPL, it remains challenging to develop TADF dendrimers and polymers inheriting these properties. Thus, the performance of solution-processed OLEDs containing TRZ-based macromolecules still lags behind small-molecule TRZ-based evaporated OLEDs. www.advancedsciencenews.com www.adpr-journal.com