Improving Processability and Efficiency of Resonant TADF Emitters: A Design Strategy

A new design strategy is introduced to address a persistent weakness with resonance thermally activated delayed fluorescence (R‐TADF) emitters to reduce aggregation‐caused quenching effects, which are identified as one of the key limiting factors. The emitter Mes3DiKTa shows an improved photoluminescence quantum yield of 80% compared to 75% for the reference DiKTa in 3.5 wt% 1,3‐bis(N‐carbazolyl)benzene. Importantly, emission from aggregates, even at high doping concentrations, is eliminated and aggregation‐caused quenching is strongly curtailed. For both molecules, triplets are almost quantitatively upconverted into singlets in electroluminescence, despite a significant (≈0.21 eV) singlet‐triplet energy gap (ΔEST), in line with correlated quantum‐chemical calculations, and a slow reverse intersystem crossing. It is speculated that the lattice stiffness responsible for the narrow fluorescence and phosphorescence emission spectra also protects the triplets against nonradiative decay. An improved maximum external quantum efficiencies (EQEmax) of 21.1% for Mes3DIKTa compared to the parent DiKTa (14.7%) and, importantly, reduced efficiency roll‐off compared to literature resonance TADF organic light‐emitting diodes (OLEDs), shows the promise of this design strategy for future design of R‐TADF emitters for OLED applications.


Introduction
Thermally activated delayed fluorescence (TADF) permits 100% internal quantum efficiency (IQE) in electroluminescent devices as this mechanism permits conversion of nonradiative triplet excited states to radiative singlet states via reverse intersystem crossing (RISC). [1] RISC is made efficient by having a small energy gap between the lowest triplet and singlet excited states (ΔE ST ). The most common molecular design for achieving small ΔE ST in organic emitters is based on a donoracceptor paradigm where the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are poorly electronically coupled and usually spatially separated, thus reducing the exchange integral. [2] Conventional donoracceptor TADF compounds emit from an excited state that is strongly charge transfer (CT) in nature. Generally, these CT emitters have emission spectra that are very broad in nature, with full-width at half-maximum (FWHM), often as high as 100 nm, leading to poor color purity. [3] In 2015, a new approach for TADF emitters was presented by Hatakeyama and co-workers. [4] Their design involved an alternating HOMO/LUMO pattern based on complimentary resonance effects of electron-donating oxygen and electron-withdrawing boron moieties (Figure 1a). The first emitter of this resonating TADF (R-TADF) family, DABOA (originally called 2a, but renamed for clarity), possessed a suitably small ΔE ST of 0.15 eV and a reasonably high photoluminescence quantum yield, Φ PL , of 72% in 1 wt% poly(methyl methacrylate) (PMMA) thin film. In this first report no devices were fabricated as the emission maxi mum, λ PL , was too high in energy at 399 nm. [4] Following this initial work, the same group modified their design and incorporated stronger nitrogen-based donor moieties ( Figure 1c). [5] Blue-emitting compounds DABNA-1 and DABNA-2 (λ PL = 460 and 469 nm, respectively), showing very high Φ PL of 88 and 90%, respectively, in 1 wt% mCBP, were reported. These emitters were incorporated into organic light-emitting diodes (OLEDs), which showed very good maximum external quantum efficiencies (EQE max ) of 13.2% and 20.2%, respectively. Though the OLEDs showed very large efficiency roll-off and did not reach a luminance of 1000 cd m −2 , DABNA-2 nevertheless represents one of the most efficient deep blue emitters reported to date with Commission Internationale d'Éclairage (CIE) coordinates of (0.12, 0.13). [6] Building from DABNA-1, the C 3 -symmetric derivative TABNA (originally named 2, renamed for clarity) likewise showed a similarly small ΔE ST of 0.21 eV along with a reasonable Φ PL of 54% in 1 wt% PMMA films; however, the undesired blueshift in the emission (λ PL = 399 nm) again led to no devices being fabricated. [7] Going beyond the simplistic orbital picture, we demonstrated, based on highly correlated quantum chemical calculations at the spin-component scaling second-order approximate coupled-cluster (SCS-CC2) level of theory, that DABNA-1 and TABNA display short-range charge transfer in both the T 1 and S 1 excited states, which is responsible for their small ΔE ST while maintaining the necessary overlap between Adv. Optical Mater. 2020, 8,1901627 Figure 1. Previously reported R-TADF emitters. a) Simplified resonating HOMO/LUMO plot for DABOA and HOMO-LUMO distributions calculated using density functional theory (DFT); b) simplified difference density plot of DABOA and singlet and triplet difference density plots calculated in the gas phase using spin-component scaling second-order approximate coupled-cluster (SCS-CC2) approach; c) structures of previously reported R-TADF emitters and related photophysical and OLED data. the hole and electron wavefunctions to guarantee a large oscillator strength, f, which explains their high observed Φ PL . [3] Increasing the number of acceptor groups (B2, B3, and B4) stabilized the LUMO energy and led to blueshifted emission with emission maxima, λ PL , of 455, 441, and 450 nm, respectively, in 1 wt% PMMA. [8] Further, an excellent device performance using B2, was reported with EQE max values as high as 18%, coupled with a slightly improved efficiency roll-off compared to the devices with the DABNA emitters. The next iteration in design involved the introduction of a donor carbazole derivative and solubilizing tert-butyl groups about the skeleton of DABNA-1 (TBN-TPA). [9] The addition of the tertbutylcarbazole unit did not adversely affect the multiresonance mechanism, with HOMO/LUMO density remaining localized on the DABNA-1 core and only a slight observed redshift in the emission at 470 nm (λ PL of DABNA-1 = 460 nm) coupled with an enhanced Φ PL of 98% in toluene (Φ PL of DABNA-1 = 89% in CH 2 Cl 2 ). The OLED using TBN-TPA showed an excellent EQE max of 32.1% and also an improved efficiency rolloff. [9] Another method to improve efficiency roll-off has been proposed recently where a tert-butyl analogue of DABNA-1, t-DABNA (λ PL = 445), was used alongside an assistant TADF dopant DMAC-DPS. [10] Improved EQE max values from 25.1% to 31.4% were observed in the assistant dopant device compared to the OLED with no assistant dopant. Vastly improved efficiency roll-off was also observed where the EQE at 100 cd m −2 decreased by only 13% in the assistant dopant device compared to 76% in the device without the assistant dopant. Hatakeyama and co-workers [11] recently reported a B/N containing pentacene decorated with peripheral diphenylamines (v-DABNA). The deep blue devices reported show outstanding EQE max and very low efficiency roll-off at 100 cd m −2 of 34.4% and 5% (EQE 100 = 32.8%), respectively, at CIE of (0.12,0.11). To the best of our knowledge, these OLEDs are the highest performing deep-blue devices reported to date. The OLEDs also show very high color purity, with a FWHM of 18 nm in a 1 wt% doped device. A summary of all major photophysical data of literature B/N R-TADF based emitters is in Table S1 (Supporting Information).
R-TADF emitters are clearly very promising, with EQE max of up to 30% possible for blue devices, but their most attractive feature is their consistently narrow emission FWHM. The sharper emission profile is particularly attractive as this translates to improved color purity in the device. [12] R-TADF compounds have small ΔE ST , high Φ PL , and small emission FWHM, making them an ideal class of emitter for OLEDs. However, as many of the examples in Figure 1c are highly planar, they are at risk of quenching due to aggregation and/or excimer formation. Further, nearly all are based on a B/N design, which limits the structural diversity and scope of these emitters.
Mes 3 DiKTa (Figure 2) was designed in order to mitigate the undesirable quenching by aggregation frequently observed in R-TADF emitters. The mesityl groups were chosen as they would (1) adopt an orthogonal conformation and thus not affect the multiresonance mechanism and (2) their steric bulk will inhibit aggregation-caused quenching. [13] DiKTa, acting as a reference emitter, and derivatives have been previously reported; however, their potential as TADF emitters was Adv. Optical Mater. 2020, 8,1901627  not identified until very recently, being initially categorized as normal fluorescent emitters. [14] Since commencing our study, DiKTa has been reported as a R-TADF emitter under the name of QAO. [15] It has a high Φ PL of 72% and narrow blue emission (λ PL = 468 nm; FWHM = 39 nm) in 5 wt% 1,3-bis(N-carbazolyl) benzene (mCP), and a high EQE max of 19.4% (at <1 cd m −2 ). However, the maximum luminance was only 1100 cd m −2 and only limited theoretical work was performed. A second emitter in the same report, QAO-DAd, is much better described as a donor-acceptor system and showed none of the photophysical characteristics emblematic of R-TADF emitters. In addition, two phenyl substituted derivatives of DiKTa were reported, 3-PhQAD and 7-PhQAD. [16] Near identical λ PL to DiKTa were reported at 466 and 464 nm in 2 wt% mCP, respectively, for 3-PhQAD and 7-PhQAD along with comparable Φ PL of 73% and 68%. Addition of the phenyl groups has very little impact upon the photophysics of the DiKTa luminophore nor on the device performance where comparable EQE max of 19.1% and 18.7% were reported for 3-PhQAD and 7-PhQAD, respectively. The OLED with 7-PhQAD showed a significant efficiency rolloff with EQE 100 of 5.4% compared to 10.2% for 3-PhQAD; however, no explanation was given for this observation. Further, a drop-in luminance compared to the best boron-based R-TADF emitters was observed for 3-PhQAD (4975 cd m −2 ) and 7-PhQAD (2944 cd m −2 ). There is thus a clear need for improvement in the performance of this new class of R-TADF emitter. Using in silico design and building from our previous work, we demonstrate that peripheral modification of a R-TADF triangulene-type emitter does not affect the R-TADF mechanism, and reduces quenching by aggregation. Mes 3 DiKTa shows an enhanced Φ PL of 80% compared to 75% for the reference DiKTa at 3.5 wt% in mCP. An excellent EQE max of 21.1% at a luminance of 25 cd m −2 was obtained, representing the highest for these ketone-based R-TADF emitters. Importantly, improved efficiency roll-off of 31% was also noted with EQE 100 of 14.5% and a significantly higher maximum luminance of 12 900 cd m −2 for the OLED with Mes 3 DiKTa.

Computational Studies
Based on our recent study, [3] all the results presented in the main manuscript are obtained by running higher-level SCS-CC2; for completeness, time-dependent density functional theory (TD-DFT) calculations results are presented in Figure S32 (Supporting Information). All TD-DFT flavors (namely, different functionals and full TD-DFT versus Tamm-Dancoff approximation treatment) lead to different description of both T 1 and S 1 excited states in comparison to SCS-CC2, and also to large overestimations of ΔE ST . Similar to results obtained for DABNA-1 and TABNA, including high-order electronic correlation effects is required to properly account for the short-range charge transfer character in the lowest singlet and triplet excited states of both DiKTa and Mes 3 DiKTa, revealing that the R-TADF mechanism is operative. As expected, there is very little density located on the mesityl groups in Mes 3 DiKTa ( Figure 2). However, the difference density plots show opposite patterns to the previously reported B,N R-TADF emitters because of the inverted design which places a central nitrogen-based donor and peripheral ketone acceptors. In order to further validate our computational methodology and demonstrate its robustness, calculations were performed for each of the literature R-TADF diketone emitters in Figure 1, along with other B,N R-TADF emitters, namely, DABNA-2, t-DABNA, and TBN-TPN. Predicted ΔE ST energies are in good agreement with experiment (Table S9, Supporting Information). We note that our calculations on 3-PhQAD and 7-PhQAD sensibly disagree with the interpretation proposed by Zhang and co-workers [16] based on TD-DFT calculations, where the nature of the excited states in these compounds involves CT-LE hybridization, while our SCS-CC2 calculations suggest that T 1 and S 1 are short-range CT excited states.
Calculated ΔE ST for DiKTa and Mes 3 DiKTa are 0.27 and 0.26 eV, respectively, and are expected to be small enough to permit RISC at ambient temperatures. Predicted oscillator strengths, f, of 0.20 and 0.23 for each DiKTa and Mes 3 DiKTa, respectively, resulting in k f of 1.05 × 10 8 and 1.10 × 10 8 s −1 from S 1 to S 0 , which should translate into enhanced Φ PL .

Synthesis
DiKTa was synthesized using a similar method to that reported previously ( Figure 2). [3] Diester, 1 was obtained in modest yield of 32% via a high-temperature Ullmann coupling. The subsequent ring closing reaction produced DiKTa in 86% yield. Overall a reduced yield of 26% compared to the original 31% was obtained. [14a] Mes 3 DiKTa was obtained in good yield following bromination of intermediate 2, Friedel-Craft acylation to produce brominated Br 3 DiKTa, and Suzuki-Miyaura coupling with mesityl-2-boronic acid to deliver the target Mes 3 DiKTa. 1 H and 13 C NMR, high resolution mass spectrometry, elemental analysis, and high-performance liquid chromatography were used to confirm the structures along with the purity. Single crystal X-ray diffraction data for both emitters indicated different packing motifs; although neither compound shows strong intermolecular interactions, the presence of the mesityl groups does disrupt π···π interactions in Mes 3 DiKTa [no centroid···centroid distances less than 4.030(3) Å]. The only intermolecular interactions seen in Mes 3 DiKTa are extremely weak CH···π interactions between a phenyl hydrogen and a mesitylene ring, at distances close to the van der Waals limit (2.85 and 2.06 Å), leading to the formation of weakly interacting molecular stacks along the crystallographic b-axis. In DiKTa π···π interactions are found [centroid···centroid distance of 3.8793(6) Å] between adjacent emitter molecules, giving rise to interacting stacks running along the c-axis ( Figure S17, Supporting Information). Good thermal stability was observed with 5% weight loss at 323 and 437 °C for DiKTa and Mes 3 DiKTa, respectively ( Figure S29, Supporting Information).

Optoelectronic Properties
The electrochemical behavior of DiKTa and Mes 3 DiKTa was studied by cyclic voltammetry and differential pulse voltammetry in degassed acetonitrile (MeCN) with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The cyclic voltammograms (CVs) and differential pulse voltammograms (DPVs) are shown in Figure 3a and the data are summarized in Table S3 (Supporting Information). A slightly destabilized HOMO and stabilized LUMO in Mes 3 DiKTa versus DiKTa can be attributed to the mesomeric electronwithdrawing character of the mesityl groups. The reduced ΔE H-L in turn is correlated with the observed redshifted emission (vide infra). Further, the mesityl groups contribute to the electrochemical stability of the compound with reversible oxidation and reduction waves by inhibiting an electrochemical degradation process located at the para-CH position to the nitrogen. Atmospheric photoelectron spectroscopy (APS) was also performed on neat films of the emitters to corroborate the HOMO levels inferred from CV ( Figure S18, Supporting Information). The APS values are in good agreement with those obtained by DPV (Table S3, Supporting Information). UV-vis spectra in toluene show high-energy, low-intensity bands along with a low-energy high-intensity band, the latter of which we attribute to a short-range charge-transfer S 0 -S 1 strongly allowed optical transition typical of multiresonant compounds (Figure 3b and Figure S19, Supporting Information). This is in good agreement with theory, where a high oscillator strength, f, of 0.23 and 0.20 for Mes 3 DiKTa and DiKTa is predicted for this transition. A very small positive solvatachromism is observed for this band in the ground state ( Figure S19, Supporting Information). This is again consistent with the short-range CT nature of this transition (cf. Tables S4 and S5, Supporting Information).
We next investigated the photophysical properties of the two emitters in toluene. The photoluminescence (PL) spectrum of Mes 3 DiKTa shows the expected mirror image profile to the absorption spectrum, and the small difference between the peaks of the absorption and emission of 27 nm (805 cm −1 ) suggests a weak geometry relaxation in the singlet state (Figure 3c and Figure [17] b) Absorption (Abs) and steady-state PL (SS) spectra obtained in toluene at 300 and 77 K, phosphorescence (Phos.) spectra obtained in toluene glass at 77 K after 70 ms delay for 70 ms, λ exc = 415 nm. c) solvatochromatic PL study of Mes 3 DiKTa, λ exc = 345 nm. d) Time resolved PL of Mes 3 DiKTa and DiKTa in degassed toluene, λ exc = 355 nm detected across the full spectral range, where IRF is the instrument response function. DABNA-1; the vibronic progression is less well resolved in the PL spectrum. The rigid nature of the compound is responsible for the narrow emission spectrum at RT. A similar profile but with more pronounced vibronic progression is observed for the phosphorescence spectrum obtained after 70 ms at 77 K in a toluene glass. The ΔE ST of 0.19 eV is sufficiently small to enable a RISC process at room temperature. Modest positive solvatochromism (31 nm, or 1522 cm −1 for DiKTa and 1408 cm −1 for Mes 3 DiKTa) was observed in the steady-state PL spectra (Figure 3c), in contrast to the larger positive solvatochromism typically observed for conventional D-A TADF emitters. [18] This demonstrates that the nature of the excited states of R-TADF emitters is distinct from conventional D-A TADF emitters. In R-TADF emitters, both T 1 and S 1 are short-range CT excited states, which together with the narrow FWHM of both the fluorescence and the phosphorescence spectra constitute the remarkable characteristics of R-TADF emitters.
Time-resolved PL was measured in both aerated and degassed solutions in toluene. The decays show multiexponential kinetics with both a prompt, τ p , ns PL lifetime emission and a delayed, τ d , microsecond PL lifetime. Both DiKTa and Mes 3 DiKTa have a small contribution to the delayed component (Figure 3d), consistent with previous studies of R-TADF emitters, [5] along with τ d of 33 and 23 µs, respectively. Predicted k f is larger than experimentally calculated in toluene (1.05 × 10 8 s −1 compared to 4.9 × 10 7 s −1 for DiKTa and 1.10 × 10 8 s −1 compared to 5.4 × 10 7 s −1 for Mes 3 DiKTa), which we attribute to the solvent that tends to stabilize CT-like excitations and reduces their transition dipole moment compared to the gas phase. Solution photophysical data for both emitters in toluene are shown in Table 1 (the full data for each of the solvents can be found in Tables S4-S6, Supporting Information).
We next investigated the solid-state PL behavior in thin films of 3.5 wt% emitter in mCP ( Table 2). This concentration was chosen in order to avoid aggregation of the emitters, and mCP was selected as the host matrix as its triplet energy (2.81 eV) [19] is higher than that of the emitters. Promising Φ PL values of 75% and 80% were observed for vacuum-sublimed 3.5 wt% doped films of DiKTa and Mes 3 DiKTa, respectively. To probe whether the higher Φ PL of Mes 3 DiKTa may result from the suppression of interchromophore interactions by the mesityl groups, we measured the photoluminescence of both materials in mCP as a function of concentration in spin-coated films, which gives slightly lower Φ PL than the vacuum-sublimed ones. Figure 4a shows how for a neat film of DiKTa, a distinct second, broad peak emerges at about 540 nm, likely resulting from excimer formation, while a neat film of Mes 3 DiKTa retains its narrow spectral shape. This is accompanied by a strong reduction in Φ PL for DiKTa with increasing concentration, while this tails off far more gently for Mes 3 DiKTa (Figure 4b). Clearly, the introduction of the mesityl groups alleviates close interaction, as was intended by the original chemical design. Figure 4d shows the time-resolved PL decay traces at different temperatures for Mes 3 DiKTa in mCP. There is a temperature dependence of the magnitude of the delayed lifetime, τ d , which expectedly decreases with decreasing temperature from 300 to 200 K (Figure 4d), typical of TADF emitters. The delayed emission we observe at 300 and 200 K is fluorescence from the S 1 state while the emission at 100 and 5 K is phosphorescence from the T 1 state; there is limited delayed fluorescence at these latter two temperatures ( Figure S1, Supporting Information). This same behavior is observed for DiKTa (Figures S22-S24, Supporting Information). Much like that observed in toluene, the contribution of the delayed emission to the overall emission in DiKTa is more prominent than for Mes 3 DiKTa ( Figure S25, Supporting Information), indicating higher TADF emission contribution, possibly associated with the smaller ΔE ST in DiKTa. Figure 4c shows the spectral analysis of the prompt fluorescence at 77 K and phosphorescence (time delay 20 ms) at 77 K for Mes 3 DiKTa in 3.5 wt% mCP (see Figure S24a, Supporting Information, for DiKTa). We were able to estimate the ΔE ST from the onset of these spectra. The slight redshifted emission observed in Mes 3 DiKTa, for both the S 1 and T 1 states ( Table 3) versus DiKTa can be correlated to the observed smaller electrochemical gap. Good agreement for ΔE ST between experiment and theory is observed, with DiKTa (ΔE STexp = 0.20 eV vs ΔE STtheory = 0.27 eV), while for Mes 3 DiKTa the ΔE ST was found to be 0.21 eV and computed to be 0.26 eV.
These EL spectra are significantly narrower than conventional blue D-A TADF OLEDs but similar to those EL spectra observed in R-TADF emitters (Table S5, Supporting Information). The EL spectra of our devices also showed very weak emission from the host at high voltages that likely occurs due to the slightly shallower HOMO level of mCP than the emitters (Figures S27a and S28a, Supporting Information). [20] Figure 5d shows EQE versus luminance curves. The DiKTa OLED shows an EQE max of 14.7% at 8 cd m −2 while the OLED Adv. Optical Mater. 2020, 8,1901627  The dashed line is a guide to the eye, based on fitting an exponential decay function to the data. c) Steady-state PL spectra at 300 K, at 77 K, and phosphorescence spectra in 3.5 wt% mCP, phosphorescence obtained at 77 K at a delay of 20 ms for 70 ms, λ exc = 415 nm. d) Variable temperature time-resolved PL spectra, λ exc = 355 nm in 3.5 wt% mCP, where IRF is the instrument response function. λ exc = 355 nm at 300 K under vacuum, prompt component obtained using a single exponential, delayed obtained using a stretched exponential; b) Obtained from the onset of the PL spectrum at 77 K after 70 ms, λ exc = 415 nm; c) Obtained from the onset of the steady-state spectrum at 77 K, λ exc = 335 nm; d) Obtained at 300 K, λ exc = 335 nm; f) Calculated using an integrating sphere, under N 2 , λ exc = 335 nm.
with Mes 3 DiKTa shows an EQE max of 21.1% at 25 cd m −2 . Given these EQE max values, the high Φ PL of 80% and 75% in mCP and considering charge balance as 100% and outcoupling efficiency as 25%, the theoretical EQE max for DiKTa and Mes 3 DiKTa are ≈18% and ≈20%, respectively. [21] Thus, we can infer that essentially 100% of the triplet excitons are being efficiently harvested and converted to singlet excitons via the TADF mechanism.
Though we must be careful, as there is some uncertainty on the efficiency of charge recombination and light extraction, this is a remarkable and somewhat unexpected result in view of the very low RISC rate in these molecules (see Table 1). It thus appears that, provided the triplets are protected against nonradiative loss mechanisms, a reasonably small ΔE ST value as found in DiKTa and Mes 3 DiKTa is enough to quantitatively upconvert   all triplet excitations. We thus conclude that the weak coupling between the electronic excitations and the nuclei vibrations associated with the rigid backbone structure not only results in narrow spectral emission but also appears to slow down competitive nonradiative decay channels. At 100 cd m −2 the EQE 100 value for the OLED with DiKTa is 8.3%, decreasing 44% from its maximum value. The EQE 100 for the device with Mes 3 DiKTa is 14.5%, showing an efficiency roll-off of 31%. At 1000 cd m −2 the EQE 1000 values decrease dramatically to 3.3% and 4.5% for DiKTa and Mes 3 DiKTa, which is a decrease of 78% and 79% from their maximum values. This serious efficiency roll-off at high driving voltages was observed in other R-TADF emitters such as DABNA-1 and DABNA-2, [5] and recently was also observed by Zhang and co-workers, [12] where 3-PhQAD and 7-PhQAD EQE values drop more than 85% from their maximum value at 1000 cd m −2 . Thus, we contend that the increased steric bulk afforded by the mesityl groups reduces the efficiency roll-off of the OLEDs when compared to 3-PhQAD and 7-PhQAD. Zhang and co-workers have investigated the mechanisms responsible for this strong efficiency roll-off and concluded that both triplet-triplet annihilation (TTA) and singlet exciton-polaron annihilation (SPA) play significant roles in the efficiency roll-off. Due to the similarity of 3-PhQAD and 7-PhQAD with our emitters and the similarity of the device structures, the cause of our efficiency roll-off is likely to be SPA and TTA as well. Even though the EQE values are poor at high driving voltages, the luminance levels reached by both emitters are excellent (Lum max DiKTa = 10 400 cd m −2 and Lum max Mes 3 DiKTa = 13 000 cd m −2 ), which is not often observed in TADF-based devices, including R-TADF OLED. Both devices operate at similar current densities and show low turn-on voltages of around 3 V ( Figure S7c, Supporting Information). The previously reported OLEDs with DiKTa [15] using a similar device structure to the one used in this study showed EQE max of 19.4% but at luminance lower than 1 cd m −2 . At comparable luminance of 10, 100, and 1000 cd m −2 they obtained similar EQE values of 15%, 9%, and 2% to those presented here. As such, Mes 3 DiKTa demonstrated improved device performance in comparison to previously reported R-TADF materials, showing the highest EQE value at the relevant luminance of 100 cd m −2 among the keto R-TADF emitters reported to date (Table 3).

Conclusions
Based on a new molecular engineering approach to improve the efficiency of R-TADF emitters in OLEDs, we have designed the molecule Mes 3 DiKTa. The OLED reached an EQE max of up to 21%. When comparing Mes 3 DiKTa to the parent DiKTa a ≈50% improvement in EQE max is observed due to a significant reduction of aggregation-caused quenching by addition of mesityl groups to the resonant DiKTa core. By means of spincomponent scaling second-order approximate coupled-cluster quantum-chemical calculations, we accurately predict ΔE ST , with calculated values for Mes 3 DiKTa being 0.26 eV compared to 0.21 eV determined experimentally in 3.5 wt% mCP. The modest ΔE ST value originates from the nature of the involved electronic excitations that involve short-range reorganization in the electronic density offering simultaneously large singlet radiative decay rates and small singlet-triplet exchange interactions. Remarkably, despite a very slow reverse intersystem crossing process, all triplets upconvert into singlets in electroluminescence, a result that we ascribe to the rigid nature of the emitters. This leads to improved OLED device efficiency with reduced roll-off at luminance up to 100 cd m −2 without adversely affecting the R-TADF nature of the emitter. We further note that the improvement in device performance brought about by the use of peripherical side groups in Mes 3 DiKTa has only a modest impact on the emission color, with CIE coordinates for blue emission similar to those measured for the DiKTa core. Finally, there is room for improvement at high luminance where efficiency roll-off remains large and we hope the strategy applied here can be further fine-tuned in the future to reduce bimolecular recombination, i.e., through the design of molecules with limited spectral overlap between singlet emission and charge and triplet absorption. Such work is currently in progress.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. The research data supporting this publication can be accessed at https://doi.org/10.17630/e717a3b6-25b6-4bd8-b1aa-9459da244707.