# Efficient UV Organic Solid-State Lasers with Ultra-Short Wavelengths Based on Dispirofluorene-Indenofluorene Isomers

Yusei Kaya, Hyung-Suk Kim, Denis Ari, Kevin Mall-Haidaraly, Isabelle

Hantonne, Buddhika S. B. Karunathilaka, Ayano Abe, Youhei Chitose, Sébastien Forget, Sebastien Chenais, et al.

<figure><img src="image_1.png" /></figure>

To cite this version:

Yusei Kaya, Hyung-Suk Kim, Denis Ari, Kevin Mall-Haidaraly, Isabelle Hantonne, et al. Efficient UV Organic Solid-State Lasers with Ultra-Short Wavelengths Based on Dispirofluorene-Indenofluorene Isomers. Advanced Functional Materials, 2025, pp.e14060. 10.1002/adfm.202514060. hal-05242014

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https://hal.science/hal-05242014v1

Submitted on 26 Sep 2025

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Distributed under a Creative Commons Attribution 4.0 International License
High Efficiency Blue-Emitting Multi-Resonance TADF Organic Light-Emitting Diodes constructed on Spiro Quinolinoacridine/fluorene-diphenylphosphine oxide positional isomers.

Kelvine Letellier,⁺ Yue-Jian Yang,⁺ Wei Gao, Dong-Ying Zhou, Denis Ari, Cassandre Quinton, Zuo-Quan Jiang, and Cyril Poriel*

K. Letellier,⁺ D. Ari, C. Quinton, C. Poriel
Univ Rennes, CNRS, ISCR-UMR CNRS 6226
F-35000 Rennes, France

E-mail: cyril.poriel@univ-rennes.fr

Y.-J. Yang,⁺ W. Gao, 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

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, 215123, Jiangsu, PR China

[+] Equal contribution.

Keywords: organic light-emitting diode, quinolinoacridine, phosphorescence-sensitized OLED, multi-resonance thermally activated delayed fluorescence, host material

ABSTRACT: Improving the performance of blue-emitting devices has been one of the most important challenges in the field of Organic Light-emitting Diodes (OLEDs). In recent years, the Multi-Resonance Thermally Activated Delayed Fluorescence (MR-TADF) with a phosphorescent sensitization technique has shown its potential to enhance efficiency and mitigate efficiency roll-off. We report herein our investigations on the design, the synthesis and the study of quinolinoacridine-based positional isomers (SQA-1-FPOPh₂, SQA-3-FPOPh₂ and SQA-4-FPOPh₂) as host for blue phosphorescence-sensitized MR-TADF OLEDs. The molecular design strategy involves the spatial separation of the electron rich (quinolinoacridine) and electron-poor (fluorene/diphenylphosphine oxide) fragments. The impact of the π-conjugation disruption between the fluorene and the diphenylphosphine oxide on the electronic properties of the materials is discussed. These bipolar materials were successfully incorporated as host in blue MR-TADF OLEDs employing different emitters BNCz or v-DABNA and different phosphorescent sensitizer, FIrpic, CN-Ir or fac-Ir(tpz)₃. Remarkably, when used with the v-DABNA: CN-Ir combination (0.5:20 wt% respectively), SQA-4-FPOPh₂ presents excellent electroluminescent performances. These devices possess a narrowband emission peak at 472 nm, a full-width at half-maximum of 21 nm, and a high maximum external quantum efficiency of 22.9%. The excellent molecular orientation, which maximizes the light outcoupling efficiency (η out), is proposed to be at the origin of this high performance.

Introduction

In the Organic Light-Emitting Diode (OLED) technology, the development of high triplet energy (E$_T$) materials to host blue emitting phosphorescent or thermally activated delayed fluorescent (TADF) emitter has been undoubtedly one of the driving force of this research field. Indeed, blue emission has always been a central concern in OLED technology both in term of efficiency and stability[1-6] and the three generations of OLEDs, namely fluorescent, phosphorescent and TADF OLEDs suffered from similar efficiency and stability issues.[2, 5, 7-10] The development of more and more

efficient host materials for blue emision, gathering all the necessary properties, has not only allowed to significantly increase the performance of blue OLEDs but has also contributed to new insights in the field. The high performances recently reported for the new generation of simplified Phosphorescent OLEDs (PhOLED) called single-layer can be here cited as a representative example.[11-16] In order to reach high efficiency blue OLEDs with a narrow electroluminescence spectrum, the Multi-Resonance Thermally Activated Delayed Fluorescence (MR-TADF) with a phosphorescent sensitization technique has recently garnered significant attention. This technique has shown its ability to enhance efficiency and mitigate
efficiency roll-off, tow key features in this field.[17-22] The mechanism of the host-based phosphorescence-sensitized MR-TADF OLEDs is presented elsewhere.[23, 24] To maximize the OLED performance, host materials with specific properties should be designed.

<figure><img src="image_3.png" /></figure>

Donor-Spiro-Acceptor
design

<figure><img src="image_4.png" /><figcaption>SQA-1-FPOPh₂</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>SQA-3-FPOPh₂</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>SQA-4-FPOPh₂</figcaption></figure>

Chart 1. Top. General *Donor-Spiro-Acceptor* (*D-Spiro-A*) molecular design (from reference[15]). Bottom. Molecular structures of **SQA-1-FPOPh₂**, **SQA-3-FPOPh₂** and **SQA-4-FPOPh₂**.

In the present work, we aim at reporting our investigations on the design, the synthesis and the study of bipolar quinolinoacridine-based positional isomers as host for blue emitting phosphorescence-sensitized MR-TADF OLEDs. To achieve high E<sub>T</sub> and a short HOMO/LUMO difference, the design strategy involves the spatial separation of the electron rich (quinolinoacridine) and electron-poor (fluorene/diphenylphosphine oxide) fragments. The disruption between the fluorene and the quinolinoacridine is performed thanks to the *spiro* bridge and the separation between the diphenylphosphine oxide and the fluorene by the grafting position either at C1, C3 or C4. Such a type of design, known as *Donor-Spiro-Acceptor (D-Spiro-A)*[15] allows an excellent control of the HOMO/LUMO energy levels and keeps in the meantime a high E<sub>T</sub> value.[15, 16, 25-28] The C1, C3 and C4 positions of the fluorene, oppositely to position C2, are known to efficiently disrupt the π-conjugation by either electronic feature for C3 position (*meta* linkage), steric feature for C4 position (*ortho* linkage) or both for C1 position (*meta* linkage).[29-31] The positional isomerism of fluorene provides a high efficiency molecular tool to tune the properties of the resulting materials.[32] Thus, three positional isomers were investigated: **SQA-1-FPOPh₂**,

**SQA-3-FPOPh₂** and **SQA-4-FPOPh₂** (Chart 1). These substitution patterns appear herein interesting to reach the necessary electronic and physical properties for the targeted applications. Except **SQA-1-FPOPh₂**, which displays a particular steric feature, the two other materials, **SQA-3-FPOPh₂** and **SQA-4-FPOPh₂** exhibited high E<sub>T</sub> values, short HOMO/LUMO difference and excellent thermal stability, with high decomposition (T<sub>d</sub>) and glass transition (T<sub>g</sub>) temperatures. These bipolar materials were successfully incorporated as host in blue MR-TADF OLEDs employing different emitters BNCz or v-DABNA and different phosphorescence sensitizer, Flpic, CN-Ir or *fac*-Ir(tpz)₃. Remarkably, **SQA-4-FPOPh₂** when used with the v-DABNA:CN-Ir combination (0.5:20 wt% respectively) has displayed excellent electroluminescent performances. These devices presents a narrowband emission peak at 472 nm, full-width at half-maximum (FWHM) of 21 nm, and high maximal External Quantum Efficiency (EQE<sub>max</sub>) of 22.9%. The excellent molecular orientation, which maximizes the light outcoupling efficiency (η<sub>out</sub>), is proposed to be at the origin of this high performance. This work also highlights how positional isomers can provide very different device data, showing the importance of this concept in organic electronics.

## Results and discussion

The three compounds **SQA-1-FPOPh₂**, **SQA-3-FPOPh₂** and **SQA-4-FPOPh₂** were synthesized through a short (3 steps) and high-efficiency route (overall yields comprised between 46 and 56%), which is a key feature for possible industrialization. The synthetic pathway is described in Supplementary Information (Figure S 1). The study of the NMR spectra (see Figures S 2-8) has highlighted the formation of diastereoisomers during the formation of the *spiro* carbon, caused by the nitrogen inversion of the QA unit and the monosubstitution of the fluorene fragment (see supporting information for details). Indeed, this stereochemical feature has already been reported in literature on analogue compounds possessing a quinolinophenotiazine fragment.[33] Moreover, in ¹H NMR spectroscopy, an important shift of the signals assigned to the QA unit is detected for **SQA-1-FPOPh₂** compared to the two other isomers. This can be caused by the cofacial arrangement of the QA and the POPh₂ as previously observed in literature for π-stacked materials.[34-36]

The determination of HOMO and LUMO energy levels has been performed by cyclic voltammetry (CV) in CH₂Cl₂ for oxidation and in DMF for reduction; potentials are given versus a saturated calomel electrode (SCE), Figure 1a,b. Both **SQA-3-FPOPh₂** and **SQA-4-FPOPh₂** present a first reversible wave with peak potentials of 1.04 and 1.01 V. This electron transfer is assigned to the oxidation of the QA core. This is confirmed by the electronic density of the HOMO, which is entirely spread on this fragment for the three
compounds (Figure 1c). However, **SQA-1-FPOPh₂** displays a different behavior with a first oxidation also reversible but significantly shifted to low potentials (E=0.86 V). Usually, in a π-conjugated system, the decrease of the oxidation potential is, in principle, assigned to an increase of the conjugation length between two fragments, herein the fluorene and the POPh₂. However, it is known that the C1 position of a fluorene, due to the combination of a biphenyl *meta* linkage and a sterically hindered environment provides a significant through-bond π-conjugation disruption.[32, 37-39] Thus, the conjugation cannot be extended in such a way. The particular geometry of the C1-fluorene moiety may be therefore implied in the decrease of the first oxidation potential as through-space interactions are indeed known to lead to such behaviours.[34, 36, 40] Thus, the cofacial arrangement of **SQA-1-FPOPh₂** induces through space interactions between the QA unit and the POPh₂ unit, which are known to decrease the oxidation potential, increasing in turn the HOMO energy.[34, 36, 40] This finding is in accordance with the shifts observed in ¹H NMR spectra (see above). Thus, the HOMO energy level of **SQA-1-FPOPh₂** is evaluated at -5.11 eV, significantly higher than those of both **SQA-3-FPOPh₂** and **SQA-4-FPOPh₂** (-5.32 and -5.30 eV respectively).

<figure><img src="image_7.png" /><figcaption>Figure 1. (a) Cyclic voltammetries of the three host recorded on Pt working electrode in reduction (DMF + Bu₄NPF₆ 0.1 M, sweep-rate 100 mV s⁻¹). And (b) in oxidation (CH₂Cl₂ + Bu₄NPF₆ 0.2 M, sweep-rate 100 mV s⁻¹). (c) Frontier molecular orbitals obtained by TD-DFT, B3LYP/6-311+G(d,p)</figcaption></figure>

The LUMO energy levels, determined thanks to the reduction waves, were evaluated at -2.11, -2.11 and -2.24 eV for **SQA-1-FPOPh₂**, **SQA-3-FPOPh₂** and **SQA-4-FPOPh₂**. Molecular modelling shows that the LUMO is dispersed on the fluorene core with a slight contribution of the phosphine oxide in the case of SQA-

**4-FPOPh₂** (Figure 1c). This explains the very similar values obtained for **SQA-1-FPOPh₂** and **SQA-3-FPOPh₂** and the lower one measured for **SQA-4-FPOPh₂**. This shows that the *ortho* linkage of the C4-position induces more electronic coupling with the phosphine oxide than the C1 and C3 positions (*meta* linkage for both). This result is in accordance with previously reported data on the influence of linkages in the design of materials for electronics.[29, 32] This is particularly important to well control the electronic properties of organic materials. These HOMO and LUMO energy levels provide different electrochemical gaps (ΔE<sub>El</sub>): 3.00 eV for **SQA-1-FPOPh₂**, 3.21 eV for **SQA-3-FPOPh₂** and 3.06 eV for **SQA-4-FPOPh₂**. These short gaps are of great interest for the charge injection in devices.

<figure><img src="image_8.png" /><figcaption>Figure 2. (a) UV-Vis absorption spectra in cyclohexane. (b) Emission spectra at room temperature in cyclohexane. (c) Emission spectra at 77 K in 2-MeTHF. λ<sub>exc</sub> = 306 nm for **SQA-1-FPOPh₂**, λ<sub>exc</sub> = 290 nm for **SQA-3-FPOPh₂**, λ<sub>exc</sub> = 306 nm for **SQA-4-FPOPh₂**. (d) Triplet spin density distribution (TD-DFT, b3lyp/6-311+G(d,p), isovalue 0.004).</figcaption></figure>

The UV-Vis absorption and emission spectra of **SQA-1-FPOPh₂** (black line), **SQA-3-FPOPh₂** (red line), and **SQA-4-FPOPh₂** (blue line) were recorded in cyclohexane at room temperature (Figure 2a,b). The optical properties are summarized in Table 1. The three molecules present a similar shape with three bands (at ca 275-280 nm, 290-306 nm and 312-320 nm), which can be attributed to π-π* transitions. As classically observed for materials possessing a D-*Spiro*-A design, the HOMO-LUMO transition is almost forbidden with very weak oscillator strengths of 0.001-0.002 for the three molecules (Figure 3). As above stated, the *spiro* carbon leads to an orthogonal arrangement of the HOMO localized on the QA unit and the LUMO localized on the accepting fluorene core.

The TD-DFT calculations show that, in the case of **SQA-1-FPOPh₂** and **SQA-4-FPOPh₂**, the bands found experimentally at 320 and 312 nm respectively
correspond to the HOMO-1→LUMO transition located on the fluorene core. This is in accordance with the results previously reported in literature for the positional isomer substituted at C2.[16] The band at lower energy of both SQA-1-FPOPh₂ and SQA-4-FPOPh₂ correspond to the HOMO→LUMO+8 and HOMO→LUMO+6 respectively, both located on the QA core and are also similar to previously reported results on the C2-substituted isomer.[16] The band at lower energy observed for SQA-3-FPOPh₂ is assigned to a transition located on the QA core, while the band at higher energy is assigned to a fluorene-located transition. While the two

transitions are inverted compared to those of the other positional isomers, SQA-3-FPOPh₂ tends towards the unsubstituted model compound SQA-F without the POPh₂ unit (see structure and TD-DFT diagram in SI), which possesses the same transitions assignment. This shows the higher degree of conjugation disruption between the fluorene and the acceptor fragment in the case of the C3 substitution.

<figure><img src="image_9.png" /><figcaption>Figure 3. SQA-1-FPOPh₂ (Left), SQA-3-FPOPh₂ (Middle) and SQA-4-FPOPh₂ (Right). Representation of the energy levels and the main molecular orbitals involved in the electronic transitions obtained by TD-DFT, B3LYP/6-311+G(d,p), shown with an isovalue of 0.04 [ebohr⁻³]¹/². For clarity purposes, only the major contributions of each transition are shown (see Supporting Information for details)</figcaption></figure>
<table><caption>Table 1. Selected electronic and physical data of SQA-1-FPOPh₂, SQA-3-FPOPh₂ and SQA-4-FPOPh₂</caption><thead><tr><th></th><th>SQA-1-FPOPh₂</th><th>SQA-3-FPOPh₂</th><th>SQA-4-FPOPh₂</th></tr></thead><tbody><tr><td>λ<sub>abs</sub> [nm]<sup>a</sup></td><td>320, 306, 281</td><td>312, 290, 275</td><td>319, 293, 281</td></tr><tr><td>λ<sub>em</sub> [nm]<sup>a</sup></td><td>405</td><td>354</td><td>367</td></tr><tr><td>QY<sup>a,b</sup></td><td>0.01</td><td>0.02</td><td>0.03</td></tr><tr><td>λ<sub>phophsho</sub> [nm]<sup>c</sup></td><td>469</td><td>429</td><td>443</td></tr><tr><td>τ<sub>S</sub> [ns]<sup>a</sup></td><td>5.5</td><td>1.8</td><td>4.8</td></tr><tr><td>τ<sub>T</sub> [s]<sup>c</sup></td><td>6.6</td><td>5.2</td><td>4.4</td></tr><tr><td>E<sub>S1</sub> [eV]<sup>d</sup></td><td>3.29</td><td>3.70</td><td>3.57</td></tr><tr><td>E<sub>T1</sub> [eV]<sup>e</sup></td><td>2.64</td><td>2.89</td><td>2.80</td></tr><tr><td>LUMO<sub>El</sub> [eV]<sup>f</sup></td><td>-2.11</td><td>-2.11</td><td>-2.24</td></tr><tr><td>HOMO<sub>El</sub> [eV]<sup>f</sup></td><td>-5.11</td><td>-5.32</td><td>-5.30</td></tr><tr><td>LUMO<sub>th</sub> [eV]<sup>g</sup></td><td>-1.56</td><td>-1.47</td><td>-1.65</td></tr><tr><td>HOMO<sub>th</sub> [eV]<sup>g</sup></td><td>-5.03</td><td>-5.43</td><td>-5.44</td></tr><tr><td>ΔE<sub>el</sub> [eV]<sup>f</sup></td><td>3.00</td><td>3.21</td><td>3.06</td></tr><tr><td>ΔE<sub>th</sub> [eV]<sup>g</sup></td><td>3.47</td><td>3.96</td><td>3.79</td></tr><tr><td>μ<sub>h</sub> (cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>)<sup>h</sup></td><td>0.13×10<sup>-6</sup></td><td>1.73×10<sup>-6</sup></td><td>0.25×10<sup>-6</sup></td></tr><tr><td>μ<sub>e</sub> (cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>)<sup>h</sup></td><td>2.48×10<sup>-6</sup></td><td>21.12×10<sup>-6</sup></td><td>1.76×10<sup>-6</sup></td></tr><tr><td>T<sub>d</sub> (°C)<sup>i</sup></td><td>400</td><td>404</td><td>424</td></tr><tr><td>T<sub>g</sub> (°C)<sup>j</sup></td><td>150</td><td>159</td><td>172</td></tr></tbody></table>

[a] In cyclohexane. [b] Compared to quinine sulfate. [c] in 2-MeTHF at 77 K. [d] Calculated from the onset of the highest energy band in cyclohexane at RT (1239.84/λ). [e] Calculated from the peak maximum of the highest energy phosphorescent spectrum, (1239.84/λ), at 77 K in 2-MeTHF. [f] from CVs in dichloromethane (HOMO) or DMF (LUMO). [g] Found through TD-DFT calculations, B3LYP, 6-311+G(d, p). th = theoretical, El = electrochemical. [h] determined from SCLC devices analysis [i] determined by TGA. [j] determined by DSC at the second heating cycle.

In emission, the three compounds also present similar spectra, which appear large and unresolved. There is nevertheless a gradual red shift of the maxima from SQA-3-FPOPh₂ (354 nm) to SQA-4-FPOPh₂ (367 nm), and to SQA-1-FPOPh₂ (405 nm). At a first glance, it was then highly surprising to notice that SQA-1-FPOPh₂ displayed the most red shifted spectra despite its substitution at C1. Indeed, the C1 substitution of the fluorene backbone is known to provide the most efficient π-conjugation disruption and usually leads to a fluorescent emission blue shifted compared to their C3 and C4 analogues.[29, 32] A different result is highlighted herein. We propose that this emission features arise from excimers induced by intermolecular interactions due to the interactions between the fluorene and the POPh₂

moieties in the excited state. Several groups have also reported similar behaviours in fluorescence spectroscopy, for example with oligophenyl based cruciforms,[41] ethynyltriphenylene derivatives,[42] carbazolophanes,[43] and pyrene derivatives.[44-47] It is hence clear that intramolecular

interactions exist not only in the ground state as shown above in ¹H NMR but also in the first excited state. These interactions provide singular emission properties for SQA-1-FPOPh₂ different to the majority of C1-based fluorene compounds reported in literature.[30, 32, 39, 48] It is nevertheless interesting to mention that a spirophenylacridine-based host also constructed on a C1-substituted fluorene has recently shown a similar emission feature (ie a large and red-shifted spectrum
centred at 415 nm), confirming the importance of the intramolecular interactions in the emission properties.[49]
All these molecules present a very low quantum yield (below 3%) in accordance with the spatial separation of the frontier orbitals and the resulting forbidden HOMO→LUMO transition. Note that **SQA-1-FPOPh₂** displays the lowest quantum yield in the series (and the longest singlet state lifetime τₛ, Table 1) in accordance with the intramolecular interactions above mentioned. The evolution of the emission spectra as a function of the polarity of the solvent was then investigated in order to go deeper into the polarity of the

excited states. For **SQA-1-FPOPh₂**, a red shift of 66 nm between apolar cyclohexane (405 nm) and polar MeOH (471 nm) is measured, highlighting a photoinduced intramolecular charge transfer (ICT) (Figure 4a). The intensity of the ICT is then increased in the case of **SQA-3-FPOPh₂** with a difference of 83 nm between cyclohexane (354 nm) and MeOH (437 nm). The largest ICT is finally observed with **SQA-4-FPOPh₂** presenting a difference of 107 nm between cyclohexane (367 nm) and MeOH (474 nm). This is assigned to the fact that **SQA-4-FPOPh₂** displays the lowest LUMO energy level in the series.

<figure><img src="image_10.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Figure 4. Emission spectra in various solvents of (a) **SQA-1-FPOPh₂**, (b) **SQA-3-FPOPh₂** and (c) **SQA-4-FPOPh₂**.</figcaption></figure>

The differences (Δμ) between the dipole moment at the ground state (μ) and at the first excited state (μ*) have been evaluated using the Lippert-Mataga formalism. Δμ of 19.4, 26.1 and 31.3 D have been measured. The dipole moments at the ground state have been obtained by DFT calculations, at 3.5, 4.4 and 4.5 D for **SQA-1-FPOPh₂**, **SQA-3-FPOPh₂** and **SQA-4-FPOPh₂**, respectively (see SI). With these data, we obtained the different polarity of the three molecules at the excited state (μ* = 15.9, 21.7 and 26.8 D resp.). The strong red shift of the emission maxima (Figure 4c) and the corresponding high Δμ observed for **SQA-4-FPOPh₂** show a significant photoinduced ICT. The decrease of Δμ for **SQA-3-FPOPh₂** and **SQA-1-FPOPh₂** is in line with a decrease of the photoinduced ICT. Thus, the polarity of excited states depends on the fluorene substitution (and then the nature of their linkages) as follows C4, C3 and C1 and is due to both the electronic coupling between the phosphine and the fluorene and the cofacial interaction in the case of C1.

The E<sub>T</sub> were evaluated from the phosphorescence
contribution measured at 77 K in 2Me-THF (Figure 2c).
Significant differences were detected highlighting the
importance of the fluorene substitution pattern on this
property. Thus, the highest value was obtained for SQA-
3-FPOPh<sub>2</sub>, E<sub>T</sub> = 2.89 eV (λ = 429 nm), in accordance
with a T<sub>1</sub> state fully governed by the 3-
fluorene(diphenylphosphineoxide) fragment as shown by
the triplet spin density distribution (SDD) obtained by
TD-DFT (b3lyp/6-311+g(d,p), Figure 2d). As this E<sub>T</sub> is
identical to that of model compound SQA-F, which is the
same structure witout the phosphine oxide unit (E<sub>T</sub>= 2.89

eV, Figure S 38), the impact of the phosphine oxide is
fully vanished. In addition, the E_T of SQA-3-FPOPh₂ is
also close that of fluorene (E_T = 2.93 eV),[30] showing the
efficiency of this molecular arrangement to maintain a
very high E_T. The slight decrease is mainly caused by the
interaction of the fluorene with the QA core *via* spiro-
conjugation.[48] SQA-4-FPOPh₂ also displays a high E_T
(2.80 eV, λ = 443 nm) nevertheless decreased compared
to its positional isomer SQA-3-FPOPh₂. This can be
assigned to the different coupling occurring between C3
and C4 positions. Note that for SQA-4-FPOPh₂, the
triplet SDD is also exclusively localized on the fluorene
but this feature may be dependent on the dihedral angle
between the fluorene and the phosphine oxide.

Again, the case of SQA-1-FPOPh₂ appears
particularly singular not only compared to its positional
isomers presented herein but also in the light of the
literature. Indeed, the E<sub>T</sub> of SQA-1-FPOPh₂ is measured
particularly low, E<sub>T</sub>= 2.64 eV, with a first
phosphorescene band detected at a very high wavelength,
469 nm. This is very different to other C1-fluorene based
materials known to possess a very high E<sub>T</sub> due to the
strong π-conjugation disruption induced by this
substitution position.[3, 29, 39, 50] This low value is
corroborated by the triplet spin density localized on both
the fluorene unit and the QA core (Figure 2d). This is
very different to what was discussed above for both
SQA-3-FPOPh₂ and SQA-4-FPOPh₂ and assigned to
the cofacial arrangement.

The three isomers possess long T₁ state lifetimes, 4.4, 5.2 and 6.6 s for **SQA-4-FPOPh₂**, **SQA-3-FPOPh₂** and **SQA-1-FPOPh₂** respectively in accordance with a phosphorescent contribution.
Finally, a host material should present an excellent thermal stability for vacuum deposition and device operating. Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) provide very high decomposition temperatures $T_d$ above 400°C. The glass transition temperatures $T_g$ range between 150 and 172°C (Figure S 10). The excellent thermal and morphological stability of the series is beneficial for the fabrication of OLED devices utilizing thermal evaporation techniques and improves the stability of the device.[51]

The hole-only and electron-only devices were fabricated to determine the charge transport properties, a key feature in OLED technology (Figure S 54). At low bias, the curves were well-fitted within the space-charge-limited current (SCLC) region. The three isomers demonstrate excellent charge balance performance, as summarized in Table 1. Notably, **SQA-4-FPOPh₂** exhibits the most balanced charge transport characteristics among the series ($\mu_c/\mu_h$ = 1.76×10⁻⁶/ 0.25×10⁻⁶ cm² V⁻¹ s⁻¹). Recent studies have consistently highlighted that achieving comparable hole and electron mobilities is a critical factor for optimizing OLED performance.[14, 15, 52] This bipolar transport property is important for enhancing device efficiency, as it ensures efficient recombination of electrons and holes within the emissive layer. This optimized charge balance is one of the key factors contributing to the high device efficiency reported in this study.

Finally, as both **SQA-4-FPOPh₂** and **SQA-3-FPOPh₂** present all the necessary properties, they were used as hosts in MR-TADF OLEDs. Due to its low $E_T$, **SQA-1-FPOPh₂** was not investigated. The phosphorescent sensitizers Flpic, *fac*-Ir tpz)₃ and CN-Ir[¹⁸, ⁵³, ⁵⁴] were selected as their HOMO energy levels match that of the emitters (BNCz and v-DABNA)[⁵⁵, ⁵⁶] (Table S 17, note that the photophysical, electrochemical and thermal characterisations of *fac*-Ir tpz)₃ are presented in SI). The spectral overlap between the photoluminescence spectrum of the sensitizer and the absorption spectrum of the emitters (energy transfer from $T_1$ excitons of the sensitizer to $S_1$ excitons of the emitter) were measured in diluted toluene solution (Figure S 55). A large spectral overlap was observed, which optimizes device performance by minimizing energy loss and improving emission efficiency.

Before any applications in phosphorescent sensitizer MR-TADF devices, **SQA-4-FPOPh₂** and **SQA-3-**

**FPOPh₂** were both used as host in classical emissive layer (EML) for (i) TADF OLEDs using BNCz as emitter and (ii) phosphorescent OLEDs using Flpic as emitter (Table 2, Figure S 56). The architecture of the OLEDs was: 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), Figure 5a. The $EQE_{max}$ obtained with **SQA-3-FPOPh₂** were measured at 10.4% (Device D1) and 12.7% (Device D2) for BNCz- and Flpic-based OLEDs respectively. Switching the phosphine oxide from C3 in **SQA-3-FPOPh₂** to C4 in **SQA-4-FPOPh₂** leads to an increase of efficiency with an $EQE_{max}$ recorded at 15.4% for both BNCz- and Flpic-based OLEDs (Devices D7 and D8). All devices exclusively present the emission of the emitter, either BNCz (492 nm) or Flpic (476 nm), without residual emission of the host. However, for all the devices the driving voltages at the current density of 0.01 mA cm⁻² were high, translating some difficulties for the injection of charges. In addition, in the case of the PhOLEDs, the emission is large. To improve the electrical performance and optical characteristics, phosphorescent-sensitized MR-TADF EML were fabricated using Flpic, *fac*-Ir tpz)₃, or CN-Ir as phosphorescent sensitizers and BNCz or v-DABNA as blue emitter.

First, using BNCz as emitter with either Flpic (Device D3) or *fac*-Ir tpz)₃ (Device D4) as sensitizer and **SQA-3-FPOPh₂** as host, the OLED displays an $EQE_{max}$ of ca 15%, Table 2. With **SQA-4-FPOPh₂**, the association of BNCz and *fac*-Ir tpz)₃ gives a similar $EQE_{max}$ of around 15% (Device D10), showing no increase in performance. These devices achieved narrow spectral emission with a peak wavelength of 492 nm (Figure 5b). However, the BNCz/Flpic association with **SQA-4-FPOPh₂** displays an increase in performance with an $EQE_{max}$ reaching 20.6% a very high maximal current efficiency $CE_{max}$ of 40 cd/A (Device D9). This improved efficiency may be attributed to the shorter $T_1$ lifetime of Flpic (1.8 µs) compared to *fac*-Ir tpz)₃ (2.3 µs) (Figure S 40), which facilitates the conversion of $T_1$ (sensitizer) into $S_1$ (emitter) in the EML, thereby reducing the accumulation of $T_1$ excitons in the sensitizer and minimizing potential non-radiative losses. This feature efficiently suppresses triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA).[⁵⁷, ⁵⁸] Note that the lifetime of the three Iridium complex investigated in this work have been measured in strictly identical conditions (2-MeTHF in absence of oxygen) in order to insure a precise comparison (Figure S 40-41).
<figure><img src="image_13.png" /><figcaption>**Figure 5.** Phosphorescent sensitizer MR-TADF devices performance using SQA-3-FPOPh₂ and SQA-4-FPOPh₂ as host, respectively. (a) Device structure with the energy-level diagrams and the molecular structures of materials in different layers, (b, e) Electroluminescence spectra, (c, f) EQE-Luminance curves, (d, g) J-V-L curves. The different devices EML are presented in Table 2.</figcaption></figure>

The EQE is correlated to the light outcoupling efficiency ($\eta_{out}$), 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 host. Horizontal molecular orientation relative to the substrate enhances $\eta_{out}$ and in turn EQE and is dependant of the host material used, which is the main component of the EML.[59] The horizontal molecular orientation within the EML was investigated by co-doped films identical to the EMLs of these devices (Figure S 57). The horizontal dipole ratio ($\Theta_h$) of the EML made by FIrpic/BNCz and either SQA-4-FPOPh₂ or SQA-3-FPOPh₂ are 78% and 76%, respectively, explaining, at least partially, the highest results reached by the latter. The relatively high $\Theta_h$ is assigned to the rigidity of the molecular structure due to the presence of both the spiro bridge and the methylene bridge of the QA unit. This suppresses the reorientation and randomization of the

guest and sensitizer molecules during the process of evaporation performed at high evaporation temperatures.

Furthermore, co-doped films were prepared using two host materials, SQA-3-FPOPh₂ and SQA-4-FPOPh₂, with 2 wt% BNCz:10 wt% FIrpic. The photoluminescence quantum yields (PLQYs) were measured at 94% and 93%, respectively. Despite the presence of energy transfer processes, the PLQY of the co-doped films remains high, which is crucial for achieving high device efficiency. Owing to the close alignment between the triplet energy level of FIrpic and the singlet energy level of BNCz, an additional exciton management pathway is enabled, wherein singlet excitons of BNCz are selectively transferred through a backward Dexter energy transfer from BNCz to FIrpic. This process suppresses the formation of long-lived triplet excitons in BNCz. Considering the substantially shorter exciton lifetime of FIrpic compared to that of BNCz (68.8 µs[55], this strategy effectively reduces triplet
exciton accumulation and decreases the average exciton lifetime within the system.[60]

As deep blue OLEDs are still nowadays an important challenge, we investigated a bluer emitter, namely v-DABNA. This TADF emitter was combined with CN-Ir as the phosphorescent sensitizer (Figure 5, Table 2). There is a good alignment of the $E_T$ of CN-Ir (2.70 eV), with the singlet energy of v-DABNA (2.65 eV) and this phosphor possesses a short triplet state lifetime of 2.5 µs of great interest for this work.[18] In addition, there is a significant overlap between the emission of CN-Ir and the absorption of v-DABNA, which clearly shows the efficiency of this combination (Figure S 55). First, we used 0.5 wt% v-DABNA alone dispersed in hosts. For SQA-3-FPOPh₂, the performance appears very low with an EQE of 5.8% (Figure 5, Device D5). Incorporating CN-Ir into the 0.5 wt% v-DABNA EML provides a

<table><caption>**Table 2.** Summary of the performance of the devices.</caption><thead><tr><th>Host</th><th>EML</th><th>V<sup>a</sup><br>(V)</th><th>λ<sub>EL</sub><sup>b</sup><br>(nm)</th><th>CE<sub>max</sub><sup>c</sup><br>(cd/A)</th><th>FWHM<sup>b</sup><br>(nm)</th><th>EQE<sup>d</sup><br>(%)</th></tr></thead><tbody><tr><td rowspan="6">SQA-3-FPOPh₂</td><td>2 wt% BNCz (D1)</td><td>3.55</td><td>492</td><td>15</td><td>31</td><td>10.4/-</td></tr><tr><td>15 wt% FIirpic (D2)</td><td>3.50</td><td>476</td><td>15</td><td>58</td><td>12.7/7.8</td></tr><tr><td>2 wt% BNCz: 10 wt% FIirpic (D3)</td><td>3.52</td><td>492</td><td>21</td><td>31</td><td>15.8/8.7</td></tr><tr><td>2 wt% BNCz: 20 wt% fac-Ir.tpz<sub>3</sub> (D4)</td><td>3.51</td><td>492</td><td>16</td><td>31</td><td>15.5/8.7</td></tr><tr><td>0.5 wt% v-DABNA (D5)</td><td>3.15</td><td>472</td><td>5</td><td>21</td><td>5.8/-</td></tr><tr><td>0.5 wt% v-DABNA:20 wt% CN-Ir (D6)</td><td>3.05</td><td>472</td><td>10</td><td>21</td><td>9.0/6.5</td></tr><tr><td rowspan="6">SQA-4-FPOPh₂</td><td>2 wt% BNCz (D7)</td><td>4.54</td><td>492</td><td>30</td><td>32</td><td>15.4/1.9</td></tr><tr><td>15 wt% FIirpic (D8)</td><td>4.63</td><td>476</td><td>30</td><td>57</td><td>15.4/8.5</td></tr><tr><td>2 wt% BNCz:10 wt% FIirpic (D9)</td><td>4.51</td><td>492</td><td>40</td><td>32</td><td>20.6/10.0</td></tr><tr><td>2 wt% BNCz: 20 wt% fac-Ir.tpz<sub>3</sub> (D10)</td><td>4.69</td><td>492</td><td>30</td><td>32</td><td>15.7/8.9</td></tr><tr><td>0.5 wt% v-DABNA (D11)</td><td>4.24</td><td>472</td><td>10</td><td>21</td><td>11.7/-</td></tr><tr><td>0.5 wt% v-DABNA:20 wt% CN-Ir (D12)</td><td>4.07</td><td>472</td><td>20</td><td>21</td><td>22.9/7.3</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 CE<sub>max</sub>. [d] EQE at the maximum and 1000 cd m⁻².

These higher performances were explained by the highest Θ<sub>h</sub> values measured on co-doped films, which were found to be as high as 80% for SQA-3-FPOPh₂ and 82% for SQA-4-FPOPh₂ (Figure S 58). This orientation is a crucial factor contributing to the high EQE observed in SQA-4-FPOPh₂-based devices.[61] Indeed, it is known that, at high current densities, enhanced electric fields can cause polarization effects, which modify the molecular orientation of the EML and lead to a decrease in efficiency.[62] To clarify the performance of this series of devices, thin-films were prepared by co-doping CN-Ir and v-DABNA with either SQA-3-FPOPh₂ or SQA-4-FPOPh₂ as hosts. Thus, the PLQY of these films was measured at 82% and 92%, respectively, following the

significant increase in the EQE, which remains nevertheless relatively low (9.0 %, Device D6). SQA-4-FPOPh₂ appears to be more adapted to this emissive combination. Indeed, SQA-4-FPOPh₂ displays an EQE<sub>max</sub> of 11.7% with 0.5 wt% v-DABNA EML (Device D11), which is significantly increased to 22.9% with the doping of CN-Ir (Device D12). Compared to all the systems described above, the driving voltages are also beneficially decreased for both SQA-3-FPOPh₂ and SQA-4-FPOPh₂ devices (3.05 and 4.07 V respectively, Table 2), highlighting the efficiency of this strategy in term of charge injection. These devices present a narrow blue emission with FWHMs of only 21 nm, centered at 472 nm. Thus, the additional energy transfer between CN-Ir and v-DABNA reduces exciton loss and achieves in the case of SQA-4-FPOPh₂ a high EQE<sub>max</sub>.

different device performances obtained. This shows the great importance of the substitution position on the photophysical parameters and highlights how positional isomerism is a key tool in material designs as previously shown in other works.[29, 31, 37, 63-65]

## Conclusions

In summary, we report herein spiro-configured host materials constructed on the assembly of a QA unit and different fluorene positional isomers for high performance blue MR-TADF OLEDs. The impact of the fluorene π-conjugation disruption on the electronic properties of the materials is discussed, highlighting very
different behaviours directly arising from the substitution position. These materials were used as host in blue MR-TADF OLEDs employing different emitters BNCz or v-DABNA and different phosphorescent sensitizers, FIrpic, CN-Ir or fac-Ir tpz.3. The very different device data obtained have revealed the difficulty to reach high performances and the necessity to perfectly well adapted the combination of the three component of the EML, namely the MR-TADF emitter, the phosphorescent sensitizer and the host. Particularly, the design of host materials appear very important. The v-DABNA: CN-Ir EML combination (0.5:20 wt% respectively) with SQA-4-FPOPh₂ as host has provided excellent electroluminescent performances. These devices possess a narrowband emission peak at 472 nm, a thin full-width at half-maximum of 21 nm, and a high maximum external quantum efficiency of 22.9%. The molecular orientation, which maximizes the light outcoupling efficiency is proposed to be at the origin of this high performance. This work shows the efficiency of the positional isomerism strategy to tune the electronic, physical and electrical properties of 'D-Spiro-A' molecules for such application. If this concept has been largely devlopped for phosphorescent OLEDs, it remains less evelopped for phosphorescent-sensitized MR-TADF OLEDs.[15] In the light of the wide diversity of donor and acceptor units, which can be combined in 'D-Spiro-A' molecular structures, the device performance can be surely improved in the near future for blue MR-TADF OLEDs. We are currently designing such hosts.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

This work has been financially supported by the ANR (N°24-CE50-7171- White-Break Project). KL thank the Region Bretagne (Simplify Project) and the ANR (N°22-CE07-0024-Evolution Project). We also thank the CRMPO (Rennes) for mass analyses and GENCI (Project N°AD010814136) and the CTI team at ISCR for computing time. 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.

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