## [2.2] Paracyclophane-Substituted Chiral Multiresonant Thermally Activated Delayed Fluorescence Emitters for Efficient Organic Light-Emitting Diodes

Yan Xu, Hassan Hafeez, Jasmin Seibert, Sen Wu, Jhon Sebastian Oviedo Ortiz, Jeanne Crassous, Stefan Bräse, Ifor Samuel, Eli Zysman-Colman

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### ► To cite this version:

Yan Xu, Hassan Hafeez, Jasmin Seibert, Sen Wu, Jhon Sebastian Oviedo Ortiz, et al. [2.2] Paracyclophane-Substituted Chiral Multiresonant Thermally Activated Delayed Fluorescence Emitters for Efficient Organic Light-Emitting Diodes. Advanced Functional Materials, 2024, 10.1002/adfm.202402036. hal-04529263

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Submitted on 18 Nov 2024

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Distributed under a Creative Commons Attribution 4.0 International License
[2.2]Paracyclophane-Substituted Chiral Multiresonant Thermally
Activated Delayed Fluorescence Emitters for Efficient Organic
Light-Emitting Diodes

Yan Xu,<sup>a‡</sup> Hassan Hafeez, <sup>b‡</sup> Jasmin Seibert, <sup>c</sup> Sen Wu, <sup>a</sup> Jhon Sebastian Oviedo Ortiz, <sup>d</sup> Jeanne

Crassous, <sup>d</sup> Stefan Bräse, <sup>c,e</sup> Ifor D. W. Samuel, <sup>*b</sup> Eli Zysman-Colman <sup>*a</sup>

<sup>a</sup>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](mailto:eli.zysman-colman@st-andrews.ac.uk)

<sup>b</sup>Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St
Andrews, St Andrews, Fife, KY16 9SS, UK. E-mail: [idws@st-andrews.ac.uk](mailto:idws@st-andrews.ac.uk)

<sup>c</sup>Institute of Organic Chemistry (IOC), Karlsruhe Institute of Technology (KIT), Kaiserstrasse
12, 76131 Karlsruhe, Germany. E-mail: [braese@kit.edu](mailto:braese@kit.edu)

<sup>d</sup>University of Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR
6226, F-35000 Rennes, France. E-mail: [jeanne.crassous@univ-rennes.fr](mailto:jeanne.crassous@univ-rennes.fr)

<sup>e</sup>Institute of Biological and Chemical Systems (IBCS-FMS), Karlsruhe Institute of Technology
(KIT), Kaiserstrasse 12, 76131 Karlsruhe, Germany.

<sup>‡</sup>Yan Xu and Hassan Hafeez contributed equally to this work.

**Keywords:** [2.2]Paracyclophane, multiresonant thermally activated delayed fluorescence, circularly polarized luminescence, organic light-emitting diodes
**Abstract**

We report two pairs of chiral multi-resonant thermally activated delayed fluorescence (MR-TADF) materials **PCP-DiKTa** and **Czp-DiKTa** by decorating a known MR-TADF core, **DiKTa**, with different [2.2]paracyclophane (PCP) based planar chiral groups. **PCP-DiKTa** shows narrow sky-blue emission with a full width at half maximum (FWHM) of 44 nm, while the emission of **Czp-DiKTa** is slightly broader with a FWHM of 66 nm and red-shifted. Both emitters show high photoluminescence quantum yields of 93 and 99% for **PCP-DiKTa** and **Czp-DiKTa**, respectively. Enantiomerically pure samples of both compounds show chiroptical properties in the ground state while only **Czp-DiKTa** exhibited chiroptical activity in the excited state, with dissymmetry factors (|g<sub>PL</sub>|) of 4×10<sup>-4</sup>. Organic light-emitting diodes (OLEDs) with **PCP-DiKTa** and **Czp-DiKTa** showed maximum external quantum efficiencies (EQE<sub>max</sub>) of 25.7 and 29.2%, with λ<sub>EL</sub> of 489 and 518 nm, and FWHMs of 53 and 69 nm, respectively. These EQE<sub>max</sub> values are higher than those of other reported devices employing PCP-based D-A type emitters. Our work demonstrates that the PCP moiety is not only a powerful building block to develop planar chiral emitters but one that is compatible with the fabrication of high efficiency devices.

**Introduction**

Organic light-emitting diode (OLED) research has witnessed remarkable growth and dynamism
since Tang and Van Slyke first reported an operational OLED at low turn-on voltage back in
1987.[1] OLED displays are now widely found in smartphones, smartwatches, TVs, and
automotive screens, offering a panoply of advantages over other display technologies like
displaying pure black, having a slim profile, being more energy efficient, and being able to be
fabricated on many different substrates including those that are flexible and transparent.[2]
Current OLED displays contain anti-glare filters for high-contrast viewing, resulting in an 50%
efficiency loss after passing through a linear polarizer and a quarter-wave plate. One solution
to address this issue is to migrate to circularly polarized OLEDs (CP-OLEDs), implying the use
of chiral emitters that emit circularly polarized luminescence (CPL).[3] Generally, the extent of
CPL in a chiral emitter is quantified by the luminescence dissymmetry factor, g<sub>PL</sub>, where g<sub>PL</sub> = 2 (I<sub>L</sub> - I<sub>R</sub>) / (I<sub>L</sub> + I<sub>R</sub>) = 4[|μ<sub>e</sub>| · |μ<sub>m</sub>| · cosθ] / (|μ<sub>e</sub>|<sup>2</sup> + |μ<sub>m</sub>|<sup>2</sup>), where I<sub>L</sub> and I<sub>R</sub> are the left-/right-
handed emission intensities, μ<sub>e</sub> and μ<sub>m</sub> are the electric and magnetic transition dipole moment
vectors, respectively, and θ is the angle between the two. For small organic molecules the μ<sub>m</sub> is
typically much smaller than the μ<sub>e</sub>, so the relationship between g<sub>PL</sub> and the transition dipole
moments can be simplified as g<sub>PL</sub> = 4 · μ<sub>m</sub> · cosθ / μ<sub>e</sub>.<sup>[4]</sup> This equation implies that the maximum
g<sub>PL</sub> is ±2, and that materials with magnetic dipole-allowed and electric dipole-forbidden
transitions would be beneficial to maximize the g<sub>PL</sub>. In optimized CP-OLEDs, the CPL
materials must also be able to harvest efficiently both singlet and triplet excitons, which means
that they should emit either by phosphorescence or thermally activated delayed fluorescence
(TADF). Thus, there has recently been an intense interest in the development of CP-TADF
emitters.<sup>[5]</sup>

There are two main strategies employed for the design of CP-TADF molecules. One involves
designing molecules with an intrinsically chiral TADF skeleton (utilizing point, axial or planar
chirality), while the other entails attaching chiral units to achiral TADF moieties. However, the
emission is broad in donor-acceptor CP-TADF compounds as there is large structural relaxation
in the excited state. The color purity is insufficient for high-definition display applications
hence, narrowband emitters are required. Recently, Hatakeyama and co-workers introduced
multiresonant TADF (MR-TADF) compounds that are rigid p- and n-doped polycyclic aromatic
hydrocarbon compounds that show narrowband emission.[6] The electron difference densities
of the S<sub>1</sub> and T<sub>1</sub> excited states compared to the ground state in these compounds shows a pattern
of alternating increasing and decreasing electron density that is responsible for the characteristic
short-range charge transfer excited state and the suitably small singlet-triplet energy gap, ΔE<sub>ST</sub>,
to enable TADF. Consequently, what is required within this materials space are high-
performance CP-MR-TADF emitters. However, examples of CP-MR-TADF emitters remains
limited (Figure S1), especially compared to the thousands of reported TADF emitters reported
to date. Devices based on these emitters using a single host often suffer from serious efficiency
roll-off owing to the too long exciton lifetimes. One approach to optimize the device
performance is to enhance the efficiency of the reverse intersystem crossing (RISC) process by
having low-lying excited states of mixed short-range charge-transfer (SRCT) and long-range charge transfer (LRCT) character in the CP-MR-TADF emitters. Our prior work demonstrated that decorating an MR-TADF core with various electron-donating groups can modulate the interplay between and the relative energies of the SRCT and LRCT states and their impact on the emission profile.[7] Therefore, the choice of decoration of MR-TADF cores with chiral groups to achieve CPL must also be mindful to retain the desired narrowband emission and to aid in enhancing the RISC kinetics.

[2.2] Paracyclophane (PCP) and its derivatives have emerged as a useful planar chiral skeleton, including in the construction of CP-TADF emitters.[8] Most of the examples of PCP-containing CP-TADF emitters are based on a D-A design. For example, our group integrated a PCP with a nitrogen-heterocycle to produce the bulky carbazolophane (Czp) donor and employed it within the CP-TADF emitter **CzpPhTRZ** (Figure 1).[9] The increased steric bulk of the Czp unit compared to carbazole (Cz) induced an increased torsion angle between the donor and the phenylene bridge, resulting in a relatively smaller Δ*E*<sub>ST</sub> of 0.16 eV. (**R**)-**CzpPhTrz** emits at λ<sub>PL</sub> of 470 nm and has a g<sub>PL</sub> value of 1.2 × 10<sup>-3</sup> in toluene, while *rac*-**CzpPhTrz** has a Φ<sub>PL</sub> of 69% in 10 wt% doped films in DPEPO, which translated into OLEDs that achieved an EQE of 17% at an λ<sub>EL</sub> at 480 nm; no OLEDs were fabricated using enantiopure samples of the emitter and thus no g<sub>EL</sub> was measured. Zheng, Ye, Liao *et al*. subsequently reported a structurally related CP-TADF molecule, **PXZp-Ph-TRZ**, using the same chiral paracyclophane-extended donor strategy as with **CzpPhTrz** but replacing Cz with a stronger phenoxazine (PXZ) donor.[10] The yellow emitter (λ<sub>PL</sub> = 527 nm) has a much smaller Δ*E*<sub>ST</sub> of 0.03 eV than **CzpPhTrz** and a Φ<sub>PL</sub> of 60% in 10 wt% doped films in CBP. The solution-processed CP-TADF OLEDs showed an EQE<sub>max</sub> of 7.8%, and using enantiopure emitters the CP-OLEDs displayed a g<sub>EL</sub> of 4.6×10<sup>-3</sup>. With a similar skeleton, Chen, Li, Zhang *et al*. reported a pair of D-chiral π-A type TADF emitter (**R/S**)-**PXZ-PT**, with the PCP acting as bridging unit.[11] (**R/S**)-**PXZ-PT** emits at λ<sub>PL</sub> of 565 nm in toluene, has a moderate Δ*E*<sub>ST</sub> of 0.19 eV in 2Me-THF, and a high Φ<sub>PL</sub> of 78% in 10 wt% doped films in CBP; additionally, the g<sub>PL</sub> was measured to be ±1.9×10<sup>-3</sup> in toluene. The vacuum-deposited CP-OLEDs with (**R**)-**PXZ-PT** exhibited a yellow emission [⟨λ<sub>EL</sub> of 557 nm, CIE of (0.44, 0.55)⟩ with an EQE<sub>max</sub> of 20.1%. In these examples, the emission is broad owing
to radiative decay from a LRCT excited state. Recently, Zheng, Liao, Pu *et al.* attached Czp to two different known MR-TADF cores and reported two CP-MR-TADF emitters.[12] **Czp-tBuCzB**, **Czp-POAB** emit at $\lambda_{PL}$ of 478 and 498 nm and show narrowband emissions with FWHM of 23 and 36 nm, respectively, in toluene. **Czp-tBuCzB** and **Czp-POAB** have small $\Delta E_{ST}$ of 0.09 and 0.13 eV in toluene, and high $\Phi_{PL}$ of 98 and 96% in 5 and 8% doped films in 26DCzPPy, respectively. The sky-blue CP-MR-TADF OLEDs showed $EQE_{max}$ of 32.1 and 28.7%, respectively. For the OLED device with **Czp-tBuCzB**, the efficiency roll-off was small of 3.7% at 1000 cd m⁻²; however, the device with **Czp-POAB** showed a much more severe efficiency roll-off of 28.9% at 1000 cd m⁻², which the authors rationalized as due to the planar structure and longer $\tau_d$ than that of **Czp-tBuCzB**.

Here, we report two pairs of chiral MR-TADF enantiomers, **PCP-DiKTa** and **Czp-DiKTa** by decorating a known MR-TADF core, **DiKTa**, with different [2.2]paracyclophane (PCP)-based planar chiral groups. Owing to the different donor strength of the two chiral groups, the energy of the LRCT state from these donors to the **DiKTa** acceptor will change and this will impact the photophysical and chiroptical properties of the emitters. **PCP-DiKTa** emits at $\lambda_{PL}$ of 477 nm and has a narrow profile (FWHM of 44 nm), while the emission of **Czp-DiKTa** is red-shifted at $\lambda_{PL}$ of 501 nm and is broader (FWHM of 66 nm). This implies that Czp is a stronger donor than PCP. Enantiopure **Czp-DiKTa** emitters displayed CPL activity with $g_{PL}$ of $\pm 4 \times 10^{-4}$ while no CPL was detected for **PCP-DiKTa** enantiomers. Both compounds have high $\Phi_{PL}$ of 93 and 99% as 2 wt% doped films in 26DCzPPy. The shorter delayed lifetimes, $\tau_d$, of **PCP-DiKTa** and **Czp-DiKTa** than that of the parent **DiKTa** ($\tau_d$ = 242 µs) is associated with faster reverse intersystem crossing (RISC) ($k_{RISC}$ of $3.01 \times 10^4$ s⁻¹ and $4.41 \times 10^4$ s⁻¹, respectively).[13] The OLEDs with **PCP-DiKTa** and **Czp-DiKTa** showed $EQE_{max}$ of 25.7 and 29.2%. The OLEDs with **Czp-DiKTa** showed a less pronounced efficiency roll-off with EQEs at 100/1000 cd/m² of 25.9/17.1%. This work demonstrates the impact that the electronics of the chiral group has on the photophysics of the emitter and how this ultimately affects the device performance.
<figure><img src="image_3.png" /><figcaption>Figure 1. Chemical structures, photophysical and OLED data of reported PCP-based CP-TADF materials, **PCP-DiKTa** and **Czp-DiKTa** in this work.</figcaption></figure>

## Results and discussion

### Molecular design and synthesis

The syntheses of **Czp-DiKTa** and **PCP-DiKTa** are outlined in Scheme S1. **Czp-DiKTa** was obtained in 66% yield as yellow solid following a palladium-catalyzed Buchwald-Hartwig C-N coupling between **Czp** and **DiKTa-Br**, and **PCP-DiKTa** was synthesized in 48% yield as a yellow solid following a sequence of Miyaura borylation of PCP-Br and Suzuki–Miyaura cross-coupling with **DiKTa-Bpin**. The molecular structures and purity were validated using a combination of ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectrometry (HRMS), melting point determination, elemental analysis (EA), and high-performance liquid chromatography (HPLC) (Figure S2-S14). The thermal stability of the two compounds was investigated by thermogravimetric analysis (TGA) and different scanning calorimetry (DSC) (Figure S15). Both compounds are thermally stable, with a 5% mass loss (*T*<sub>d</sub>) occurring at 390 °C for **PCP-DiKTa** and 362 °C for **Czp-DiKTa**.

### Theoretical calculations
The ground-state properties of **Czp-DiKTa** and **PCP-DiKTa** were calculated using density
functional theory (DFT) at the PBE0/6-31G(d,p) level in the gas phase starting from a geometry
generated using Chem3D,[¹⁴] while the excited-state calculations were performed using the spin
component scaling second-order approximate coupled-cluster (SCS-CC2) method in tandem
with the cc-pVDZ basis set, which we have previously shown to be a sufficient level of theory
to accurately predict Δ*E*<sub>ST</sub> in MR-TADF emitters.[¹⁵] In the ground state, the dihedral angle
between the stereogenic donor and the **DiKTa** group are 47.6° for **PCP-DiKTa** and 57.5° for
**Czp-DiKTa**, which is similar with the dihedral angle of 58° found in the optimized structure of
**CzpPhTRZ**.[⁹] For **PCP-DiKTa**, the HOMO is distributed across the entire molecule while the
LUMO is mainly localized on the **DiKTa** fragment (Figure S16). In contrast, the HOMO of
**Czp-DiKTa** is mainly located on Czp donor, with a small degree of electron density on the
adjacent arene of the **DiKTa**, while the LUMO is localized on the **DiKTa** fragment. The
difference in the HOMO distributions reflects the electron-donating strength of the chiral donor
groups. Compared to **DiKTa** (HOMO/LUMO = -6.20/-2.23 eV),[⁷a] **PCP-DiKTa** (-5.94/-2.24
eV) has a similar LUMO level and a shallower HOMO level while **Czp-DiKTa** (-5.57/-2.36
eV) has a much shallower HOMO level and a deeper LUMO, leading to a much smaller
HOMO-LUMO gap. The calculated S₁/T₁ energies are 3.38/3.11 eV for **PCP-DiKTa** and
3.30/3.06 eV for **Czp-DiKTa**, with corresponding Δ*E*<sub>ST</sub> values of 0.27 and 0.24 eV, respectively.
These values are similar to the calculated S₁/T₁ energies (3.46/3.20 eV) and Δ*E*<sub>ST</sub> value (0.26
eV) of **DiKTa**.[¹³] The oscillator strengths for the S₀-S₁ transitions for **PCP-DiKTa** (0.24) and
**Czp-DiKTa** (0.22) are larger than that of **DiKTa** (0.20), implying that these two emitters will
be brighter. The difference density plots for the S₁, S₂, T₁ and T₂ excited states are shown in
Figure 2. The first singlet and triplet excited states show a pattern of alternating increasing and
decreasing electron density that is typical of excited states of short-range charge transfer
(SRCT) character; there is a small contribution to the S₁ difference density plot originating from
the chiral donors. The S₂ state for both molecules has n-π* character, while the electron density
of the T₂ is located mainly on the donor moieties. The different orbitals involved in the S₂/T₂
states compared to the S₁/T₁ states contribute to the spin-orbit coupling. The SOC matrix
elements (SOCME) at the optimized T₁ geometry between S₁ and T₂ are 4.15 and 0.45 cm⁻¹ for
**PCP-DiKTa** and **Czp-DiKTa**, respectively, which are significantly larger than that between S₁
and T₁ (0.27 and 0.26 cm⁻¹, respectively, Figure S16).

<figure><img src="image_4.png" /></figure>

Figure 2. Difference density plots and energies for the S₁, S₂, T₁ and T₂ of **PCP-DiKTa** and **Czp-DiKTa** calculated at SCS-CC2/cc-pVDZ in the gas phase (ISO value = 0.02). The blue color represents an area of decreasing electron density, and the yellow color represents an increased electron density between the ground and excited states.

## Optoelectronic properties

The electrochemical properties of the two emitters were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in deaerated DCM with 0.1 M tetra-*n*-butylammonium hexafluorophosphate as the supporting electrolyte (Figure S17, Table S1). Both compounds show irreversible oxidation waves, which are assigned to the oxidation of either whole skeleton for **PCP-DiKTa** or the **Czp** for **Czp-DiKTa** according to an analysis of the HOMO distribution from DFT calculation. Both compounds show reversible reduction waves, which correspond to the reduction of the **DiKTa** core according to DFT calculations.[16] The *E*<sub>ox</sub>/*E*<sub>red</sub> values were determined from the peaks of the DPVs. Compared to **DiKTa** (*E*<sub>ox</sub>/*E*<sub>red</sub> = 1.78/-1.34 V), both **PCP-DiKTa** (1.58/-1.29 V) and **Czp-DiKTa** (1.14/-1.27 V) have similar, albeit slightly anodically shifted reduction potentials while their oxidation potentials are cathodically shifted. The corresponding HOMO-LUMO gaps are 2.87 and 2.41 eV, respectively, and the trends in the electrochemical data largely mirror those in the DFT study.
The UV-vis absorption and steady-state photoluminescence (PL) spectra recorded at room temperature in dilute toluene are shown in Figure 3a. There is a similar band at 445 nm in the absorption spectra, to the SRCT transitions centered on the **DiKTa** core. Compared to **DiKTa** ($\lambda_{abs}$ = 433 nm),[13] the SRCT band of **PCP-DiKTa** and **Czp-DiKTa** is slightly red-shifted and broader, indicating that this transition has an admixture of LRCT and SRCT character. The lower molar extinction coefficient ($\varepsilon$) of $7.56 \times 10^3$ M$^{-1}$ cm$^{-1}$ for this band in **Czp-DiKTa** is due to the more twisted geometry between the **Czp** donor and **DiKTa** compared to **PCP-DiKTa** ($\varepsilon$ of $28.43 \times 10^3$ M$^{-1}$ cm$^{-1}$), which itself is similar in magnitude to the SRCT band in **DiKTa** ($\varepsilon$ of $22.06 \times 10^3$ M$^{-1}$ cm$^{-1}$).[17] This is analogous behavior to that observed in **Cz-DiKTa** and **DMAC-DiKTa**, both of which display a similarly twisted structure to **Czp-DiKTa** ($\varepsilon$ of $16.92 \times 10^3$ and $12.50 \times 10^3$ M$^{-1}$ cm$^{-1}$, respectively),[7a] while the CT band in **CzpPhTrz** has a lower $\varepsilon$ of $4.65 \times 10^3$ M$^{-1}$ cm$^{-1}$ as the **Czp** group is more poorly electronically coupled to the **TRZ** acceptor.

Both compounds have unstructured PL spectra in toluene with $\lambda_{PL}$ of 469 nm for **PCP-DiKTa** and 505 nm for **Czp-DiKTa**. Compared to the PL spectra of **DiKTa**, **Cz-DiKTa**, **DMAC-DiKTa**, and **CzpPhTrz** (FWHMs of 27, 54, 94, and 80 nm, respectively),[7a] that of **PCP-DiKTa** (FWHM of 37 nm) is slightly broader than that of **DiKTa**, while that of **Czp-DiKTa** (FWHM of 66 nm) is slightly broader than that **Cz-DiKTa**, and narrower than that **DMAC-DiKTa** and **CzpPhTrz**. The PL spectra of **PCP-DiKTa** show a small degree of positive solvatochromism (Figure S18), which is consistent with an excited state of SRCT character that is emblematic of MR-TADF emitters. By contrast, there is a much stronger positive solvatochromism in the PL spectra of **Czp-DiKTa**, reflecting an excited state with significant LRCT character. Indeed, in tetrahydrofuran and ethyl acetate, **Czp-DiKTa** shows dual emission emanating from the LRCT and SRCT states. The $S_1$ and $T_1$ energies of both compounds were determined from the onsets of the steady-state PL and phosphorescence spectra at 77 K in toluene (Figure 3c-3d). These are 2.67/2.51 eV for **PCP-DiKTa** and 2.57/2.44 eV for **Czp-DiKTa**, resulting in $\Delta E_{ST}$ of 0.16 and 0.13 eV for **PCP-DiKTa** and **Czp-DiKTa**, respectively. These values are smaller than those of **DiKTa** (0.22 eV), **Cz-DiKTa** (0.20 eV) and **DMAC-DiKTa** (0.21 eV). The $\Phi_{PL}$ of **PCP-DiKTa** and **Czp-DiKTa** in degassed toluene solutions are
36 and 37%, respectively, which are higher than that of DiKTa in toluene (26%); these decrease to 32 and 30% upon exposure to oxygen. The time-resolved emission decays do not show any delayed fluorescence, with lifetimes, τ<sub>PL</sub>, of 6.2 and 13.9 ns (Figure S19). In contrast, DiKTa shows multiexponential kinetics containing both prompt and delayed PL lifetimes in toluene, with τ<sub>p</sub> and τ<sub>d</sub> of 5.1 ns and 23 µs, respectively.[17]

<figure><img src="image_5.png" /><figcaption>Figure 3. (a) UV-vis absorption and steady-state PL spectra recorded in aerated toluene at 298 K ($\lambda_{exc}$ = 340 nm). (b) Steady-state PL spectra ($\lambda_{exc}$ = 343 nm) in 2 wt% doped films. (c) Steady-state PL spectra of PCP-DiKTa at 77 K and room temperature ($\lambda_{exc}$ = 353 nm), and phosphorescence spectra at 77 K (1-10 ms, $\lambda_{exc}$ = 353 nm) in toluene. (d) Steady-state PL spectra of Czp-DiKTa at 77 K and room temperature ($\lambda_{exc}$ = 365 nm), and phosphorescence spectra at 77 K (1-10 ms, $\lambda_{exc}$ = 365 nm) in toluene; concentration: 1 × 10<sup>-5</sup> M.</figcaption></figure>

We then explored the photophysical properties of the two emitters in an OLED-relevant host,
2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy), which is one that is bipolar and thus can mediate both the transport of holes and electrons within the emissive layer of the device;[18] additionally, it has a suitably high triplet energy of 2.71 eV to confine the excitons onto the emitter.[19] We studied the photophysical properties of these films as a function of emitter doping concentrations from 2 to 20 wt% (Figure S20). Both compounds displayed only modest red-shifting of their PL in this doping range, which implies little aggregation. Both emitters showed the highest Φ<sub>PL</sub> at 2 wt% doped films at 93% for **PCP-DiKTa** and 99% for **Czp-DiKTa** under N<sub>2</sub>, values which then reduced to 66 and 73% in air, respectively (Tables S2-S3). Therefore, we used 2 wt% emitter doped 26DCzPPy films for the subsequent photophysical measurements. The steady-state emissions of **PCP-DiKTa** and **Czp-DiKTa** peaks at λ<sub>PL</sub> at 477 and 501 nm, with FWHMs of 44 and 66 nm, respectively (Figure 3b, Table 1). Multiexponential decay kinetics under vacuum were observed, with prompt fluorescence lifetimes, τ<sub>P</sub>, for **PCP-DiKTa** and **Czp-DiKTa** of 6.4 and 11.7 ns, respectively, and delayed fluorescence lifetimes, τ<sub>DF</sub>, of 179 and 140 µs, respectively (Figure 4). The k<sub>RISC</sub> of **PCP-DiKTa** and **Czp-DiKTa** are 3.01×10<sup>4</sup> and 4.41×10<sup>4</sup> s<sup>-1</sup>, respectively, which are faster than that of **DiKTa** (2.52×10<sup>4</sup> s<sup>-1</sup>).[13] Full kinetics parameters are summarized in Table S4. Temperature-dependent time-resolved PL decay behavior revealed the expected increase in the contribution from the delayed emission with increasing temperature that defines TADF (Figure 4). The ΔE<sub>ST</sub> values estimated from the difference in energy between the onsets of the steady-state PL and phosphorescence spectra at 77 K are 0.19 eV for **PCP-DiKTa** and 0.17 eV for **Czp-DiKTa**, which are slightly larger than those in toluene (Figure S21).
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

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

<figure><img src="image_9.png" /><figcaption>Figure 4. Time-resolved PL decays ($\lambda_{exc}$ = 343 nm) of (a) **PCP-DiKTa** and (b) **Czp-DiKTa** in 2 wt% doped film in 26DCzPPy (inset figures are the time-resolved PL decays of the prompt component, $\lambda_{exc}$ = 375 nm). Temperature-dependent time-resolved PL decay of (c) **PCP-DiKTa** and (d) **Czp-DiKTa** in 2 wt% doped film in 26DCzPPy, $\lambda_{exc}$ = 343 nm.</figcaption></figure>

<table><caption>Table 1. Photophysical properties of **PCP-DiKTa** and **Czp-DiKTa**.</caption><thead><tr><th></th><th>λ<sub>abs</sub><sup>a</sup></th><th>λ<sub>PL</sub><sup>a</sup></th><th>λ<sub>PL</sub><sup>b</sup></th><th>FWHM<sup>b</sup> / nm</th><th>Φ<sub>PL</sub><sup>a</sup> / %</th><th>Φ<sub>PL</sub><sup>b</sup> / %</th><th>τ<sub>p</sub><sup>b,c</sup> / ns</th><th>τ<sub>d</sub><sup>b,c</sup> / μs</th><th>S<sub>1</sub> / T<sub>1</sub><sup>d</sup> / eV</th><th>ΔE<sub>ST</sub><sup>d</sup> / eV</th></tr></thead><tbody><tr><td><b>PCP-DiKTa</b></td><td>444 / nm</td><td>469 / nm</td><td>477 / nm</td><td>44 nm</td><td>36(32)</td><td>93(66)</td><td>6.4</td><td>179</td><td>2.67/2.51</td><td>0.16</td></tr><tr><td><b>Czp-DiKTa</b></td><td>446 / nm</td><td>505 / nm</td><td>501 / nm</td><td>66 nm</td><td>37(30)</td><td>99(73)</td><td>11.7</td><td>140</td><td>2.57/2.44</td><td>0.13</td></tr></tbody></table>

<footnote><sup>a</sup> In toluene at 298 K. <sup>b</sup> spin-coated 2 wt% thin films in 26DCzPPy ($\lambda_{exc}$ = 343 nm). <sup>c</sup> Average lifetime ($\tau_{avg}$ = $\Sigma A_i \tau_i^2 / \Sigma A_i \tau_i^2$, where $A_i$ is preexponential for lifetime $\tau_d$). Prompt and delayed</footnote>
emission were measured by TCSPC ($\lambda_{exc}$ = 379 nm) and MCS ($\lambda_{exc}$ = 343 nm), respectively. d
Determined from the onset of the SS PL and phosphorescence spectra in toluene at 77 K.

Chiroptical properties

The experimental electronic circular dichroism (ECD) spectra of the enantiomeric compounds
were measured in toluene solutions (10⁻⁴ M); the (1st)/(2nd)-enantiomers refer to the 1st and 2nd
enantiomers according to retention time that are detected by chiral HPLC (Figures S5 and S13).
They displayed the typical mirror-image relationship (Figures 5a and b). In the case of (2nd)-
PCP-DiKTa, Cotton effects were observed at 309 (Δε = −46 M⁻¹ cm⁻¹), 343 (+1.3), and 371
nm (+3.2). For (2nd)-Czp-DiKTa enantiomer, Cotton effects were observed at 287 (Δε = −13.7
M⁻¹ cm⁻¹), 306 (+10), 318 (−4.9), 354 (−1.9), 367 (−1.7), 377 (+2), 393 (+6.8), and 452 nm
(+2.5).

The circularly polarized luminescence (CPL) was then examined. The CPL spectra were
recorded in dilute solutions of toluene at r.t and are shown in Figure 5c for (1st)/(2nd)-Czp-
DiKTa enantiomers. These were found to display mirror-image CPL signals at 507 nm with
photoluminescence dissymmetry factors gPL of around ±4 × 10⁻⁴ and with signs similar to the
low-energy ECD-active bands, i.e., positive (negative) for (1st)/(2nd)-Czp-DiKTa (see gPL
plots in Figure 5d). However, no clear CPL signals could be measured for (1st)/(2nd)-PCP-
DiKTa. Since the emission process mirrors the low-energy absorption one, the absence of CPL
signal may be explained by the fact that for PCP-DiKTa the band at 445 nm, corresponding to
the HOMO-LUMO transition, does not display ECD activity (Figure 5a). This may be
explained by the fact that, in this case, the low-energy excited state at 445 nm is not ECD-
active. The magnitude of the gPL of Czp-DiKTa is similar to those reported for other CP-MR-
TADF emitters; for example, (R)/(S)-Czp-tBuCzB and (R)/(S)-Czp-POAB have gPL of 5.4/-
5.1 × 10⁻⁴ and 4.8/-4.6 × 10⁻⁴ in toluene, respectively. The gPL values are, however, slightly
lower than other PCP-based D-A TADF emitters, which usually have dissymmetry values on
the order of 10⁻³.[12]
<figure><img src="image_10.png" /><figcaption>Figure 5. Experimental ECD spectra of (a) (1st)/(2nd)-PCP-DiKTa and (b) (1st)/(2nd)-Czp-DiKTa in toluene. Experimental CPL spectra (c) and g<sub>PL</sub> values (d) of (1st)/(2nd)-Czp-DiKTa in toluene (λ<sub>exc</sub> = 444 nm). (1st)/(2nd)-enantiomers refer to the 1<sup>st</sup> and 2<sup>nd</sup> enantiomers eluted by chiral HPLC.</figcaption></figure>

## Organic light-emitting diodes (OLEDs)

We next fabricated vacuum-deposited OLEDs. The OLED device stack and the chemical structures of the organic layers are shown in Figures 5a and b, respectively. The OLEDs consist 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)/ 1,3-bis(N-carbazolyl)benzene (mCP, 5 nm)/ 26DCzPPy as host (20 nm) with PCP-DiKTa, and Czp-DiKTa 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 devices were made with 2, 5, 10, and
15 wt% doping of **PCP-DiKTa** or **Czp-DiKTa**, with the lower concentrations of emitter giving higher efficiency, helped by higher Φ<sub>PL</sub> (Table S3). The current density-voltage-luminance (JVL) characteristics of the OLEDs with 2 and 5 wt% doping of **PCP-DiKTa** and **Czp-DiKTa** are shown in Figure 5c and Table 2. All devices had turn-on voltages of between 3.8-4.1 V. For the OLED with **PCP-DiKTa**, the 2 wt% device showed the highest EQE<sub>max</sub> of 25.7% in accordance with the high Φ<sub>PL</sub> for this concentration. The EQE decreased to 19.1% at 100 cd/m<sup>2</sup> and to 8.7% at 1000 cd/m<sup>2</sup> (Figure 5d). The OLED emitted at λ<sub>EL</sub> at 489 nm (Figure 5e) with a FWHM of 53 nm, which is close to the PL (λ<sub>PL</sub> of 477 nm; FWHM of 44 nm) of the emitter molecule. The CIE coordinates are (0.162, 0.450), Figure 5f. With an increase in the doping concentration of the emitter to 5 wt%, the EQE<sub>max</sub> of the device dropped to 20.5%, which is consistent with the drop in Φ<sub>PL</sub> from 93% (for the 2 wt% doped film) to 83% (for the 5 wt% doped film). The devices with higher concentrations of the emitter are shown in Figure S22.

The OLEDs with **Czp-DiKTa** (5 wt%) showed a very high EQE<sub>max</sub> of 29.2%, which decreased to 25.9% at 100 cd/m<sup>2</sup> and a high EQE of 17.1% at 1000 cd/m<sup>2</sup> as shown in Figure 5d and Table 2. Thus, compared to the device with **PCP-DiKTa**, the EQE<sub>max</sub> was both higher and the efficiency roll-off less severe in the device with **Czp-DiKTa**. The device emitted at λ<sub>EL</sub> at 518 nm (FWHM of 69 nm), Figure 5e, which was slightly red-shifted compared to the PL (λ<sub>PL</sub> = 501 nm; FWHM = 66 nm). The corresponding CIE coordinates are (0.256, 0.610). With 2 wt% of the emitter concentration, a slightly lower EQE<sub>max</sub> of 27.8% was recorded despite the higher Φ<sub>PL</sub> (99 and 97% for the 2 and 5 wt% doped films, respectively). The EQE<sub>100</sub>/EQE<sub>1000</sub> were 23.1/15.0%, reflecting a more severe efficiency roll-off in this device. Overall, the efficiency roll-off for **Czp-DiKTa** devices at all concentrations (Figure S22) was less pronounced. Photographs of the OLEDs with **PCP-DiKTa** and **Czp-DiKTa** are shown in Figure 5e, respectively.
<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" /></figure>

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

<figure><img src="image_16.png" /><figcaption>**Figure 5.** (a) Schematic of the OLED stack; (b) Chemical structures of the materials used in the device; (c) Current density-voltage-luminescence (JVL) characteristics; (d) External quantum efficiency (EQE) vs luminescence; (e) Electroluminescence spectra, inset figures are photographs of the OLEDs fabricated with **PCP-DiKTa** 2 wt% and **Czp-DiKTa** 2 wt%; (f) CIE coordinates of the devices.</figcaption></figure>
<table><caption>Table 2. OLED performance.</caption><thead><tr><th></th><th>x<sup>a</sup><br>/ wt%</th><th>λ<sub>EL</sub><sup>b</sup><br>/ nm</th><th>FWHM<sup>c</sup><br>nm</th><th>V<sub>on</sub><sup>d</sup><br>/ V</th><th>L<sub>max</sub><sup>e</sup><br>/ cd m<sup>-2</sup></th><th>EQE<sub>max/100/1000</sub><sup>f</sup><br>/ %</th><th>CIE<sup>g</sup><br>/ x, y</th></tr></thead><tbody><tr><td rowspan="2">PCP-DiKTa</td><td>2</td><td>489</td><td>53</td><td>3.9</td><td>5558</td><td>25.7/19.1/8.7</td><td>0.162, 0.450</td></tr><tr><td>5</td><td>490</td><td>61</td><td>3.8</td><td>3248</td><td>20.5/16.3/6.9</td><td>0.225, 0.480</td></tr><tr><td rowspan="2">Czp-DiKTa</td><td>2</td><td>505</td><td>63</td><td>4.1</td><td>11536</td><td>27.8/23.1/15.0</td><td>0.214, 0.563</td></tr><tr><td>5</td><td>518</td><td>69</td><td>4.2</td><td>10689</td><td>29.2/25.9/17.1</td><td>0.256, 0.610</td></tr></tbody></table>

a Emitter doping concentration in the EML. b The electroluminescence peak; c Full width at half-maximum; d Turn-on voltage; e Maximum luminance; f Maximum external quantum efficiency/EQE at 100 cd m⁻²/EQE at 1000 cd m⁻²; g Commission Internationale de l'Éclairage (CIE) coordinates.

## Conclusion

In summary, we have successfully developed two chiral MR-TADF emitters, **PCP-DiKTa** and **Czp-DiKTa**, by decorating **DiKTa** with two different chiral groups Czp and PCP, and explored how the donor strength modulated the energy of mixed SRCT and LRCT state, thereby influencing the photophysical properties and corresponding device performance. The resulting compounds **PCP-DiKTa** and **Czp-DiKTa** showed high Φ<sub>PL</sub> of 93% and 99%, moderate ΔE<sub>ST</sub> of 0.19 and 0.17 eV, and shorter delayed lifetimes of 179 and 140 µs in 2 wt% doped films in 26DCzPPy than that of the parent DiKTa. Meanwhile, (1st)/(2nd)-PCP-DiKTa and (1st)/(2nd)-Czp-DiKTa displayed mirror-image ECD spectra in toluene solutions and CPL activity was found for (1st)/(2nd)-Czp-DiKTa only, with g<sub>PL</sub> values of ±4 × 10<sup>-4</sup> in toluene solutions. OLEDs with **PCP-DiKTa** and **Czp-DiKTa** showed narrow EL spectra with FWHMs of 53 nm and 69 nm, and high EQE<sub>max</sub> of 25.7% and 29.2%. Notably, device with **Czp-DiKTa** showed the less pronounced efficiency roll-off, with EQEs at 100/1000 cd/m² of 25.9/17.1%. This study serves as an illustrative example of how a CP-TADF emitter can be tailored to achieve narrow emission and superior device performance, mitigating efficiency roll-off concerns.
**Supporting information**

Supporting Information is available from the Wiley Online Library or from the author.

**Acknowledgements**

We are grateful to the EPSRC for financial support (grant EP/W0151371/1 and EP/W007517/1).

Yan Xu thanks the China Scholarship Council (202106310038) for support. J. S. thanks the Jürgen Manchot Foundation, the Karlsruhe School of Optics and Photonics and Dr. Erik Strandberg and Bianca Posselt, for their help in ECD measurements. We thank the European Commission Research Executive Agency (Grant Agreement number: 101073045–TADFsolutions – HORIZON-MSCA-2021-DN) for financial support. We acknowledge the Ministère de l’Enseignement Supérieur et de la Recherche and the Centre National de la Recherche Scientifique (CNRS). Dr. Ludovic Favereau is thanked for his technical support.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The research data supporting this publication can be accessed at XX.

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

Through decorating the MR-TADF core **DiKTa** with different PCP based chiral groups, two
pairs of chiral MR-TADF emitters, **PCP-DiKTa** and **Czp-DiKTa**, were synthesized. When
applied these two emitters into OLEDs, both of them show both narrow emission at 489 and
518 nm and high EQE<sub>max</sub> of 25.7 and 29.2%.