# Chiral Selenium-Integrated Multi-Resonant Thermally Activated Delayed Fluorescent Emitters Showing Improved Reverse Intersystem Crossing Rate

Jingxiang Wang, Hassan Hafeez, Dongyang Chen, Jhon Sebastian Oviedo

Ortiz, Yan Xu, Aidan P. Mckay, David B. Cordes, Jeanne Crassous, Ifor D.

W. Samuel, Eli Zysman-Colman

► To cite this version:

Jingxiang Wang, Hassan Hafeez, Dongyang Chen, Jhon Sebastian Oviedo Ortiz, Yan Xu, et al. Chiral Selenium-Integrated Multi-Resonant Thermally Activated Delayed Fluorescent Emitters Showing Improved Reverse Intersystem Crossing Rate. Angewandte Chemie International Edition, 2025, 64 (33), pp.e202506999. 10.1002/anie.202506999. hal-05167307

HAL Id: hal-05167307

https://hal.science/hal-05167307v1

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# Chiral Selenium-Integrated Multi-Resonant Thermally Activated Delayed Fluorescent Emitters Showing Improved Reverse Intersystem Crossing Rate

Jingxiang Wang<sup>+</sup>, Hassan Hafeez<sup>+</sup>, Dongyang Chen, Jhon Sebastian Oviedo Ortiz, Yan Xu, Aidan P. McKay, David B. Cordes, Jeanne Crassous,* Ifor D. W. Samuel,* and Eli Zysman-Colman*

**Abstract:** Nitrogen/carbonyl (N/C=O) based multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters are attractive due to their bright, narrowband emission and the ease with which they can be synthesized. However, their photophysics typically suffer from slow reverse intersystem crossing (RISC) because of their relatively large singlet-triplet energy gap ($\Delta E_{ST}$). Thus, the organic light-emitting diodes (OLEDs) with these emitters typically show severe efficiency roll-off. Here, two MR-TADF emitters **DiKTaSe** and **tBuCz-DiKTaSe** have been designed and synthesized. The introduction of selenium in the form of an annelated benzoselenophene enhances spin-orbit coupling and increases the RISC rate. The twisted *ortho*-substituted *tert*-butylcarbazole moiety in **tBuCz-DiKTaSe** helps to suppress aggregation-caused quenching of the emission in films. In addition, the large size of the selenium atom and long C–Se bonds induce helical chirality in both **DiKTaSe** and **tBuCz-DiKTaSe**. Finally, the OLEDs with **DiKTaSe** showed maximum external quantum efficiency (EQE<sub>max</sub>) of 22.7% while OLEDs with **tBuCz-DiKTaSe** showed a higher EQE<sub>max</sub> of 27.8% and less-pronounced efficiency roll-off, with EQE at 100 cd m<sup>-2</sup> (EQE<sub>100</sub>)/ EQE at 1000 cd m<sup>-2</sup> (EQE<sub>1000</sub>) of 23.5/12.5%. These efficiency values are amongst the highest of devices employing DiKTa-based emitters. Our work provides key insight into how to judiciously employ heavy atoms to increase the performance of the emitter and the device.

## Introduction

Multi-resonant thermally activated delayed fluorescence (MR-TADF) materials have generated considerable interest as emitters in organic light-emitting diodes (OLEDs). The short-range charge transfer (SRCT) character for the emissive S<sub>1</sub> state induced by the *ortho* or *para* localized election-

donating and electron-withdrawing atoms in the polycyclic aromatic framework is responsible for the TADF in these compounds.[1] Their rigid structure endows the MR-TADF emitter with narrowband emission and high photoluminescence (PL) quantum yields ($\Phi_{PL}$). Thus, these attractive properties permit MR-TADF OLEDs to achieve simultaneously high external quantum efficiency (EQE) and saturated color. Hatakeyama et al. reported the first MR-TADF emitter, **DABNA-1**, which contained a boron atom as the electron-withdrawing group.[2] It emits at peak wavelength ($\lambda_{PL}$) of 460 nm and has a full width at half maximum (FWHM) of 30 nm, a $\Phi_{PL}$ of 88% and a singlet-triplet energy gap ($\Delta E_{ST}$) of 0.18 eV in 1 wt% doped films in 3,3'-Di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP). The OLEDs with **DABNA-1** as an emitter exhibited a narrow blue emission at 459 nm (FWHM of 28 nm) and a maximum EQE (EQE<sub>max</sub>) of 13.5%. Since this pioneering report, a large number of boron-based MR-TADF emitters have been developed and used in high-performance OLEDs.[3,4] However, the key borylation step frequently requires high temperature and pyrophoric reagents, and the borylation is often not well controlled in terms of the chemoselectivity, resulting in generally moderate yields.[5,6]

Jiang, Liao and co-workers[7,8] and our group[9] reported another category of MR-TADF emitter (**DiKTa**, also known as **QAO**, **QAD**) where the boron acceptor atoms are replaced with carbonyl-based moieties. These compounds typically form under mild and high-yielding conditions, usually via a Friedel-Crafts acylation to produce the cyclized product.[9]

[*] J. Wang<sup>+</sup>, D. Chen, Y. Xu, A. P. McKay, D. B. Cordes, E. Zysman-Colman
Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, UK
E-mail: eli.zysman-colman@st-andrews.ac.uk

H. Hafeez<sup>+</sup>, I. D. W. Samuel
Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, UK
E-mail: idws@st-andrews.ac.uk

J. S. O. Ortiz, J. Crassous
University of Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Rennes F-35000, France
E-mail: jeanne.crassous@univ-rennes.fr

[+] Both authors contributed equally to this work.

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Additional supporting information can be found online in the Supporting Information section

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© 2025 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH. This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
**DiKTa** shows a narrowband blue emission at $\lambda_{PL}$ of 463 nm (FWHM of 37 nm) and has a $\Phi_{PL}$ of 75% and $\Delta E_{ST}$ of 0.20 eV in 3.5 wt% doped films in 1,3-bis(N-carbazolyl)benzene (mCP). Depending on the device configuration, the OLEDs showed an $EQE_{max}$ of 19.4% at the electroluminescence peak wavelength ($\lambda_{EL}$) of 468 nm[7] or an $EQE_{max}$ of 14.7% at $\lambda_{EL}$ of 465 nm.[9] Although many **DiKTa**-based MR-TADF emitters have been reported, including examples of devices showing very high $EQE_{max}$ of >30%, their efficiency roll-off is severe at high current density due to the slow reverse intersystem crossing (RISC) linked to the relatively large $\Delta E_{ST}$ (Figure S40, Table S5).[8,10–31]

According to Fermi's Golden rule, the rate constant of RISC ($k_{RISC}$) can be expressed as Equation 1:[32]

$$k_{RISC} \propto |H_{SO}|^2 \exp\left(-\frac{\Delta E_{ST}}{k_B T}\right) \quad (1)$$

where $k_B$, $H_{SO}$ and $T$ refer to Boltzmann's constant, the spin-orbit coupling (SOC) matrix element, and temperature, respectively. Thus, despite a large $\Delta E_{ST}$ a fast $k_{RISC}$ can be achieved by strengthening the SOC. As the value of SOC is approximately proportional to the fourth power of the nuclear charge of the atoms involved in the emissive transition, heavy atoms can be introduced to the **DiKTa** core to enhance $k_{RISC}$.[33,34] Jiao et al. reported JY-2-Cl, a **DiKTa** derivative decorated with six chlorine atoms (Figure S40).[35] It exhibits monomer and dimer dual emission at 509 and 545 nm with high $\Phi_{PL}$ of 82.6% in 3 wt% doped film in 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP). Due to the heavy atom effect (HAE) linked to the chlorine atoms, it has much shorter delayed fluorescence lifetimes ($\tau_d$) of 48.1 and 206.6 µs measured at 480 and 570 nm, respectively, compared to JY-2, a derivative without any chlorine atoms ($\tau_d$ of 628 µs). The OLEDs with JY-2-Cl showed high $EQE_{max}$ of 29.1% yet still showed severe efficiency roll-off, with EQE at 100 cd m⁻² ($EQE_{100}$) of around 10% and EQE at 1000 cd m⁻² ($EQE_{1000}$) of around 1%. Yu et al. embedded oxygen, sulfur and selenium atoms into the **DiKTa** (QAO) core in the spiro-locked analogs SOQ, SSQ and SSeQ (Figure S40).[6] These three compounds emit at 457, 458 and 456 nm, with narrow FWHMs of 32, 30 and 34 nm, and have high $\Phi_{PL}$ values of 95%, 92% and 96%, respectively, in 3 wt% doped film in mCBP. Although they all have comparably moderate $\Delta E_{ST}$ of 0.21–0.22 eV, the $k_{RISC}$ calculated from the time-resolved PL decays in doped films steadily improved from 2.06 × 10⁴ to 2.69 × 10⁴ and 4.54 × 10⁴ s⁻¹ for SOQ, SSQ, and SSeQ, respectively. The SSQ-based device showed the highest $EQE_{max}$ of 25.5% at 456 nm (FWHM of 31 nm) and lowest efficiency roll-off, with $EQE_{100}$ of 13.4% and $EQE_{1000}$ of 3.5%. Jiang et al. integrated S-containing spiro structures into the DiKTa (QAO) unit in SpiroS-QAO, SpiroSO2-QAO, SpiroO-QAO and SpiroOSO2-QAO.[36] These compounds emit at 494, 458, 484 and 485 nm, have narrow FWHMs of 43, 32, 34 and 33 nm and moderate $\Delta E_{ST}$ of 0.21, 0.21, 0.23 and 0.23 eV in toluene. Their doped films in PPF showed high $\Phi_{PL}$ values of 94%, 88%, 88% and 89% and $k_{RISC}$ of 2.0 × 10⁴, 1.4 × 10⁴, 6.1 × 10³ and 4.9 × 10³ s⁻¹, respectively. Of these

four, SpiroS-QAO displayed the highest $\Phi_{PL}$ and fastest $k_{RISC}$ and the device exhibited the highest $EQE_{max}$ of 27.3%.

We previously reported the brominated DiKTa derivatives, **dBr-tBu-DiKTa** and **tBr-DiKTa**, containing two and three peripheral bromine atoms substituted about the DiKTa core (Figure S40).[37] Compared to the reference bromine-less emitter **Mes₃-DiKTa**,[9] the $k_{RISC}$ values increased progressively from 2.92 × 10³ to 1.93 × 10⁴ and 1.97 × 10⁵ s⁻¹, while the $\Phi_{PL}$ decreased from 90%, 82% to 61% in 1% doped films in mCP as a function of increasing bromine content. The OLED with **dBr-tBu-DiKTa** showed the same $EQE_{max}$ of 21% at low luminance but showed decreased efficiency roll-off compared to the device with **Mes₃-DiKTa**. However, due to the poor stability resulting from the weaker C-Br bonds and the stronger propensity to aggregate of **dBr-tBu-DiKTa** in films, the device showed a much worse $EQE_{max}$ than that of the device with **Mes₃-DiKTa** at higher luminance. These examples document that it is not simply a case of incorporating heavier atoms to lead to improved device performance but that considerations as to photochemical stability and $\Phi_{PL}$ also need to be taken into account in the quest to design higher performance MR-TADF emitters.

Here, we report two new MR-TADF emitters, **DiKTaSe** and **tBuCz-DiKTaSe**. The internal selenium atom was introduced to strengthen the SOC and maintain the stability of the molecules. A twisted *tert*-butyl carbazole substituent in **tBuCz-DiKTaSe** acts as a spacer to suppress aggregation. In addition, the large size of the selenium atom and long C-Se bonds induce the formation of a configurationally stable helicene structure, which endows the emitters with chiroptical properties observable at room temperature. As a result, **DiKTaSe** and **tBuCz-DiKTaSe** exhibit narrowband blue emission at 464 nm (FWHM of 34 nm) and 470 nm (FWHM of 35 nm) in toluene, respectively. In 2 wt% doped film in 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (2,6-DCzPPy), **DiKTaSe** emits at 484 nm (FWHM of 60 nm), with $\Phi_{PL}$ of 62%. By contrast, **tBuCz-DiKTaSe** shows narrower emission at 489 nm, with a FWHM of 50 nm, and has a higher $\Phi_{PL}$ of 82%. The red-shift and broadening of the PL spectra of the 2 wt% doped films compared to those in toluene can be attributed to the intermolecular interaction between host and guest as well as aggregation of emitters. Thanks to the internal selenium atom, both **DiKTaSe** and **tBuCz-DiKTaSe** show fast $k_{RISC}$ of 9.97 × 10⁴ and 16.5 × 10⁴ s⁻¹ (Table 1). The separated enantiomers of **DiKTaSe** and **tBuCz-DiKTaSe** show symmetric circular dichroism (CD) signals in toluene with absorption dissymmetry factors (lg<sub>abs</sub>) of 1.7 × 10⁻³ and 1.8 × 10⁻³, respectively. Due to their poor $\Phi_{PL}$ in toluene, we were unable to detect their CPL signals in solution. However, as a 2 wt% doped film in PMMA, it was possible to measure CPL activity in the case of the enantiomers of **tBuCz-DiKTaSe**, with an emission dissymmetry factor (lg<sub>PL</sub>) of around 2.3 × 10⁻³. The OLEDs with **DiKTaSe** as an emitter showed an $EQE_{max}$ of 22.7% at $\lambda_{EL}$ of 480 nm and FWHM of 63 nm. The devices with **tBuCz-DiKTaSe** showed a higher $EQE_{max}$ of 27.8% at $\lambda_{EL}$ of 490 nm and a slightly narrower FWHM of 57 nm. In addition, the efficiency roll-off for the device with **tBuCz-DiKTaSe** was alleviated as compared to that with **DiKTaSe**, showing $EQE_{100}/EQE_{1000}$ of 23.5%/12.5% ($EQE_{100}/EQE_{1000}$
<table><caption>**Table 1:** Photophysical data of DiKTaSe and tBuCz-DiKTaSe.</caption><thead><tr><th>Compound</th><th>λ<sub>abs</sub><sup>a)</sup><br>(nm)</th><th>λ<sub>PL</sub><sup>b)</sup><br>(FWHM)<br>(nm)</th><th>ΔE<sub>ST</sub><sup>c)</sup><br>(eV)</th><th>λ<sub>PL</sub><sup>d)</sup><br>(FWHM)<br>(nm)</th><th>Φ<sub>PL</sub><sup>e)</sup><br>(%)</th><th>τ<sub>p</sub><sup>f)</sup><br>(ns)</th><th>τ<sub>d</sub><sup>f)</sup><br>(μs)</th><th>k<sub>RISC</sub><sup>f)</sup><br>(10<sup>4</sup> s<sup>-1</sup>)</th><th>HOMO/<br>LUMO<sup>g)</sup><br>(eV)</th></tr></thead><tbody><tr><td>DiKTaSe</td><td>439</td><td>464 (34)</td><td>0.22</td><td>484 (60)</td><td>62</td><td>0.4</td><td>178</td><td>9.97</td><td>-5.98/-3.02</td></tr><tr><td>tBuCz-DiKTaSe</td><td>448</td><td>470 (35)</td><td>0.22</td><td>489 (50)</td><td>82</td><td>0.8</td><td>183</td><td>16.5</td><td>-5.57/-3.01</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Synthesis of DiKTaSe and tBuCz-DiKTaSe.</figcaption></figure>

of 16.7%/7.8% for DiKTaSe). The devices with these two emitters showed improved EQE<sub>max</sub> and reduced efficiency roll-off compared to the device with Mes<sub>3</sub>DiKTa (EQE<sub>max</sub>/EQE<sub>100</sub>/EQE<sub>1000</sub> of 21.1%/14.5%/4.5%), an emitter whose structure mitigates ACQ but does not contain heavy atoms.[9]

## Results and Discussion

The syntheses of DiKTaSe and tBuCz-DiKTaSe are outlined in Figure 1. The intermediates **1** and **3** were prepared via a palladium-catalyzed Buchwald-Hartwig amination in 63% and 52% yield, respectively. These intermediates were reacted with 2-bromoisophthalic acid dimethyl ester under Ullmann coupling conditions to afford **2** and **4** in 61% and 62% yield, respectively, which were then hydrolyzed to their diacids, activated using thionyl chloride and ultimately cyclized under Friedel-Crafts acylation conditions to afford racemic DiKTaSe and tBuCz-DiKTaSe in yields of 66% and 38%, respectively. The identity and purity of DiKTaSe and tBuCz-DiKTaSe were characterized by melting point determination, ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectrometry (HRMS), high-performance liquid chromatography (HPLC), elemental analysis (Figures S1-S22) and single crystal X-ray diffraction analysis (Figure 2, Table S1). The enantiomers of DiKTaSe

and tBuCz-DiKTaSe were separated using chiral HPLC to afford (P)-DiKTaSe (ee. of 100%), (M)-DiKTaSe (ee. of 99.8%), (P)-tBuCz-DiKTaSe (ee of 100%) and (M)-tBuCz-DiKTaSe (ee of 98.2%) (Figures S23, S24). The absolute configurations (P and M) were tentatively assigned by comparison of the measured and simulated CD spectra in toluene solution (Figure S25). Their thermal stability was assessed by thermogravimetric analysis (TGA) under a nitrogen atmosphere. Both DiKTaSe and tBuCz-DiKTaSe have high thermal decomposition temperatures (T<sub>d</sub>, corresponding to 5% weight loss) of 372 °C and 448 °C, respectively, which are sufficiently high to permit sublimation of the material without decomposition during the vacuum deposition process of OLED fabrication. No glass transition temperature was observed in the differential scanning calorimetry (DSC) measurement of either compound (Figure S26).

The optimized ground-state geometries and frontier molecular orbitals of DiKTaSe and tBuCz-DiKTaSe were first simulated using density functional theory (DFT) at the PBE0/6-31G(d,p) level (Figure 2).[38,39] The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of DiKTaSe are distributed over the entire polycyclic aromatic system. The HOMO and LUMO energies are calculated to be -6.11/-2.30 eV, with an energy gap (ΔE) of 3.81 eV. The slightly shallower HOMO, deeper LUMO and narrower ΔE compared to those of the parent

<footnote>a) In toluene. b) In toluene at room temperature, λ<sub>exc</sub> = 340 nm. c) In 2-MeTHF at 77 K, λ<sub>exc</sub> = 340 nm, ΔE<sub>ST</sub> = E(S<sub>1</sub>) - E(T<sub>1</sub>). d) In 2 wt% doped films in 2,6-DCzPPy at 300 K under vacuum, λ<sub>exc</sub> = 340 nm. e) In 2 wt% doped films in 2,6-DCzPPy measured using an integrating sphere, under N<sub>2</sub> at λ<sub>exc</sub> = 430 nm. f) In 2 wt% doped films in 2,6-DCzPPy at 300 K under vacuum, λ<sub>exc</sub> = 375 nm. g) In degassed DCM with 0.1 M [nBu<sub>4</sub>N]PF<sub>6</sub> as the supporting electrolyte and Fc/Fc<sup>+</sup> as the internal reference (0.46 V vs. SCE).[45]</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Calculated electron density distribution (isovalue: 0.02) of the HOMO, LUMO and energy levels in the gas phase at the PBE0/6-31G(d,p) level, difference density plots and energies of S<sub>1</sub> and T<sub>1</sub> calculated in the gas phase at SCS-(ADC)2/cc-pVDZ level (Blue indicates an area of decreased electron density while yellow indicates increased electronic density between the ground and excited states), and thermal ellipsoid plot of the single crystal structures, respectively, for a) **DiKTaSe** and b) **tBuCz-DiKTaSe** (ellipsoids are drawn at the 50% probability level and solvent molecules are omitted for clarity).</figcaption></figure>

compound **DiKTa** (HOMO of −6.20 eV, LUMO of −2.23 eV and Δ*E* of 3.97 eV)[16] are attributed to a combination of the introduction of the electron-donating selenium atom and the extension of π-conjugation within the framework. The LUMO distribution of **tBuCz-DiKTaSe** is similar to that of **DiKTaSe** while the HOMO is mainly localized on the tert-butyl carbazole moiety due to its stronger electron-donating ability, which results in shallower HOMO and similar LUMO energies (−5.56/−2.24 eV) compared to those of **DiKTaSe**.

The excited-state energies were firstly calculated using time-dependent DFT (TD-DFT) within the Tamm-Dancoff approximation (TDA) at the PBE0/6-31G(d,p) level (Figure S27). The S<sub>1</sub> and T<sub>1</sub> energies are calculated to be 3.21/2.61 and 2.78/2.53 eV for **DiKTaSe** and **tBuCz-DiKTaSe** and the corresponding Δ*E*<sub>ST</sub> values are 0.60 and 0.25 eV, respectively, which, not surprisingly, are overestimated given the level of theory employed.[40,41] Nonetheless, it is clear that the TADF efficiency of **tBuCz-DiKTaSe** will be higher than of **DiKTaSe**. The SOC matrix elements (SOCME) were then calculated based on the optimized T<sub>1</sub> geometry at the PBE0/def2-TZVP level (Figure S27). There are five low-lying triplet states below the S<sub>1</sub> of **DiKTaSe**, which may be implicated in facilitating RISC.[42-44] The SOCME for S<sub>1</sub>-T<sub>1</sub>, S<sub>1</sub>-T<sub>2</sub>, S<sub>1</sub>-T<sub>3</sub>, S<sub>1</sub>-T<sub>4</sub> and S<sub>1</sub>-T<sub>5</sub> transitions are 3.22, 13.3, 16.9, 10.8 and 3.97 cm<sup>-1</sup>, respectively, for **DiKTaSe**. By contrast, the S<sub>1</sub> state of **tBuCz-DiKTaSe** shows long-range charge transfer (LRCT) character. There are only two triplet states that lie below the S<sub>1</sub> state and the SOCME values are

2.31 and 10.2 cm⁻¹ for the S₁-T₁ and S₁-T₂ transitions. The larger SOCME values compared to those of **DiKTa** (0.68, 7.29, 9.86 and 7.70 cm⁻¹, respectively) can thus be directly attributed to the HAE linked to the presence of selenium. The energies and difference densities of the excited singlet and triplet states were also calculated using Spin-Component Scaling second-order algebraic diagrammatic construction (SCS-(ADC)2/cc-pVDZ), which we have previously shown to provide very accurate predictions of the state energies in MR-TADF compounds.[40,41] The difference density plots for the transitions to both S₁ and T₁ states of **DiKTaSe** and **tBuCz-DiKTaSe** show the typical alternating increasing and decreasing electron density patterns on the adjacent atoms of the MR-TADF core transitions, which indicate these states have SRCT character (Figure 2). The S₁ and T₁ energies are calculated to be 3.19/2.92 and 3.06/2.83 eV and the corresponding Δ*E*<sub>ST</sub> values are calculated to be 0.27 and 0.23 eV, respectively. These values are comparable to those of other **DiKTa**-based MR-TADF emitters[16,26,31] including **DiKTa** (Δ*E*<sub>ST</sub> = 0.26 eV),[26] which indicates that these two compounds are likely to be TADF. The more stabilized S₁ and T₁ energies compared to those of **DiKTa** (S₁ = 3.46 eV and T₁ = 3.20 eV)[26] indicate that their emission should be red-shifted compared to the parent.

The HOMO and LUMO energy levels of **DiKTaSe** and **tBuCz-DiKTaSe** were then inferred from cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements in degassed DCM, with 0.1 M [nBu₄N]PF₆ as the
<figure><img src="image_6.png" /><figcaption>**Figure 4.** a) Steady-state PL spectra at 300 K of 2 wt% doped film of DiKTaSe and tBuCz-DiKTaSe in 2,6-DCzPPy ($\lambda_{exc}$ = 340 nm). Variable temperature time-resolved PL decays of 2 wt% doped film of b) DiKTaSe and c) tBuCz-DiKTaSe in 2,6-DCzPPy ($\lambda_{exc}$ = 375 nm).</figcaption></figure>

the strong SOC originating from the heavy selenium, the ISC is enhanced and competes with direct fluorescence, making it impossible to determine the prompt fluorescence lifetime in solution for each of the two emitters. Moreover, RISC rates are also accelerated and the delayed lifetimes, $\tau_d$, are 1.9 and 1.6 µs for DiKTaSe and tBuCz-DiKTaSe, respectively, despite their large $\Delta E_{ST}$ values. This PL decay behavior was not observed in toluene solutions of DiKTa.[26] Due to competing non-radiative decay processes, the $\Phi_{PL}$ values of DiKTaSe and tBuCz-DiKTaSe in degassed toluene are only 1.0% and 1.5%, respectively.

The photophysical properties of the emitters in films were then investigated. The bipolar host material 2,6-DCzPPy was used because of its high triplet energy ($E_T$ = 2.71 eV) and capacity to balance charge transport within the layer in OLEDs.[46] The highest $\Phi_{PL}$ values of 69% and 88% for DiKTaSe and tBuCz-DiKTaSe were obtained in 1 wt% doped films in 2,6-DCzPPy. Increasing the doping concentration to 2, 5 and 10 wt% for the films of both DiKTaSe and tBuCz-DiKTaSe results in a modestly red-shifted emission (from 477 to 494 nm for DiKTaSe and from 481 to 496 nm for tBuCz-DiKTaSe) that becomes steadily broader (FWHMs from 58 to 76 nm for DiKTaSe and from 51 to 67 nm for tBuCz-DiKTaSe), which is the result of increasing contributions from aggregates (Figure S31). This also leads to a decrease in $\Phi_{PL}$ of DiKTaSe and tBuCz-DiKTaSe to 62%, 53% and 44% and 82%, 69% and 62%, respectively, for the 2, 5 and 10 wt% doped films (Table S3).

Steady-state PL spectra of 2 wt% doped films of DiKTaSe and tBuCz-DiKTaSe in 2,6-DCzPPy peak at $\lambda_{PL}$ of 484 and 489 nm and have FWHMs of 60 nm/0.30 eV and 50 nm/0.25 eV, respectively (Figure 4a, Table 1). The small bathochromic shifting of the PL spectra compared to that of DiKTa ($\lambda_{PL}$ of 469 nm) is due to the extension of the $\pi$-conjugation within the framework, which aligns well with the calculated $\Delta E$ values (Figure S32a). The narrower emission band of tBuCz-DiKTaSe is attributed to the introduction of the tert-butyl carbazole, which suppresses the aggregation of the emitters and also conformational motion. The 77 K SS PL and Ph spectra of the doped films indicate that the $S_1/T_1$ energies (Figure S33) are 2.71/2.50 and 2.67/2.48 eV. The $\Delta E_{ST}$ values for DiKTaSe and tBuCz-DiKTaSe are thus 0.21 and 0.19 eV, respectively, slightly smaller than those in toluene.

The time-resolved PL decays under vacuum (Figure 4b,c) reveal prompt fluorescence lifetimes, $\tau_p$, of 0.4 and 0.8 ns (Figure S33) and $\tau_d$ of 178 and 183 µs at 300 K, respectively. There is an increase in the delayed emission intensity with increasing temperature that confirms the TADF properties of DiKTaSe and tBuCz-DiKTaSe. The calculated photophysical kinetics parameters are summarized in Table S4. The rate constants for ISC ($k_{ISC}$) for DiKTaSe and tBuCz-DiKTaSe ($1.66 \times 10^9$ and $0.90 \times 10^9$ s$^{-1}$, respectively) are much faster than the radiative decay rate constant from $S_1$, $k_r^S$ ($5.31 \times 10^7$ and $2.26 \times 10^7$ s$^{-1}$, respectively), which explains the very weak prompt emission in the films. The $k_{RISC}$ for DiKTaSe and tBuCz-DiKTaSe are $9.97 \times 10^4$ and $16.5 \times 10^4$ s$^{-1}$, respectively. The larger proportion of delayed emission to the total $\Phi_{PL}$ (Table S4) compared to that of DiKTa (Figure S32b) is mainly responsible for the improved $k_{RISC}$ compared to those of DiKTa ($k_{RISC}$ of $1.25 \times 10^4$ s$^{-1}$) and S$S$eQ ($k_{RISC}$ of $4.54 \times 10^4$ s$^{-1}$),[6] and the similar $k_{RISC}$ compared to that of tBr-DiKTa ($k_{RISC}$ of $19.7 \times 10^4$ s$^{-1}$)[37] can all be attributed to the strong HAE from the selenium atom adjacent to the DiKTa core.

Time-resolved PL decays of the films of DiKTaSe and tBuCz-DiKTaSe at higher doping concentrations in 2,6-DCzPPy were also measured. As shown in Figure S34, biexponential delayed decays were observed, with $\tau_d$ of 94/349 and 79/414 µs for the 5 and 10 wt% doped films of DiKTaSe, respectively, and 130/500 and 77/338 µs for 5 and 10 wt% doped films of tBuCz-DiKTaSe, respectively. The longer lifetime component of the delayed emission may be due to the PL decay from aggregates (Figure S31). The $k_{RISC}$ values are calculated to be $18.5 \times 10^4$ and $13.1 \times 10^4$ s$^{-1}$ for DiKTaSe and $15.4 \times 10^4$ and $13.3 \times 10^4$ s$^{-1}$ for tBuCz-DiKTaSe for the 5 and 10 wt% doped films, respectively, which are similar to the $k_{RISC}$ values of $9.97 \times 10^4$ and $16.5 \times 10^4$ s$^{-1}$ for 2 wt% doped films of DiKTaSe and tBuCz-DiKTaSe.

To explore the stereochemical stability of DiKTaSe and tBuCz-DiKTaSe, the racemization energy barriers of the two helicenes were calculated using DFT at the M06-2X/def2-TZVP level of theory (Figure 5). The M06-2X functional was chosen here because it accurately predicts the SRCT character of the $S_1$ state of tBuCz-DiKTaSe instead of the LRCT character predicted by PBE0 (Figure S35). Generally, [n]helicenes ($n$ = 5–8) have transition states (TS) (with only
<figure><img src="image_7.png" /><figcaption>Figure 5. Calculated racemization energy barriers of DiKTaSe and tBuCz-DiKTaSe at the M06-2X/def2-TZVP level.</figcaption></figure>

one imaginary frequency) belonging to the C_s point group during racemization.[47] As DiKTaSe and tBuCz-DiKTaSe have asymmetric structures, the TS cannot strictly satisfy the C_s symmetry. The large size of Se and relatively long C-Se bonds contribute to DiKTaSe and tBuCz-DiKTaSe having relatively high racemization energy barriers of ΔG‡ = 35.4 and 33.2 kcal mol⁻¹. The rate constant k of a chemical reaction against temperature can be described using the Eyring equation (Equation 2) and the half-life t₁/₂ is defined according to Equation 3:

$$k = \frac{\kappa k_B T}{h} e^{-\frac{\Delta G^\ddagger}{RT}} = 2.1 \times 10^{10} T e^{-\frac{1000 \Delta G^\ddagger}{1.986 T}} \quad (2)$$

$$t_{1/2} = \frac{\ln 2}{k} \quad (3)$$

Where κ is the transmission coefficient (which is often assumed to be equal to one), k_B is the Boltzmann constant, T is the temperature and h is the Planck constant. A ΔG‡ of 30 kcal mol⁻¹ corresponds to a half-life of more than 35 years at room temperature (298.15 K). Therefore, the calculated high energy barrier to racemization in DiKTaSe and tBuCz-DiKTaSe enables the chiral separation of the enantiomers and indicates high stereochemical stability.[48–51]

The chiroptical properties of both enantiomers of DiK-TaSe and tBuCz-DiKTaSe were then investigated by CD in toluene. As shown in Figure 6a and c, the CD spectra of both emitters display mirror image Cotton effects. The gabs values are + 1.6 × 10⁻³ and -1.7 × 10⁻³ at 327 nm for (P)/(M)-DiKTaSe and -1.7 × 10⁻³ and + 1.8 × 10⁻³ at 373 nm for (P)/(M)-tBuCz-DiKTaSe. The gabs at the lowest-energy absorption bands (439 and 448 nm for DiKTaSe and tBuCz-DiKTaSe, respectively) are 9.5 × 10⁻⁴/-9.6 × 10⁻⁴ for (P)/(M)-DiKTaSe and 6.3 × 10⁻⁴/-6.7 × 10⁻⁴ for (P)/(M)-tBuCz-DiKTaSe (Figure 6b and d). Due to their weak ΦPL in toluene, we were unable to detect the CPL signals in solution (Figure S36). However, it was possible to obtain CPL activity in the case of the 2 wt% doped films of the enantiomers of tBuCz-DiKTaSe in PMMA, with an emission dissymmetry

factor |g<sub>PL</sub>| of around 2.3 × 10⁻³ (Figure S37). Unfortunately, we did not detect a mirror image CPL signal for DiKTaSe because of its weak emission and some anisotropy effects resulting from some aggregation in the solid-state sample.

## Organic Light-Emitting Diodes

Based on their attractive photophysical properties, we next fabricated vacuum-deposited OLEDs. The device stack and the chemical structures of the organic layers are shown in Figure 7a and b, respectively. The OLED structure consisted of: indium tin oxide (ITO)/1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN, 5 nm)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 45 nm)/mCP (5 nm)/2,6-DCzPPy as host (20 nm) with DiKTaSe and tBuCz-DiKTaSe as emitter dopants/ 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB, 45 nm)/lithium fluoride (LiF, 1 nm)/aluminum (Al, 100 nm). Here, HATCN was used as the hole injection layer, TAPC as the hole transport layer, mCP as an exciton blocking layer, TmPyPB as electron transport layer, and LiF to reduce the work function of the top Al electrode. The OLEDs were fabricated with 2 and 5 wt% of the emitter concentrations. Figure 7c shows the current density-voltage-luminance characteristics of the OLEDs where all devices presented a turn-on voltage of 3.8–4 V.

The devices with 2 and 5 wt% of DiKTaSe showed similar efficiency trends, with EQE<sub>max</sub> of 22.7% and 21.4%, respectively (Figure 7d and Table 2). The EQE<sub>100</sub> (at 100 cd m⁻²) for the 2 wt% OLEDs was 15.8%, which dropped to 6.5% at 1000 cd m⁻². Despite the fast k<sub>RISC</sub> of the emitter, significant efficiency roll-off remains. This is because k<sub>ISC</sub> (Table S4) is also large, and efficiency roll-off is determined by a combination of k<sub>RISC</sub>, k<sub>ISC</sub> and k<sub>r</sub><sup>S.[52]</sup> The electroluminescence maximum (λ<sub>EL</sub>) for the 2 wt% device is 480 nm with a FWHM of 63 nm (Figure 7e) and these properties are similar to those of the thin films (Table 1).
<figure><img src="image_8.png" /><figcaption>**Figure 6.** a) CD spectra and b) $g_{abs}$ values of DiKTaSe recorded in toluene solution. c) CD spectra and d) $g_{abs}$ values of tBuCz-DiKTaSe recorded in toluene solution.</figcaption></figure>

<table><caption>**Table 2:** Device performance for OLEDs with DiKTaSe and tBuCz-DiKTaSe.</caption><thead><tr><th></th><th>x<br>(wt%)</th><th>λ<sub>EL</sub><br>(nm)</th><th>FWHM<br>(nm)</th><th>Von<br>(V)</th><th>EQE<sub>max/100/1000</sub><br>(%)</th><th>CIE<br>/ x, y</th></tr></thead><tbody><tr><td rowspan="2">DiKTaSe</td><td>2</td><td>480</td><td>63</td><td>4.0</td><td>22.7/15.8/6.5</td><td>0.169, 0.373</td></tr><tr><td>5</td><td>489</td><td>70</td><td>3.9</td><td>21.4/16.7/7.8</td><td>0.201, 0.436</td></tr><tr><td>tBuCz-</td><td>2</td><td>480</td><td>55</td><td>3.9</td><td>23.8/19/10.2</td><td>0.163, 0.379</td></tr><tr><td>DiKTaSe</td><td>5</td><td>490</td><td>57</td><td>3.8</td><td>27.8/23.5/12.5</td><td>0.172, 0.461</td></tr></tbody></table>

The Commission Internationale de l'Éclairage chromaticity coordinates, CIE(x, y), for the 2 wt% OLED are (0.169, 0.373), indicating an emission in the sky-blue region of the spectrum (Figure 7f).

The tBuCz-DiKTaSe OLEDs showed a slightly higher EQE<sub>max</sub> of 23.8% using 2 wt% of the emitter, compared to OLEDs with DiKTaSe. The efficiency roll-off of the devices with tBuCz-DiKTaSe was less-pronounced with an EQE<sub>100</sub> of 19% and an EQE<sub>1000</sub> of 10.2% (Figure 7d and Table 2). This higher efficiency of the tBuCz-DiKTaSe OLEDs compared to the ones with DiKTaSe is attributed to the higher Φ<sub>PL</sub> (82% vs. 62%) of the former. The 2 wt% tBuCz-DiKTaSe OLEDs showed an λ<sub>EL</sub> at 480 nm with a narrower FWHM of 55 nm as compared to that of the device with DiKTaSe; and the CIE coordinates for the devices with tBuCz-DiKTaS are (0.163, 0.379). Higher efficiencies at all luminances were observed with an increase in concentration of tBuCz-DiKTaSe in the EML of the OLED to 5 wt%, with EQE<sub>max</sub>/EQE<sub>100</sub>/EQE<sub>1000</sub>

of 27.8%, 23.5% and 12.5%. The devices emitted at λ<sub>EL</sub> of 490 nm (FWHM of 57 nm), with corresponding CIE coordinates of (0.172, 0.461). Figure 8a and b evidence that the OLEDs with 5 wt% of tBuCz-DiKTaSe showed an outstanding performance that is amongst the highest EQE<sub>max</sub> and EQE<sub>100</sub> compared to reported DiKTa-based MR-TADF OLEDs. This improved performance is attributed to a combination of suppressed aggregation due to the presence of the ortho-substituted tert-butylcarbazole and improved k<sub>RISC</sub> arising from the HAE. The increase in efficiency for the 5 wt% OLEDs occurs in spite of the lower Φ<sub>PL</sub> of the tBuCz-DiKTaSe emitter for 5 wt% doped film compared to 2 wt% doped film (69% vs. 82%, respectively. This suggests improved charge balance could arise from charge injection directly onto the emitter, and possibly charge transport between emitter molecules, which is supported by the higher current density of the 5 wt% devices (Figure 7c). The barrier to hole injection into the tBuCz-DiKTaSe emitter (HOMO = -5.57 eV) is much smaller than injection into the host (HOMO = -6.05 eV). OLEDs with higher concentrations (7.5, 10 and 15 wt%) were also fabricated (Figure S38), which all showed slightly lower EQE values and broader EL compared to the 5 wt% doped device. A preliminary assessment of the operational stability of the OLEDs with Se-based emitters (DiKTaSe and tBuCz-DiKTaSe) was compared with DiKTa devices at 100 cd m<sup>-2</sup> as shown in Figure S39. The results show that although lifetimes are short, the addition of Se into the emitter design not only increases the efficiency but also improves the operational stability of the devices.
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 7.** a) Schematic of the OLED stack; b) Chemical structures of the materials used in the device; c) Current density-voltage-luminance (J/VL) characteristics; d) EQE versus luminance; e) Electroluminescence spectra, inset figures are photographs of the OLEDs fabricated with 5 wt% of DiKTaSe and tBuCz DiKTaSe; f) CIE coordinates of the devices.</figcaption></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 8.** a) EQE<sub>max</sub> vs λ<sub>EL</sub> comparison and b) EQE<sub>100</sub> vs λ<sub>EL</sub> comparison for reported MR-TADF DiKTa-based OLEDs versus devices with DiKTaSe and tBuCz-DiKTaSe (detailed chemical structures and device data are provided in Figure S40 and Table S5, respectively).</figcaption></figure>
## Conclusions

Two new selenium-containing MR-TADF emitters **DiKTaSe** and **tBuCz-DiKTaSe** have been designed and synthesized. The strong HAE provided by the selenium accelerates RISC with $k_{RISC}$ reaching $9.97 \times 10^4$ and $16.5 \times 10^4$ s⁻¹ in 2 wt% doped film in 2,6-DCzPPy, respectively. The presence of the twisted *ortho*-substituted *tert*-butylcarbazole suppresses emitter aggregation in films of **tBuCz-DiKTaSe**, leading to higher $\Phi_{PL}$ value of 82% and narrower emission in 2 wt% doped film in 2,6-DCzPPy compared to those of **DiKTaSe**. The OLEDs with **DiKTaSe** showed $EQE_{max}$ of 22.7% at $\lambda_{EL}$ of 480 nm (FWHM of 63 nm), while the devices with **tBuCz-DiKTaSe** showed higher $EQE_{max}$ of 27.8% at $\lambda_{EL}$ of 490 nm and a slightly narrower FWHM of 57 nm. In addition to high $EQE_{max}$, the efficiency roll-off of the devices with **tBuCz-DiKTaSe** was less-pronounced as compared to the devices with both **DiKTaSe** and other reported **DiKTa**-based OLEDs, showing an impressive $EQE_{100}/EQE_{1000}$ of 23.5%/12.5%. The higher efficiencies for the OLEDs with **tBuCz-DiKTaSe** attest to the superior emitter design.

## Supporting Information

¹H and ¹³C NMR spectra, HRMS, HPLC and EA of all target compounds; supplementary computational data, photophysical data and devices data; the structures and properties of all the chiral MR-TADF emitters.

## Author Contributions

J.W. designed the emitters, performed part of the synthesis, carried out the theoretical and optoelectronics characterization. H.H. fabricated the OLEDs. D.C. performed part of the synthesis. J.S.O.O. measured the chiroptical properties. Y.X. performed some of the separation of enantiomers. A.P.M. and D.B.C carried out the single crystal characterization. J.C. supervised J.S.O.O., I.D.W.S. supervised H.H., and E.Z.-C. supervised J.W., D.C. and Y.X. All authors contributed to the writing and editing of the manuscript.

## Acknowledgements

Jingxiang Wang thanks the China Scholarship Council (202006250026). This work was funded by the EPSRC through grant numbers EP/R00188X/1, EP/W007517/1, EP/W015137/1 and EP/Z535291/1. This project has been partly funded by the European Union Horizon 2021 research and innovation program under grant agreement no. 101073045 (TADF solutions) and the EPSRC grant EP/X026175/1.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The research data supporting this publication can be accessed at https://doi.org/10.17630/4d3f056f-bf62-4b92-bb0f-629db8997a67.

**Keywords:** Aggregation-caused quenching • Heavy atom effect • Helicene • Multi-resonant thermally activated delayed fluorescence • Organic light-emitting diode

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**Research Article**

**Organic Materials**

J. Wang, H. Hafeez, D. Chen, J. S. O. Ortiz, Y. Xu, A. P. McKay, D. B. Cordes, J. Crassous*, I. D. W. Samuel*, E. Zysman-Colman* e202506999

Chiral Selenium-Integrated Multi-
Resonant Thermally Activated Delayed
Fluorescent Emitters Showing Improved
Reverse Intersystem Crossing Rate

The selenium-based tBuCz-DiKTaSe
emitter containing a twisted ortho-
disposed *tert*-butylcarbazole shows high
Φ<sub>PL</sub>, suppressed ACQ, fast k<sub>RISC</sub> and is
chiral. The OLEDs show high EQE<sub>max</sub> and
alleviated efficiency roll-off compared to
devices with carbonyl-based MR-TADF
emitters.

<figure><img src="image_17.png" /></figure>