# Pure Hydrocarbon Hosts Enabling Efficient Multi-Resonance TADF Blue-Emitting Organic Light-Emitting Diodes

Yue-Jian Yang, Denis Ari, Zhe-Hong Yu, Kelvine Lettelier, Olivier Jeannin, Qi Zheng, Aziz Khan, Cassandre Quinton, Dong-Ying Zhou, Jiang Zuo-Quan, et al.

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Yue-Jian Yang, Denis Ari, Zhe-Hong Yu, Kelvine Lettelier, Olivier Jeannin, et al. Pure Hydrocarbon Hosts Enabling Efficient Multi-Resonance TADF Blue-Emitting Organic Light-Emitting Diodes. Angewandte Chemie International Edition, In press, 10.1002/anie.202501895. hal-04987850

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# Pure Hydrocarbon Hosts Enabling Efficient Multi-Resonance TADF Blue-Emitting Organic Light-Emitting Diodes

Yue-Jian Yang,[a,b]+ Denis Ari,[c]+ Zhe-Hong Yu,[a,b]+ Kelvine Lettelier,[c] Olivier Jeannin,[c] Qi Zheng,[a,b]
Aziz Khan,[a,b] Cassandre Quinton,[c] Dong-Ying Zhou,[*a,b] Zuo-Quan Jiang*[a,b] and Cyril Poriel*[c]

**Abstract:** Pure hydrocarbon (PHC) materials are a class of highly efficient and stable host materials for organic light-emitting diodes (OLEDs), composed solely of carbon and hydrogen atoms. Despite recent great advancements in PHC research, their applications are still mainly limited to phosphorescent OLEDs (PHOLEDs). High-performance blue OLEDs still pose a considerable challenge to achieve. Thus, expanding PHC materials into other types of OLEDs is critical for advancing organic electronic technologies. In this study, we designed a series of original high-triplet PHC materials based on a multi-substitution approach of the 9,9'-spirobifluorene (SBF) backbone and used them, for the first time, as host in phosphorescence-sensitized multi-resonance thermally activated delayed fluorescence (MR-TADF) OLEDs. Devices based on 2,6-Bis(3,6-di-tert-butyl-9H-carbazol-9-yl)boron (DtBuCzB) emitter, using FIrpic or *fac*-Ir tpz₃ as the sensitizer, achieved high maximum external quantum efficiency (EQE<sub>max</sub>) values ranging from 29.1% to 33.9%. Additionally, blue MR-TADF OLED devices based on v-DABNA with a phosphorescent sensitizer (CN-Ir), demonstrated outstanding electroluminescent performance, with EQE<sub>max</sub> of approximately 31% notably due to an excellent molecular orientation induced by the PHC hosts. All devices exhibited narrow full-width at half-maximum spectra and minimal efficiency roll-off. This study marks the first application of PHC materials as hosts in phosphorescence-sensitized MR-TADF OLEDs, highlighting their potential as promising candidates for next-generation blue OLEDs and offering a viable pathway to achieve high-performance devices.

## Introduction

The development of organic electronics, particularly organic light-emitting diode devices (OLEDs) technology, is beginning to impact daily life significantly.[1] Based on different photophysical principles, three generations of primary luminescent materials have been developed: fluorescent materials, phosphorescent materials,[2-7] and thermally activated delayed fluorescence (TADF) materials.[8-11] Despite considerable progress in the past decades, blue light emission still needs to be improved in terms of efficiency, stability and color purity.[2, 12-13] This is a major challenge for the future of lighting and display. Stability has always been an important issue

in blue OLED technology, which has significantly slowed down its development and commercialization. The bond dissociation energy plays a critical role in OLED degradation, a process known as the *exciton-induced degradation process*.[14] As C-N, C-P and C-S bonds are known to be significantly more fragile than C-C bonds,[15-18] pure hydrocarbon (PHC) materials,[19] only constituted of hydrogen and carbon atoms, have appeared as a possible solution for the next generation of blue OLEDs. Removing heteroatoms from the molecular structure of host materials for phosphorescent OLEDs (PHOLEDs) not only reduces structural synthetic complexity and production costs but also simplifies technological processes, which is particularly important given the current focus on ecological transformation.[20, 21]

Although the PHC strategy was introduced as early as 2005 and used in various optoelectronic fields,[22] progress has been relatively slow[23-26] and the first high-performance devices have only appeared in recent years.[27-28] Nowadays, several PHC materials have provided high-efficiency blue PHOLEDs and have even surpassed the performance of their heteroatom-based counterparts, clearly showing their real potential.[29-33] Despite these advances in OLED research using PHC host materials, the broad electroluminescent (EL) emission spectra and relatively moderate efficiency have hindered their application in display technologies. Recently, the advent of multiple resonance thermally activated delayed fluorescence (MR-TADF) has transformed the development of OLED materials by enabling both high color purity and high efficiency, which are essential for high-definition OLED displays.[34-36] Based on the very promising previous results obtained with PHC in PHOLEDs,[29-33] it would be highly appealing to combine novel PHC host materials with MR-TADF emitting materials through the phosphorescent sensitization technique, which has garnered significant attention for its ability to enhance efficiency and mitigate efficiency roll-off.[37-42] Although the application of PHC-based host materials in phosphorescence-sensitized devices has not yet been demonstrated, their ability to meet the practical requirements of this technology makes them worthy of exploration in this study. In **Figure 1**, the mechanism of PHC host-based phosphorescence-sensitized MR-TADF OLEDs is presented. The initial first 25% singlet excited state (S₁) and first

<footnote>[a] Y.-J. Yang,⁺ Z.-H. Yu,⁺ Q. Zheng, A. Khan, D.-Y. Zhou, Z.-Q. Jiang Institute of Functional Nano & Soft Materials (FUNSOM), Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, 215123, Jiangsu, PR China.</footnote>

<footnote>[b] Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, 215123, Jiangsu, PR China.
E-mail: zqjiang@suda.edu.cn; dyzhou@suda.edu.cn</footnote>

<footnote>[c] D. Ari,⁺ K. Lettelier, O. Jeannin, C. Quinton, C. Poriel
Univ Rennes, CNRS, ISCR-UMR CNRS 6226, F-35000 Rennes,
E-mail: cyril.poriel@univ-rennes.fr</footnote>

<footnote>[+] Equal contribution</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** a) Molecular structures of four pure hydrocarbon isomers; b) The energy transfer process of the phosphorescence-sensitized system.</figcaption></figure>

75% triplet excited state ($T_1$) excitons are transferred from the host to the phosphorescent sensitizer via Förster resonance energy transfer (FRET) and Dexter energy transfer (DET) processes, respectively. The phosphorescent sensitizer leverages the spin-orbit coupling effect of heavy metal atoms to efficiently convert $S_1$ excitons into $T_1$ excitons via intersystem crossing (ISC). The $T_1$ excitons are then transferred to the $S_1$ state of the emitter through the FRET process.[43] The key step in this energy transfer cascade is the transfer of $T_1$ exciton energy from the sensitizer to the $S_1$ excitons of the emitter via FRET, thereby reducing the generation of long-lived $T_1$ excitons in the TADF emitter.[44] Additionally, selecting the phosphorescent sensitizer with a triplet state energy level ($E_T$) closely matching the singlet energy level ($E_s$) of the MR-TADF emitter enables filtering the $S_1$ excitons of the emitter through reverse DET, further minimizing $T_1$ exciton accumulation in the emitter.[38] Since the $T_1$ exciton lifetime of the phosphorescent sensitizer is typically shorter than that of the MR-TADF emitters, this method effectively reduces $T_1$ exciton density and shortens exciton lifetimes while preserving the high external quantum efficiency (EQE) and pure color emission characteristic of the MR-TADF emitters.[45-48]

In this work, we present the first PHC host-based phosphorescence-sensitized MR-TADF OLEDs. To achieve high $E_T$ and a wide energy gap ($E_g$), the design strategy involves disrupting the π-conjugation of the 9,9'-spirobifluorene (SBF) skeleton by introducing peripheral meta-terphenyl (mtp) units. Four positional isomers of SBF were investigated: **1,6-dimtp-SBF**, **1,8-dimtp-SBF**, **3,3-dimtp-SBF** and **4,4-dimtp-SBF** (Scheme 1). These substitution patterns are very rarely reported in the literature but appear herein interesting to reach the necessary electronic and physical properties for the targeted applications. All PHC host materials exhibited high $E_T$ values, wide $E_g$ and excellent thermal stability, with high decomposition ($T_d$) and glass transition ($T_g$) temperatures. When used as host materials in blue MR-TADF OLEDs employing 2,6-Bis(3,6-di-tert-butyl-9H-carbazol-9-yl)boron (DtBuCzB, see structure in Figure S55) as the emitter, these isomers demonstrated outstanding EL performance. Devices sensitized with Flrpic and *fac*-Ir tpz₃ (see structure in Figure S55) produced narrowband emission peak at 488 nm, full-width at half-

maximum (FWHM) of 29-30 nm and high maximum EQE ($EQE_{max}$) ranging from 29.1% to 33.9%, with minimal efficiency roll-off. Among the isomers, **1,8-dimtp-SBF** and **4,4-dimtp-SBF** showed significantly enhanced current and power efficiencies (CE and PE). Additionally, blue MR-TADF OLEDs incorporating v-DABNA (see structure in Figure S55) and using **1,8-dimtp-SBF** and **4,4-dimtp-SBF** as host, in combination with CN-Ir as sensitizer, achieved narrowband emissions at 468 nm and short FWHMs of 21 nm. These devices also reached a very high $EQE_{max}$ of approximately 31% owing to their excellent molecular orientation, which maximizes the light outcoupling efficiency ($\eta_{out}$). These findings show the versatility of PHC materials and their great potential for achieving the new generation of high-efficiency OLEDs.

## Results and Discussion

The synthetic route is shown in Scheme 1. The synthesis of **1,6-dimtp-SBF** and **1,8-dimtp-SBF** involves first the synthesis of the corresponding fluorenones, possessing the anchor points at C1 & C6 for **1,6-diBr-FO** and at C1 & C8 for **1,8-diBr-FO**. These substitution patterns are very rarely reported in the literature and are difficult to reach from a synthetic point of view. These compounds were synthesized using a one-pot carbon-hydrogen activation and cyclization approach, first reported by Sorensen and coworkers in 2017.[49-50] This approach is interesting as it allows to form in a single step two sophisticated fluorenyl cores with a 1,6 and 1,8-substitution pattern. With both **1,6-diBr-FO** and **1,8-diBr-FO** prepared, the grafting of the mtp fragment was performed via a Suzuki-Miyaura cross-coupling reaction yielding **1,6-dimtp-FO** and **1,8-dimtp-FO**. The spirofluorene unit was finally introduced with a classical sequence involving a nucleophilic addition reaction of either **1,6-dimtp-FO** or **1,8-dimtp-FO** with the lithiated biphenyl followed by an intramolecular cyclization reaction giving **1,6-dimtp-SBF** and **1,8-dimtp-SBF**. The synthesis of **3,3-dimtp-SBF** and **4,4-dimtp-SBF** was achieved via Suzuki-Miyaura coupling between the corresponding bromo-SBF and mtp-boronic acid. This synthetic route is straightforward, with yields exceeding 70% and it supports
<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Synthesis of 1,6-dimtp-SBF, 1,8-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF.</figcaption></figure>

reactions on a gram scale. Additionally, it offers a new synthetic strategy for the functionalization of the spirofluorene unit at opposite fluorene positions. The four regioisomers can only be differentiated by the SBF substitution patterns (C1, C3, or C4). These substitution sites limit the electronic coupling between the external mtp groups and the central SBF. Such design features are expected to optimize the hosting properties in OLEDs.

To better understand the configurations of the four isomers, single-crystal X-ray diffraction analyses were performed (Figure 2, Table S6-S9). The analysis revealed that all isomers possess a rigid orthogonal configuration in the SBF core, with a dihedral angle of approximately 90° between the two fluorene units (Figure S37). The dihedral angles between the SBF core and the mtp groups were also measured (Figure 2) and summarized in Table S10, with further discussion provided below. The number of intramolecular short contacts of π...π, C-H...π and C-H...H-C types is substantial across all PHC molecules studied (Figure S38-S42). These contacts contribute to structural rigidity and stabilize specific

conformations, highlighting the pronounced steric hindrance within these systems, particularly at the C1 and C4 positions known for their high steric demands.[53] Distinct patterns of intermolecular interactions are also observed across the molecules (Figure S43-S47). For 1,6-dimtp-SBF, only a single short C-H...H-C contact is present, influencing the packing arrangement and leading to a less dense crystal structure. In contrast, 1,8-dimtp-SBF exhibits a relatively greater number of intermolecular short contacts, with C-H...π interactions contributing to the packing. 3,3-dimtp-SBF displays three types of intermolecular interactions C-H...π, π...π and C-H...H-C resulting in a more compact crystal structure than those of 1,6-dimtp-SBF and 1,8-dimtp-SBF. The crystal structure of 4,4-dimtp-SBF is particularly noteworthy, as it includes two distinct conformers. The first conformer exhibits a remarkably high number of intermolecular short contacts, involving three types of interactions: C-H...π, π...π and C-H...H-C,

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Single crystal configuration of 1,6-dimtp-SBF, 1,8-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF. Hydrogen atoms have been omitted for clarity.</figcaption></figure>
producing an extremely dense and compact crystal structure. Conversely, the second conformer shows significantly fewer interactions, with only C-H ... π contacts. This reveals the +coexistence of two very distinct conformers in the crystal: one with a high number of short intermolecular contacts and another with a low number of interactions, which suggests the possibility of polymorphism in the crystalline structure of **4,4-dimtp-SBF**. These intermolecular interactions drive molecular packing, which can substantially influence device performance by promoting charge transfer and improving thermal stability. Additionally, owing to the presence of the large and twisted mtp substituents, all four isomers exhibit a loose molecular packing arrangement (Figure S48), which significantly reduces strong π...π interactions between molecules, thereby minimizing intermolecular quenching. This loose packing facilitates the horizontal orientation of molecules in films, enhancing η<sub>out</sub> and improving OLED performance.[51] This will be demonstrated below. These findings reveal the potential of this series of isomers as promising host materials for OLED applications.

As the PHC concept is based on the stability of the C-H and C-C bonds vs C-N, C-O or C-S bonds, Bond Dissociation Energies (BDE) were evaluated on different SBF-based models (Figure S54). Independently of the substitution position (C1, C3 or C4), the BDEs of C-H and C-C bonds were always higher than those of C-N, C-O or C-S bonds in corresponding model compounds.

As shown in **Figure 3a**, the UV-visible absorption spectra of the four isomers in dilute cyclohexane solution (10⁻⁵ M) exhibit significant differences translating the importance of the substitution pattern. Time-dependent density functional theory (TD-DFT) calculations, using the B3LYP functional and def2svp basis set, were performed on the optimized ground (S₀) state geometries (Figure S49, Table S11-S14). All four isomers display a characteristic absorption band between 308 and 316 nm, corresponding to the SBF backbone.[52, 53] Additionally, red-shifted absorption bands at longer wavelengths were observed, indicating

electronic coupling between the fluorene fragment and the terphenyl substituents.[53] Among the isomers, the absorption spectrum of **1,8-dimtp-SBF** is the most similar to that of **SBF**. TD-DFT studies reveal that while the bathochromic-shifted absorption band remains present in **1,8-dimtp-SBF**, its intensity is relatively low, indicating that the C1 position has a smaller influence on absorption properties compared to positions C3 and C4. Indeed, the C1 position presents the advantages of both C3 and C4 positions, namely an electronic decoupling thanks to the *meta*-linkage (as for C3) and a high steric hindrance (as for C4). The high sterically hindered environment of **1,8-dimtp-SBF** is materialized by the large dihedral angles, measured in the crystallographic structure (80.6°/81.0°, Figure 2 and Table S10), between the fluorene and the mtp. In contrast, **3,3-dimtp-SBF** exhibits a particularly intense bathochromic-shifted absorption band, reflecting an extension of the π-conjugation. This π-extension at the C3 position has been discussed elsewhere.[52-53] As a result, the absorption intensity of **1,6-dimtp-SBF** lies between that of **3,3-dimtp-SBF** and **1,8-dimtp-SBF** with a shoulder at ca 316 nm translating the electronic coupling induced by the position C3. The spectrum of **4,4-dimtp-SBF** also shows this shoulder at ca. 316 nm, which reflects a certain degree of π-conjugation between the fluorene and mtp units. However, the large angle formed between the fluorene and the mtp at the C4 position (66.3°/68.4°/74.7°, Figure 2) significantly disrupts π-conjugation in **4,4-dimtp-SBF**. This strong steric congestion is the main characteristic of the C4 position.[4, 54] To further validate the steric hindrance effects in the four isomers, we performed the Independent Gradient Model based on Hirshfeld (IGMH) analysis (Figure S53). In summary, the C1 position effectively disrupts the π-conjugation because of the cumulative effect of a strong steric hindrance and *meta*-substitution, while the C3 and C4 positions cause only partial disruption of π-conjugation due to only one of these effects (*meta*-linkage for C3 and steric hindrance for C4). Based on the onset of the absorption

<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) Absorption and (b) fluorescence spectra at room temperature in cyclohexane (c) phosphorescence spectra (left) in 2-methyl-THF at 77 K and (d) spin density distribution (SDD, right) of the triplet state of **1,6-dimtp-SBF**, **1,8-dimtp-SBF**, **3,3-dimtp-SBF** and **4,4-dimtp-SBF**.</figcaption></figure>
spectra, the $E_g^{opt}$ of 1,8-dimtp-SBF, 4,4-dimtp-SBF, 3,3-dimtp-SBF and 1,6-dimtp-SBF are estimated to be 3.85, 3.79, 3.76 and 3.72 eV, respectively (Table 1). Compared to the $E_g^{opt}$ of SBF (3.97 eV), there is hence a significant reduction of $E_g^{opt}$, reflecting the varying degrees of π-conjugation disruption as a function of the substitution pattern. These results suggest that these materials are promising candidates as host materials for OLEDs.

The emission characteristics are also significantly influenced by the substitution pattern (Figure 3b). The 1,8-dimtp-SBF displays the bluest emission spectrum ($\lambda_{max}$ = 317 and 332 nm) with only a slight bathochromic shift compared to SBF ($\lambda_{max}$ = 310 and 323 nm), as also observed above in absorption. Surprisingly, the fluorescence spectra of 1,6-dimtp-SBF and 3,3-dimtp-SBF are nearly identical, both exhibiting a moderate bathochromic shift ($\lambda_{max}$ = 338 nm). It is well established that the position C3 of a fluorene (similar to the C2 position) leads to a planification of $S_1$, [53] which can erase the linkage effects and usually induces a bathochromic shift of the emission and a relatively narrow spectrum as observed for both 1,6-dimtp-SBF and 3,3-dimtp-SBF. The root-mean-square deviation (RMSD) of 4,4-dimtp-SBF is calculated to be 1.06 Å, revealing that the deformation and fluctuation of geometric configurations between $S_0$ and $S_1$ state of 4,4-dimtp-SBF are large during the electronic transition process (Figure S51). Additionally, the calculated reorganization energies for 1,6-dimtp-SBF, 1,8-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF are 0.78, 0.59, 0.56 and 1.09 eV, respectively. Among the four isomers, 4,4-dimtp-SBF exhibits the largest reorganization energy (Table S15),

resulting in the largest Stokes shift and the largest bathochromic shift in fluorescence emission, consistent with experimental observations ($\lambda_{max}$ = 361 nm). C4-substituted SBFs have previously been the subject of several studies to shed light on their unusual fluorescence properties and the present findings are following other C4-substituted SBFs reported in the literature. [4, 53-54] The quantum yields ($\Phi$) were measured at 0.65, 0.24, 0.60 and 0.61 for 1,6-dimtp-SBF, 1,8-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF respectively. Except for 1,8-dimtp-SBF, all the quantum yields are similar to that of SBF (0.55). The highest value observed for 1,6-dimtp-SBF can be related to the combination of a low non-radiative constant $k_{nr}$ and a relatively high radiative constant $k_r$ (4.69 and 8.70 × 10⁷ s⁻¹ respectively, Table 1). The two smallest $k_{nr}$ in the series are detected for the two compounds possessing relaxed C3 linkages, 1,6-dimtp-SBF and 3,3-dimtp-SBF, without strong steric hindrance. Oppositely, in both 1,8-dimtp-SBF and 4,4-dimtp-SBF, the steric congestion induced by the position C1 or C4 increases $k_{nr}$ to 9.15 × 10⁷ and 1.07 × 10⁸ s⁻¹ respectively. In the case of 4,4-dimtp-SBF, $\Phi$ is maintained high since $k_r$ is also measured very high, 1.68 × 10⁸ s⁻¹, in accordance with a high oscillator strength of HOMO/ LUMO transition ($f$ = 0.0996) but for 1,8-dimtp-SBF, the combination of low $k_r$ (2.28 × 10⁷ s⁻¹) and high $k_{nr}$ (9.15 × 10⁷ s⁻¹) leads to a dramatic drop of $\Phi$. In the latter, the C1 position is at the origin of this feature with an almost null oscillator strength for the HOMO/LUMO transition ($f$ = 0.0001) and a highly sterically hindered environment. This is in line with previously reported results on C1-substituted SBFs. [27,53]

<table><caption>Table 1 Selected properties of 1,6-dimtp-SBF, 1,8-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF. SBF is reported herein for comparison. [53, 55]</caption><thead><tr><th></th><th>1,6-dimtp-SBF</th><th>1,8-dimtp-SBF</th><th>3,3-dimtp-SBF</th><th>4,4-dimtp-SBF</th><th>SBF</th></tr></thead><tbody><tr><td>λ<sub>abs</sub> [nm]<sup>a</sup></td><td>311, 318</td><td>312</td><td>306, 316</td><td>308</td><td>308</td></tr><tr><td>λ<sub>em</sub> [nm]<sup>a</sup></td><td>338</td><td>317, 332</td><td>338</td><td>361</td><td>310, 323</td></tr><tr><td>Φ</td><td>0.65<sup>b</sup></td><td>0.24<sup>b</sup></td><td>0.60<sup>b</sup></td><td>0.61<sup>b</sup></td><td>0.55<sup>c</sup></td></tr><tr><td>λ<sub>phos</sub> [nm]<sup>d</sup></td><td>438</td><td>433</td><td>444</td><td>446</td><td>429</td></tr><tr><td>τ<sub>f</sub> [ns]<sup>a</sup></td><td>7.47</td><td>8.30</td><td>7.58</td><td>3.63</td><td>4.6</td></tr><tr><td>k<sub>r</sub> (×10<sup>7</sup>) [s<sup>-1</sup>]</td><td>8.70</td><td>2.89</td><td>7.91</td><td>16.80</td><td>12</td></tr><tr><td>k<sub>nr</sub> (×10<sup>7</sup>) [s<sup>-1</sup>]</td><td>4.69</td><td>9.15</td><td>5.28</td><td>10.74</td><td>10</td></tr><tr><td>τ<sub>p</sub> (s)</td><td>5.33</td><td>5.70</td><td>4.46</td><td>4.14</td><td>5.4</td></tr><tr><td>E<sub>S</sub> [eV]<sup>e</sup></td><td>3.86</td><td>4.04</td><td>3.87</td><td>3.82</td><td>4.05</td></tr><tr><td>E<sub>T</sub> [eV]<sup>f</sup></td><td>2.83</td><td>2.86</td><td>2.82</td><td>2.78</td><td>2.89</td></tr><tr><td>LUMO<sub>Ei</sub> [eV]<sup>g</sup></td><td>-2.08</td><td>-1.93</td><td>-2.09</td><td>-2.12</td><td>-1.74</td></tr><tr><td>LUMO<sub>th</sub> [eV]</td><td>-1.35</td><td>-1.32</td><td>-1.35</td><td>-1.38</td><td>-1.26</td></tr><tr><td>HOMO<sub>Ei</sub> [eV]<sup>h</sup></td><td>-5.93</td><td>-5.84</td><td>-5.97</td><td>-6.01</td><td>-5.95</td></tr><tr><td>HOMO<sub>th</sub> [eV]</td><td>-5.80</td><td>-5.71</td><td>-5.88</td><td>-5.97</td><td>-5.99</td></tr><tr><td>E<sub>g</sub><sup>el</sup> [eV]</td><td>3.85</td><td>3.91</td><td>3.88</td><td>3.89</td><td>4.14</td></tr><tr><td>E<sub>g</sub><sup>opt</sup> [eV]<sup>i</sup></td><td>3.72</td><td>3.85</td><td>3.76</td><td>3.79</td><td>3.97</td></tr><tr><td>T<sub>g</sub> [°C]</td><td>150</td><td>127</td><td>172</td><td>164</td><td>-</td></tr><tr><td>T<sub>d</sub> [°C]</td><td>435</td><td>407</td><td>490</td><td>463</td><td>234</td></tr><tr><td>μ<sub>h</sub> (×10<sup>-6</sup>) [cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>]<sup>j</sup></td><td>9.26</td><td>6.94</td><td>8.75</td><td>7.58</td><td>-</td></tr><tr><td>μ<sub>e</sub> (×10<sup>-6</sup>) [cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>]<sup>j</sup></td><td>4.04</td><td>3.23</td><td>2.52</td><td>6.81</td><td>-</td></tr><tr><td>μ<sub>h</sub> / μ<sub>e</sub></td><td>2.29</td><td>2.14</td><td>3.47</td><td>1.13</td><td>-</td></tr></tbody></table>

[a] In cyclohexane. [b] Compared to SBF. [c] Compared to quinine sulfate. [d] In 2-methyl-THF at 77 K. [e] Calculated from the onset of the lowest energy band in cyclohexane (1239.84/λ). [f] Calculated from the peak maximum of the highest energy phosphorescent band, (1239.84/λ), at 77 K in 2-MeTHF. [g] From CVs in DMF. [h] From CVs in DCM. [i] Calculated from the onset of the UV/Vis absorption spectrum in cyclohexane. [j] Hole/electron mobility. th = theoretical, el = electrochemical, opt = optical.
The phosphorescence emission spectra recorded at 77 K in 2-MeTHF (Figure 3c) provide the $E_T$ values for 1,8-dimtp-SBF, 1,6-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF, which are estimated to be 2.86, 2.83, 2.82 and 2.78 eV, respectively. Contrary to our observations from absorption and fluorescence, the nature of the linkage fully drives the $E_T$ values. 1,8-dimtp-SBF exhibits the highest $E_T$, reflecting its effective disruption of π-conjugation, with an $E_T$ value close to that of unsubstituted SBF (2.89 eV). This results from the combination of two meta linkages and a constrained environment, with dihedral angles between the SBF core and the mtp units of 89.2°/89.3° in the T₁ state (Figure S52 and Table S16). In contrast, 1,6-dimtp-SBF and 3,3-dimtp-SBF, which both possess C3 substitution patterns, display lower $E_T$ values of 2.83 eV and 2.82 eV, respectively, with T₁-state dihedral angles of 5.0°/73.8° and 7.1°/39.1°. These $E_T$ values are nevertheless higher than that of 4,4-dimtp-SBF, which features ortho linkages (C4) and has an $E_T$ value of 2.78 eV, with T₁-state dihedral angles of 33.5°/62.4°. This feature can be illustrated by the spin density distribution (SDD, Figure 3d). Indeed, 1,8-dimtp-SBF shows spin density fully localized on a single fluorene unit, while 4,4-dimtp-SBF exhibits partial delocalization on the mtp unit, explaining the lower $E_T$ value. The C3 linkage of 1,6-dimtp-SBF and 3,3-dimtp-SBF provides spin densities concentrated on two phenyl rings, one of the fluorene and one of the mtp, resulting in $E_T$ that are intermediate between those of the two other isomers. Additionally, these compounds exhibit relatively long T₁ state lifetimes, estimated to range from 4.14 to 5.70 s. In summary, the $E_T$ values of these four isomers can be finely modulated through different substitution patterns but are, for all, maintained very high, which is an essential feature for achieving efficient energy transfers in OLEDs.

The electrochemical properties of the four isomers were examined using cyclic voltammetry (CV) in CH₂Cl₂ (for oxidation) and DMF (for reduction), as presented in Figure S26-S35 and summarized in Table 1. From the onset oxidation potentials, HOMO energy levels for 1,6-dimtp-SBF, 1,8-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF were evaluated at -5.93, -5.84, -5.97 and -6.01 eV, respectively. The HOMO energy levels can be classified in two sets: one possessing HOMO energy levels higher than that of SBF

(-5.95 eV), namely 1,6-dimtp-SBF and 1,8-dimtp-SBF (-5.93, -5.84 eV) and one surprisingly lower than that of SBF, namely 3,3-dimtp-SBF and 4,4-dimtp-SBF (-5.97 and -6.01 eV). The couple 3,3-dimtp-SBF/4,4-dimtp-SBF appears to be more difficult to oxidize than the building unit SBF, indicating an electron-withdrawing effect of the mtp fragment on the SBF core. This electron-withdrawing effect of the mtp core seems to have more influence than the π-conjugation extension induced by the fluorene/terphenyl connection. Their HOMO energies are therefore lowered compared to that of SBF. The strongest effect on the HOMO energy is thus logically detected for 4,4-dimtp-SBF, which possesses two ortho linkages.

Compounds 1,6-dimtp-SBF and 1,8-dimtp-SBF display drastically different behavior. Indeed, in the case of 1,8-dimtp-SBF, the presence of the two mtp at C1 induces strong cofacial interactions.[56-57] Such interactions are known to significantly decrease the HOMO energy levels.[58-62] This can be seen by the electronic density born by the two mtp in the HOMO (Figure 4), which is characteristic of such types of interactions. For 1,6-dimtp-SBF, which possesses only one mtp fragment at C1, this effect is logically diminished and the HOMO is then lowered. Thus, three different effects are involved in this series: (i) Through-space interaction because of the substitution at C1, (ii) through bond extension of the π-conjugation and (iii) electron-withdrawing effect of the mtp fragment.

The reduction potentials were measured in DMF providing peak potentials of -2.48, -2.32, -2.31 and -2.28 V for 1,8-dimtp-SBF, 1,6-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF respectively. Corresponding LUMO energy levels were evaluated at -1.93, -2.08, -2.09 and -2.12 eV. Oppositely to what was observed for the HOMO energies, the electron-withdrawing effect of the mtp groups is predominant in the LUMOs, depressing for all molecules the LUMO energy levels compared to SBF (-1.74 eV). Specifically, the LUMO of 1,6-dimtp-SBF and 1,8-dimtp-SBF is mainly localized on the substituted fluorene (because of the substitution on a single fluorene), while the LUMO of 3,3-dimtp-SBF and 4,4-dimtp-SBF is distributed across both the SBF backbone and the attached benzene rings (owing to the substitution on two fluorenes).

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Frontier molecular orbitals of 1,6-dimtp-SBF, 1,8-dimtp-SBF, 3,3-dimtp-SBF and 4,4-dimtp-SBF. (top: LUMO, bottom: HOMO, B3LYP, def2svp, isovalue of 0.035).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Device performance of D1-B2 using **1,6-dimtp-SBF**, **1,8-dimtp-SBF**, **3,3-dimtp-SBF** and **4,4-dimtp-SBF** as host, respectively. Current density and luminance versus driving voltage (J·V·L) characteristics for a) D1-D4; b) D5-D8; c) B1 and B2. d) EQE versus luminance characteristics for D1-D4 (top), D5-D8 (middle) and B1-B2 (bottom). e) CE versus luminance characteristics for D1-D4 (top), D5-D8 (middle) and B1-B2 (bottom).</figcaption></figure>

For the latter, this leads to a deepening of the LUMO energy. The deep HOMO and high LUMO energy levels provide wide $E_g$ of 3.91, 3.89, 3.88 and 3.85 eV for **1,8-dimtp-SBF**, **4,4-dimtp-SBF**, **3,3-dimtp-SBF** and **1,6-dimtp-SBF** respectively. This wide $E_g$ effectively prevents holes and electrons from recombining directly within the host material, which facilitates efficient energy transfer of the sensitization system and enhances emission efficiency. This is crucial for incorporating emitter and phosphorescent sensitizers into the OLED emission layer (EML).

The PHC constructed on the SBF skeleton offers significant morphological stability and high thermal stability, which can reduce efficiency roll-off owing to heat accumulation and material degradation. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to assess the thermal and morphological properties of the four isomers. The results show that these compounds exhibit excellent thermal stability (Figure S36). The $T_d$ measured at 5% mass loss, are 435, 407, 490 and 463 °C for these isomers, respectively, indicating their robust thermal stability. Additionally, the $T_g$ are significantly high, recorded at 150, 127, 170 and 164 °C, respectively. The variations are primarily attributed to the different molecular arrangements of the isomers, highlighting the impact of the substitution patterns on this characteristic. In contrast, the thermal stability of the **SBF** building block is comparatively low, with a $T_d$ of 234 °C.[53, 55] These findings show that these host materials display high thermal stabilities while maintaining the favorable electronic properties of **SBF**, including high $E_T$ and wide $E_g$.

Before examining the EL characteristics, we first analyzed hole- only and electron-only devices (HOD and EOD) to measure charge

mobility (Figure S56). At low bias, the curves were well-fitted within the space-charge-limited current (SCLC) region. The calculated zero-field mobilities of the compounds are summarized in Table 1. Consistent with other PHC systems reported in the literature, all compounds demonstrated good charge balance owing to their comparable hole and electron mobilities. This charge balance is crucial for enhancing OLED performance, as it facilitates efficient recombination of holes and electrons, contributing to the high efficiency observed in subsequent studies, particularly for the new generations of simplified OLEDs called Single-Layer.[21, 63-64] This behavior is partially attributed to the inherent PHC properties of these host materials as previously observed in the literature.[31-33] Indeed, for hosts containing heteroatoms, achieving balanced charge transport can be challenging due to the intrinsic properties of their functional units, which may be either electron-rich or electron-deficient.

Finally, as the four isomers displayed all the prerequisites, they were employed as hosts in MR-TADF OLEDs. To reduce the likelihood of exciton formation through direct charge trapping on the emitter, phosphorescent sensitizers (Flpic, fac-Ir(tpz)₃, CN-Ir)[38, 65- 66] were selected with HOMO energy levels closely matching that of the emitters (DtBuCzB and v-DABNA)[67-68] (Table S22). Additionally, to maximize the spectral overlap between the photoluminescence (PL) of the sensitizer and the absorption spectrum of the emitter, thereby enhancing FRET efficiency of energy transfer from $T_1$ excitons of the sensitizer to $S_1$ excitons of the emitter, we measured the overlap between the PL spectra of the sensitizers and the absorption spectra of the emitters in dilute toluene solution (Figure S57). The results indicate the large
spectral overlap, which effectively optimizes device performance by minimizing energy loss and improving emission efficiency. Furthermore, to suppress T₁ exciton exchange from the T₁ state of the sensitizer to the T₁ state of the emitter via DET, the emitter doping concentration was maintained between 0.5 and 2 wt%, ensuring sufficient spatial separation between the sensitizer and the emitter.[69-72] The OLEDs investigated present the following structure: ITO/HAT-CN (10 nm)/TAPC (40 nm)/TCTA (10 nm)/mCP (10 nm)/ EML (20 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al (80 nm). In this configuration, HAT-CN (1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile) acts as the hole injection layer, TAPC (1,1-bis[(di-4-tolylamino)phenyl]cyclohexane) and TCTA (Tris(4-carbazoyl-9-ylphenyl)amine) serve as hole transport layers, mCP (1,3-bis-(N-carbazolyl)benzene)) functions as an electron-blocking layer and TmPyPB (1,3,5-tri(m-pyridin-3-ylphenyl)benzene) and Liq (lithium-8-hydroxyquinolinolate) are used as the electron transport and electron injection layers, respectively. The EML consists of the host material mixed with 2 wt% of DtBuCzB and 10 wt% of Flpic. The EL performance of the OLEDs is depicted in **Figure 5** and Figure S58, while the device structure, energy level diagram and molecular structures of the functional materials are presented in Figure S55. The device performances are summarized in **Table 2**.

As shown in Figure 5 and Figure S58, devices D1–D4 exhibit similar EL spectra, with a maximum of 488 nm and a FWHM of 30 nm. Their driving voltages are respectively 3.71, 3.72, 3.79 and 3.79 V at 0.01 mA cm⁻², indicating excellent charge injection properties. Remarkably, all devices achieve exceptionally high EQE<sub>max</sub>, ranging from 29.1% to 33.9%. Devices D2 and D4, based on **1,8-dimtp-SBF** and **4,4-dimtp-SBF** respectively, exhibit both an extremely high EQE<sub>max</sub> of 33.9% and a maximum CE (CE<sub>max</sub>) of 60 cd A<sup>-1</sup>. However, D2 demonstrates a slightly higher maximum PE (PE<sub>max</sub>) of 54.1 lm W<sup>-1</sup> compared to 49.7 lm W<sup>-1</sup> for D4, likely owing to the lower driving voltage of D2. These results represent state-of-the-art performance for OLED devices utilizing DtBuCzB. It is noteworthy that the exceptional charge balance demonstrated by **4,4-dimtp-SBF** significantly contributes to its high efficiency. Additionally, devices D1 (**1,6-dimtp-SBF**) and D3 (**3,3-dimtp-SBF**) also achieve very high EQE<sub>max</sub> values respectively measured at 29.1% and at 29.3%. This shows the high efficiency of the whole series of materials. In OLED devices, EQE is directly influenced by the η<sub>out</sub>, which depends not only on the thickness and refractive index of the materials but also on the orientation of the transition dipole moment of the emitter in the amorphous host. Specifically, horizontal molecular orientation relative to the substrate enhances both η<sub>out</sub> and EQE. The orientation of MR molecules in the EML can affect efficiency. While altering the structure of MR molecules can adjust the orientation, modifying the host material, which is the main component of the EML, provides a more direct approach to achieving efficient OLEDs.[73] In order to elucidate the very high performance of the PHC series, the horizontal molecular orientation within the EML has been investigated. Thus, co-doped films (film<sub>1</sub>-film<sub>4</sub>) based on the EMLs of devices D1-D4 were prepared. The measurement results are presented in Figure S59.

The results indicate that the horizontal dipole ratio (Θ<sub>h</sub>) of Flpic and DtBuCzB in **1,6-dimtp-SBF**, **1,8-dimtp-SBF**, **3,3-dimtp-SBF** and **4,4-dimtp-SBF** are 82%, 84%, 84% and 89%, respectively. These high Θ<sub>h</sub> are attributed to the rigidity of the host structure and the suppression of reorientation and randomization of the guest and sensitizer molecules during deposition, owing to the high evaporation temperature. These findings further enhance the EL performance of OLED devices based on PHC hosts and explain the very high EQE values reached.

To further validate the present design strategy, we used the phosphorescent material fac-Ir tpz₃ as a sensitizer for DtBuCzB to fabricate devices (D5-D8). The structure of these devices is identical to that of the previously mentioned D1-D4 devices, except that the EML doping was changed from 10 wt% Flpic to 20 wt% fac-Ir tpz₃. The device structures and energy level diagrams are shown in Figure S55 and the EL performance is illustrated in Figure 5 and summarized in Table 2. Similar to the devices using Flpic as a sensitizer, devices D5-D8 exhibited extremely high performance. These devices achieved narrow spectral emission with a maximum at 488 nm and their EQE<sub>max</sub> ranged from 29.9% to 32.9%. Devices D6 and D8 demonstrated outstanding performance, with D6 achieving an EQE<sub>max</sub> of 32.0%, a CE<sub>max</sub> of 55 cd A<sup>-1</sup> and a PE<sub>max</sub> of 51.1 lm W<sup>-1</sup>. Device D8 exhibited an EQE<sub>max</sub> of 32.9%, a CE<sub>max</sub> of 60 cd A<sup>-1</sup> and a PE<sub>max</sub> of 52.2 lm W<sup>-1</sup>. Devices D5 and D7 also exhibited excellent performance, with the corresponding device characteristics summarized in Table 2. Notably, at high luminance levels of 1000 cd m<sup>-2</sup>, D5-D8 exhibited outstanding low efficiency roll-off of 20.6%, 35.6%, 23.3% and 27.0% respectively. This is attributed to the excellent carrier balance capability of the PHC host and the short T<sub>1</sub> lifetime of the phosphorescent sensitizer. fac-Ir tpz₃ could efficiently facilitate the conversion of T<sub>1</sub> into S<sub>1</sub> in the EML, thereby reducing the accumulation of T<sub>1</sub> excitons within the device. This reduction suppresses triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), effectively mitigating efficiency roll-off.[71, 74] Consistent with previous findings, we also prepared co-doped films (film<sub>5</sub>-film<sub>8</sub>) based on the EML of devices D5-D8 to investigate the Θ<sub>h</sub> within the EML (Figure S60). The measured Θ<sub>h</sub> were 81%, 82%, 84% and 88%, respectively. These results show only a little variation from the data presented above, suggesting that the structure and physical properties of the host material (and not the dopant or the sensitizer) largely determine molecular arrangement within the EML. As the molecular orientation in the EML often plays a dominant role in the OLED performance, this clearly emphasizes herein the high efficiency of the overall PHC concept rather than that of individual molecules.
<table><caption>**Table 2.** Summary of OLED device performance.</caption><thead><tr><th>EML</th><th>Host</th><th>V<sup>a</sup><br>(V)</th><th>λ<sub>EL</sub><sup>b</sup><br>(nm)</th><th>FWHM<sup>b</sup><br>(nm)</th><th>CE<sup>c</sup><br>(cd A<sup>-1</sup>)</th><th>EQE<sup>d</sup><br>(%)</th><th>Θ<sub>h</sub><sup>e</sup></th></tr></thead><tbody><tr><td rowspan="4">Host:<br>10 wt% Flpic:<br>2 wt% DtBuCzB</td><td>1,6-dimtp-SBF (D1)</td><td>3.71</td><td>488</td><td>30</td><td>50</td><td>29.1/15.5</td><td>82</td></tr><tr><td>1,8-dimtp-SBF (D2)</td><td>3.72</td><td>488</td><td>30</td><td>60</td><td>33.9/20.0</td><td>84</td></tr><tr><td>3,3-dimtp-SBF (D3)</td><td>3.79</td><td>488</td><td>30</td><td>50</td><td>29.3/16.7</td><td>84</td></tr><tr><td>4,4-dimtp-SBF (D4)</td><td>3.79</td><td>488</td><td>30</td><td>60</td><td>33.9/17.6</td><td>89</td></tr><tr><td rowspan="4">Host:<br>20 wt% fac-Ir(tpz)<sub>3</sub>:<br>2 wt% DtBuCzB</td><td>1,6-dimtp-SBF (D5)</td><td>3.46</td><td>488</td><td>30</td><td>55</td><td>29.9/23.8</td><td>81</td></tr><tr><td>1,8-dimtp-SBF (D6)</td><td>3.29</td><td>488</td><td>29</td><td>55</td><td>32.0/20.6</td><td>82</td></tr><tr><td>3,3-dimtp-SBF (D7)</td><td>3.55</td><td>488</td><td>29</td><td>55</td><td>31.0/23.8</td><td>84</td></tr><tr><td>4,4-dimtp-SBF (D8)</td><td>3.61</td><td>488</td><td>30</td><td>60</td><td>32.9/24.2</td><td>89</td></tr><tr><td rowspan="2">Host:<br>20 wt% CN-Ir:<br>0.5 wt% v-DABNA</td><td>1,8-dimtp-SBF (B1)</td><td>3.61</td><td>468</td><td>21</td><td>30</td><td>30.9/23.6</td><td>85</td></tr><tr><td>4,4-dimtp-SBF (B2)</td><td>3.38</td><td>468</td><td>21</td><td>30</td><td>31.1/22.2</td><td>93</td></tr></tbody></table>

[a] Driving voltages at the current density of 0.01 mA cm⁻². [b] Measured at a driving current density of 40 mA cm⁻². [c] Values of CEmax. [d] EQE at the maximum and 1,000 cd m⁻². [e] The horizontal dipole ratio. Note that the non-sensitized devices are presented in Table S24 and Figure S63.

As blue OLEDs remain one of the most challenging areas in the field, we further explored the potential of PHC hosts with another EML association, namely CN-Ir as the phosphorescent sensitizer and v-DABNA as the blue MR-TADF emitter. CN-Ir exhibits an E<sub>T</sub> (2.70 eV) comparable to the singlet energy of v-DABNA (2.65 eV) and possesses a relatively short phosphorescence lifetime (1.8 µs).[38] Furthermore, there is a significant overlap between the emission of CN-Ir and the absorption of v-DABNA, which renders it an ideal sensitizer (Figure S57). Owing to its high E<sub>T</sub> and wide E<sub>g</sub>, 1,8-dimtp-SBF exhibits superior excitonic properties, while 4,4-dimtp-SBF ensures optimal charge balance and horizontal orientation. Consequently, these materials outperform the other two hosts in device performance, making them the primary choices for blue MR-TADF OLEDs in this study. The optimized device structures (B1 and B2) are as follows: ITO/HAT (10 nm)/TAPC (40 nm)/TCTA (10 nm)/mCP (10 nm)/0.5 wt% v-DABNA: 20 wt% CN-Ir: Host (20 nm)/TmPyPb (40 nm)/Liq (2 nm)/Al (80 nm). The EL spectra and device performance are shown in Figure 5, with the data summarized in Table 2. Incorporating CN-Ir into the device reduces the energy gaps between the host and emitter, as its HOMO and LUMO levels are positioned between them, facilitating carrier injection and transport within the EML (Table S22). As a result, both B1 (with 1,8-dimtp-SBF as the host) and B2 (with 4,4-dimtp-SBF as the host) devices exhibited low driving voltages. Additionally, these devices showed narrow blue EL spectra (FWHMs of only 21 nm), centered at 468 nm. The additional energy transfer between CN-Ir and v-DABNA can reduce exciton loss and achieve a very high EQE<sub>max</sub> of approximately 31%. Devices B1 and B2 also demonstrated high current CE<sub>max</sub> and PE<sub>max</sub>. Furthermore, similar to the devices above, devices B1 and B2 also exhibit excellent efficiency roll-off values of 21.8% and 26.7%, respectively. Co-doped films (film<sub>g</sub> and film<sub>10</sub>) were prepared to measure Θ<sub>h</sub>, which were found to be as high as 85% and 93% (Figure S61). This orientation is one important reason involved in the excellent efficiency and low roll-off observed in these devices.[75] At high current densities, enhanced electric fields usually cause polarization effects, altering molecular orientation within the EML and leading to efficiency degradation.[76] However, using PHC

materials as hosts can stabilize molecular orientation, mitigating polarization effects and maintaining high EQE levels even at high luminance.

To further elucidate the high performance of this series of devices, transient PL measurements were conducted to examine the photophysical properties of the EML (Figure S62). Films (film<sub>11</sub> and film<sub>12</sub>) were prepared by co-doping CN-Ir and v-DABNA with 1,8-dimtp-SBF and 4,4-dimtp-SBF as host materials, respectively. The PL quantum yields (PLQY) of film<sub>11</sub> and film<sub>12</sub> were measured at 95% and 96%, respectively. Importantly, the PLQY of the co-doped films remained high despite energy transfer processes, which is critical for achieving high device efficiency. Using the PLQY values and decay times, the reverse intersystem crossing (k<sub>RISC</sub>) rates were calculated to be 2.71 × 10<sup>6</sup> s<sup>-1</sup> and 3.67 × 10<sup>6</sup> s<sup>-1</sup> (film parameters summarized in Table S23). This evidence reinforces the soundness of our molecular design and the viability of PHC materials as host materials for phosphorescent-sensitized devices.

## Conclusion

In the present work, we report a series of four PHC host materials constructed on the original and uncommon di-substitution of the SBF backbone. By strategically disrupting the π-conjugation of the SBF backbone, high E<sub>T</sub> and wide E<sub>g</sub> were achieved, which are essential characteristics for efficient energy transfers in the EML. The introduction of mtp substituents was particularly efficient in fine-tuning the electronic and optical properties of these regioisomers. The whole series of devices displays very high performances with different phosphorescent sensitizers (Flpic, fac-Ir(tpz)<sub>3</sub>, CN-Ir) and emitters (DtBuCzB and v-DABNA). Particularly, v-DABNA-based blue OLEDs (468 nm) have reached impressive performance with EQE<sub>max</sub> above 30% and narrow FWHMs of 21 nm. We have demonstrated that PHC hosts stabilize molecular orientation and significantly reduce efficiency roll-off, which are two key features in OLEDs. This study marks the first application of PHC materials as hosts in phosphorescence-sensitized MR-TADF OLEDs, highlighting their
great potential as promising candidates for next-generation blue
OLEDs and offering a viable pathway to achieve high-
performance devices.

Acknowledgements

The authors acknowledge financial findings from the National Natural Science Foundation of China (Nos. 22175124, 62175171 and 61961160731), the Natural Science Foundation of Jiangsu Province of China (BK20220057). This work is also supported by Suzhou Key Laboratory of Functional Nano & Soft Materials, Collaborative Innovation Center of Suzhou Nano Science & Technology, the 111 Project. This work has also been financially supported by the ANR (N22-CE07-0024-Evolution Project). DA thanks the EUR LUMOMAT project and the Investments for the Future program ANR-18-EURE-0012 (PIPPIN) for a Ph.D. grant. KL thanks the Region Bretagne for a PhD Grant (Project SIMPLIFY). We also thank the CRMPO (Rennes) for mass analyses and GENCI (Project N° AD010814136) for computing time.

**Keywords:** pure hydrocarbon host, organic light-emitting diode, phosphorescence-sensitized, multi-resonance thermally activated delayed fluorescen

Reference

[1] G. Hong, X. Gan, C. Leonhardt, Z. Zhang, J. Seibert, J. M. Busch, S. Bräse, "A Brief History of OLEDs—Emitter Development and Industry Milestones" *Adv. Mater.* **2021**, *33*, 2005630.

[2] C. Poriel, J. Rault-Berthelot, "Blue Single-Layer Organic Light-Emitting Diodes Using Fluorescent Materials: A Molecular Design View Point" *Adv. Funct. Mater.* **2020**, *30*, 1910040.

[3] X. Wu, D.-G. Chen, D. Liu, S.-H. Liu, S.-W. Shen, C.-I. Wu, G. Xie, J. Zhou, Z.-X. Huang, C.-Y. Huang, S.-J. Su, W. Zhu, P.-T. Chou, "Highly Emissive Dinuclear Platinum(III) Complexes" *J. Am. Chem. Soc.* **2020**, *142*, 7469-7479.

[4] C. Poriel, J. Rault-Berthelot, "Structure–property relationship of 4-substituted-spirobifluorenes as hosts for phosphorescent organic light emitting diodes: an overview" *J. Mater. Chem. C* **2017**, *5*, 3869-3897.

[5] Y.-L. Zhang, S.-Y. Yang, Z.-Q. Feng, Y.-K. Qu, D.-Y. Zhou, C. Zhong, L.-S. Liao, Z.-Q. Jiang, "r-Stacked host materials based on spirofluorene scaffolds for warm white OLEDs achieving 94.7 lm W⁻¹ at 1,000 cd m⁻²" *Sci. Chi. Chem.* **2022**, *65*, 2219-2230.

[6] M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, "Highly efficient phosphorescent emission from organic electroluminescent devices" *Nature* **1998**, *395*, 151-154.

[7] X. Wu, C.-Y. Huang, D.-G. Chen, D. Liu, C. Wu, K.-J. Chou, B. Zhang, Y. Wang, Y. Liu, E. Y. Li, W. Zhu, P.-T. Chou, "Exploiting racismism enhanced organic room-temperature phosphorescence to demonstrate Wallach's rule in the lighting chiral chromophores" *Nat. Commun.* **2020**, *11*, 2145.

[8] X. Tang, L.-S. Cui, H.-C. Li, A. J. Gillett, F. Auras, Y.-K. Qu, C. Zhong, S. T. E. Jones, Z.-Q. Jiang, R. H. Friend, L.-S. Liao, "Highly efficient luminescence from space-confined charge-transfer emitters" *Nat. Mater.* **2020**, *19*, 1332-1338.

[9] S.-Y. Yang, Y.-K. Qu, L.-S. Liao, Z.-Q. Jiang, S.-T. Lee, "Research Progress of Intramolecular π-Stacked Small Molecules for Device Applications" *Adv. Mater.* **2022**, *34*, 2104125.

[10] Y.-K. Qu, Q. Zheng, J. Fan, L.-S. Liao, Z.-Q. Jiang, "Spiro Compounds for Organic Light-Emitting Diodes" *Acc. Mater. Res.* **2021**, *2*, 1261-1271.

[11] X. Wu, C.-H. Wang, S. Ni, C.-C. Wu, Y.-D. Lin, H.-T. Qu, Z.-H. Wu, D. Liu, M.-Z. Yang, S.-J. Su, W. Zhu, K. Chen, Z.-C. Jiang, S.-D. Yang, W.-Y. Hung, P.-T. Chou, "Multiple Enol–Keto Isomerization and Excited-State Unidirectional Intramolecular Proton Transfer Generate Intense, Narrowband Red OLEDs" *J. Am. Chem. Soc.* **2024**, *146*, 24526-24536.

[12] J.-H. Lee, C.-H. Chen, P.-H. Lee, H.-Y. Lin, M.-k. Leung, T.-L. Chiu, C.-F. Lin, "Blue organic light-emitting diodes: current status, challenges and future outlook" *J. Mater. Chem. C* **2019**, *7*, 5874-5888.

[13] Y. Wang, J. H. Yun, L. Wang, J. Y. Lee, "High Triplet Energy Hosts for Blue Organic Light-Emitting Diodes" *Adv. Funct. Mater.* **2021**, *31*, 2008332.

[14] S. Schmidbauer, A. Hohenleutner, B. König, "Chemical Degradation in Organic Light-Emitting Devices: Mechanisms and Implications for the Design of New Materials" *Adv. Mater.* **2013**, *25*, 2114-2129.

[15] H. Li, M. Hong, A. Scarpaci, X. He, C. Risko, J. S. Sears, S. Barlow, P. Winget, S. R. Marder, D. Kim, J.-L. Brédas, "Chemical Stabilities of the Lowest Triplet State in Aryl Sulfones and Aryl Phosphine Oxides Relevant to OLED Applications" *Chem. Mater.* **2019**, *31*, 1507-1519.

[16] N. Lin, J. Qiao, L. Duan, L. Wang, Y. Qiu, "Molecular Understanding of the Chemical Stability of Organic Materials for OLEDs: A Comparative Study on Sulfonyl, Phosphine-Oxide and Carbonyl-Containing Host Materials" *J. Phys. Chem. C* **2014**, *118*, 7569-7578.

[17] D. Y. Kondakov, W. C. Lenhart, W. F. Nichols, "Operational degradation of organic light-emitting diodes: Mechanism and identification of chemical products" *J. Appl. Phys.* **2007**, *101*, 024512.

[18] N. Lin, J. Qiao, L. Duan, H. Li, L. Wang, Y. Qiu, "Achilles Heels of Phosphine Oxide Materials for OLEDs: Chemical Stability and Degradation Mechanism of a Bipolar Phosphine Oxide/Carbazole Hybrid Host Material" *J. Phys. Chem. C* **2012**, *116*, 19451-19457.

[19] C. Poriel, J. Rault-Berthelot, "Pure Hydrocarbons: An Efficient Molecular Design Strategy for the Next Generation of Host Materials for Phosphorescent Organic Light-Emitting Diodes" *Acc. Mater. Res.* **2022**, *3*, 379-390.

[20] Y. Morinaka, H. Ito, K. J. Fujimoto, T. Yanai, Y. Ono, T. Tanaka, K. Itami, "Nonplanar Nanographene: A Hydrocarbon Hole-Transporting Material That Competes with Triarylamines" *Angew. Chem. Int. Ed.* **2024**, *63*, e202409619.

[21] C. Poriel, J. Rault-Berthelot, "Designing Host Materials for the Emissive Layer of Single-Layer Phosphorescent Organic Light-Emitting Diodes: Toward Simplified Organic Devices" *Adv. Funct. Mater.* **2021**, *31*, 2010547.

[22] K.-T. Wong, Y.-L. Liao, Y.-T. Lin, H.-C. Su, C.-c. Wu, "Spiro-Configured Bifluorenes: Highly Efficient Emitter for UV Organic Light-Emitting Device and Host Material for Red Electrophosphorescence" *Org. Lett.* **2005**, *7*, 5131-5134.

[23] L.-C. Chi, W.-Y. Hung, H.-C. Chiu, K.-T. Wong, "A high-efficiency and low-operating-voltage green electrophosphorescent device employing a pure-hydrocarbon host material" *Chem. Commun.* **2009**, 3892-3894.

[24] M. Zhuo, W. Sun, G. Liu, J. Wang, L. Guo, C. Liu, B. Mi, J. Song, Z. Gao, "Pure aromatic hydrocarbons with rigid and bulky substituents as bipolar hosts for blue phosphorescent OLEDs" *J. Mater. Chem. C* **2015**, *3*, 9137-9144.
[25] S. Ye, Y. Liu, C.-a. Di, H. Xi, W. Wu, Y. Wen, K. Lu, C. Du, Y. Liu, G. Yu, "Wide-Energy-Gap Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes" *Chem. Mater.* **2009**, *21*, 1333-1342.

[26] C. Poriel, R. Métivier, J. Rault-Berthelot, D. Thirion, F. Barrière, O. Jeannin, "A robust pure hydrocarbon derivative based on the (2,1-b)-indenofluorenyl core with high triplet energy level" *Chem. Commun.* **2011**, *47*, 11703-11705.

[27] L. J. Sicard, H.-C. Li, Q. Wang, X.-Y. Liu, O. Jeannin, J. Rault-Berthelot, L.-S. Liao, Z.-Q. Jiang, C. Poriel, "C1-Linked Spirobifluorene Dimers: Pure Hydrocarbon Hosts for High-Performance Blue Phosphorescent OLEDs" *Angew. Chem. Int. Ed.* **2019**, *58*, 3848-3853.

[28] C. Poriel, J. Rault-Berthelot, Z.-Q. Jiang, "Are pure hydrocarbons the future of host materials for blue phosphorescent organic light-emitting diodes?" *Mater. Chem. Front.* **2022**, *6*, 1246-1252.

[29] Y. Luo, Z. Liu, G. Yang, T. Wang, Z. Bin, J. Lan, D. Wu, J. You, "Iridium(III)-Catalyzed Diarylation/Annulation of Benzoic Acids: Facile Access to Multi-Aryl Spirobifluorenes as Pure Hydrocarbon Hosts for High-Performance OLEDs" *Angew. Chem. Int. Ed.* **2021**, *60*, 18852-18859.

[30] Q. Li, Z. Yu, Q. Liu, Y. Guo, Z. Fu, Y. Yang, Z. Bin, D. Wu, J. Lan, "Crafting 1,4-diaryl spirobifluorene hosts in OLEDs via interannular C–H arylation: synergistic effects of molecular linearity and orthogonality" *Chem. Sci.* **2024**, *15*, 10547-10555.

[31] D. Ari, Y.-J. Yang, C. Quinton, Z.-Q. Jiang, D.-Y. Zhou, C. Poriel, "Spirobifluorene Trimers: High Triplet Pure Hydrocarbon Hosts for Highly Efficient Blue Phosphorescent Organic Light-Emitting Diodes" *Angew. Chem. Int. Ed.* **2024**, *63*, e202403066.

[32] F.-C. Kong, Y.-L. Zhang, C. Quinton, N. McIntosh, S.-Y. Yang, J. Rault-Berthelot, F. Lucas, C. Brouillac, O. Jeannin, J. Cornil, Z.-Q. Jiang, L.-S. Liao, C. Poriel, "Pure Hydrocarbon Materials as Highly Efficient Host for White Phosphorescent Organic Light-Emitting Diodes: A New Molecular Design Approach" *Angew. Chem. Int. Ed.* **2022**, *61*, e202207204.

[33] Q. Wang, F. Lucas, C. Quinton, Y.-K. Qu, J. Rault-Berthelot, O. Jeannin, S.-Y. Yang, F.-C. Kong, S. Kumar, L.-S. Liao, C. Poriel, Z.-Q. Jiang, "Evolution of pure hydrocarbon hosts: simpler structure, higher performance and universal application in RGB phosphorescent organic light-emitting diodes" *Chem. Sci.* **2020**, *11*, 4887-4894.

[34] X. Wu, C.-H. Wang, S. Ni, C.-C. Wu, Y.-D. Lin, H.-T. Qu, Z.-H. Wu, D. Liu, M.-Z. Yang, S.-J. Su, W. Zhu, K. Chen, Z.-C. Jiang, S.-D. Yang, W.-Y. Hung, P.-T. Chou, "Multiple Enol–Keto Isomerization and Excited-State Unidirectional Intramolecular Proton Transfer Generate Intense, Narrowband Red OLEDs" *J. Am. Chem. Soc.* **2024**, *146*, 24526-24536.

[35] Y. Yuan, X. Tang, X.-Y. Du, Y. Hu, Y.-J. Yu, Z.-Q. Jiang, L.-S. Liao, S.-T. Lee, "The Design of Fused Amine/Carbonyl System for Efficient Thermally Activated Delayed Fluorescence: Novel Multiple Resonance Core and Electron Acceptor" *Adv. Opt. Mater.* **2019**, *7*, 1801536.

[36] X. Wu, B.-K. Su, D.-G. Chen, D. Liu, C.-C. Wu, Z.-X. Huang, T.-C. Lin, C.-H. Wu, M. Zhu, E. Y. Li, W.-Y. Hung, W. Zhu, P.-T. Chou, "The role of host–guest interactions in organic emitters employing MR-TADF" *Nat. Photonics* **2021**, *15*, 780-786.

[37] S. Nam, J. W. Kim, H. J. Bae, Y. M. Maruyama, D. Jeong, J. Kim, J. S. Kim, W.-J. Son, H. Jeong, J. Lee, S.-G. Ihn, H. Choi, "Improved Efficiency and Lifetime of Deep-Blue Hyperfluorescent Organic Light-Emitting Diode using Pt(II) Complex as Phosphorescent Sensitizer" *Adv. Sci.* **2021**, *8*, 2100586.

[38] W. J. Chung, K. H. Lee, M. Jung, K. M. Lee, H. C. Park, M.-S. Eum, J. Y. Lee, "Over 30 000 h Device Lifetime in Deep Blue Organic Light-Emitting

Diodes with y Color Coordinate of 0.086 and Current Efficiency of 37.0 cd A⁻¹" *Adv. Opt. Mater.* **2021**, *9*, 2100203.

[39] X. Yang, X. Zhou, Y.-X. Zhang, D. Li, C. Li, C. You, T.-C. Chou, S.-J. Su, P.-T. Chou, Y. Chi, "Blue Phosphorescence and Hyperluminescence Generated from Imidazo[4,5-b]pyridin-2-ylidene-Based Iridium(III) Phosphors" *Adv. Sci.* **2022**, *9*, 2201150.

[40] C. You, X.-Q. Wang, X. Zhou, Y. Yuan, L.-S. Liao, Y.-C. Liao, P.-T. Chou, Y. Chi, "Homoleptic Ir(III) Phosphors with 2-Phenyl-1,2,4-triazol-3-ylidene Chelates for Efficient Blue Organic Light-Emitting Diodes" *ACS Appl. Mater. Interfaces* **2021**, *13*, 59023-59034.

[41] K. H. Lee, J. Y. Lee, "Phosphor sensitized thermally activated delayed fluorescence organic light-emitting diodes with ideal deep blue device performances" *J. Mater. Chem. C* **2019**, *7*, 8562-8568.

[42] H. Nakanotani, T. Higuchi, T. Furukawa, K. Masui, K. Morimoto, M. Numata, H. Tanaka, Y. Sagara, T. Yasuda, C. Adachi, "High-efficiency organic light-emitting diodes with fluorescent emitters" *Nat. Commun.* **2014**, *5*, 4016.

[43] M. A. Baldo, M. E. Thompson, S. R. Forrest, "High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer" *Nature* **2000**, *403*, 750-753.

[44] E. Kim, J. Park, M. Jun, H. Shin, J. Baek, T. Kim, S. Kim, J. Lee, H. Ahn, J. Sun, S.-B. Ko, S.-H. Hwang, J. Y. Lee, C. Chu, S. Kim, "Highly efficient and stable deep-blue organic light-emitting diode using phosphor-sensitized thermally activated delayed fluorescence" *Sci. Adv.*, *8*, eabq1641.

[45] J. Lee, C. Jeong, T. Batagoda, C. Coburn, M. E. Thompson, S. R. Forrest, "Hot excited state management for long-lived blue phosphorescent organic light-emitting diodes" *Nat. Commun.* **2017**, *8*, 15566.

[46] K. Klimes, Z.-Q. Zhu, J. Li, "Efficient Blue Phosphorescent OLEDs with Improved Stability and Color Purity through Judicious Triplet Exciton Management" *Adv. Funct. Mater.* **2019**, *29*, 1903068.

[47] B. Sim, J. S. Kim, H. Bae, S. Nam, E. Kwon, J. W. Kim, H.-Y. Cho, S. Kim, J.-J. Kim, "Comprehensive Model of the Degradation of Organic Light-Emitting Diodes and Application for Efficient, Stable Blue Phosphorescent Devices with Reduced Influence of Polarons" *Phys. Rev. Appl.* **2020**, *14*, 024002.

[48] S. K. Jeon, J. Y. Lee, "Four times lifetime improvement of blue phosphorescent organic light-emitting diodes by managing recombination zone" *Org. Electron.* **2015**, *27*, 202-206.

[49] X.-Y. Chen, S. Ozturk, E. J. Sorensen, "Synthesis of Fluorenones from Benzaldehydes and Aryl Iodides: Dual C–H Functionalizations Using a Transient Directing Group" *Org. Lett.* **2017**, *19*, 1140-1143.

[50] C. Poriel, L. Sicard, J. Rault-Berthelot, "New generations of spirobifluorene regioisomers for organic electronics: tuning electronic properties with the substitution pattern" *Chem. Commun.* **2019**, *55*, 14238-14254.

[51] Q. Li, Z. Li, "Molecular Packing: Another Key Point for the Performance of Organic and Polymeric Optoelectronic Materials" *Acc. Chem. Res.* **2020**, *53*, 962-973.

[52] L. Sicard, C. Quinton, J.-D. Peltier, D. Tondelier, B. Geffroy, U. Biapo, R. Métivier, O. Jeannin, J. Rault-Berthelot, C. Poriel, "Spirobifluorene Regioisomerism: A Structure–Property Relationship Study" *Chem. Eur. J.* **2017**, *23*, 7719-7727.

[53] C. Poriel, C. Quinton, F. Lucas, J. Rault-Berthelot, Z.-Q. Jiang, O. Jeannin, "Spirobifluorene Dimers: Understanding How The Molecular Assemblies Drive The Electronic Properties" *Adv. Funct. Mater.* **2021**, *31*, 2104980.
[54] C. Quinton, S. Thiery, O. Jeannin, D. Tondelier, B. Geffroy, E. Jacques, J. Rault-Berthelot, C. Poriel, "Electron-Rich 4-Substituted Spirobifluorenes: Toward a New Family of High Triplet Energy Host Materials for High-Efficiency Green and Sky Blue Phosphorescent OLEDs" ACS Appl. Mater. Interfaces **2017**, 9, 6194-6206.

[55] S. Thiery, D. Tondelier, C. Declairieux, G. Seo, B. Geffroy, O. Jeannin, J. Rault-Berthelot, R. Métivier, C. Poriel, "9,9' -Spirobifluorene and 4-phenyl-9,9' - Spirobifluorene: pure hydrocarbon small molecules as hosts for efficient green and blue PhOLEDs" J. Mater. Chem. C **2014**, 2, 4156-4166.

[56] L. Sicard, C. Brouillac, N. Leclerc, S. Fall, N. Zimmerman, O. Jeannin, J. Rault-Berthelot, C. Quinton, C. Poriel, "Influence of the pendant substituent at the C1 position of a spirobifluorene scaffold on the electronic properties" Mater. Chem. Front. **2024**, 8, 1349-1361.

[57] L. Sicard, C. Quinton, F. Lucas, O. Jeannin, J. Rault-Berthelot, C. Poriel, "1-Carbazolyl Spirobifluorene: Synthesis, Structural, Electrochemical and Photophysical Properties" J. Phys. Chem. C **2019**, 123, 19094-19104.

[58] C. Brouillac, F. Lucas, D. Ari, D. Tondelier, J. Meot, M. Malvaux, C. Jadaud, C. Lebreton, J. Rault-Berthelot, C. Quinton, E. Jacques, C. Poriel, "Quinolinoacridine as High Efficiency Building Unit in Single-Layer Phosphorescent Organic Light-Emitting Diodes" Adv. Electron. Mater. **2024**, 10, 2300582.

[59] V. J. Chebny, R. Shukla, S. V. Lindeman, R. Rathore, "Molecular Actuator: Redox-Controlled Clam-Like Motion in a Bichromophoric Electron Donor" Org. Lett. **2009**, 11, 1939-1942.

[60] R. Rathore, S. H. Abdelwahed, I. A. Guzei, "Synthesis, Structure and Evaluation of the Effect of Multiple Stacking on the Electron-Donor Properties of π-Stacked Polyfluorenes" J. Am. Chem. Soc. **2003**, 125, 8712-8713.

[61] D. Thirion, C. Poriel, F. Barrière, R. Métivier, O. Jeannin, J. Rault-Berthelot, "Tuning the Optical Properties of Aryl-Substituted Dispirofluorene-Indenofluorene Isomers through Intramolecular Excimer Formation" Org. Lett. **2009**, 11, 4794-4797.

[62] D. Thirion, C. Poriel, R. Métivier, J. Rault-Berthelot, F. Barrière, O. Jeannin, "Violet-to-Blue Tunable Emission of Aryl-Substituted Dispirofluorene-Indenofluorene Isomers by Conformationally-Controllable Intramolecular Excimer Formation" Chem. Eur. J. **2011**, 17, 10272-10287.

[63] F. Lucas, D. Tondelier, B. Geffroy, T. Heiser, O. A. Ibraikulov, C. Quinton, C. Brouillac, N. Leclerc, J. Rault-Berthelot, C. Poriel, "Quinolinophenothiazine as an electron rich fragment for high efficiency RGB single-layer phosphorescent organic light-emitting diodes" Mater. Chem. Front. **2021**, 5, 8066-8077.

[64] F. Lucas, C. Brouillac, S. Fall, N. Zimmerman, D. Tondelier, B. Geffroy, N. Leclerc, T. Heiser, C. Lebreton, E. Jacques, C. Quinton, J. Rault-Berthelot, C. Poriel, "Simplified Green-Emitting Single-Layer Phosphorescent Organic Light-Emitting Diodes with an External Quantum Efficiency > 22%" Chem. Mater. **2022**, 34, 8345-8355.

[65] M. Idris, S. C. Kapper, A. C. Tadle, T. Batagoda, D. S. Muthiah Ravinson, O. Abimbola, P. I. Djurovich, J. Kim, C. Coburn, S. R. Forrest, M. E. Thompson, "Blue Emissive fac/mer-Iridium (III) NHC Carbene Complexes and their Application in OLEDs" Adv. Opt. Mater. **2021**, 9, 2001994.

[66] E. Baranoff, B. F. E. Curchod, "Flrpic: archetypal blue phosphorescent emitter for electroluminescence" Dalton Trans. **2015**, 44, 8318-8329.

[67] Y. Xu, Z. Cheng, Z. Li, B. Liang, J. Wang, J. Wei, Z. Zhang, Y. Wang, "Molecular-Structure and Device-Configuration Optimizations toward Highly Efficient Green Electroluminescence with Narrowband Emission and High Color Purity" Adv. Opt. Mater. **2020**, 8, 1902142.

[68] Y. Kondo, K. Yoshiura, S. Kitera, H. Nishi, S. Oda, H. Gotoh, Y. Sasada, M. Yanai, T. Hatakeyama, "Narrowband deep-blue organic light-emitting diode featuring an organoboron-based emitter" Nat. Photonics **2019**, 13, 678-682.

[69] S. H. Han, J. H. Jeong, J. W. Yoo, J. Y. Lee, "Ideal blue thermally activated delayed fluorescence emission assisted by a thermally activated delayed fluorescence assistant dopant through a fast reverse intersystem crossing mediated cascade energy transfer process" J. Mater. Chem. C **2019**, 7, 3082-3089.

[70] H.-G. Kim, H. Shin, Y. H. Ha, R. Kim, S.-K. Kwon, Y.-H. Kim, J.-J. Kim, "Triplet Harvesting by a Fluorescent Emitter Using a Phosphorescent Sensitizer for Blue Organic-Light-Emitting Diodes" ACS Appl. Mater. Interfaces **2019**, 11, 26-30.

[71] X. Song, D. Zhang, Y. Lu, C. Yin, L. Duan, "Understanding and Manipulating the Interplay of Wide-Energy-Gap Host and TADF Sensitizer in High-Performance Fluorescence OLEDs" Adv. Mater. **2019**, 31, 1901923.

[72] D. Zhang, X. Song, M. Cai, L. Duan, "Blocking Energy-Loss Pathways for Ideal Fluorescent Organic Light-Emitting Diodes with Thermally Activated Delayed Fluorescent Sensitizers" Adv. Mater. **2018**, 30, 1705250.

[73] B. Madushani, M. Mamada, K. Goushi, H. Katagiri, H. Nakanotani, T. Hatakeyama, C. Adachi, "Hexacarbazolylbenzene: An Excellent Host Molecule Causing Strong Guest Molecular Orientation and the High-Performance OLEDs" Adv. Mater. **2024**, 36, 2402275.

[74] Y.-T. Lee, C.-Y. Chan, M. Tanaka, M. Mamada, U. Balijapalli, Y. Tsuchiya, H. Nakanotani, T. Hatakeyama, C. Adachi, "Investigating HOMO Energy Levels of Terminal Emitters for Realizing High-Brightness and Stable TADF-Assisted Fluorescence Organic Light-Emitting Diodes" Adv. Electron. Mater. **2021**, 7, 2001090.

[75] U. Balijapalli, Y.-T. Lee, B. S. B. Karunathilaka, G. Tumen-Ulzii, M. Auffray, Y. Tsuchiya, H. Nakanotani, C. Adachi, "Tetrabenzo[a,c]phenazine Backbone for Highly Efficient Orange-Red Thermally Activated Delayed Fluorescence with Completely Horizontal Molecular Orientation" Angew. Chem. Int. Ed. **2021**, 60, 19364-19373.

[76] E. Pakhomenko, S. He, R. J. Holmes, "Polarization-Induced Exciton-Polaron Quenching in Organic Light-Emitting Devices and Its Control by Dipolar Doping" Adv. Opt. Mater. **2022**, 10, 2201348.
**Table of Contents**

<figure><img src="image_8.png" /></figure>

The first application of pure hydrocarbon materials as hosts in phosphorescence-sensitized multi-resonance thermally activated delayed fluorescence (MR-TADF) OLEDs is reported. Blue MR-TADF OLED using either DtBuCzB or v-DABNA as emitter and either Flpic, fac-Ir tpz or CN-Ir as sensitizer, achieved very high maximum external quantum efficiencies ranging from 29.1% to 33.9%, due to an excellent molecular orientation induced by the PHC hosts.

X account: @CyrilPoriel