Aggregation-induced emission-active thermally activated delayed fluorescent materials for solution-processed organic light-emitting diodes: a review

Abstract

Thermally activated delayed fluorescence (TADF) materials with aggregation-induced emission (AIE) properties have attracted great attention recently. Specifically, multiple-resonance TADF (MR-TADF) emitters are of particular interest as next-generation narrowband luminophores for organic light-emitting diodes (OLEDs) due to their intrinsically narrow emission bands, high photoluminescence efficiencies, and facilely tunable emission colors. The incorporation of AIE behavior into TADF or MR-TADF platforms has been a successful approach towards maximizing exciton utilization in the solid state. In contrast to traditional planar luminophores plagued by aggregation-caused quenching (ACQ), AIE-active emitters are characterized by emission enhancement upon aggregation, enabling the realization of OLEDs with high external quantum efficiencies and low efficiency roll-off. In this review, we present recent progress in AIE-active traditional TADF emitters that are classified based on their emission color (blue, green, and yellow/red) with a focus on non-doped OLED device structures. Additionally, AIE-active MR-TADF materials are presented and compared with their traditional TADF counterparts. Part of the special emphasis is placed on molecular design tenets, structure–property correlations, photophysical phenomena, and device performance optimization. This review tries to give insights into rational molecular design strategies for the construction of next-generation AIE-active TADF and MR-TADF emitters for high-performance OLED applications.

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Introduction

Over the past three decades, intensive research has been carried out on the development of high efficiency, low cost and more stable OLEDs for extensive use in solid-light emitting and flat-panel displays.^1,2 Since the advent of fluorescent and phosphorescent materials, OLEDs have reached the level of commercial implementation. Despite such developments, both fluorescent OLEDs (FOLEDs) and phosphorescent OLEDs (PHOLEDs) have significant drawbacks.^3–6 FOLEDs are plagued by an intrinsic limitation of internal quantum efficiency (IQE), with a limit set at around 25%, because of the formation of singlet excitons only through electrical excitation (Fig. 1(A)).⁷ On the other hand, PHOLEDs employ heavy metal complexes, mainly based on iridium (Ir) or platinum (Pt), to increase spin–orbit coupling (SOC), thus facilitating the spin–forbidden radiative decay of triplet excitons; hence, their IQE is 100% (Fig. 1(B)).^8–11 However, PHOLEDs suffer from difficulty in achieving stable and appropriate color purity and the issues of toxicity, cost, and availability of metal-based materials. To make non-emissive triplet excitons more useful, a number of other approaches have been developed. These involve the utilization of hybridized local and charge-transfer (HLCT) states, the so-called “hot exciton” processes,¹² triplet–triplet annihilation¹³ (TTA, or P-type delayed fluorescence), and E-type delayed fluorescence. E-Type delayed fluorescence, a photophysical mechanism first identified in 1961 by Parker and coworkers while characterizing Eosin,¹⁴ was subsequently revisited in relation to OLEDs. In 2012, Adachi and co-authors implemented this mechanism in OLEDs and proposed the phenomenon of thermally activated delayed fluorescence (TADF).^15–19 TADF materials facilitate the efficient up-conversion of triplet excitons to emissive singlet states by means of thermal activation and provide a promising class of emitters for future OLED technologies. Similar to other phosphorescent materials, TADF materials can trap both singlet and triplet excitons using a highly efficient reverse intersystem crossing (RISC) process (Fig. 1(C)). Thermal activation allows TADF materials to achieve 100% internal quantum efficiency (IQE). Together, these strategies work towards eliminating the inherent inefficiencies of traditional OLED systems and opening the door for high-performance, cost-efficient, and environmentally friendly light-emitting devices.

Fig. 1 Illustration of the mechanism of traditional fluorescent OLEDs (A), phosphorescence OLEDs (B) and TADF OLEDs (C). Abbreviations: F = fluorescence; P = phosphorescence; ISC = intersystem crossing; RISC = reverse intersystem crossing; ΔE_ST = singlet–triplet energy gap, KF = radiative fluorescence rate; and Knr = non-radiative decay rate.

Boltzmann statistics indicate that when the energy gap between the singlet and triplet excited states ΔE_ST is small enough, RISC becomes thermodynamically beneficial.²⁰ The exchange interaction between the two unpaired electrons in the excited states determines ΔE_ST in molecular terms.²¹ By spatially separating the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) through rational molecular design, a reduced ΔE_ST can be engineered. Based on this idea, a variety of effective TADF materials have recently been created, including structural motifs such as triazine,²² benzophenone,^23,24 sulfone,^25,26 xanthone^27,28 and spiro-fluorene.^29,30 OLEDs with these TADF emitters have shown outstanding electroluminescent (EL) characteristics comparable to those of the latest phosphorescent OLEDs (PHOLEDs). This method has allowed for remarkable external quantum efficiencies (EQEs) in OLEDs with blue, green, yellow, orange, and red TADF materials.^31–35

The design criteria of TADF emitters are inherently controlled by the electronic and spatial modulation of the donor (D) and acceptor (A) units in a single molecular construct (Fig. 22C). The essence is to realize effective separation of the frontier molecular orbitals, i.e., the HOMO and the LUMO, such that ΔE_ST is minimized while ensuring adequate orbital overlap for effective radiative transition. A limited ΔE_ST provides effective RISC from the triplet towards the singlet excited state and thus allows for the harvesting of both singlet and triplet excitons for light emission and eventually enhances the internal quantum efficiency. In molecular design, a balanced charge transfer (CT) and locally excited (LE) character are optimal for inhibiting nonradiative decay channels and enhancing radiation decay rates. This fine balance directly affects the PLQY, a key parameter governing the overall device efficiency. Molecular architectures with too much donor–acceptor separation tend to have poor orbital overlap, leading to low oscillator strength and reduced rates of radiative decay. Too little separation, however, increases ΔE_ST and inhibits RISC and delayed fluorescence efficiency. Therefore, a meticulously designed intermediate level of orbital interaction is required to attain both a small ΔE_ST and a high PLQY. Torsioned donor–acceptor molecular geometry has been recognized as a popular strategy to attain this balance (Fig. 22C). The torsional configuration between the acceptor and donor units spatially separates the HOMO and LUMO distributions, resulting in a minimized ΔE_ST that enables the thermally activated up-conversion of triplet excitons. Nonetheless, over-twisting leads to over-isolated orbitals with dramatically lower radiative transition probabilities. Thus, the precise adjustment of the dihedral angle between the acceptor and donor pieces is vital for maximizing the charge-transfer efficiency and emission intensity. Systematic molecular design and computational guidance have allowed ΔE_ST values down to 0.01 eV, allowing for external quantum efficiencies (EQEs) nearing and even surpassing 40% in state-of-the-art TADF emitters.^36–45

However, in the case of TADF emitters, which have dominated the literature, such emitters are normally found to have broad emission bands and large Stokes shifts, with FWHM values generally >70 nm (Fig. 22A), thus sacrificing color purity. This broadening of the spectrum is a universal drawback of traditional TADF emitters. In such systems, intense ICT interactions between the donor and acceptor units and large structural relaxation in the excited state cause strong vibronic coupling between the first excited singlet (S₁) and ground (S₀) states. As a result, the reorganization energy is enhanced, causing broad emission bands and less spectral sharpness. Narrowband emission is thus essential for achieving high-color-purity OLEDs for use in advanced displays. Particularly, emission spectra need to meet or get close to BT.2020 (Rec. 2020) standard promulgated by the International Telecommunication Union (ITU) for ultra-high-definition (UHD) television screens, requiring exceptionally pure and saturated red, green, and blue color coordinates.^46–50

Although optical engineering methods may be utilized to reduce the emission bandwidth and improve color purity, these measures tend to bring with them some practical disadvantages, such as decreased external quantum efficiency (EQE), intricate device architecture, higher fabrication costs, and increased power consumption.^51,52 Consequently, the most viable and sustainable strategy is the creation of intrinsic narrowband TADF materials that can simultaneously impart high emission efficiency and enhance color purity without the use of external optical modulation. These emitters need to accomplish a fine balance of photophysical and structural parameters, reducing reorganization energy and quenching nonradiative transitions while ensuring efficient RISC and high radiative decay. Advances along these lines, notably by molecular design approaches like multiple-resonance (MR) frameworks and rigid polycyclic architectures, are likely to lead the way for high-performing OLEDs that satisfy the stringent criteria of next-generation display technologies.

The fabrication process is pivotal in dictating the performance, reproducibility, and scalability of OLEDs. Vacuum thermal evaporation (VTE) and solution processing are the two main fabrication pathways that hold great interest in OLED research and fabrication (Fig. 2). Both methods have differential merits and drawbacks that not only affect the efficiency of the device but also manufacturability at large scales and affordability. VTE has been the best-known method for high-performance OLEDs since its development by Tang and Van Slyke in 1987.⁵³ The sublimation of low-molecular-weight organic material under high vacuum (10⁻⁶–10⁻⁷ Torr) and its subsequent deposition onto pre-patterned substrates is the process involved. VTE facilitates nanometer-scale control over the film thickness and enables the formation of complex multilayer architectures with well-defined interfaces. Such structural accuracy promotes balanced charge injection and transport, leading to superior efficiency, stability, and reproducibility of the devices.^54–56 In addition, VTE-based devices possess high color purity and excellent device uniformity as a result of the tight, pinhole-free films of evaporated films. Nevertheless, this technique has significant limitations in terms of scalability and cost. The need for ultra-high vacuum chambers, high thermal energy, and fine metal masks greatly complicates production and increases energy consumption.^57–60 The material utilization efficiency is also generally less than 30% since a significant portion of the evaporated material condenses on chamber walls and not on the substrate. The limited shadow mask capability also severely limits the pixel density and color patterning in large-area displays. Contrarily, solution-processing methods have come forward as viable substitutes for cost-effective, large-area OLED manufacturing.^61–64 Such techniques encompassing spin coating, inkjet printing, blade coating, and slot-die coating rely on soluble organic semiconductors and polymers to coat liquid solutions on thin films (Fig. 2). Solution processing also has some inherent benefits: (i) high material utilization efficiency (>80%); (ii) compatibility with flexible and stretchable substrates; (iii) low-temperature operation, which is suitable for roll-to-roll manufacturing; and (iv) ease of compositional tuning for doped or blend systems. Of these, inkjet printing has been particularly promising for high-resolution patterning with resolutions of up to 1200 dpi without the use of shadow masks, qualifying it for next-generation display technologies.^65,66 Issues remain in the creation of well-defined multilayer architectures since subsequent solution deposition can partially dissolve or blend underlayers. Additionally, film uniformity, phase purity, and controlled morphology are non-trivial to attain, especially for small-molecule materials. From the perspective of performance, OLEDs produced using thermal evaporation tend to have better EQE, operation stability, and color uniformity mainly because of better interfacial control and layer purity. However, solution-processed OLEDs, although sometimes lower in efficiency by a margin, have tremendous advantages related to scalability, cost-effectiveness, and process simplicity. The development of solution-processable TADF emitters and AIE-active materials has further diminished the gap in performance between the two fabrication methods by reducing exciton quenching and enhancing film quality in solution-deposited systems. A comparative overview of both methods is given in Table 1, with an emphasis on their major differences with respect to process parameters, device structure, and commercial feasibility.

Fig. 2 Schematic of the vacuum thermal evaporation and solution-processing OLEDs.

Table 1 Comparative overview of the vacuum thermal evaporation (VTE) and solution-processing OLED device fabrication procedures

Parameter	Vacuum thermal evaporation (VTE)	Solution-processing
Process environment	High vacuum (10^−6–10^−7 Torr)	Ambient or inert atmosphere
Operating temperature	Elevated (150–300 °C)	Low to moderate (<100 °C)
Material utilization	<30%	>80%
Device architecture	Precise multilayer, sharp interfaces	Limited to single or few layers
Patterning technique	Metal shadow masks	Inkjet or screen printing
Film morphology	Dense, pinhole-free	Dependent on the solvent and drying kinetics
Scalability	Limited to small or medium areas	Suitable for large-area and flexible devices
Production cost	High (complex vacuum system)	Low (solution-based)
Device efficiency/stability	High	Moderate to high (improving)

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Overall, though VTE is still the gold standard for laboratory-scale high-performance OLED production, solution processing provides an appealing route to low-cost, large-area, and flexible optoelectronics. The further advancement of solution-processable emitters, especially AIE-TADF and AIE-MR-TADF systems, along with advancements in interfacial engineering and solvent orthogonality, is likely to close the remaining performance gap and drive the evolution of OLED technology from vacuum-based to fully printable, scalable manufacturing platforms.

Aggregation-induced emission (AIE) is a peculiar photophysical phenomenon whereby luminogenic materials show weak or even no fluorescence in diluted solutions but emit strong luminescence in the aggregated or solid state (Fig. 3).^67–74 It is contrary to the normal aggregation-caused quenching (ACQ) behavior common to traditional π-conjugated molecules. The AIE effect mainly comes from the hindrance of intramolecular motions (RIM), both rotation and vibration, upon aggregation. In solution, active intramolecular motions allow for nonradiative relaxation from the excited to the ground state, which leads to weak emission. During aggregation, motions are impeded, hindering nonradiative channels and enhancing efficient radiative processes. Accordingly, AIE offers an effective means to reverse concentration quenching and exciton–exciton annihilation, allowing for strong luminescence in the solid state. Efficient emission in neat solid films can be achieved by AIE-active materials and has displayed outstanding optoelectronic performance in OLEDs.^74–83 AIE-active molecules possess superior solid-state emission efficiency, morphological stability, and processability compared to traditional emitters, which render them particularly suitable for large-area solution-processed devices. The construction of AIE-active units into TADF scaffolds has proved to be a highly promising strategy for maximizing exciton utilization efficiency in the solid state. In TADF systems, the efficient harvesting of singlet and triplet excitons via the RISC process is vital for high electroluminescence efficiency. However, normal donor–acceptor TADF emitters tend to experience aggregation-induced quenching (AIQ) and exciton diffusion losses at high doping levels or in pure films. The incorporation of AIE functionality efficiently alleviates these issues by limiting intermolecular π–π stacking and silencing exciton quenching routes. By means of rational molecular design, scientists have reported a family of AIE-active TADF emitters that combine rigid multiple-resonance (MR) or donor–acceptor (D–A) cores with flexible or bulky peripheral substituents (Fig. 22). The hybrid structure enables the combinability of narrowband emission, high PLQYs, and good film-forming processes. In addition, the synergy between AIE and TADF processes provides further benefits, such as lower energy loss, during exciton conversion, improved stability at high current densities, and compatibility with non-doped or solution-processed device structures. All these combined aspects make AIE-TADF emitters the most desirable for next-generation high-performance OLEDs with high efficiency, superior color purity, and excellent stability. In summary, the combination of AIE and TADF or MR-TADF concepts is a significant breakthrough in organic optoelectronics. The creation of AIE-active TADF or MR-TADF emitters effectively addresses the issue of the efficient utilization of excitons and stable solid-state emission, offering a universal platform for the design of high-performance materials that are applicable to both display and lighting technologies. Further advancement in this area, especially through molecular engineering of rigidity, conjugation, and donor–acceptor alignment, will likely speed up the achievement of energy-efficient OLED devices, meeting the rigorous requirements of ultrahigh-definition (UHD) displays and other newer photonic technologies.

Fig. 3 Aggregation-induced emission (AIE) behavior of luminogenic materials: these compounds have weak or no fluorescence in diluted solution (THF) (left) but are highly emissive upon aggregation (right).

In this review, we present recent progress in AIE-active traditional TADF emitters classified based on emission color (blue, green, and yellow/red) with a focus on non-doped OLED device structures. Additionally, AIE-active MR-TADF materials are presented and contrasted with their traditional TADF counterparts. A portion of special emphasis is placed on molecular design tenets, structure–property correlations, photophysical phenomena, and device performance optimization. This review attempts to provide insight into rational molecular design strategies for the construction of next-generation AIE-active TADF and MR-TADF emitters for high-performance OLED applications. Particular emphasis is placed on the generic molecular design principles underlying AIE and TADF cooperation, including steric engineering, conformational restriction, donor–acceptor modulation, heteroatom incorporation, and multiple-resonance core construction. The interconnection of molecular structure, excited-state kinetics, emission width, reverse intersystem crossing (RISC) efficiency, and aggregation behavior is examined in detail in order to emphasize significant structure–property correlations. In addition, the impact of these photophysics on device performance metrics, including external quantum efficiency, color purity, efficiency roll-off, and operational stability, is addressed across typical state-of-the-art OLED systems. Overall, the present review intends to provide a systematic and thought-provoking overview of rational molecular engineering approaches toward the development of next-generation AIE-active TADF and MR-TADF emitters. By bridging the concepts of molecular design with solid-state photophysics and device optimization, we intend to provide directions for the development of high-performance, narrowband, and efficiency-stable OLED technologies.

Blue AIE-active traditional TADF materials for solution-processed OLEDs

Blue light-emitting materials incorporating AIE-active TADF OLEDs are becoming a more essential and hard topic in industry and academia. All OLED displays require a large number of blue pixels to produce appropriate display brightness. Though blue AIE-active TADF materials have a higher band gap than other hues (green, yellow, red, etc.), researchers face a challenge in developing high band gap materials with good stability. This section provides a comprehensive explanation of blue AIE-active TADF materials, including their molecular structures, photophysical properties, and solution-processed OLED device performance.

C. Yang et al. described the design and synthesis of two isomeric blue-emitting compounds, o-ACSO₂ 2 and m-ACSO₂ 3, derived from a DMAC-DPS 1 core framework (Fig. 5).⁸⁴ These positional isomers showed marked AIE features together with efficient spatial charge transfer, as well as maintaining favorable TADF properties. Out of the two, m-ACSO₂ 3 exhibited excellent photophysical properties with a high PLQY of 76%, a low singlet–triplet energy gap (ΔE_ST) of 0.07 eV, and good film-forming properties. Consequently, a non-doped, solution-processed OLED employing m-ACSO₂ 3 as the emissive material displayed efficient sky-blue electroluminescence with a peak emission wavelength (λ_em) of 486 nm and Commission International de l'Éclairage (CIE) coordinates of (0.21, 0.34). The device had a maximum current efficiency (CE_max) of 37.9 cd A⁻¹, a power efficiency (PE_max) of 23.8 lm W⁻¹, and an external quantum efficiency (EQE_max) of 17.2% (Table 2a). In addition, using 2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (DPEPO) as the electron-transporting material, which is known to be inert to intermolecular interactions, efficiently prevented unwanted exciplex formation between the emissive layer (EML) and the electron transport layer (ETL). Prevention of such exciton losses decreases exciton losses and improves emission color purity. Moreover, with adaptive electron-blocking layer (EBL) engineering, the emission properties may be controlled from a common PN heterojunction-type usually undesirable in blue OLEDs to a homojunction-like exciton recombination zone (Fig. SI1), thus maximizing the performance of the AIE-active m-ACSO₂ 3 emitter.

Table 2 a) Device performances of blue AIE-DADF-OLEDs 1–10. (b) Device performances of blue AIE-DADF-OLEDs 11–23

(a)
Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
DMAC-DPS 1	464	74	474/—	(0.18, 0.26)	5.0	—	4.6/1.8/2.6	84
o-ACSO2 2	508	66	492/—	(0.23, 0.40)	4.4	—	14.1/7.8/5.9
m-ACSO2 3	475	76	486/—	(0.21, 0.34)	4.1	—	37.9/23.8/17.2
CBM-DMAC 4	501	46.7	499/—	(0.28, 0.47)	4.5	2599	14.3/6.4/6.7	85
TTT-Ph-Cz 5	422	42	426/74	(0.16, 0.07)	5.2	107	1.61/—/3.45	86
TTT-Ph-AC 6	470	79	480/80	(0.16, 0.27)	4.0	1257	18.06/—/9.73
TTT-Ph-BAC 7	556	32	498/89	(0.21, 0.39)	3.6	1382	15.47/—/6.77
DPAC-BPCTPA 8	480	39.8	482/—	(0.19, 0.30)	4.4	615	5.8/3.6/2.9	87
DMAC-BPCTPA 9	490	55.7	494/—	(0.24, 0.41)	4.1	2821	8.1/3.7/3.7
3DMAC-BPCTPA 10	487	64.8	486/—	(0.22, 0.31)	4.4	1267	13.3/7.3/4.8

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(b)
Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
3TCPM 11	478	35	487/—	(0.27, 0.43)	3.9	315	8.0/4.2/3.4	88
2TCPM 12	464	20	471/—	(0.21, 0.29)	4.4	568	3.9/1.7/1.8
1TCPM 13	510	50	489/—	(0.26, 0.42)	3.8	1472	17.9/11.2/7.6
TCTA-BP-DMAC 14	558	57.6	488/—	(0.21, 0.41)	3.1	2040	22.8/14.9/9.8	89
Ib-0 15 + mCP (10 wt%)	437	22.8	474/—	(0.18, 0.29)	4.0	260.7	13.12/—/6.86	90
P-Cz4CzCN 16	470	65	492/—	(0.24, 0.47)	5.0	6289	26.6/12.7/11.5	92
P-Cz4CzCN 16 + red dopant (1.8%)	—	—	484, 604	(0.46, 0.40)	3.3	2837	20.1/12.6/9.5
P4CzCN-PA 17 + 4TCzBN (40%)	436	—	484/—	(0.18, 0.26)	5.3	—	28.8/13.7/13.4	93
P4CzCN-BCz 18 + 4TCzBN (60%)	435, 355	—	495/—	(0.25, 0.37)	5.8	—	14.4/6.5/6.9
P4CzCN-BCz 19 + 4TCzBN (60%)	435, 355	—	490/—	(0.24, 0.39)	5.7	—	19.8/7.6/7.6
P4CzCN-BCz 20 + 4TCzBN (60%)	435, 355	—	493/—	(0.24, 0.39)	5.8	—	17.1/8.3/8.2
3phCN-Cz 21 + 4TCzBN (20%)	394	52.6	481/—	(0.15, 0.28)	4.9	6036	21.91/—/11.53	94
3phCN-ICz 22 + 4TCzBN (20%)	437	56.3	481/—	(0.16, 0.30)	3.5	5071	41.88/—/22.04
3phCN-tCzICz 23 + 4TCzBN (20%)	440	52.8	479/—	(0.15, 0.26)	3.6	3546	28.21/—/16.6

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Z. Qi et al. synthesized a series of AIE-active TADF materials, including CBM-DMAC 4, 3CPyM-DMAC 46, 2CPyM-DMAC 47, CBM-PXZ 51, 3CPyM-PXZ 103, and 2CPyM-PXZ 104, using tert-butylcarbazole (tBuCz), 9,9-dimethyl-9,10-dihydroacridine (DMAC), and phenoxazine (PXZ) as electron-donating moieties, coupled with pyridine-substituted benzophenone as the electron-accepting core (Fig. 5, 11 and 21).⁸⁵ One of the most important molecular design strategies was to incorporate intramolecular hydrogen bonding, which largely cut off nonradiative decay channels and improved solid-state luminescent efficiency. The steric bulk of the DMAC unit in these materials (CBM-DMAC 4, 3CPyM-DMAC 46, and 2CPyM-DMAC 47) effectively hindered detrimental π–π stacking interactions unlike their planar PXZ analogues (CBM-PXZ 51, 3CPyM-PXZ 103, and 2CPyM-PXZ 104), which tend to be more aggregation-caused quenching-prone. To verify their AIE behavior, the photoluminescence properties were examined in tetrahydrofuran THF/water mixtures. In pure THF, all the compounds showed weak emission, but as the proportion of water increased to 30–80%, the PL intensity increased gradually. Particularly, at 90% water content, the intensity of the PL was ∼40-fold higher than in pure THF, affirming strong AIE behavior. Additional supporting evidence was provided through PLQY measurements, while all the materials had low PLQYs (0.5–3.4%) in solution, and heavily increased PLQYs were noted in the solid state, marking their excellent AIE features.⁸⁵ The materials were then used in solution-processed, undoped OLEDs with the structure: ITO/PEDOT:PSS (40 nm)/Emissive Material (40 nm)/TPBi (30 nm)/Cs₂CO₃ (2 nm)/Al. Among them, the 3CPyM-DMAC 46-based device had the highest EQE_max of 11.4%, which is higher than other analogues (Tables 2a, 5 and 10). This work highlights the efficacy of incorporating intramolecular hydrogen bonding into AIE-TADF emitters as an efficient means of achieving non-doped solution-processing OLEDs.

In another study, Y. Wang et al. prepared three blue-emitting TADF compounds (TTT-Ph-Cz 5, TTT-Ph-AC 6, and TTT-Ph-BAC 7) by employing carbazole (Cz) and acridine (AC, BAC) derivatives as donor fragments and a tristriazolotriazine (TTA) core as the acceptor (Fig. 5).⁸⁶ The design strategy for the three blue AIE-active TADF compounds is shown in Fig. 4. Among the three, BAC-functionalized derivative TTT-Ph-BAC 7 showed the most intensive AIE behavior. Even though no emission was noticed in pure THF, a progressive increase in PL intensity was realized upon increasing the proportion of water, notably being highly significant at a 50% water fraction. With 90% water, a remarkable PL intensity enhancement was observed, where emission at its peak occurred at 491 nm just about 197 times that in THF, exhibiting strong AIE behavior (Fig. SI2). Devices prepared through solution processing with these materials showed blue TADF emissions at 460–470 nm in the solid film form. Among them, the device of TTT-Ph-AC 6 showed the highest performance, with a maximum EQE of ∼10% at an emission peak of 480 nm (Table 2a).

Fig. 4 Design strategy for blue AIE-active TADF TTT-Ph-Cz 5, TTT-Ph-AC 6, and TTT-Ph-BAC 7 compounds.

TADF materials with AIE and superior film-forming properties are pivotal for the development of efficient, solution-processed, non-doped OLEDs. In this context, Z. Qi et al. communicated a new family of AIE-active TADF emitters that feature acridine and phenoxazine as electron donors and phenyl ketone units as acceptors. The resulting compounds are DPAC-BPCTPA 8, DMAC-BPCTPA 9, 3DMAC-BPCTPA 10, PXZ-BPCTPA 48, 3DPAC-BPCTPA 49, and 3PXZ-BPCTPA 50 (Fig. 5 and 11).⁸⁷ These materials were tuned to have highly twisted molecular structures, which indeed minimized intermolecular packing forces in the solid state, leading to enhanced luminescence efficiency and efficient RISC. Additionally, the incorporation of branched alkyl chains into the molecular backbones enhanced the free volume, which favored film formation by inhibiting pinhole formation upon solution processing. The AIE features of these compounds were confirmed by PL measurements in DMF/water mixtures with increasing water content (Fig. SI3). For instance, DPAC-BPCTPA 8 showed weak emission in neat DMF, but its PL intensity notably increased when the water content was 50%. Likewise, 3DPAC-BPCTPA 49 also showed a significant enhancement in PL even at 10% water composition.⁸⁷ Other derivatives showed similar trends, unmistakably proving their AIE features. These results suggest that the synthesized compounds exhibit good AIE features with high levels of AIE activity, rendering them excellent candidates for non-doped OLED devices. Solution-processed OLED devices fabricated from these materials demonstrated good electroluminescent performance. Devices that used PXZ-BPCTPA 48 as the emissive layer displayed blue emission at 486 nm with CIE values of (0.22, 0.31) and an EQE_max of 4.8% (Table 5). Conversely, 3PXZ-BPCTPA 50-based devices showed green emission at 535 nm with CIE coordinates of (0.38, 0.56) and a considerably enhanced EQE_max of 12.1%. This article highlights an efficient molecular design route in which AIE and TADF characteristics are merged with enhanced film-forming properties, providing a stable route for the production of high-performing, solution-processed, non-doped OLEDs.

Fig. 5 Chemical structures of blue AIE-TADF materials 1–10.

H. Zhu et al. prepared a series of AIE-active TADF molecules (3TCPM 11, 2TCPM 12, 1TCPM 13, and 1TCPM-Cz 52) with a phenyl ketone acceptor and triphenylamine donor unit (Fig. 7 and 11).⁸⁸ Selectively fixing triphenylamine to the C1, C2, and C3 positions of the carbazole spacer modulated the charge transfer mechanism from through-bond charge transfer (TBCT) to through-space charge transfer (TSCT). The TSCT-type materials possessed stronger AIE features owing to their highly twisted molecular geometries and hindered intramolecular motions, which efficiently hindered nonradiative decay. Undoped, solution-processed OLEDs were fabricated using the following device structure: ITO/PEDOT:PSS (40 nm)/emitter (40 nm)/TPBi (30 nm)/Cs₂CO₃ (2 nm)/Al. The 1TCPM-Cz 52 device displayed green EL with CIE coordinates of (0.29, 0.48), and the maximum EQE was 13.3% and CE was 35.5 cd A⁻¹ (Table 5). Moreover, 1TCPM 21 produced blue EL with CIE coordinates of (0.26, 0.42) and recorded the highest EQE of 7.6% and CE of 17.9 cd A⁻¹ (Table 2b). These results demonstrate a promising strategy for designing high-performance TSCT-type AIE-TADF emitters through accurate donor–acceptor conformation modulation.

B. Z. Tang et al. designed two AIE-active TADF emitters, TCTA-BP-DMAC 14 and TCTA-BP-PXZ 106 using a donor–acceptor–donor arrangement with benzoyl (acceptor), dimethylacridine (DMAC) or phenoxazine (PXZ) (donors), and a dendritic TCTA unit to facilitate charge transport and film-forming efficiency (Fig. 7 and 20).⁸⁹ Both emitters exhibited considerably higher PLQYs in neat films (53.5% and 57.6%, respectively) than in solution (4.9% and 7.2%) because of the AIE effect. Vacuum-deposited undoped and doped TCTA-BP-PXZ 106-based OLEDs had EQEs of 6.9% and 17.6%, respectively, with low efficiency roll-off (<0.6%) due to small ΔE_ST and effective RISC. A yellow light-emitting (CIE: 0.40, 0.56) solution-processed OLED based on this molecule exhibited a maximum EQE of 10.7% with stable luminance at high efficiency (Table 10). Concurrently, TCTA-BP-DMAC 14 exhibited sky-blue EL (CIE: 0.21, 0.41) with an EQE of 9.8% (Table 2b). The data substantiate the success of the molecular design in formulating efficient and stable solution-processed and vacuum-deposited OLEDs.

Y. Wang et al. prepared multifunctional AIE-TADF materials Ib-0 15, Ib-1 67, Ib-2 68, and Ib-3 69 using diphenyl sulfone and DMAC units (Fig. 7 and 16).⁹⁰ The TSCT and TBCT characteristics were demonstrated by the ortho-substituted derivatives, with solid-state PLQYs of 88–100%, which is significantly higher than that of DMAc-BPSB (47%).⁹¹ The design strategy for TADF active multifunctional emitters and their respective molecular structures are depicted in Fig. 6. These materials showed AIE, TADF, RTP, and triboluminescence (TL) behaviors at the same time. Among solution-processed OLEDs, the Ib-2 68 device performed best at a doping level of 15 wt%, with a maximum EQE of 20.1% and a CE of 56.73 cd A⁻¹ (Table 7). The Ib-1 67 device provided moderate performance (EQE: 12.34%, CE: 32.45 cd A⁻¹ at 5 wt%). Devices incorporating Ib-0 15 and Ib-3 69 were less efficient (with EQEs of 6.86% and 11.28%, respectively) due to their lower PLQYs. This work forms the basis for the design of multifunctional high-efficiency organic emitters.

Fig. 6 Design strategy for thermally activated delayed fluorescence (TADF)-active multifunctional emitters and their respective molecular structures.

Fig. 7 Chemical structure of blue AIE-TADF materials 11–23.

Two AIE-active TADF polymers, P-Cz4CzCN 16 and P-4CzCN 53, with twisted structures consisting of carbazole donors and cyanophenyl acceptors in a propeller-like geometry were reported by W. Jiang et al. (Fig. 7 and 11).⁹² This structure facilitated solid-state AIE activity and minimized exciton quenching. The P-Cz4CzCN 16 polymer had a smaller ΔE_ST (0.12 eV) and a larger PLQY (65%) compared to P-4CzCN 53 (37%). P-Cz4CzCN 16-based OLEDs emitted sky-blue EL at 492 nm (CIE: 0.24, 0.47) and a peak EQE of 11.5% (Table 2b). Due to its dense polymer network and solvent stability, the material also acted as a blue host in purely solution-processed hybrid white OLEDs with Ir(MDQ)₂(acac) red emitter. With 1.8 wt% doping, the device showed white emission, maximum EQE of 9.5%, CE of 20.1 cd A⁻¹, and PE of 12.6 lm W⁻¹, with stable spectra over voltages and a high CRI of 76 (Fig. SI4). Extending to polymer hosts, Jiang et al. also designed four benzonitrile-based AIE-TADF polymers (P4CzCN-PA 17 and P4CzCN-BCz (18–20)) with high triplet energies (>2.68 eV) and intensive AIE behavior (Fig. 7).⁹³ PL spectra of THF/water mixtures established their AIE features. With the use of 4TCzBN as a TADF dopant, the P4CzCN-PA 17 host supported blue EL at 484 nm with an EQE of 13.4%. However, P4CzCN-BCz 20 resulted in sky-blue EL (493 nm) but with inferior efficiency (EQE: 8.2%, Table 2b). The lower performance was attributed to the large ΔE_ST and orbital shielding effects of the large BCz group, which arrested energy transfer and radiative decay (Fig. SI5). Jiang et al. disclosed three twisted D–A designed blue TADF host materials (3phCN-Cz 21, 3phCN-ICz 22, and 3phCN-tCzICz 23) with high triplet energy levels (>2.8 eV) and strong AIE features (Fig. 7).⁹⁴ Their AIE characteristics were confirmed through PL spectra of THF/water mixtures. OLEDs with these hosts and 4TCzBN as the dopant had high efficiencies. The best performance was of the device with 3phCN-ICz 21, which provided a maximum EQE of 22.04% and a CE of 41.88 cd A⁻¹ (Table 2b). This indicates the potential of high-triplet-energy AIE materials as being suitable for solution-processed blue TADF OLEDs.

X. Ban et al. designed and developed two AIE-active TADF emitters, TRZCz-Cz 24 and TRZCz-Bz 25, with a donor–acceptor molecular design (Fig. 9).⁹⁵ In these molecules, the donor is carbazole and the acceptor is phenylbenzimidazole. Photophysical analyses showed that both compounds exhibited small ΔE_ST and clear AIE behavior, which is conducive to improving exciton utilization in electroluminescent devices. The AIE behavior was explored through PL measurements in THF/water mixtures with different water percentages. The PL intensity first decreased with an increase in water content to 40% because of the twisted intramolecular charge transfer (TICT) effect. When the water content was further increased, the PL intensity became higher, indicating the AIE nature of both materials (Fig. SI6). Non-doped solution-processed blue OLEDs were prepared utilizing both compounds as emitters. The TRZCz-Cz 24 device showed the highest EQE of 8.7%, which was more than twice that of TRZCz-Bz 25 (EQE = 3.9%) and exhibited a high performance of TRZCz-Cz 24. Moreover, by doping the blue phosphorescent emitter Iridium(III)bis[2-(4,6-difluorophenyl)pyridyl-N,C2′]picolinate (FIrpic), the TRZCz-Cz 24-based PHOLED achieved an EQE of up to 18.4% and a CE of 38.6 cd A⁻¹. Taking advantage of the high efficiency of TRZCz-Cz 24 in blue non-doped OLEDs, the authors also prepared phosphorescent white PHOLEDs (WPHOLEDs) with bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)iridium(III) (Ir(MDQ)₂(acac)) as a red dopant and TRZCz-Cz 24 as blue host (Fig. SI7). The obtained WPHOLED exhibited outstanding performance, with EQE of 16.5%, CE of 33.8 cd A⁻¹, and CIE coordinates of (0.33, 0.38) (Table 3), showing high color purity and device efficacy. This article is a prime example of a successful molecular design strategy for achieving high-performance AIE-TADF materials applicable to OLED.

Table 3 Device performances of blue AIE-DADF-OLEDs 24–32

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
TRZCz-Cz 24	439	—	468/—	(0.17, 0.28)	3.6	6977	14.4/9.0/8.7	95
TRZCz-Cz 24 + FIrpic (10%)	—	—	505/—	(0.23, 0.48)	5.0	10 966	38.6/20.2/18.4
TRZCz-Bz 25	438	—	477/—	(0.18, 0.27)	4.0	3786	7.3/4.2/3.9
TRZCz-Bz 25 + FIrpic (10%)	—	—	505/—	(0.23, 0.44)	5.4	8620	28.8/13.9/11.9
TRZCz-Bz 25 + Ir(MDQ)2(acac) (W2)	—	—	—	(0.33, 0.38)	3.2	9548	33.8/21.3/16.5
Compound 26 + DMAC-TRZ (30%)	461	54	486/—	(0.18, 0.32)	3.78	19 986	45.15/32.95/22.35	96
32clCBP 27 + DPEPO (20%)	490	80	478/—	(0.18, 0.32)	3	3350	35.5/34.3/16.3	98
32clCXT 28 + DPEPO (20%)	492	87	484/—	(0.18, 0.32)	2.9	9360	50.8/54.3/21.2
32PclCXT 29 + DPEPO (20%)	506	99	500/—	(0.18, 0.32)	2.8	22 040	88.3/85.3/29.9
Cz-4CzCN 30	452	—	479/81	(0.19, 0.32)	3.5	5435	27.03/16.97/4.35	99
mCP-4CzCN 31	450	—	469/67	(0.17, 0.25)	3.0	6436	31.72/24.9/15.86
TCz-4CzCN 32	450	74	473/66	(0.17, 0.27)	3.4	5806	26.01/16.33/12.63

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Despite rapid progress in OLED technologies, efficiency remains limited in devices composed of multiple functional components due to complex design and fabrication challenges. To address this issue, V. Nutalapati et al. employed a successive functionalization strategy to develop a novel terpyridine-based host material (compound 26, Fig. 9).⁹⁶ Scanning electron microscopy (SEM) and photophysical analysis confirmed that the synthesized compound exhibited strong AIE characteristics. Doped blue OLEDs were fabricated using DMAC-TRZ⁹⁷ as the emitter and compound 26 as the host. Among the various doping concentrations tested, the 30 wt% doped device demonstrated the best performance, emitting at 486 nm with CIE coordinates of (0.18, 0.32). This device achieved a maximum EQE of 22.35%, CE of 45.15 cd A⁻¹, and PE of 32.95 lm W⁻¹ (Table 3), confirming the effectiveness of this host-dopant system for high-performance solution-processed blue OLEDs.

In another study, Z. Chi et al. reported a set of three high-performance blue AIE-active TADF emitters: 32clCBP 27, 32clCXT 28, and 32PclCXT 29 (Fig. 9).⁹⁸ These molecules were constructed with indolocarbazole-based donor units and electron-accepting moieties of xanthenone (XT) or benzophenone (BP). Compared to BP, the XT unit provided greater planarity and rigidity, which suppressed structural relaxation in the excited state and significantly improved PLQYs in the aggregated state. Photoluminescence measurements in DMSO/water mixtures demonstrated a marked increase in emission intensity upon water addition, indicating prominent AIE behavior (Fig. SI8). The incorporation of indolocarbazole enhanced AIE and enabled effective TADF characteristics in these molecules. Among the solution-processed doped OLEDs fabricated at a 20 wt% dopant concentration, the device based on 32PclCXT 29 delivered outstanding performance with a maximum EQE of 29.9%, CE of 88.3 cd A⁻¹, and PE of 85.3 lm W⁻¹ (Table 3). All devices exhibited low turn-on voltages and blue to sky-blue EL emissions, demonstrating the significant potential of indolocarbazole-based emitters for efficient AIE-TADF OLED applications.

Recently, X. Ban et al. reported a series of dendritic AIE-active TADF materials (Cz-4CzCN 30, mCP-4CzCN 31, TCz-4CzCN 32, and 4OCzCN 97) by incorporating flexible carbazole units and multiple carbazole dendrons on the periphery of the parent TADF fluorescent core (Fig. 9 and 19).⁹⁹ Encapsulation by carbazole dendrons drastically minimizes the free volume of the molecular structure and augments intermolecular interactions, leading to a compact and rigid framework. Such structural confinement efficiently inhibits non-radiative relaxation channels, which convert traditional ACQ into AIE, thus increasing radiative transition efficiency, emission intensity, and solid-state emissive layer stability. The peripheral wrapping groups also shield the TADF core against exciton quenching triggered by intermolecular interactions, allowing the stretchable film to retain a high PLQY of up to 72%. The fluorescence properties of the emitters in poor solvents were also studied systematically. The emission intensity shows dramatically increased values with increasing water content, verifying the clear-cut AIE features (Fig. SI9). The materials were then used in solution-processed (Fig. 8b), non-doped TADF OLEDs with the structure: ITO/PEDOT:PSS (40 nm)/emissive layer (40 nm)/TmPyPB (40 nm)/Cs₂CO₃ (1 nm)/Al (100 nm) (Fig. 8c). The EL peaks of Cz-4CzCN 30, mCP-4CzCN 31, and TCz-4CzCN 32 are centered at 479 nm, 469 nm, and 473 nm, respectively (Table 3). The blue shift and the decrease in FWHM with the increase in the number of peripheral carbazole units are found to be in line with the evolution of the PL spectrum. Contrarily, the unencapsulated TADF core 4OCzCN 97 emitter shows a significant red shift (EL peaks) at 511 nm (Table 9) and broadening of the spectrum, which is due to exciton build-up and quenching from intermolecular interactions. The unencapsulated parent core 4OCzCN 97 device was plagued by heavy concentration quenching, with a best luminance of only 3155 cd m⁻², a relatively high turn-on voltage of 4.0 V and a moderate maximum EQE of 4.35% (Table 9). Comparatively, the structurally optimized Cz-4CzCN 30, which involves carbazole–alkyl chain encapsulation, showed much better performance with a maximum luminance of 5435 cd m⁻² at a lower turn-on voltage of 3.5 V and a maximum EQE of 11.65%. Optimization with mCP-4CzCN 31 led to better device properties in terms of delivering a low turn-on voltage of 3.0 V, a high maximum luminance of 6436 cd m⁻², and an excellent maximum EQE of 15.86% (Table 4). These findings confirm that the integration of extra peripheral carbazole units efficiently improves luminescence efficiency and that the encapsulation of flexible alkyl chains provides a useful route to achieving high-performance TADF emitters. The excessive inclusion of carbazole units, as in TCz-4CzCN 32, resulted in lower performance, with a lower maximum luminance of 5806 cd m⁻², a maximum EQE of 12.63%, and a higher turn-on voltage of 3.4 V (Table 3). However, the OLED from a 20% stretchable composite film (Fig. 8d) has a high EQE of 8.62%, maximum luminance of 2542 cd m⁻² (Fig. 8e and f), and low turn-on voltage at 4.5 V and shows stable electroluminescence after 500 bending cycles with a curvature radius of 0.78 cm. Such degradation can be attributed to unbalanced carrier transport due to excess carbazole moieties. Thus, the exact control of peripheral carbazole unit numbers is necessary to match luminescent efficiency with charge-transport performance in dendritic TADF molecular designs. This research provides a general molecular design approach for designing stretchable small-molecule semiconductors and substantially enhances the viable application of TADF materials in future-generation wearable and flexible optoelectronic devices.

Fig. 8 Stretchable light-emitting complex with TADF and deformation stability characterization of flexible OLEDs. (a) Chemical structure of TADF material encapsulated by a flexible alkyl chain with different number of carbazole units. (b) Schematic of the spin-coating process and subsequent stretching of a stretchable luminescent layer. (c) Schematic of light emitting layer design for stretchable devices. (d) Emission images of the PET-based OLEDs captured at high luminance under random deformation. (e) Current density–voltage–brightness (J–V–L) curves of transfer devices with different stretching degrees. (f) EQE value curve of the transfer device. Reproduced with permission from ref. 99. Copyright 2025, Elsevier.

Fig. 9 Chemical structures of blue AIE-TADF materials 24–32.

Green AIE-active traditional TADF materials for solution-processed OLEDs

In contrast to blue AIE-active TADF materials, the molecular design of green-emitting analogs generally requires the extension of π-conjugation or enhancement of donor–acceptor interaction strength, which leads to a red shift in the emission spectrum. The present study investigates different classes of green AIE-active TADF emitters, such as dendrimers, polymers, and single-molecule systems, and evaluates their photophysical properties and OLED device performance.

Y. Tao et al. prepared two AIE-active TADF emitters, m-DTPACO 33 and p-DTPACO 34, using triphenylamine (TPA) as the hole-transporting moiety and phthaloyl as the electron-acceptor unit, with a positional difference on the central phenyl ring (Fig. 10).¹⁰⁰ The two emitters showed both AIE and TADF characteristics, with PLQYs increasing from 2% and 46% in solution to 39% and 75% in solid films for m-DTPACO 33 and p-DTPACO 34, respectively. In solution-processed doped OLEDs, the devices gave green EL and had maximum EQEs of 13.0% (m-DTPACO 33) and 9.0% (p-DTPACO 34), and CE/PE values of 43.5 cd A⁻¹/33.3 lm W⁻¹ and 29.3 cd A⁻¹/25.4 lm W⁻¹ (Table 4).

Fig. 10 Chemical structures of green AIE-TADF materials 33–45.

Table 4 Device performances of green AIE-DADF-OLEDs 33–45

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
m-DTPACO 33 + 4CzCNPy	510	75	520/—	(0.26, 0.48)	3.5	—	29.3/25.4/9.0	100
p-DTPACO 34 + 4CzCNPy	608	39	520/—	(0.26, 0.48)	3.2	—	43.5/33.3/13.0
DBT-BZ-DMAC 35	532	8.3	508/—	(0.26, 0.55)	2.7	27 270	43.3/35.7/14.2	101
DBT-BZ-DMAC 35 + CBP 6 wt%	492	65.9	—	(0.23, 0.51)	3.3	11 200	51.7/50.7/17.9
DBT-BZ-PXZ 36	594	57.5	557/—	(0.43, 0.54)	2.9	—	26.6/27.9/9.2	102
DBT-BZ-PXZ 36 + CBP 6 wt%	—	—	528/—	(0.34, 0.57)	7.0	—	60.6/59.2/19.2
DBT-BZ-PTZ 37	604	76.8	563/—	(0.44, 0.53)	2.7	—	26.5/29.1/9.7
DBT-BZ-PTZ 37 + CBP 10 wt%	—	—	538/—	(0.37, 0.56)	6.8	—	46.0/43.3/15.1
G2B 39	461	33.4	500/—	(0.26, 0.48)	3.4	—	14.0/11.5/5.7	103
G3B 40	461	21.1	516/—	(0.31, 0.50)	3.7	—	7.7/5.7/2.9
tBuG2B 41	473	74	502/—	(0.27, 0.52)	2.7	4922	46.6/40.7/17.0	104
MeG2B 42	473	48	500/—	(0.28, 0.48)	2.6	661	23.5/25.0/9.0
PhG2B 43	464	41	500/—	(0.30, 0.48)	2.7	965	22.7/20.0/8.8
TATC-BP 44	550	22	549/—	(0.41, 0.54)	2.6	—	17.8/20.0/5.9	105
TATC-BP 44 (30 wt%): H2	—	—	—	(0.37, 0.53)	2.8	—	48.1/47.8/15.9
TATP-BP-45	543	24	541/—	(0.38, 0.55)	2.7	—	18.9/19.2/6.0
TATP-BP-45 (30 wt%): H2	—	—	—	(0.34, 0.53)	2.8	—	46.4/47.2/15.4

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B. Z. Tang et al. presented a multifunctional emitter, DBT-BZ-DMAC 35, with AIE, TADF, and mechanoluminescence (Fig. 10).¹⁰¹ The molecule utilized benzoyl (BZ) as the acceptor and DMAC and dibenzothiophene (DBT) as the donors. A twisted D–A conformation facilitated both the AIE and TADF effects. The PLQY of the neat film Was up to 80.2%, which surpassed 65.9% in the case of the CBP-doped films. Non-doped OLEDs exhibited superior efficiency (EQE = 14.2%), while doped devices optimized further to EQE = 17.9%, CE = 51.7 cd A⁻¹, and PE = 50.7 lm W⁻¹ (Table 4). The same group extended this platform further by replacing DMAC with phenoxazine (PXZ) or phenothiazine (PTZ) to give DBT-BZ-PXZ 36 and DBT-BZ-PTZ 37, respectively (Fig. 10).¹⁰² The increased rigidity of PXZ/PTZ donors enhanced charge transport and D–A separation. The PLQYs for the CBP-doped films were 57.5% and 76.8% for the PXZ and PTZ derivatives, respectively. The calculated ΔE_ST values were still small: 0.09 eV (PXZ) and 0.05 eV (PTZ), similar to that for the DMAC analogue (0.08 eV). Moreover, non-doped devices had moderate (EQE ≈ 9.2–9.7%) performance, doped OLEDs had better efficiency, and DBT-BZ-PXZ 36 had EQE = 19.2%, CE = 60.6 cd A⁻¹, and PE = 59.2 lm W⁻¹, while DBT-BZ-PTZ 37 had EQE = 15.1%, CE = 46.0 cd A⁻¹, and PE = 43.3 lm W⁻¹ (Table 4).

Yamamoto's group synthesized high-molecular-weight green TADF dendrimers of a benzophenone core and carbazole arms: G1B 38, G2B 39, and G3B 40 (Fig. 10).¹⁰³ The PLQYs of these dendrimers increased in the solid state through intramolecular motion suppression through π–π interactions. For instance, G2B 39 exhibited a PLQY of 33.4% and G3B 40 exhibited a PLQY of 21.1%, both of which were much greater than those in toluene. Non-doped solution-processed OLEDs with TPBi as ETL produced maximum EQEs of 5.7% (G2B 39) and 2.9% (G3B 40) through spin-coating, while vacuum-deposited TPBi produced 4.8% and 3.6%, respectively (Table 4). For further performance optimization, Yamamoto's group substituted G2B 39 with different substituents to produce tBuG2B 41, MeG2B 42, PhG2B 43, and MeOG2B 98 (Fig. 10 and 20).¹⁰⁴ PLQYs in neat films increased to 74% (tBu), 34% (Me), 17% (MeO), and 41% (Ph), respectively. In nondoped OLEDs, the tBuG2B 41 device exhibited superior performance with EQE = 17.0%, even retaining 13.8% EQE at 1000 cd m⁻². In contrast, vacuum-deposited devices exhibited lower efficiencies (EQE = 14.5%) for tBuG2B 41 (Table 4). These findings indicate that tBuG2B 41 is the most viable candidate for high-performance all-solution-processed green OLEDs.

Table 5 Device performances of green AIE-DADF-OLEDs 46–53

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
3CPyM-DMAC 46	514	66.8	532/—	(0.34, 0.51)	4.2	6726	35.4/15.9/11.4	85
2CPyM-DMAC 47	536	53.3	544/—	(0.39, 0.56)	4.3	3832	23.8/10.7/9.1
PXZ-BPCTPA 48	528	58.6	529/—	(0.36, 0.56)	4.0	5508	13.0/7.0/4.3	87
3DPAC-BPCTPA 49	475	46.6	502/—	(0.27, 0.46)	4.5	3913	13.3/12.2/8.2
3PXZ-BPCTPA 50	525	66.5	535/—	(0.38, 0.56)	4.2	13 708	37.2/14.6/12.1
CBM-PXZ 51	540	33.2	545/—	(0.41, 0.55)	4.8	3658	12.4/4.3/4.8	85
1TCPM-Cz 52	521	57	505/—	(0.29, 0.48)	3.8	1738	35.5/22.3/13.3	88
P-4CzCN 53	447	37	515/—	(0.23, 0.39)	5.5	302	8.7/4.2/3.6	92

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L. Wang et al. prepared two green-emitting multifunctional emitters, TATC-BP 44 and TATP-BP 45, with AIE, TADF, and mechanoluminescence (Fig. 10).¹⁰⁵ Both had benzophenone as the acceptor and triazatruxene as the donor, differing in having hexyl or phenyl side chains. PLQYs were low in solution (0.8–1.9%) but reached 22–24% in neat films. Both emitters exhibited clear AIE properties upon the addition of water (Fig. SI10). Un-doped OLEDs based on these emitters had maximum EQEs ∼6%, while doped devices based on 30 wt% TATC-BP 44 in a dendritic polycarbazole host (H2) attained EQE = 15.9% and green emission with CIE = (0.37, 0.53) (Table 4).

Fig. 11 Chemical structures of blue AIE-TADF materials 46–53.

D. H. Cho et al. synthesized two new three-armed AIE-active TADF emitters, IAcTr-out 54 and IAcTr-in 55, using indenoacridine as the electron-donating core unit and triazine as the acceptor unit (Fig. 13).¹⁰⁶ Both molecules displayed typical TADF behavior in addition to AIE properties, which were confirmed by PL measurement in the THF/water mixtures with different water contents. As shown in Fig. SI11(a) and (c), the PL intensities of both materials also increased as the water content increased, verifying the AIE character.¹⁰⁶ Emission enhancement in aggregation is caused by hindered intramolecular dynamics, inhibiting nonradiative decay channels. Non-doped, fully solution-processed OLEDs using IAcTr-out 54 and IAcTr-in 55 yielded maximum EQEs of 4.10% and 10.93%, respectively (Fig. 12 and Table 6). In mCP-doped devices, higher enhanced EQEs of 17.53% (IAcTr-out 54) and 18.43% (IAcTr-in 55) were obtained (Table 6). The better performance of IAcTr-in 55 in various device architectures is attributed to its AIE activity being more intense and improved charge balance.

Fig. 12 (a) Device structures and fabrication methods, and (b) all-solution-processed device structures. Reproduced with permission from ref. 106. Copyright 2018, the American Chemical Society.

Table 6 Device performances of green AIE-DADF-OLEDs 54–64

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
IAcTr-out 54	507	47.7	524/—	(0.35, 0.56)	3.6	—	12.98/6.86/4.1	106
IAcTr-out 54:mCP	—	—	—	(0.29, 0.54)	3.9	—	54.3/37.9/17.5
IAcTr-in 55	515	64.5	532/—	(0.38, 0.57)	3.3	—	35.8/18.7/10.9
IAcTr-in 55:mCP	—	—	—	(0.33, 0.58)	4.0	—	60.7/37.1/18.4
CzTAZPO 56	512	71	537/—	(0.37, 0.56)	4.5	9776	29.1/—/12.8	107
sCzTAZPO 57	502	57	531/—	(0.36, 0.56)	4.1	8283	20.4/—/9.6
Ac3TRZ3 58	505	36	520/—	(0.30, 0.54)	3.4	6910	11.4/—/3.5	108
Ac6 + Ac3TRZ3 58 (10 wt%)	—	—	492/—	(0.22, 0.42)	2.9	6175	30.3/—/11.0
TAc3TRZ3 59	518	63	538/—	(0.26, 0.48)	2.9	7860	10.2/—/3.1
Ac6 + TAc3TRZ3 59 (10 wt%)	—	—	503/—	(0.25, 0.47)	2.9	9689	40.6/—/14.2
TAT-BP 60	501	51	530/—	—	2.5	—	20.9/21.8/6.4	109
TAT-2BP 61	498	44	530/—	—	2.5	—	32.3/33.0/9.8
PyB-DPAC 62	530	49	538/—	(0.30, 0.49)	6	—	31.6/12.4/11.1	110
PyB-DPAC 63	509	65	522/—	—	—	—	—
4CzPhIPN-MO 64	489	86	536/—	(0.34, 0.56)	3.4	16 682	45.1/36.0/14.5	111

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W. Wang et al. presented two asymmetrical AIE-active TADF emitters, CzTAZPO 56 and sCzTAZPO 57, that combine carbazole-derived donor units, triazine acceptors, and phosphoryl (P=O) groups to maximize charge transport (Fig. 13).¹⁰⁷ The molecular designs provide strongly twisted geometries, inhibiting π–π stacking and allowing for AIE and TADF behavior. PL of THF/water mixtures of the two compounds established their AIE behavior; both had very weak emissions in neat THF, while a substantial increase in PL intensity was noted upon 99% addition of water. The latter is a result of aggregate formation, which limits nonradiative intramolecular motions and hence enhances radiative decay. Solution-processed, undoped OLEDs prepared using these emitters as an emissive layer emitted green EL with remarkable efficiencies. The CzTAZPO 56-based device attained the highest EQE of 12.8% and CE of 29.1 cd A⁻¹, while sCzTAZPO 57 obtained EQE of 9.6% and CE of 20.6 cd A⁻¹ (Table 6). Interestingly, both devices exhibited minimal efficiency roll-off at a luminance of 1000 cd m⁻², which demonstrates outstanding operation stability and reversible charge transport. These findings support the importance of introducing twisted donor–acceptor architectures and electron-withdrawing groups (such as phosphoryl units) into the molecular design of green AIE-TADF materials for solution-processed OLEDs with high efficiency and low efficiency degradation at high brightness.

Fig. 13 Chemical structures of green AIE-TADF materials 54–57.

F. Wang et al. described two AIE-active TADF materials, Ac₃TRZ₃ 58 and TAc₃TRZ₃ 59 (Fig. 15), assembled based on triazine-based electron acceptors and circularly arranged acridan or teracridan donors surrounding a central hexaphenylbenzene core. The compounds show both AIE and TADF properties, making them ideal candidates for efficient solution-processed OLEDs.¹⁰⁸

The molecular design of materials has the following major advantages.

• Spatially separated donor and acceptor units enable the through-space charge transfer (TSCT) mechanism, effectively minimizing the singlet–triplet energy gap (ΔE_ST) to support efficient TADF.

• Donor–acceptor pair proximity increases through-space electronic interaction, supporting radiative transitions.

• The propeller-shaped, nonplanar hexaarylbenzene core hinders intramolecular motions, activating the AIE mechanism and enhancing solid-state emission efficiencies.

It was found to have a strong AIE effect, with PL intensity enhanced by 6–17 fold upon going from solution to aggregated state in the THF/water mixtures. Of the two, TAc₃TRZ₃ 59, with the stronger teracridan donor, showed PLQYs much higher in neat and doped films (36% and 63%, respectively) than Ac₃TRZ₃ 58. Solution-processed OLEDs with TAc₃TRZ₃ 59 as the emissive layer showed a maximum EQE of 14.2% (Table 6), proving the effectiveness of this design approach for the construction of high-performance AIE-TADF materials.

L. Wang et al. synthesized two AIE-active TADF compounds, TAT-BP 60 and TAT-2BP 61, where benzophenone (BP) acted as the electron acceptor and triazatruxene (TAT) as the electron donor (Fig. 15).¹⁰⁹ Their strongly twisted molecular architectures with large dihedral angles resulted in effective frontier orbital separation and small ΔE_ST values (<0.1 eV), producing short delayed fluorescence lifetimes (<1 µs) and strong TADF performance. Both materials exhibited clear AIE characteristics and high PLQYs in the solid state. Solution-processed non-doped OLEDs were fabricated using these compounds as emissive layers. Among them, the device based on TAT-2BP 61 showed superior performance with green EL at 530 nm, a maximum EQE of 9.8%, CE of 32.3 cd A⁻¹, and PE of 33.0 lm W⁻¹ (Table 6). Also notable was a minimal efficiency roll-off of 1.0% at 1000 cd m⁻² and 2.0% at 2000 cd m⁻², indicating that these devices are very stable. These results make them prime candidates for non-doped OLED emitters.

C. Yang et al. prepared two AIE-active TADF compounds, PyB-DPAC 62 and PyB-DMAC 63, containing an acridine-based donor and a 4-benzoylpyridine electron acceptor (Fig. 15).¹¹⁰ Both compounds showed good thermal and morphological stability, suitable HOMO/LUMO energy levels (Fig. SI12), and large torsion angles between donor and acceptor units. This conformation allowed for effective AIE and TADF behavior in addition to high PLQYs in neat films. A non-doped, solution-processed OLED with emitter PyB-DPAC 62 (with device structure: ITO/PEDOT:PSS/PyB-DPAC 68/DPEPO/TmPyPB/Liq/Al) exhibited superior device performance with a maximum EQE of 11.1%, CE of 31.6 cd A⁻¹, and PE of 12.4 lm W⁻¹ (Table 6). Importantly, the solution-processed device showed better performance than its vacuum-deposited counterpart, which is attributed to the thicker emissive layer allowing wider carrier recombination and better exciton utilization. This study supports the feasibility of merging AIE and TADF in a sole molecular scaffold to produce efficient, non-doped emitters for OLEDs.

Y. Sun et al. synthesized two AIE-active TADF compounds, 4CzPhIPN-MO 64 and 4CzIPN-MO 105 (Fig. 15 and 20), through structural modification of the traditional 4CzIPN core with methoxy substituents and a phenyl spacer (Fig. 14).¹¹¹ The structural adjustments were designed to enhance the photophysical properties by molecular topology engineering. Specifically, 4CzPhIPN-MO 64 exhibited characteristic AIE behavior, and both compounds showed well-defined TADF features. The inclusion of a phenyl spacer enhanced steric hindrance around the donor units, thus limiting intramolecular movements and quenching nonradiative deactivation. The structural change effectively improved solid-state PLQYs and imparted AIE properties to the molecule. The methoxy groups also helped in lowering the HOMO level, minimizing the hole injection barrier in the devices. Solution-processed non-doped OLEDs were fabricated using both emitters. The 4CzPhIPN-MO 64-based device showed green EL with a peak of 536 nm and CIE coordinates (0.34, 0.56) with a maximum EQE of 14.5%, CE of 45.1 cd A⁻¹, and PE of 36.0 lm W⁻¹ (Table 6). In contrast, the 4CzIPN-MO 105-based device exhibited yellow EL emission at 574 nm with CIE coordinates of (0.48, 0.54) and much poorer performance, with a maximum EQE of 5.6%, CE of 15.6 cd A⁻¹, and PE of 12.8 lm W⁻¹ (10). In addition, the 4CzPhIPN-MO 64 device showed remarkably low efficiency roll-off with losses of only 3.8% and 3.9% at luminances of 100 and 1000 cd m⁻², respectively, which distinctively illustrates the benefit of molecular topology control as a rational approach to enable high-efficiency and stability in solution-processed, non-doped OLEDs.

Fig. 14 Molecular structures of the target compounds: 4CzIPN-MO 64 and 4CzPhIPN-MO 105.

Fig. 15 Chemical structure of green AIE-TADF materials 54–64.

Y. Sun et al. developed and synthesized two AIE-active TADF materials with dual-emitting cores of acceptor–donor–donor–acceptor (A–D–D–A) architecture, i.e., 2DBT-BZ-2Cz 65 and 2DFT-BZ-2Cz 66, to study their luminescence characteristics (Fig. 16).¹¹² Both compounds 2DBT-BZ-2Cz 65 and 2DFT-BZ-2Cz 66 use carbazole as the donor units, and dibenzo[b,d]thiophen-2-yl(phenyl)methanone and dibenzo[b,d]furan-2-yl(phenyl)methanone act as the acceptor units. Systematic probing of their AIE and TADF properties was achieved via thermal, photophysical, electrochemical, and DFT studies. Efficient FMO separation provided a small ΔE_ST and allowed TADF behavior as indicated by delayed fluorescence lifetimes. Moreover, the deeply twisted molecular structures of the dually emitting cores impeded π–π stacking interactions, thus maximizing the AIE property. Solution-processed, undoped OLED devices were prepared with both of these materials and exhibited green electroluminescence with peaks at 536 nm for 2DBT-BZ-2Cz 65 and 540 nm for 2DFT-BZ-2Cz 66 (Table 7). The 2DBT-BZ-2Cz 65-based device displayed better performance with a maximum EQE of 6.8%, CE of 20.7 cd A⁻¹, and PE of 12.4 lm W⁻¹, while the 2DFT-BZ-2Cz 65-based device had a maximum EQE of 4.5%, CE of 14.5 cd A⁻¹, and PE of 7.6 lm W⁻¹ (Table 7). This study illustrates an auspicious molecular design strategy for designing high-performance OLED materials with integrated TADF and AIE properties.

Fig. 16 Chemical structures of green AIE-TADF materials 65–75.

Table 7 Device performances of green AIE-DADF-OLEDs 65–75

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
2DBT-BZ-2Cz 65	539	—	536/—	(0.37, 0.58)	3.6	10 000	20.7/12.4/6.8	112
2DFT-BZ-2Cz 66	535	—	540/—	(0.38, 0.59)	3.9	6700	14.5/7.6/4.5
Ib-1 67 + mCP (5 wt%)	52	96.7	506/—	(0.23, 0.42)	4.4	154.4	32.45/—/12.34	90
Ib-2 68 + mCP (15 wt%)	467	99.7	508/—	(0.24, 0.47)	4.0	518.2	56.73/—/20.07
Ib-3 69 + mCP (10 wt%)	465	88.8	514/—	(0.26, 0.49)	4.4	426.7	32.99/—/11.28
2PXZ-2TRZ 70	509	94	518/—	(0.31, 0.58)	3.3	26 420	32.1/23.3/10.2	113
2PXZ-2TRZ 70 + mCP (30 wt%)	—	—	508/—	(0.26, 0.54)	3.0	21 620	65.0/68.1/27.1
TTT-PTZ 71 + CBP (10 wt%)	510	56.2	564/—	(0.39, 0.53)	4.2	3350	12.4/24.5/—	114
TTT-PTZ-Me 72 + CBP (10 wt%)	493	13.5	406/—	(0.20, 0.20)	3.0	479	1.7/6.4/—
TTT-CH3-PTZ 73 + CBP (10 wt%)	517	12.3	556/—	(0.34, 0.46)	3.9	711	7.4/3.9/—
(S)-N-5-TPA 74	522	28.9	504/—	(0.24, 0.52)	4.3	19 381	7.4/—/2.7	115
(S)-N-4-TPA 75	539	44.3	512/—	(0.30, 0.54)	4.2	23 701	29.6/—/10.2

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J. X. Tang et al. documented an AIE-active through-space charge transfer (TSCT) TADF compound, 2PXZ-2TRZ 70, built through a three-dimensional architecture consisting of a 2,4,6-triphenyl-1,3,5-triazine (TRZ) acceptor and a biphenazine (2PXZ) donor, connected through two spiro-fluorene bridges (Fig. 16).¹¹³ This twin-locking framework triggers numerous TSCT channels and efficiently limits intramolecular rotation between the fragments, forming a tiny singlet–triplet energy gap (ΔE_ST), an efficient RISC process, and high PLQY. Accordingly, 2PXZ-2TRZ 70 demonstrates both AIE and TADF properties. To determine EL performance, OLEDs were prepared with the following device structure: ITO/HAT-CN (10 nm)/TAPC (40 nm)/TCTA (10 nm)/mCP (8 nm)/EML (30 nm)/PPF (8 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm). The EML was PPF doped with 10–40 wt% of 2PXZ-2TRZ 70, as well as a non-doped structure. The devices emitted green EL, with peak wavelengths at 508 nm for the 30 wt%-doped device and 518 nm for the non-doped one, accompanied by the corresponding CIE coordinates (0.26, 0.56) and (0.31, 0.58), respectively. The doped device demonstrated excellent performance, with a maximum EQE of 27.1%, CE of 65.0 cd A⁻¹, and PE of 68.1 lm W⁻¹. Comparatively, the non-doped device had a maximum EQE of 10.2%, CE of 32.1 cd A⁻¹, and PE of 23.3 lm W⁻¹ (Table 7). These results prove the notable potential of large polycyclic aromatic hydrocarbons (PAHs) with multi-locked structures for highly efficient TSCT-TADF materials.

P. Data et al. synthesized three multifunctional materials TTT-PTZ 71, TTT-PTZ-Me 72, and TTT-CH₃-PTZ 73 with AIE, TADF, and RTP properties (Fig. 16).¹¹⁴ By introducing methyl groups at various positions on the donor and acceptor units, the photophysical behavior could be controlled to achieve a switch between TADF and RTP through host selection and ΔE_ST control. Upon application in solution-processed OLEDs, TTT-PTZ 71 and TTT-PTZ-Me 72 emitted single yellow-green and yellowish-green EL at CIE coordinates of (0.39, 0.53) and (0.34, 0.46), respectively, with maximum EQEs of 12.4% and 7.4%, respectively (Table 7). The third compound, TTT-CH₃-PTZ 73, with a weak intramolecular charge transfer (ICT) character, exhibited a decreased EQE_max, illustrating the electronic structure effect on device performance. This work presents a new approach to designing next-generation multifunctional TADF materials.

Most recently, K. Y. Zhang et al. prepared two AIE-active TADF CPL enantiomers, (S)-N-5-TPA 74 and (S)-N-4-TPA 75, (Fig. 16)¹¹⁵ in which the triphenylamine (TPA) donors are linked at the 5th and 4th positions of the phthalimide acceptor core, respectively. Both enantiomers possessed AIE features. Interestingly, (S)-N-4-TPA 75 exhibited better PLQY values in neat films (44.39%) than (S)-N-5-TPA 74 (28.92%). OLEDs prepared using these materials as solution-processed emissive layers exhibited blue-green EL emissions with CIE coordinates (0.24, 0.52) and (0.30, 0.54), respectively. The devices achieved a maximum EQE of 2.7% and CE of 7.4 cd A⁻¹ for (S)-N-5-TPA 74, and improved EQE of 10.2% and CE of 29.6 cd A⁻¹ for (S)-N-4-TPA 75 (Table 7). Notably, the two devices showed high EQEs of 2.6% and 8.5%, respectively, even when the luminance reached 1000 cd m⁻², implying a low efficiency roll-off. This stability results from their twisted molecular structures and inhibited π–π stacking. In addition, the CP-TADF device based on (S)-N-4-TPA 75, with its greater π–π interaction, showed greater circularly polarized electroluminescence (CP-EL). These results offer a promising strategy for next-generation circularly polarized OLEDs with AIE-active CP-TADF materials.

The combination of TADF and AIE properties provides an effective method for preparing highly emissive materials for OLEDs. B. Z. Tang et al. effectively converted 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN), a common ACQ compound, into an effective AIE-active luminogen (AIEgen) through the introduction of flexible, alkyl chain-linked spirobifluorene (SP) dendron. This resulted in the preparation of 5CzBN-SSP 76, 5CzBN-DSP 77, and 5CzBN-PSP 78 (Fig. 18).¹¹⁶ Adding more SP dendrons gave distinct AIE-TADF characteristics, enhanced film-forming properties, and increased isopropyl alcohol resistance, a valuable property for solution processing of OLEDs (Fig. 17). Schematic illustration of the core–dendron system concept utilized in nondoped emissive layers, together with the chemical structures of 5CzBN-SSP 76, 5CzBN-DSP 77, and 5CzBN-PSP 78. PL analysis showed that the pure 5CzBN material had normal ACQ behavior, as evidenced by the steady reduction of PL intensity with increasing water fraction (f_w) in the THF solution (Fig. SI13). This quenching was due to an ISC rate that favored nonradiative decay. In contrast, the dendronized derivatives, 5CzBN-SSP 76, 5CzBN-DSP 77, and 5CzBN-PSP 78, exhibited good AIE behavior, demonstrating a strong enhancement in PL intensity with increasing f_w up to 90% (Fig. SI13). Solution-processed nondoped OLEDs prepared from these emitters showed green electroluminescence with emission maxima at 508 nm. Among the devices, 5CzBN-PSP 78 performed the best with a maximum EQE of 20.1%, a CE of 58.7 cd A⁻¹, and a PE of 46.2 lm W⁻¹. When compared to devices using 5CzBN-SSP 76, 5CzBN-DSP 77, EQEs of 7.3% and 13.9%, CEs of 21.9 and 40.7 cd A⁻¹, and PEs of 17.2 and 31.9 lm W⁻¹ were obtained, respectively (Table 8). These results provide a new molecular design principle for high-performance AIE-TADF materials.

Fig. 17 Schematic of the core–dendron system concept utilized in non-doped emissive layers, together with the chemical structures of 5CzBN-SSP 76, 5CzBN-DSP 77, and 5CzBN-PSP 78.

Table 8 Device performances of green AIE-DADF-OLEDs 76–86

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
5CzBN-SSP 76	473	38	508/90	(0.28, 0.54)	3.4	5400	2.1.9/17.2/7.3	116
5CzBN-DSP 77	476	45.7	508/88	(0.27, 0.54)	3.2	14 800	40.7/31.9/13.9
5CzBN-PSP 78	480	69.6	508/84	(0.27, 0.53)	3.1	13 700	58.7/46.2/20.1
5CzBN-SPhCz 79	471	72	532/—	(0.32, 0.55)	2.9	26 500	41.6/34.1/14.0	117
5CzBN-DPhCz 80	474	83	532/—	(0.32, 0.54)	3.0	26 500	43.4/34.2/14.5
5CzBN-TPhCz 81	482	88	524/—	(0.32, 0.54)	3.1	27 800	49.3/36.4/15.7
5CzBN-QPhCz 82	485	90	524/—	(0.32, 0.54)	3.5	36 800	53.7/37.5/17.1
5CzBNPPhCz 83	489	93	512/—	(0.26, 0.50)	3.2	32 100	61.4/42.8/21.8
phCz-2-5CzBN 84	509	87	512/—	(0.29, 0.54)	3.3	21 960	54.8/40.0/18.9	118
phCz-2-5CzBN 84 + 2 wt% S-Cz-BN	—	—	496/—	(0.16, 0.49)	3.1	8181	41.9/37.6/18.8
phCz-3-5CzBN 85	523	76	523/—	(0.35, 0.58)	3.1	9072	45.8/37.8/14.5
phCz-3-5CzBN 85 + 2 wt% S-Cz-BN	—	—	500/—	(0.30, 0.56)	3.4	7630	31.5/24.7/10.9
phCz-4-5CzBN 86	503	73	506/—	(0.26, 0.52)	3.4	24 876	63.4/44.3/22.5
phCz-4-5CzBN 86 + 2 wt% S-Cz-BN	—	—	496/—	(0.16, 0.49)	3.0	11 288	47.3/42.4/20.8

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Later, the same team generalized this idea by constructing a series of core–shell structured AIE-TADF materials based on the 5CzBN core, which was functionalized with flexible alkyl-linked bulky fragments as protective outer shells (Fig. 18).¹¹⁷ This shell architecture plays several functions: (i) it stretches out the intermolecular distance, (ii) offers 3D spatial segregation to inhibit TTA, and (iii) shields the triplet excitons from oxygen quenching by creating nanoaggregates. These changes ensure long-lived excited states while improving photophysical stability. The large molecular weight and flexible chains of the shell architectures also promote homogeneous film formation in solution-processing methods. OLEDs prepared with these core–shell-type emitters exhibited green EL emission with appropriate CIE coordinates. Notably, the device based on 5CzBNPPhCz 83 realized a maximum luminance of 32 100 cd m⁻², EQE of 21.8%, CE of 61.4 cd A⁻¹, and PE of 42.8 lm W⁻¹, outperforming the performance of devices derived from other analogs in the series (Table 8). These data unequivocally illustrate the potentiality and originality of the core–shell approach toward assembling high-performance AIE-TADF materials that are applicable to solution-processed nondoped OLEDs.

Fig. 18 Chemical structures of green AIE-TADF materials 76–83.

Moreover, Y. Sun et al. prepared three regioisomeric AIE-TADF compounds (phCz-2-5CzBN 84, phCz-3-5CzBN 85, and phCz-4-5CzBN 86) through the conjugation of flexible dendrons to diverse positions (C2, C3, and C4) on the carbazole units through ether linkages (Fig. 19).¹¹⁸ The regioisomers showed significant AIE and TADF activities with reduced intermolecular interactions because of their twisted molecular conformations and further shielding by the bulky substituents. This study emphasizes the positional influence of functional groups on photophysical performance and reaffirms the significance of molecular conformation in AIE-TADF systems. The nondoped device consisting of the synthesized regioisomeric materials as active layers for fully solution-processed OLED devices showed green EL, while the doped devices emitted blue-green to green EL emissions (Table 8). The nondoped device using phCz-4-5CzBN 86 showed excellent performance with a maximum EQE of 22.5%, CE of 63.4 cd A⁻¹, and PE of 44.3 lm W⁻¹. In contrast, phCz-2-5CzBN 84, and phCz-3-5CzBN 85 devices reported maximum EQEs of 18.9% and 14.5%, CEs of 54.8 and 45.8 cd A⁻¹, and PEs of 40.0 and 37.8 lm W⁻¹, respectively. These findings provide new revelations of the regioisomeric design effects on TADF behavior and prove the promising suitability of these materials as emitters or TADF sensitizers for solution-processable, high-efficiency OLEDs.

Fig. 19 Chemical structures of green AIE-TADF materials 84–97.

In a follow-up study, the same researchers reported two new AIE-active TADF compounds, 5CzBN-9CSP 87 and 5CzBN-12CSP 88, by installing long alkyl chains onto the 5CzBN core, as peripheral dendrons, in conjunction with bipolar spirobifluorene (SP) units (Fig. 19).¹¹⁹ SP units ensure balanced charge mobility and transport while inhibiting crystallization, thereby reducing intermolecular aggregation and increasing PLQY in neat films. The longer alkyl chains facilitate not only good solubility and film homogeneity but also good TADF characteristics, like low ΔE_ST and efficient RISC rates. Nondoped, solution-processed OLEDs built from these emitters showed green EL with emission maxima at 508 nm. The 5CzBN-9CSP 87 device obtained a top EQE of 21.1%, CE of 59.3 cd A⁻¹, and PE of 42.3 lm W⁻¹, while that of 5CzBN-12CSP 88 had an EQE of 17.6%, CE of 48.0 cd A⁻¹, and PE of 34.2 lm W⁻¹ (Table 9). Surprisingly, the doped devices with identical materials exhibited blueshifted emission with ultra-pure green EL peaks at 496 nm. The 5CzBN-9CSP 87-doped device provided a maximum EQE of 22.3%, CE of 35.5 cd A⁻¹, and PE of 45.2 lm W⁻¹, while 5CzBN-12CSP 88 surpassed this with an EQE of 25.9%, CE of 39.5 cd A⁻¹, and PE of 56.5 lm W⁻¹. In addition, the team studied the influence of flexible chain length and composition on the photophysical and device properties of 5CzBN-based materials.¹²⁰ They developed a series of derivatives by appending alkyl chains with different lengths (from ethane to dodecane) and oligo(ethylene glycol) (OEG) units onto the TADF-active 5CzBN core. Although all materials maintained similar emission characteristics in the toluene solution, indicating retained AIE and TADF properties, there were tremendous differences in their solid-state behavior. Increased alkyl chain length gave better shielding of the TADF core, enhancing the solid-state PL quantum yield from 41% to 63%. Consequently, the nondoped OLED device of 5CzBN-Hex 92 reached a high maximum EQE of 18.7%, which is approximately double that of 5CzBN-Eth 90 (9.8%), and significantly higher than that of 5CzBN-Dod 95, which had a maximum EQE of just 0.24%. These results highlight the pivotal function of flexible chain engineering in controlling thin-film behavior and device performance, providing a helpful approach for the rational design of advanced AIE-TADF materials for efficient OLEDs.

Table 9 Device performances of green AIE-DADF-OLEDs 87–97

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
5CzBN-9CSP 87	489	92	508/—	(0.23, 0.50)	3.6	9969	59.3/42.3/21.1	119
5CzBN-9CSP 87 + 2 wt% S-Cz-BN	—	—	496/—	(0.12, 0.43)	3.5	11 880	35.5/45.2/22.3
5CzBN-12CSP 88	488	92	508/—	(0.23, 0.50)	3.7	8876	48.0/34.2/17.6
5CzBN-12CSP 88 + 2 wt% S-Cz-BN	—	—	596/—	(0.15, 0.46)	3.5	13 024	39.5/56.5/25.9
5CzBN-Eth 90	524	41	520/—	(0.33, 0.57)	2.6	5332	30.5/27.4/9.8	120
5CzBN-Pro 91	520	43	516/—	(0.30, 0.55)	2.5	4538	31.4/28.2/10.4
5CzBN-Hex 92	507	55	508/—	(0.28, 0.54)	2.9	8853	54.7/57.3/18.7
5CzBN-Hep 93	507	57	508/—	(0.28, 0.53)	2.9	6838	44.7/40.1/15.9
5CzBN-Non 94	502	59	506/—	(0.26, 0.53)	3.5	3447	26.3/18.4/9.0
5CzBN-Dod 95	499	63	506/—	(0.25, 0.51)	4.6	168	0.67/0.30/0.24
5CzBN-OEG 96	517	42	516/—	(0.30, 0.56)	3.1	1215	6.5/5.9/2.1
4OCzCN 97	450	32	511/88	(0.27, 0.52)	4.0	3155	11.8/7.41/4.35	99

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Yellow/red AIE-active traditional TADF materials for solution-processed OLEDs

Compared with blue and green AIE-active TADF materials, red materials require a small singlet energy for light emission, which is usually achieved by escalating the CT character via changing the acceptor and donor strengths. From the molecular structure design principle, red materials have either strong acceptors or donors, which can be increased for a further shift in the emission wavelength. This section covers AIE-active TADF materials for solution-processed red OLEDs.

Y. Sun et al. designed and synthesized two AIE-active TADF materials, namely Cz-AQ 99 and TPA-AQ 100, by integrating Cz and TPA (electron donor) into the anthracene-9,10-dione (AQ) core (electron acceptor) (Fig. 20).¹²¹ Both the synthesized materials exhibited AIE and TADF properties. AIE properties of the synthesized material were examined in THF/water mixtures with different f_w. Both materials exhibited weak emissions in the pure THF solution. As the water fraction increased, the PL emission intensity enhanced significantly, which can be attributed to material aggregation.¹²¹ From these results, it can be confirmed that the better AIE features of synthesized materials might be suitable for high-efficiency solution-processed OLEDs. Finally, the solution-processed OLED devices using Cz-AQ 99 and TPA-AQ 100 as the dopants displayed orange and red emission peaks at 572 and 612 nm with CIE coordinates of (0.50, 0.49) and (0.60, 0.40), maximum EQEs of 5.8% and 7.8%, and CEs of 10.8 and 10.6 cd A⁻¹, respectively (Table 10). These results may be useful for the design and development of new efficient AIE-active TADF materials for solution-processed red OLEDs.

Fig. 20 Chemical structured of red AIE-TADF materials 98–106.

Table 10 Device performances of red AIE-DADF-OLEDs 98–106

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
MeOG2B 98	497	74	570/—	(0.44, 0.51)	2.5	1017	17.7/19.0/6.4
Cz-AQ 99	635	28	572/—	(0.50, 0.49)	3.5	3200	10.8/—/5.8	121
TPA-AQ 100	642	52	612/—	(0.60, 0.40)	3.8	2200	106/—/7.5
ND-AC 101	556	66	558/—	(0.38, 0.57)	—	—	38.5/30.2/12.0	122
ND-AC 101 + CBP 9 wt%	—	—	542/—	(0.47, 0.49)	—	—	58.1/50.7/16.8
CND-AC 102 + CBP 1.5 wt%	643	32	588/—	(0.43, 0.54)	—	—	21.3/14.6/8.4
3CPyM-PXZ 103	547	48.4	572/—	(0.47, 0.51)	4.6	8085	21.6/6.8/9.4	85
2CPyM-PXZ 104	578	35.3	597/—	(0.53, 0.46)	4.7	6551	14.5/4.2/6.9
4CzIPN-MO 105	531	35	574/—	(0.48, 0.54)	3.1	7615	15.6/12.8/5.6	111
TCTA-BP-PXZ 106	547	53	546/—	(0.40, 0.56)	3.0	12 524	34.4/24.6/10.7	89
TAT-DBPZ 107 + CBP 20 wt%	572	10.5	604/—	(0.40, 0.56)	3.2	—	29.7/23.3/15.4	123
TAT-FDBPZ 108 + CBP 20 wt%	601	8.3	611/—	(0.40, 0.56)	3.0	—	15.6/14.0/9.2
TATC-TRZ 109	525	22	560/—	—	3.0	—	19.9/15.6/7.5	124
TATC-TRZ 109 + mCP 5 wt%	—	—	530/—	—	5.0	—	23.7/9.3/8.6
TATP-TRZ 110	538	44	564/—	—	3.0	—	7.4/5.8/2.8
TATP-TRZ 110 + mCP 5 wt%	—	—	516/—	—	6.0	—	29.7/13.4/10.9
4DMAC-TPPQ 111	650	13.7	685/—	—	4.4	—	0.3/—/—	125
4PXT-TPPQ 112	762	0.8	780/—	—	4.6	—	0.04/—/—

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C. Yang et al. also synthesized two other AIE-active TADF compounds, ND-AC 101 and CND-AC 102, with naphthyridine or cyano-naphthyridine acceptors and acridine donors (Fig. 20).¹²² The highly orthogonal molecular geometries facilitated both the AIE and TADF processes through small singlet–triplet energy gaps. Of these, ND-AC 101 showed a high PLQY of 66% in a CBP host and 55% in neat films. Solution-processed doped OLEDs with ND-AC 101 and CND-AC 102 emitted yellow and orange-red EL at 542 nm and 588 nm, and corresponding CIE coordinates of (0.38, 0.57) and (0.47, 0.49), respectively. The ND-AC 101-based device offered a high EQE of 16.8%, CE of 58.1 cd A⁻¹, and PE of 50.7 lm W⁻¹, which is much better than that of the CND-AC 102-based device (EQE of 8.4%, CE of 21.3 cd A⁻¹, and PE of 14.6 lm W⁻¹). Moreover, a nondoped OLED derived from ND-AC 104 showed yellow emission at 558 nm (CIE: 0.43, 0.54) with an EQE of 12.0%, CE of 38.5 cd A⁻¹, and PE of 30.2 lm W⁻¹ (Table 10), validating the applicability of naphthyridine-based structures to high-efficiency OLEDs.

L. Wang et al. prepared two new AIE-active TADF compounds, TAT-DBPZ 107 and TAT-FDBPZ 108, by pairing triazatruxene (TAT) with electron-donating capabilities and dibenzo[a,c]phenazine (DBPZ) or fluorinated DBPZ (FDBPZ) with acceptor capabilities (Fig. 21).¹²³ Both compounds showed excellent AIE features and bright red emission in the solid state. The planar, rigid π-conjugated backbone of TAT and DBPZ yielded small ΔE_ST and high PLQYs. Both materials emitted weak PL in pure THF, whereas their PL intensities increased steadily with increasing fractions of water, as expected for AIE (Fig. SI14). This is due to aggregation that hinders nonradiative decay processes through suppressed intramolecular motions. Solution-processed doped OLEDs from TAT-DBPZ 107 and TAT-FDBPZ 108 delivered EL peaks at 604 nm and 611 nm with maximum EQEs of 15.4% and 9.2%, respectively (Table 10). Interestingly, the two devices showed low efficiency roll-off, highlighting the potential of TAT-based architectures for red-emitting, solution-processable TADF OLEDs.

Fig. 21 Chemical structures of red AIE-TADF materials 107–112.

C. Yang et al. synthesized and designed two star-shaped AIE-active TADF emitters, TATC-TRZ 109 and TATP-TRZ 110, using triazatruxene (TAT) as the electron-donating core and triazine as the electron-accepting unit (Fig. 21).¹²⁴ They have a similar twisted molecular structure, differing only in the substituents on the TAT unit: dodecyl chains in TATC-TRZ 109 and phenyl groups in TATP-TRZ 110. Both emitters possessed small singlet–triplet energy gaps ΔE_ST, enabling fast RISC as well as increased PLQYs in neat films versus dilute toluene solution characteristics of AIE behavior. Bulk-processed nondoped OLEDs were prepared from these materials as emitters. The TATC-TRZ 109-based device emitted yellow light at 560 nm with a maximum EQE of 7.5% and CE of 19.9 cd A⁻¹. The TATP-TRZ 110-based device emitted at 564 nm, but it showed much lower performance, with an EQE of 2.8% and a CE of 7.4 cd A⁻¹ (Table 10). These results emphasize the positive function of alkyl flexible substituents in increasing molecular packing, exciton utilization, and, consequently, the efficiency of nondoped, solution-processed TADF OLEDs.

Recently, E. Z. Colman et al. described two AIE-active TADF compounds, 4DMAC-TPPQ 111 and 4PXT-TPPQ 112, derived from an extended π-conjugated electron-deficient pyrazino[2,3-g]quinoxaline (PQ) core functionalized with four electron-donating units 9,9-dimethyl-9,10-dihydroacridine (DMAC) or phenoxathiin (PXT), each connected through a phenylene bridge (Fig. 21).¹²⁵ Both compounds exhibited outstanding AIE and TADF properties with very small ΔE_ST values of 0.01 eV (4DMAC-TPPQ 111) and 0.02 eV (4PXT-TPPQ 112), facilitating highly efficient triplet harvesting through RISC. Solution-processed OLEDs were prepared from both nondoped and doped devices. The nondoped device from 4DMAC-TPPQ 111 showed pure red emission at 685 nm, while that of 4PXT-TPPQ 112 emitted deep red light at 640 nm. Upon doping into a CBP host, the 10 wt%-doped device of 4DMAC-TPPQ 111 had a red-shifted EL peak at 635 nm, while the 5 wt%-doped device of 4PXT-TPPQ 112 had an EL peak at 686 nm for even deeper red emission (Table 10). These findings highlight the promise of multi-donor–acceptor π-conjugated systems in realizing long-wavelength, efficient emission in both nondoped and doped solution-processed OLEDs.

AIE-active MR-TADF materials for solution-processed OLEDs

As noted earlier, typical traditional TADF emitters have broad emission maxima due to the extensive molecular relaxation of the excited singlet (S₁) state. This is due to the inherently strong ICT nature of their D–A architecture, which causes marked geometrical reorganization and vibronic coupling between the ground and excited states. Consequently, such systems tend to have high reorganization energies and broad FWHM values in excess of 70 nm, thus compromising spectral stability and color purity (Fig. 22B). To overcome the intrinsic bottleneck of traditional TADF emitters, Hatakeyama et al.¹²⁶ came up with a brilliant molecular design approach according to the multi-resonance (MR) effect. In MR-type TADF emitters, donor and acceptor atoms or groups are placed strategically in ortho- and para-positions of a rigid polycyclic aromatic hydrocarbon (PAH) skeleton to generate short-range resonance interactions.¹²⁷ This arrangement leads to alternating distribution of frontier molecular orbitals (FMOs) over the π-conjugated backbone, where the HOMO is largely localized on electron-donating atoms and surrounding carbon atoms at their ortho/para positions, while the LUMO is predominantly found on the electron-accepting atoms and corresponding meta carbon atoms with respect to the donors (Fig. 1(D)).^128,129 The reverse resonance effects of electron-rich and electron-deficient centers produce an atomic-level splitting between HOMO and LUMO, essentially reducing ΔE_ST, with the partial orbital overlap maintained to conserve oscillator strength in the S₁ state essential for efficient radiative decay and high TADF performance.^130–132 In addition, the planar and rigid π-framework of MR emitters enables short-range charge transfer and diminishes bonding/antibonding character in the excited state, which effectively suppresses molecular relaxation and reorganization (Fig. 1B), resulting in narrow FWHM emission and negligible Stokes shift.^133–135 Thus, MR-TADF emitters show superior color purity and high PLQY.^136–138 In MR-TADF systems, boron-emitters have risen to become the most dominant class of narrowband materials based on their unique electronic structure and ability to create elongated π-skeletons with variable multi-resonance cores and peripheral groups. However, the majority of para-B–π–D-type MR architectures mainly emit blue. In contrast, molecular architectures containing both para-B–π–B and para-N–π–N linkages can significantly improve ICT properties in intramolecular charge transfer, thus allowing bathochromic-shifted emission without compromising spectral purity. In addition, full RGB emissions can be attained by precisely tuning the excited-state distribution of MR frameworks. These molecular tuning capabilities enable MR-based OLEDs to access a broad color gamut in the CIE chromaticity diagram. Overall, the continued development of high-efficiency MR-TADF emitters with narrow FWHMs offers a highly promising route toward meeting the stringent BT.2020 standard for next-generation ultra-high-definition (UHD) displays, without the need for color filters.^139,140

Fig. 22 (A) and (B) Schematic showing the fundamental principle for achieving narrowband emission in organic materials, typically comparing conventional TADF emitters and MR-TADF emitters. (C) Molecular design strategy for solution processable AIE-active TADF materials: (a) donor–acceptor type materials, (b) polymer-type materials, and (c) dendrimer-type materials. (D) HOMO–LUMO separation in MR-TADF emitters.

The present section presents a detailed account of recent advances in solution-processed AIE-active MR-TADF OLEDs with special focus on their molecular design approaches, structural development, and photophysical, electrochemical, and electroluminescent properties. The discussion summarizes the underlying principles of molecular engineering for color purity and efficiency and presents a critical analysis of the merits and drawbacks of different classes of emitters.

D. H. Cho et al. documented the synthesis of the three blue-emitting compounds, TB-3Cz 113, TB-P3Cz 114, and TB-DACz 115; all the emitters possess AIE and TADF properties (Fig. 24).¹⁴¹ In such molecules, the electron-donating segments are carbazole derivatives, and the electron acceptor is a fused core that contains boron, which creates a donor–acceptor molecular structure that is conductive to TADF. The rigid boron-fused core also assists in inhibiting non-radiative decay by hindering intramolecular rotations, which is a critical component in the attainment of high PLQYs in the solid state. Photophysical characterization in THF/water mixtures demonstrated typical AIE behavior for each of the three emitters. Weak PL was observed in a neat THF solution because of the active intramolecular motions that enabled non-radiative decay. With increasing water content to 90%, a significant PL intensity enhancement was noted, which arose from aggregation-induced restriction of intramolecular motion, decreased non-radiative relaxation, and stabilization of the excited state in the aggregated phase (Fig. SI15). These findings reveal that these materials are very appropriate for solution-processed, non-doped OLED preparation, where solid-state emission efficiency is essential. OLED devices built with TB-3Cz 113, TB-P3Cz 114, and TB-DACz 115 as emitters showed deep-blue EL with outstanding device performance. The TB-3Cz 113 device resulted in an EQE_max of 9.90% with deep-blue emission wavelengths at CIE coordinates of (0.17, 0.07), closely approaching the NTSC standard for blue (0.14, 0.08) (Table 11). The TB-P3Cz 114 and TB-DACz 115 devices showed slightly lower EQEs of 6.13% and 6.04%, respectively, with CIE coordinates of (0.15, 0.08) and (0.18, 0.40) (Table 11), indicating slight emission shifts due to variations in donor substituents. The somewhat decreased efficiency of TB-P3Cz 114 and TB-DACz 115 could be attributed to differences in molecular packing, intermolecular contacts, and charge-transport balance due to the distinct substitution patterns of the carbazole units. In conclusion, these findings emphasize that molecular design carefully combining carbazole donors and a rigid boron-fused acceptor efficiently balances AIE and TADF characteristics to allow solution-processable, non-doped OLEDs with high efficiency and pure colors. This report underscores the significance of donor selection and molecular rigidity in realizing ultra-deep-blue emission with high photoluminescence efficiency in both solution and solid states.

Table 11 Device performances of blue AIE-TADF-OLEDs 113-122

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm cm^−1)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
TB-3Cz 113	413	52	424/2960	(0.17, 0.07)	3.5	1224	4.0/3.76/9.9	141
TB-P3Cz 114	433	51	448/2565	(0.15, 0.08)	3.0	3205	4.59/4.12/6.13
TB-DACz 115	493	53	492/3117	(0.18, 0.40)	2.5	2355	15.0/15.7/6.04
TB-tCz 116	415	53	416/44.4	(0.17, 0.06)	3.6	—	2.19/1.97/8.21	142
TB-tPCz 117	422	56	428/42.2	(0.16, 0.05)	3.5	—	5.61/5.03/15.8
BCz 118	405	61	412/33	(0.18, 0.10)	3.8	510	1.14/0.9/3.44	143
BBFCz 119	405	86.4	416/42	(0.17, 0.07)	3.5	803	2.88/2.58/6.78
BICz 120	407	75.5	424/61	(0.16, 0.08)	3.5	1610	6.13/5.51/10.1
R-D2 + 1 wt% DBNS 121 + H2	631	80	613/66	(0.64, 0.34)	3.7	4345	7.2/—/5.8	144
R-D2 + 3 wt% DBNS 121 + H2	—	—	617/68	(0.63, 0.35)	3.5	4237	5.2/—/4.2
R-D2 + 1 wt% DBNS-tBu 122 + H2	641	85	616/65	(0.65, 0.34)	3.5	5694	9.2/—/7.8
R-D2 + 3 wt% DBNS-tBu 122 + H2	—	—	621/66	(0.65, 0.34)	3.5	4823	8.4/—/7.1

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Building upon this approach, the same group synthesized two ultra-deep-blue AIE-active TADF emitters, TB-tCz 116 and TB-tPCz 117, using an oxygen-bridged boron (TB) core as the electron acceptor and 3,6-substituted carbazole derivatives as the electron donors (Fig. 24).¹⁴² Both emitters showed ultra-deep-blue emission in toluene and thin films. Maximum emission wavelengths were 415/433 nm for TB-tCz 116 and 422/445 nm for TB-tPCz 117 in the solution and film states, respectively. The solution FWHM was 44.9 nm for TB-tCz 116 and 46.3 nm for TB-tPCz 117. The narrow emission bands are due to inhibited molecular vibrations attributed to the rigid TB core, the employment of uncomplicated carbazole donors, and low conformational reorganization between the ground and excited singlet states. Both emitters exhibited distinct AIE behaviors, as verified through PL measurements in THF/water mixtures. Both compounds showed poor PL in the neat solvent and strong emission augmentation with rising water content (Fig. SI16). Absolute PLQYs for non-doped films were 41.4% for TB-tCz 116 and 51.9% for TB-tPCz 117. Solution-processed non-doped OLEDs of TB-tCz 116 and TB-tPCz 117 showed deep-blue electroluminescence with peak wavelengths and associated CIE coordinates of 416 nm/(0.17, 0.06) and 428 nm/(0.16, 0.05), respectively, which approach the NTSC standard blue coordinates (0.14, 0.08) (Table 11). The maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of 2.19 cd A⁻¹, 1.97 lm W⁻¹, and 8.21% were achieved for TB-tCz 116 and 5.61 cd A⁻¹, 5.03 lm W⁻¹, and 15.8% were achieved for TB-tPCz 117, respectively (Table 11). The two devices both had narrow emissions with FWHMs of 44.4 nm and 42.2 nm, respectively. OLEDs that used doped films with mCP as the host exhibited somewhat blue-shifted electroluminescence (412–420 nm) due to the nonpolar surroundings of the mCP matrix. Doped devices showed values of CE/PE/EQE equal to 2.72 cd A⁻¹/2.14 lm W⁻¹/15.9% for TB-tCz 116 and 3.53 cd A⁻¹/2.77 lm W⁻¹/14.1% for TB-tPCz 117, with FWHM values of 44.2 nm and 43.5 nm, respectively. Interestingly, TB-tCz 116, which also displayed comparatively weaker AIE properties, presented almost twice the efficiency in the doped films thanks to its greater PLQY, improved reverse intersystem crossing (RISC), better charge balance, and better TADF behaviour.

The same group described three new narrow-bandwidth deep-blue TADF emitters (BCz 118, BBFCz 119, and BICz 120) using donor engineering. A boron-fused unit (TDBA) was used as an electron acceptor, and the donor groups rationally varied between carbazole (Cz) and its derivatives, benzofurocarbazole (BFCz) and indenocarbazole (ICz). Two donor units were placed at the ortho- and para-positions of a benzene core to form an A–π–2D molecular structure (Fig. 24).¹⁴³BCz 118, BBFCz 119, and BICz 120 displayed emission maxima and FWHM in the film state of 414 nm (37 nm), 423 nm (51 nm), and 431 nm (53 nm), respectively. The measured PLQYs in toluene and in the solid state were 26.4/61.0% for BCz 118, 44.2/86.4% for BBFCz 119, and 20.7/75.5% for BICz 120. The dramatically increased PLQY values in the solid state over solution signify aggregation-induced emission (AIE) behaviour. All the emitters possessed considerable hole and electron current densities owing to the electron-donating carbazole-type donors and the TDBA acceptor, respectively. The electron current densities were very similar because of the shared electron-transporting unit, while the hole current densities differed significantly with respect to the donor strength. As anticipated, the indenocarbazole-substituted BICz possessed the maximum hole current density, resulting in highly balanced charge transport within the series. Non-doped OLEDs composed of BCz 118, BBFCz 119, and BICz 120 as emitters showed deep-blue electroluminescence at peak wavelengths of 412, 416, and 424 nm, respectively. Corresponding CIE coordinates were (0.18, 0.10) for BCz 118, (0.17, 0.07) for BBFCz 119, and (0.16, 0.08) for BICz 120, which are very close to the NTSC standard blue coordinates (0.14, 0.08). FWHM values were 33, 42, and 61 nm for BCz 118, BBFCz 119, and BICz 120, respectively. Maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) were 1.14 cd A⁻¹, 0.90 lm W⁻¹, and 3.44% for BCz 118; 2.88 cd A⁻¹, 2.58 lm W⁻¹, and 6.78% for BBFCz 119; and 6.13 cd A⁻¹, 5.51 lm W⁻¹, and 10.1% for BICz 120, respectively. Among them, BICz 120, with the most efficient donor, showed the best efficiency because of its strong TADF property (Fig. 23) and balanced charge-transport properties originating from the indenocarbazole donor structure. However, BCz 118, following a fluorescence route, exhibited a comparatively low EQE of 3.44% (Table 11). These results highlight the role of donor architecture in the emission mechanism and total OLED efficiency.

Fig. 23 Schematic of the anticipated luminescence processes of BCz 118, BBFCz 119, and BICz 120, emphasizing their excited-state behavior, charge-transfer features, and potential TADF or fluorescence routes. Reproduced with permission from ref. 143. Copyright 2021, Elsevier.

Fig. 24 Chemical structures of blue AIE-TADF materials 113–120.

L. Wang et al. designed and synthesized two sulfur-bridged AIE-active MR-TADF emitters, namely DBNS 121 and DBNS-tBu 122, by strategically substituting the oxygen atoms of the parent BNO1 core with sulfur (Fig. 27).¹⁴⁴ The synthetic route was similar to that of BNO1, with thiophenol derivatives employed instead of phenols to give the final compounds in good yields. Theoretical and experimental studies showed that sulfur atoms, with greater orbital contribution to the HOMO than nitrogen, significantly increase spin–orbit coupling and allow for more efficient RISC. Consequently, both emitters showed high RISC rate constants of the order 2.2 × 10⁵ s⁻¹, validating the efficacy of the sulfur-bridging strategy toward facilitating TADF processes. DBNS 121 showed a PL peak at 631 nm, while tert-butyl-substituted emitter DBNS-tBu 122 showed a red-shifted emission of 641 nm. The 10 nm bathochromic shift is due to the electron-donating character of the tBu groups, which stabilize the excited state and reduce the energy gap. Both emitters exhibited remarkably high PLQYs of ca. 85% and limited emission bandwidths (FWHM ≈ 40 nm) though slightly higher than that for the oxygen-bridged analogue BNO1. This FWHM rise is largely attributed to the heavier sulfur atoms and the accompanying augmented molecular twisting, which account for modest vibronic coupling. Surprisingly, even with distorted geometries, the trigonal planar arrangement of the MR core in DBNS 121 facilitated efficient molecular packing, resulting in AIE properties in the solid state. The rigid structure precludes intramolecular motions, inhibiting non-radiative decay pathways and boosting radiative transitions. However, partial relief of the extent of aggregation through the incorporation of bulky tBu substituents somewhat diminished the AIE effect but maintained narrowband emission and high luminescence efficiency. Solution-processed OLED devices were prepared by employing a ternary emissive layer (EML) with host H₂,¹⁴⁵ 10 wt% of R-D₂,¹⁴⁶ and 1–3 wt% DBNS 121 or DBNS-tBu 122 as the guest emitter (Fig. 25a). Out of the two emitters, DBNS-tBu 122 exhibited better device performance with a maximum EQE of 7.8%, peak EL intensity of 616 nm, and low turn-on voltage of 4.5 V (Fig. 25). The EL emission was marginally blue-shifted in comparison to the PL spectra due to the greater S₁ energy levels of the H₂ host matrix compared to the dichloromethane solution environment. In addition, the devices showed excellent operation stability under ambient conditions, verifying the technological feasibility of sulfur-bridged AIE-active MR-TADF emitters for solution-processed OLEDs. In summary, this molecular design approach highlights the success of sulfur atom incorporation into MR scaffolds to realize improved spin–orbit coupling, facilitate efficient RISC, and maintain high-efficiency, narrowband emission. This study offers an important structure–property–performance correlation that can aid in the rational design of next-generation solution-processable AIE-active MR-TADF emitters for high-performance and energy-efficient optoelectronic devices.

Fig. 25 Device configurations (a), J–V–L characteristics (b), EQE versus luminance plots (c), and EL spectra (d) for solution-processed OLEDs using DBNS 121 and DBNS-tBu 122 as emitters. Reproduced with permission from ref. 144. Copyright 2022, John Wiley and Sons.

Ban et al. also disclosed a series of deep-blue AIE-active MR-TADF dendrimers (QAO-0 123, QAO-1 124, QAO-2 125, and QAO-3 126), wherein flexible alkyl-linked functional dendrons were progressively introduced to the periphery of the emissive core for control over the photophysical and film-forming properties (Fig. 27).¹⁴⁷ The design strategy for deep-blue AIE-active MR-TADF dendrimers is shown in Fig. 26. Photophysical studies showed that the introduction of bulky dendritic substituents actually strengthens the PLQY, enhances film morphology, and triggers significant AIE behavior. Intermolecular π–π interactions are minimized by dendritic encapsulation, thus reducing exciton concentration quenching and suppressing bathochromic spectral shifts in the solid state. The highest PL peaks of QAO-0 123, QAO-1 124, QAO-2 125, and QAO-3 126 were at 437 nm, 437 nm, 438 nm, and 438 nm, respectively (Table 12). Despite the progressive peripheral dendritic substituent size increase, all the emitters still exhibited narrowband blue emission, verifying that the non-conjugated linkers effectively conserve the electronic structure of the MR-TADF core. The PLQYs of the dendrimers at low doping levels were about 60% for all the compounds. As the doping concentration increased, the PLQY decreased gradually due to aggregation-caused quenching (ACQ). However, the incorporation of bulky dendrons decelerated the rate of decrease appreciably, which reflects the encapsulation efficacy of the dendritic shell. Moreover, in pure films, the PLQYs significantly increase with dendron generation to 1.9%, 3.0%, 14.1%, and 20.5% for QAO-0 123, QAO-1 124, QAO-2 125, and QAO-3 126, respectively. The findings affirm that increased dendritic volume confers good resistance against exciton quenching and enhances solid-state emission efficiency. To further understand the aggregation behavior, photoluminescence experiments were performed in THF/water mixtures with adjustable water fractions (fw). All dendrimers exhibited AIE behavior, with the intensity of emission increasing in aggregated form and achieving maxima at fw values of 80%, 80%, 70%, and 60% for QAO-0 123, QAO-1 124, QAO-2 125, and QAO-3 126, respectively (Fig. SI17).

Fig. 26 Molecular design strategy of AIE-active MR-TADF dendrimers. Reproduced with permission from ref. 147. Copyright 2024, Elsevier.

Table 12 Device performances of blue AIE-TADF-OLEDs 123–126

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
10 wt% QAO-0 123	437	60	448/47	(0.18, 0.24)	4.5	788	4.1/2.6/3.0	147
10 wt% QAO-1 124	437	60	448/52	(0.16, 0.14)	4.2	967	10.8/7.5/8.5
10 wt% QAO-2 125	438	60	447/52	(0.15, 0.12)	4.1	1464	17/11.8/13.5
10 wt% QAO-3 126	438	60	447/52	(0.16, 0.19)	4.1	1019	11.2/8.8/10.0
10 wt% QAO-2: 1 wt% DCJTB	—	—	—	(0.28, 0.24)	4.2	2723	37.7/19.7/16.8
10 wt% QAO-2 125: 2 wt% DCJTB	—	—	—	(0.36, 0.30)	4.2	3185	43.7/19.6/18.9
10 wt% QAO-2 125: 3 wt% DCJTB	—	—	—	(0.43, 0.38)	4.2	2737	42.9/20.7/17.5
10 wt% QAO-2 125: 2 wt% DCJTB: 1 wt% S-Cz-BN	—	—	—	(0.38, 0.38)	4.5	2487	35.3/14.7/16.2
10 wt% QAO-2 125: 2 wt% DCJTB: 3 wt% S-Cz-BN	—	—	—	(0.38, 0.44)	4.8	2587	36.4/16.3/17.8
10 wt% QAO-2 125: 2 wt% DCJTB: 1 wt% S-Cz-BN	—	—	—	(0.28, 0.44)	5.0	2777	35.1/14.6/15.6

---

The findings validate that dendritic functionalization efficiently increases AIE activity with the retention of the MR-TADF character of the emissive core. Of these emitters, the second-generation dendrimer QAO-2 125 with an optimized ratio of carbazole and fluorene dendrons achieved optimal device performance overall. A non-doped OLED made with QAO-2 125 had a maximum EQE of 13.5%, deep-blue emission at 447 nm, and CIE coordinates of (0.14, 0.12) (Table 12). In addition, all-fluorescence WOLEDs using these narrowband MR-TADF emitters as sensitizers were first successfully achieved with maximum EQEs of 18.9% and a high colour rendering index (CRI) of 81.1 for two-component and three-component devices, respectively.¹⁴⁷ This research proves that peripheral dendritic engineering is a successful molecular design strategy for obtaining high-efficiency solution-processable AIE-active MR-TADF emitters with deep-blue colour purity and good film stability. Additionally, the demonstration of successful MR-TADF-sensitized WOLEDs highlights the promising potential of this strategy for future next-generation high-colour-gamut display and lighting technology.

Fig. 27 Chemical structures of blue AIE-TADF materials 121–126.

Two AIE-active MR-TADF emitters, QAOM 127 and QAM 128, were designed by Hudson et al. by replacing the carbonyl acceptor within QAO with one or two malononitrile units (Fig. 29).¹⁴⁸ Both compounds showed great thermal stability with decomposition temperatures of 292 °C (QAOM 127) and 362 °C (QAM 128). Electrochemical examination showed the same HOMO levels since the central nitrogen atom acts as the donor, with the more electron-withdrawing malononitrile groups in QAM 128 decreasing its LUMO energy. In solution, both QAOM 127 and QAM 128 exhibited red-shifted PL compared to QAO with PL maxima at 520 and 553 nm and FWHM values of 52 and 42 nm, respectively (Table 13). Comparable spectral characteristics were retained in the doped PMMA films, verifying the retained MR-TADF features. Unlike traditional MR-TADF emitters plagued by ACQ, both QAOM 127 and QAM 128 showed clear AIE behavior in THF/water mixtures. PL intensity also grew with water fraction (f_w), reaching 70% (QAM 128) and 90% (QAOM 127), with QAOM 127 exhibiting a 11.7-fold higher enhancement than pure THF (Fig. SI18). The concurrent bathochromic shift was attributed to enhanced solvent polarity and J-aggregate formation. In the solid phase, QAM 128 had a narrow emission at 642 nm (FWHM = 40 nm) with an additional band at 711 nm, while QAOM 127 had a narrow emission at 625 nm (FWHM = 105 nm). Both had PLQYs of 9% (QAM 128) and 11% (QAOM 127) in powder (Table 13), validating aggregation-enhanced radiative decay. This research proves that substitution with malononitrile efficiently pushes emissions to the red part of the spectrum while maintaining narrowband MR-TADF features and enhancing AIE, offering a favorable design pathway for efficient red-emitting OLEDs.

Table 13 Device performances of blue AIE-TADF-OLEDs 127–131

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
QAOM 127	520	9	—	—	—	—	—	148
QAM 128	553	11	—	—	—	—	—
6TBN 129	491	95	496/25	(0.08, 0.51)	3.45	7894	42.8/35.8/23.0	149
6TBN 129 + mCP	—	—	496/27	(0.11, 0.51)	3.30	4856	24.2/22.8/12.3
BNCz-TPE 130 + 1 wt% mCP	492	83	496/30	(0.11, 0.51)	3.8	7170	12.2/10.1/4.95	150
BNCz-TPE 130 + 3 wt% mCP	—	—	500/48	(0.17, 0.61)	3.7	7910	15.0/12.4/4.90
BNCz-TPE 130 + 5 wt% mCP	—	—	502/50	(0.18, 0.61)	3.7	7900	15.6/13.6/5.00
BNCz-TPE 130 + 10 wt% mCP	—	—	514/61	(0.23, 0.64)	3.5	8440	16.3/14.2/4.90
BNCz-DPATPE 131 + 1 wt% mCP	487	80	492/29	(0.10, 0.44)	4.0	7660	10.2/8.1/4.85
BNCz-DPATPE 131 + 3 wt% mCP	—	—	494/31	(0.11, 0.49)	3.9	8600	11.1/8.7/4.80
BNCz-DPATPE 131 + 5 wt% mCP	—	—	496/35	(0.13, 0.54)	3.9	6830	12.1/9.5/4.70
BNCz-DPATPE 131 + 10 wt% mCP	—	—	498/44	(0.16, 0.58)	3.9	6320	12.4/9.8/4.40

---

Z. X. Tang et al. reported an AIE-active MR-TADF emitter, referred to as 6TBN 129, consisting of a rigid B,N-holding polycyclic aromatic hydrocarbon core covalently bonded to a 4TCzBN unit with four carbazole groups (Fig. 29).¹⁴⁹ The design strategy for AIE-active MR-TADF emitters and their respective molecular structures is depicted in Fig. 28. To enhance solubility and prevent aggregation-caused quenching (ACQ), twelve tert-butyl substituents were added at the molecular periphery. These large groups are enough to generate adequate steric hindrance, resulting in a multidimensional, conformationally fixed framework that effectively minimizes intermolecular π–π stacking interactions. Therefore, 6TBN 129 exhibits a narrowband emission typical of MR-type emitters even in the solid state.

Fig. 28 Molecular design strategy of the AIE-active MR-TADF 6TBN 129 emitter.

Signaling a narrow ΔE_ST (0.025 eV) favors efficient reverse intersystem crossing (RISC). In addition, photoluminescence spectra in THF/H₂O mixtures exhibited a strong AIE effect, indicating that restricted intramolecular motion is another factor contributing to improved solid-state emission efficiency. Atomic force microscopy (AFM) characterization showed that the doped and undoped films had smooth surface morphologies, as indicated by root-mean-square (RMS) roughness values of 0.62 nm and 0.58 nm, respectively, demonstrating high-quality, uniform solution-processed film formation. The good film-forming capability and surface uniformity of the 6TBN 129-based layers are anticipated to facilitate balanced charge transport and stable exciton recombination in the emissive layer. Solution-processed OLEDs were prepared. The doped device, utilizing mCP as a host material, had a peak wavelength of emission at 496 nm with a very narrow FWHM value of 25 nm, validating the unique MR-type emission feature. The device exhibited excellent performance, with the highest CE, PE, and EQE of 42.8 cd A⁻¹, 35.8 lm W⁻¹, and 23.0%, respectively. In contrast, the efficiencies were relatively low for non-doped OLEDs at CE, PE, and EQE values of 24.2 cd A⁻¹, 22.8 lm W⁻¹, and 12.3%, respectively. The poorer performance of the non-doped devices was attributed mainly to the weak electrical conductivity and unbalanced charge transport in the neat emissive layer. In total, the rational molecular design of 6TBN 129 successfully amalgamates the MR-TADF and AIE properties to realize a delicate balance between rigidity, solubility, and inhibited aggregation. The observed concomitant narrowband emission, high photophysical efficiency, and superb film quality illustrate the amenability of 6TBN 129 as an effective emitter for high-performance solution-processable OLED applications.

Y. Lin et al. developed and prepared two AIE-active MR-TADF emitters, BNCz-TPE 130 and BNCz-DPATPE 131, by incorporating flexible AIE-active units, tetraphenylethylene (TPE) and diphenyl amino tetraphenylethylene (DPATPE), into a rigid MR molecular framework (Fig. 29).¹⁵⁰ The presence of these AIE-active groups introduces flexible spatial conformations and inhibits intermolecular π–π stacking, with the retained MR emission properties. Both the BNCz-TPE 130 and BNCz-DPATPE 131 compounds show good thermal stability with high decomposition temperatures (T_d) at 485 and 520 °C, respectively. Both emitters in the toluene solution show strong sky-blue photoluminescence at maximum peaks of 487 nm and 492 nm, along with narrow FWHMs of 24 nm (Table 13). Upon doped film formation, their PL emission maxima are observed at 496 nm for BNCz-TPE 130 and 493 nm for BNCz-DPATPE 131, with minor bathochromic shifts compared to their emission maxima in the solution phase, while retaining very narrow FWHMs of 30–31 nm. The PLQYs of 59% and 61% for BNCz-TPE 130 and BNCz-DPATPE 131, respectively, verify their high radiative efficiencies (Table 13). To explore further the AIE behavior, emission intensities of the two compounds were measured in THF/water mixtures with varying fw. The emitters show poor photoluminescence in pure THF owing to free intramolecular motion. With an increasing water fraction, molecular aggregation is brought about, and the emission intensity increases steadily to a maximum at fw = 40%. This phenomenon unequivocally establishes the AIE-active character of both emitters. Solution-processed OLEDs were also made from these emitters at a doping level of 1 wt%. The devices emit sky-blue EL with peaks of emission at 492–496 nm and FWHMs of narrow widths of 29–30 nm. Luminance values at the maximum are as high as 7170–7660 cd m⁻² (Table 13). Interestingly, the EL maximum of the BNCz-DPATPE 131-based device is blue-shifted by 4 nm relative to that of BNCz-TPE 130 in accordance with their respective PL behaviors. The devices provide encouraging performance, with maximum EQEs of 4.85–4.95%, CEs of 10.2–12.2 cd A⁻¹, and PEs of 8.1–10.1 lm W⁻¹ (Table 13). Surprisingly, even at higher doping levels (3–10 wt%), maximum EQEs are robust at 4.40–5.00% (Table 13), indicating the concentration-quenching-resistant characteristics of these emitters. This study broadens the molecular design methodologies of high-color-purity AIE-active MR-TADF emitters and proves to be a sustainable method for combining structural rigidity with molecular flexibility. The ideal integration of MR-TADF and AIE functionalities in BNCz-TPE 130 and BNCz-DPATPE 131 offers significant guidance for the development of efficient, stable, and solution-processable OLED materials with narrowband emission and outstanding device performance.

Fig. 29 Chemical structures of blue AIE-TADF materials 127–131.

Conclusion

OLEDs have evolved very quickly and are presently considered one of the most hopeful technologies for next-generation display and lighting. TADF materials have been central to this progress by facilitating the harvesting of singlet and triplet excitons together, which greatly enhances internal quantum efficiency without the use of noble metal dopants. However, most traditional TADF emitters show large emission bandwidths because of their ICT characteristics, which restrict their use in high color-purity OLEDs. To overcome these issues, MR-TADF materials have been developed as a novel and highly effective class of emitters, which are defined by rigid polycyclic π-frameworks confining electron density through resonance effects. This leads to intrinsically narrowband emission and high color purity, with MR-TADF emitters being extremely attractive for ultra-high-definition displays. However, TADF and MR-TADF materials generally suffer from aggregation-induced issues in the solid state, especially in solution-processed OLEDs, where intermolecular interactions are challenging to manage. Classic TADF emitters usually experience ACQ, while MR-TADF materials, in their planar molecular design, are susceptible to strong π–π stacking that causes broadening of emission and efficiency loss. Therefore, the incorporation of AIE characteristics into TADF and MR-TADF systems has been extremely useful. AIE-active TADF materials inhibit ACQ and preserve high radiative efficiency under the aggregated state to minimize exciton annihilation and prevent efficiency roll-off. Concurrently, AIE-active MR-TADF emitters preserve the inherent narrowband emission benefit of MR systems yet lower harmful π–π aggregation, enhancing electroluminescent efficiency and color purity. Herein, we comprehensively review recent developments in AIE-active TADF and AIE-active MR-TADF emitters, especially emphasizing solution-processed and narrow-emitting OLEDs. We emphasize molecular design strategies, i.e., steric modulation, donor/acceptor placement, resonance core tuning, and conformational locking, and comment on their influence on excited-state behavior, RISC, emission bandwidth (FWHM), and device performance. Table 14 presents an overview of recent developments in AIE-active TADF and MR-TADF OLEDs, pinpointing important photophysical and device performance metrics, such as photoluminescence quantum yield (PLQY), electroluminescence (EL) properties, full width at half maximum (FWHM), and related device metrics. Most of these advances come through the rational molecular design and targeted synthesis of new emitter molecules with the ability to have optimized control of excited-state properties. Significantly, the data show that MR-TADF materials always yield narrow EL emissions with notably smaller FWHM than traditional TADF emitters, highlighting their promise for high-color-purity OLED use. This trend corresponds to the benefit of a rigid, multi-resonance molecular structure to reduce vibrational relaxation and inhibit spectral broadening, which is indispensable for realizing efficient and high-performance OLED devices.

Table 14 Summary of the photophysical properties and electroluminescence performance of reported efficient AIE-active TADF and MR-TADF emitters in solution-processed OLEDs

Materials	λ PL [nm]	Φ PL [%]	λ EL [nm]/FWHM (nm)	CIE^d	V on [V]	L max [cd m^−1]	CEmax^g/PEmax^h/EQEmax^i [cd A^−1/lm W^−1/%]	Ref.^j
a Photoluminescence. b Photoluminescence quantum yield. c Electroluminescence. d CIE coordinates. e Turn-on voltage. f Luminance_max. g Current efficiency_max. h Power efficiency_max. i External quantum efficiency_max. j Reference.
Traditional TADF emitters
TTT-Ph-Cz 5	422	42	426/74	(0.16, 0.07)	5.2	107	1.61/—/3.45	86
TTT-Ph-AC 6	470	79	480/80	(0.16, 0.27)	4.0	1257	18.06/—/9.73
TTT-Ph-BAC 7	556	32	498/89	(0.21, 0.39)	3.6	1382	15.47/—/6.77
Cz-4CzCN 30	452	—	479/81	(0.19, 0.32)	3.5	5435	27.0/16.9/4.35	99
mCP-4CzCN 31	450	—	469/67	(0.17, 0.25)	3.0	6436	31.7/24.9/15.8
TCz-4CzCN 32	450	74	473/66	(0.17, 0.27)	3.4	5806	26.0/16.3/12.6
4OCzCN 97	450	32	511/88	(0.27, 0.52)	4.0	3155	11.8/7.41/4.35
5CzBN-SSP 76	473	38	508/90	(0.28, 0.54)	3.4	5400	2.1.9/17.2/7.3	116
5CzBN-DSP 77	476	45.7	508/88	(0.27, 0.54)	3.2	14 800	40.7/31.9/13.9
5CzBN-PSP 78	480	69.6	508/84	(0.27, 0.53)	3.1	13 700	58.7/46.2/20.1
MR-TADF emitters
TB-tCz 116	415	53	416/44.4	(0.17, 0.06)	3.6	—	2.19/1.97/8.21	142
TB-tPCz 117	422	56	428/42.2	(0.16, 0.05)	3.5	—	5.61/5.03/15.8
BCz 118	405	61	412/33	(0.18, 0.10)	3.8	510	1.14/0.9/3.44	143
BBFCz 119	405	86.4	416/42	(0.17, 0.07)	3.5	803	2.88/2.58/6.78
BICz 120	407	75.5	424/61	(0.16, 0.08)	3.5	1610	6.13/5.51/10.1
6TBN 129	491	95	496/25	(0.08, 0.51)	3.45	7894	42.8/35.8/23.0	149
BNCz-TPE 130 + 1 wt% mCP	492	83	496/30	(0.11, 0.51)	3.8	7170	12.2/10.1/4.95	150
BNCz-DPATPE 131 + 1 wt% mCP	487	80	492/29	(0.10, 0.44)	4.0	7660	10.2/8.1/4.85

---

Though steric shielding architectures (such as bulky peripheral, face-to-edge, and face-to-face shielding) have greatly mitigated π–π stacking in MR-TADF emitters, they continue to need optimization for the realization of purely monochromatic RGB emitters and FWHM < 20 nm. Future material breakthroughs will increasingly rely on the continued development of efficient shielding motifs and AIE-promoting substituents. In summary, the integration of AIE behavior with TADF and MR-TADF photophysical processes constitutes a potential pathway for achieving high-efficiency, narrowband, solution-processable OLEDs with lower efficiency roll-off. This review constitutes an attempt to develop a consistent molecular design platform that will direct future development toward highly stable, color-pure, and energy-efficient emission systems.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article. Raw data that support the findings of this study are available from the corresponding author upon reasonable request.

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5tc02758f.

Acknowledgements

This work was supported by the Ministry of Trade, Industry and Energy of the Republic of Korea (RS-2024-00418716) and by the Technology Innovation Program (RS-2025-25452683, Development of Core Technologies for High-Efficiency and High-Performance OLEDs via Molecular and Device Structure Optimization) through the Korea Planning & Evaluation Institute of Industrial Technology (KEIT) funded by the Ministry of Trade, Industry & Resources (MOTIR, Korea). This work was also supported by a Korea Basic Science Institute (NFEC) grant funded by the Ministry of Education (2020R1A6C101A184).

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