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The poor operational stability of non-fullerene electron acceptor (NFA) organic solar cells (OSCs) currently limits their commercial application. While previous studies have primarily focused on the degradation of the NFA component, we also consider here the electron donor material. We examine the stability of three representative donor polymers, PM6, D18, and PTQ10, paired with the benchmark NFA, Y6. After light soaking PM6 and D18 in air, we find an enhanced conversion of singlet excitons into trapped interchain polaron pairs on sub-100 femtosecond timescales. This process outcompetes electron transfer to Y6, significantly reducing the charge generation yield. However, this pathway is absent in PTQ10. We identify twisting in the benzo[1,2-b:4,5-b′]dithiophene (BDT)-thiophene motif shared by PM6 and D18 as the cause. By contrast, PTQ10 does not contain this structural motif and has improved stability. Thus, we show that the donor polymer can be a weak link for OSC stability, which must be addressed collectively with the NFA.

This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 812872 (TADFlife) and Grant Agreement No. 838885 (NarrowbandSSL). S.B. acknowledges support from the German Science Foundation (392306670/HU2362). The St Andrews team would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. E.Z.-C. is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). S.M.S. acknowledges support from the Marie Skłodowska-Curie Individual Fellowship. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under grant no. F.4534.21 (MIS-IMAGINE). D.B. is an FNRS Research Director. The development of efficient blue donor–acceptor thermally activated delayed fluorescence (TADF) emitters remains a challenge. To enhance the efficiency of TADF-related processes of the emitter, we targeted a molecular design that would introduce a large number of intermediate triplet states between the lowest energy excited triplet (T1) and singlet (S1) excited states. Here, we introduce an oligomer approach using repetitive donor–acceptor units to gradually increase the number of quasi-degenerate states. In our design, benzonitrile (BN) moieties were selected as acceptors that are connected together via the amine donors, acting as bridges to adjacent BN acceptors. To preserve the photoluminescence emission wavelength across the series, we employed a design based on an ortho substitution pattern of the donors about the BN acceptor that induces a highly twisted conformation of the emitters, limiting the conjugation. Via a systematic photophysical study, we show that increasing the oligomer size allows for enhancement of the intersystem crossing and reverse intersystem crossing rates. We attribute the increasing intersystem crossing rate to the increasing number of intermediate triplet states along the series, confirmed by the time-dependent density functional theory. Overall, we report Publisher PDF

Funding: The St Andrews team would like to thank EPSRC (EP/P010482/1) and the Leverhulme Trust (RPG-2016-047) for financial support. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F. R. S.-FNRS) under Grant no. 2.5020.11. Y. O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant no. F.4534.21 (MIS-IMAGINE). Two novel π-extended amine/carbonyl-based multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been designed and synthesized. The two emitters are isomeric, composed of nine fused rings and show green-yellow emission. Sym-DiDiKTa and Asym-DiDiKTa possess tert-butyl groups distributed in a symmetrical and asymmetrical fashion, respectively, which significantly impact the single-crystal packing structure. The two compounds possess similar singlet–triplet energy gaps, ΔEST, of around 0.23 eV, narrowband emission characterized by a full-width at half-maximum, FWHM, of 29 nm and a photoluminescence quantum yield, ΦPL, of 70% and 53% for the symmetric and asymmetric counterparts, respectively, in toluene. Investigation in OLEDs demonstrated that the devices with Sym-DiDiKTa and Asym-DiDiKTa displayed electroluminescence maxima of 543 and 544 nm, and maximum external quantum efficiencies (EQEmax) of 9.8% and 10.5%, respectively. The maximum EQE was further improved to 19.9% by employing a hyperfluorescence strategy. We further present the first example of a neutral MR-TADF emitter incorporated in a LEC device where Sym-DiDiKTa acts as the emitter. The LEC shows a λEL at 551 nm and FWHM of 60 nm with luminance of 300 cd m−2 and a fast turn-on time of less than 2 s to 100 cd m−2. Publisher PDF

The importance of intermediate triplet states and the nature of excited states has gained interest in recent years for the thermally activated delayed fluorescence (TADF) mechanism. It is widely accepted that simple conversion between charge transfer (CT) triplet and singlet excited states is too crude, and a more complex route involving higher-lying locally excited triplet excited states has to be invoked to witness the magnitude of the rate of reverse inter-system crossing (RISC) rates. The increased complexity has challenged the reliability of computational methods to accurately predict the relative energy between excited states as well as their nature. Here, we compare the results of widely used density functional theory (DFT) functionals, CAM-B3LYP, LC-ωPBE, LC-ω*PBE, LC-ω*HPBE, B3LYP, PBE0, and M06-2X, against a wavefunction-based reference method, Spin-Component Scaling second-order approximate Coupled Cluster (SCS-CC2), in 14 known TADF emitters possessing a diversity of chemical structures. Overall, the use of the Tamm–Dancoff Approximation (TDA) together with CAM-B3LYP, M06-2X, and the two ω-tuned range-separated functionals LC-ω*PBE and LC-ω*HPBE demonstrated the best agreement with SCS-CC2 calculations in predicting the absolute energy of the singlet S1, and triplet T1 and T2 excited states and their energy differences. However, consistently across the series and irrespective of the functional or the use of TDA, the nature of T1 and T2 is not as accurately captured as compared to S1. We also investigated the impact of the optimization of S1 and T1 excited states on ΔEST and the nature of these states for three different functionals (PBE0, CAM-B3LYP, and M06-2X). We observed large changes in ΔEST using CAM-B3LYP and PBE0 functionals associated with a large stabilization of T1 with CAM-B3LYP and a large stabilization of S1 with PBE0, while ΔEST is much less affected considering the M06-2X functional. The nature of the S1 state barely evolves after geometry optimization essentially because this state is CT by nature for the three functionals tested. However, the prediction of the T1 nature is more problematic since these functionals for some compounds interpret the nature of T1 very differently. SCS-CC2 calculations on top of the TDA-DFT optimized geometries lead to a large variation in terms of ΔEST and the excited-state nature depending on the chosen functionals, further stressing the large dependence of the excited-state features on the excited-state geometries. The presented work highlights that despite good agreement of energies, the description of the exact nature of the triplet states should be undertaken with caution. The St Andrews team would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie–Bruxelles, infrastructure funded by the Walloon Region under the Grant Agreement n1117545. Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant no. F.4534.21 (MIS-IMAGINE). D.B. is a FNRS Research Director. J.C.S.-G. acknowledges funding from the “Ministerio de Ciencia e Innovación” through the PID2019-106114GB-I00 project.

Funding: Engineering and Physical Sciences Research Council - EP/P010482/1; Leverhulme Trust - RPG-2016-047; Royal Society - SRF\R1\201089; China Scholarship Council - 201603780001; Horizon 2020 Framework Programme - 838885; Fonds De La Recherche Scientifique - FNRS - 2.5020.11; Fonds De La Recherche Scientifique - FNRS - F.4534.21; Horizon 2020 Framework Programme - 101023743. The use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi-resonance TADF (MR-TADF) emitter, DiKTa-LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green-emitting (λEL = 492 nm) solution-processed OLEDs based on DiKTa-LC showed an EQEmax of 13.6%. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed OLEDs. Publisher PDF

The design of semiconducting polymers with optimal charge transport characteristics has been at the crux of scientific research during the recent decades. While increase in crystalline order and planar conjugated backbones were demonstrated to be the key to success, they are not always mandatory. Sometimes, the charge carrier mobility can be enhanced by selecting conjugated backbones that are resilient to thermal fluctuations, despite leading to poor structural order. Herein, by coupling all-atom molecular dynamics simulations, electronic structure calculations, and kinetic Monte Carlo charge transport simulations, we demonstrate that the charge carrier mobility in amorphous donor-acceptor conjugated polymers is controlled by the density and quality of close-contact points between the chains and that the latter varies with the size of the donor block and the resulting alkyl side-chain density. We show an application of this strategy to the high-mobility poly(indacenodithiophene-alt-benzothiadiazole) (IDTBT) and poly(dithiopheneindenofluorene-alt-benzothiadiazole) (TIFBT) copolymers.

Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No 838885 (NarrowbandSSL). S.M.S. acknowledges support from the Marie Skłodowska-Curie Individual Fellowship (grant agreement No 838885 NarrowbandSSL). We would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. E.Z-C. and I.D.W.S acknowledge support from EPSRC (EP/L017008, EP/P010482/1). Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F. R. S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant n° F.4534.21 (MIS-IMAGINE). G. R. acknowledges a grant from the ‘‘Fonds pour la formation a la Recherche dans l’Industrie et dans l’Agriculture’’ (FRIA) of the FRS-FNRS. We present a p- and n-doped nonacene compound, NOBNacene, that represents a rare example of a linearly extended ladder-type multiresonant thermally activated delayed fluorescence (MR-TADF) emitter. This compound shows efficient narrow deep blue emission, with a λPL of 410 nm, full width at half maximum, FWHM, of 38 nm, photoluminescence quantum yield, ΦPL of 71 %, and a delayed lifetime, τd of 1.18 ms in 1.5 wt % TSPO1 thin film. The organic light-emitting diode (OLED) using this compound as the emitter shows a comparable electroluminescence spectrum peaked at 409 nm (FWHM=37 nm) and a maximum external quantum efficiency (EQEmax) of 8.5 % at Commission Internationale de l’Éclairage (CIE) coordinates of (0.173, 0.055). The EQEmax values were increased to 11.2 % at 3 wt % doping of the emitter within the emissive layer of the device. At this concentration, the electroluminescence spectrum broadened slightly, leading to CIE coordinates of (0.176, 0.068). Publisher PDF

The importance of intermediate triplet states and the nature of excited states has gained interest in recent years for the thermally activated delayed fluorescence (TADF) mechanism. It is widely accepted that simple conversion between charge transfer (CT) triplet and singlet excited states is too crude, and a more complex route involving higher-lying locally excited triplet excited states has to be invoked to witness the magnitude of the rate of reverse intersystem crossing (RISC) rates. The increased complexity has challenged the reliability of computational methods to accurately predict the relative energy between excited states as well as their nature. Here we compare the results of widely used DFT functionals, CAM-B3LYP, LC-ꞷPBE, LC-ꞷ*PBE, LC-ꞷ*HPBE, B3LYP, PBE0 and M06-2X against a wavefunction-based reference method, Spin-Component Scaling second-order approximate Coupled-Cluster (SCS-CC2) in fourteen known TADF emitters possessing a diversity of chemical structures. Overall, the use of the Tamm-Dancoff Approximation (TDA) together with the CAM-B3LYP, M06-2X and the two ꞷ-tuned range-separated functionals LC-ꞷ*PBE and LC-ꞷ*HPBE demonstrated the best agreement with SCS-CC2 calculations in predicting the absolute energy of the singlet S1, and triplet T1 and T2 excited states and their energy differences. However, consistently across the series and irrespective of the functional or the use of TDA, the nature of T1 and T2 is not as accurately captured as compared to S1. The presented work highlights that despite good agreement of energies, the description of the exact nature of the triplet states should be undertaken with caution.

We present a p- and n-doped nonacene compound, NOBNacene, that represents a rare example of a linearly extended ladder-type multiresonant thermally activated delayed fluorescence (MR-TADF) emitter. This compound shows efficient narrow deep blue emission (PL = 410 nm, FWHM = 38 nm, ФPL = 71%, d = 1.18 ms) in 1.5 wt% TSPO1 thin film. The organic light-emitting diode (OLED) using this compound as the emitter shows a comparable electrolumines-cence spectrum (EL = 409 nm, FWHM = 37 nm) and a maximum external quantum efficiency (EQEmax) of 8.5% at CIE coordinates of (0.173, 0.055). The EQEmax values were increased to 11.2% at 3 wt% doping of the emitter within the emissive layer of the device. At this concentra-tion, the electroluminescence spectrum broadened slightly, leading to CIE coordinates of (0.176, 0.068).

We show how borylation of an acceptor-donor-acceptor (A-D-A) thermally activated delayed fluorescence (TADF) emitter, DIDOBNA-N, transforms the compound into a multi-resonant TADF (MR-TADF) emitter, MesB-DIDOBNA-N. DIDOBNA-N emits bright blue light (𝜆PL = 444 nm, FWHM = 64 nm, 𝛷PL = 81%, 𝜏d = 23 𝜇s, 1.5 wt% in TSPO1). The deep blue organic light-emitting diode (OLED) based on this compound shows a very high maximum external quantum efficiency (EQEmax) of 15.3% for a device with CIEy of 0.073. The MR-TADF emitter, MesB-DIDOBNA-N shows efficient and narrowband violet emission (𝜆PL = 402 nm, FWHM = 19 nm, 𝛷PL = 74.7%, 𝜏d = 133 𝜇s, 1.5 wt% in TSPO1). The OLED with MesB-DIDOBNA-N shows out-standing efficiency for a violet OLED at 9.3% and CIEy = 0.044, which is the bluest EL reported for a MR-TADF OLED to date. Noteworthy are the CIE coordinates of (0.166, 0.045), which are very close to the Rec.2020 standard for blue (0.131, 0.046). The EQEmax values were improved from 9.3% to 13.6% by increasing the concentration of the emitter in the host from 1.5 wt% to 5 wt%.

Funding: Walloon Region, Grant/Award Number:n1117545; Leverhulme Trust, Grant/Award Numbers: RPG‐2016047,SRF\R1\201089; Natural Sciences and Engineering Research Council Canada, Grant/Award Numbers: DG RGPIN‐2018‐06556, SPG STPGP‐2016‐493924; Fonds dela Recherche Scientifiques de Belgique, Grant/Award Numbers: 2.5020.11,F.4534.21; Engineering and Physical Sciences Research Council, Grant/Award Number: EP/P010482/1. The electrochemiluminescence (ECL) behavior of a multiresonance thermally activated delayed fluorescence molecule has been investigated for the first time by means of ECL-voltage curves, newly designed ECL-time observatory, and ECL spectroscopy. The compound, Mes3DiKTa, shows complex ECL behavior, including a delayed onset time of 5 ms for ECL generation in both the annihilation pathway and the coreactant route, which we attribute to organic long-persistent ECL (OLECL). Triplet-triplet annihilation, thermally activated delayed fluorescence and uncompensated solution resistance cannot be ruled out as contributing mechanisms to the ECL. A very long ECL emission decay was attributed to OLECL as well. The absolute ECL efficiencies of Mes3DiKTa were enhanced and reached 0.0013% in annihilation route and 1.1% for the coreactant system, which are superior to those of most other organic ECL materials. It is plausible that ECL materials with comparable behavior as Mes3DiKTa are desirable in applications such as ECL sensing, imaging, and light-emitting devices. Publisher PDF

This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No 838885 (NarrowbandSSL) and under the Marie Skłodowska Curie grant agreement No 812872 (TADFlife). S.M.S. acknowledges support from the Marie Skłodowska-Curie Individual Fellowship (grant agreement No 838885 NarrowbandSSL). The St. Andrews team would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. E. Z.-C. is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F. R. S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n 1117545. Developing high-efficiency purely organic blue organic light-emitting diodes (OLEDs) that meet the stringent industry standards is a major current research challenge. Hyperfluorescent device approaches achieve in large measure the desired high performance by combining the advantages of a high-efficiency thermally activated delayed fluorescence (TADF) assistant dopant with a narrowband deep-blue multi-resonant TADF (MR-TADF) terminal emitter. However, this approach requires suitable spectral overlap to support Förster resonance energy transfer (FRET) between the two. Here, a color tuning of a recently reported MR-TADF B,N-heptacene core through control of the boron substituents is demonstrated. While there is little impact on the intrinsic TADF properties—as both singlet and triplet energies decrease in tandem—this approach improves the emission color coordinate as well as the spectral overlap for blue hyperfluorescence OLEDs (HF OLEDs). Crucially, the red-shifted and more intense absorption allows the new MR-TADF emitter to pair with a high-performance TADF assistant dopant and achieve maximum external quantum efficiency (EQEmax) of 15% at color coordinates of (0.15 and 0.10). The efficiency values recorded for the device at a practical luminance of 100 cd m–2 are among the highest reported for HF TADF OLEDs with CIEy ≤ 0.1. Publisher PDF

Herein, we reported an ultrapure blue multiple-resonance-induced thermally activated delayed fluorescence (MR-TADF) material (ν-DABNA-O-Me) with a high photoluminescence quantum yield and a large rate constant for reverse intersystem crossing. Because of restricted π-conjugation of the HOMO rather than the LUMO induced by oxygen atom incorporation, ν-DABNA-O-Me shows a hypsochromic shift compared to the parent MR-TADF material (ν-DABNA). An organic light-emitting diode based on this material exhibits an emission at 465 nm, with a small full-width at half-maximum of 23 nm and Commission Internationale de l'Eclairage coordinates of (0.13, 0.10), and a high maximum external quantum efficiency of 29.5 %. Moreover, ν-DABNA-O-Me facilitates a drastically improved efficiency roll-off and a device lifetime compared to ν-DABNA, which demonstrates significant potential of the oxygen atom incorporation strategy.

Two new deep-blue narrowband multi-resonant emitters, 1B-DTACrs and 2B-DTACrs, one of which shows thermally activated delayed fluorescence (TADF), based on boron, nitrogen, and oxygen doped nanographenes are reported. Devices based on 2B-DTACrs showed an EQE

The inversion of the lowest singlet and triplet excited state energy gap, in fully organic triangle-based compounds, can give rise to a new exergonic pathway to enhance the Organic Light Emitting Diodes (OLEDs) performance, going beyond the novel yet promising Thermally Activated Delayed Fluorescence (TADF) mechanism. If, on one hand, the origin of this inversion, arising from the interplay between exchange and electron correlation effects, has been extensively investigated in last years, identifying the wavefunction methods as key to predict the excited-state inversion, on the other hand a proper picture of the structure-property relationships characterizing these systems is still missing. In this work, we thus assess the effect of different symmetry point groups (D3h, C2v, C3h and C3v) on the orbital localization to shed light on the role that the symmetry has in determining the optical features of the triangulene systems (on both S1-T1 inversion and oscillator strengths). The presence of the C_2 axis and the σ_v plane (as it happens for the D3h, C2v and C3v groups) turned out to be critical for ensuring the proper orbital localization aimed at minimizing the exchange interaction and therefore favouring the inversion. In particular, adopting a C2v (and its subgroups) symmetry, either through the proper doping pattern, by introducing substituents, or by merging two triangulene cores, is the only way to conciliate a negative ΔEST and a non-zero oscillator strength. Finally, we gathered the lessons learnt from this analysis to establish a series of design rules, aimed at helping the identification of inverted singlet-triplet (INVEST) emitters for applications in the next generation of OLEDs. Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant n° F.4534.21 (MIS-IMAGINE). G. R. acknowledges a grant from the ‘‘Fonds pour la formation a la Recherche dans l’Industrie et dans l’Agriculture’’ (FRIA) of the FRS-FNRS. Computational resources were also provided by the ‘‘Consortium des Équipements de Calcul Intensif’’ (CÉCI), funded by the ‘‘Fonds de la Recherche Scientifiques de Belgique’’ (FRS-FNRS) under Grant No. 2.5020.11. The work in Alicante is supported by the Ministry of Science and Innovation of Spain (project PID2019-106114GB-I00).

This work was supported financially by the JSPS Core-to-Core Program (grant number: JPJSCCA20180005) and Kyulux Inc. S. M. S. acknowledges support from the Marie Skłodowska-Curie Individual Fellowship (grant agreement No 838885 NarrowbandSSL). We would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. E. Z.-C. is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). We thank the EPRSC for funding (EP/R035164/1). Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F. R. S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. Y. O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant no. F.4534.21 (MIS-IMAGINE). D. B. is a FNRS Research Director. Two new deep-blue narrowband multi-resonant emitters, 1B-DTACrs and 2B-DTACrs, one of which shows thermally activated delayed fluorescence (TADF), based on boron, nitrogen, and oxygen doped nanographenes are reported. Devices based on 2B-DTACrs showed an EQEmax of 14.8% and CIE coordinates of (0.150, 0.044), which are very close to the BT.2020 requirement for blue pixels. Publisher PDF

The St Andrews team would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. E. Z.-C. is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. G.R. acknowledges a grant from the “Fonds pour la formation à la Recherche dans l’Industrie et dans l’Agriculture” (F.R.I.A.) of the F.R.S.-F.N.R.S. Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant n° F.4534.21 (MIS-IMAGINE). D.B. is a FNRS Research Director. J.C.S.G. acknowledges “Ministerio de Ciecia e Innovación” of Spain (PID2019-106114GB-I00). With the surge of interest in multiresonant thermally activated delayed fluorescent (MR-TADF) materials, it is important that there exist computational methods to accurately model their excited states. Here, building on our previous work, we demonstrate how the spin-component scaling second-order approximate coupled-cluster (SCS-CC2), a wavefunction-based method, is robust at predicting the ΔEST (i.e., the energy difference between the lowest singlet S1 and triplet T1 excited states) of a large number of MR-TADF materials, with a mean average deviation (MAD) of 0.04 eV compared to experimental data. Time-dependent density functional theory calculations with the most common DFT functionals as well as the consideration of the Tamm-Dancoff approximation (TDA) consistently predict a much larger ΔEST as a result of a poorer account of Coulomb correlation as compared to SCS-CC2. Very interestingly, the use of a metric to assess the importance of higher order excitations in the SCS-CC2 wavefunctions shows that Coulomb correlation effects are substantially larger in the lowest singlet compared to the corresponding triplet and need to be accounted for a balanced description of the relevant electronic excited states. This is further highlighted with coupled cluster singles-only calculations, which predict very different S1 energies as compared to SCS-CC2 while T1 energies remain similar, leading to very large ΔEST, in complete disagreement with the experiments. We compared our SCS-CC2/cc-pVDZ with other wavefunction approaches, namely, CC2/cc-pVDZ and SOS-CC2/cc-pVDZ leading to similar performances. Using SCS-CC2, we investigate the excited-state properties of MR-TADF emitters showcasing large ΔET2T1 for the majority of emitters, while π-electron extension emerges as the best strategy to minimize ΔEST. We also employed SCS-CC2 to evaluate donor–acceptor systems that contain a MR-TADF moiety acting as the acceptor and show that the broad emission observed for some of these compounds arises from the solvent-promoted stabilization of a higher-lying charge-transfer singlet state (S2). This work highlights the importance of using wavefunction methods in relation to MR-TADF emitter design and associated photophysics. Postprint

Two new deep-blue narrowband multi-resonant emitters, 1B-DTACrs and 2B-DTACrs, one of which showing thermally activated delayed fluorescence (TADF), based on boron, nitrogen, and oxygen doped nanographenes were designed and synthesized in good yields. Precise control of the extent of borylation was achieved by modulating the reaction temperature and stoichiometry of the Lewis acid in the reaction. A comprehensive structural, theoretical, photophysical, electrochemical, and electroluminescent study is presented. Both emitters possess high photoluminescence quantum yields of up to 83%, small full width at half maximums (FWHMs) of around 24 nm, and deep blue emissions of around 450 nm in mCBP films. Devices based on 2B-DTACrs showed high maximum external quantum effi-ciencies of 14.8% and a deep-blue emission at 447 nm together with Commission Internatio-nale de l’Éclairage coordinates of (0.150, 0.044), which are very close to the BT.2020 re-quirement for blue pixels.

The inversion of the lowest singlet and triplet excited state energy gap, in fully organic triangle-based compounds, can give rise to a new exergonic pathway to enhance the Organic Light Emitting Diodes (OLEDs) performance, going beyond the novel yet promising Thermally Activated Delayed Fluorescence (TADF) mechanism. If, on one hand, the origin of this inversion, arising from the interplay between exchange and electron correlation effects, has been extensively investigated in last years, identifying the wavefunction methods as key to predict the excited-state inversion, on the other hand a proper picture of the structure-property relationships characterizing these systems is still missing. In this work, we thus assess the effect of different symmetry point groups (D3h, C2v, C3h and C3v) on the orbital localization to shed light on the role that the symmetry has in determining the optical features of the triangulene systems (on both S1-T1 inversion and oscillator strengths). The presence of the C_2 axis and the σ_v plane (as it happens for the D3h, C2v and C3v groups) turned out to be critical for ensuring the proper orbital localization aimed at minimizing the exchange interaction and therefore favouring the inversion. In particular, adopting a C2v (and its subgroups) symmetry, either through the proper doping pattern, by introducing substituents, or by merging two triangulene cores, is the only way to conciliate a negative ΔEST and a non-zero oscillator strength. Finally, we gathered the lessons learnt from this analysis to establish a series of design rules, aimed at helping the identification of inverted singlet-triplet (INVEST) emitters for applications in the next generation of OLEDs.

S.W. thanks the China Scholarship Council (201906250199). EZ-C and IDWS acknowledge support from EPSRC (EP/L017008, EP/P010482/1). We are also grateful for financial support from the University of St Andrews Restarting Research Funding Scheme (SARRF) which is funded through the Scottish Funding Council grant reference SFC/AN/08/020. EZ-C is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). We would also like to thank the Leverhulme Trust (RPG-2016-047) for financial support. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No 838885 (NarrowbandSSL). S.M.S. acknowledges support from the Marie Skłodowska-Curie Individual Fellowship. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant n° F.4534.21 (MIS-IMAGINE). D.B. is a FNRS Research Director. Strategies to tune the emission of multiresonant thermally activated delayed fluorescence (MR-TADF) emitters remain rare. Here, we explore the effect of donor substitution about a MR-TADF core on the emission energy and the nature of the excited state. We decorate different numbers and types of electron-donors about a central MR-TADF core, DiKTa. Depending on the identity and number of donor groups, the excited state either remains short-range charge transfer (SRCT) and thus characteristic of an MR-TADF emitter or becomes a long-range charge transfer (LRCT) that is typically observed in donor–acceptor TADF emitters. The impact is that in three examples that emit from a SRCT state, Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa, the emission remains narrow, while in four examples that emit via a LRCT state, TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa, the emission broadens significantly. Through this strategy, the organic light-emitting diodes fabricated with the three MR-TADF emitters show maximum electroluminescence emission wavelengths, λEL, of 511, 492, and 547 nm with moderate full width at half-maxima (fwhm) of 62, 61, and 54 nm, respectively. Importantly, each of these devices show high maximum external quantum efficiencies (EQEmax) of 24.4, 23.0, and 24.4%, which are among the highest reported with ketone-based MR-TADF emitters. OLEDs with D–A type emitters, DMAC-DiKTa and TMCz-DiKTa, also show high efficiencies, with EQEmax of 23.8 and 20.2%, but accompanied by broad emission at λEL of 549 and 527 nm, respectively. Notably, the DMAC-DiKTa-based OLED shows very small efficiency roll-off, and its EQE remains 18.5% at 1000 cd m–2. Therefore, this work demonstrates that manipulating the nature and numbers of donor groups decorating a central MR-TADF core is a promising strategy for both red-shifting the emission and improving the performance of the OLEDs. Publisher PDF

Authors thank the Leverhulme Trust (RPG-2016-047). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No 838885 (NarrowbandSSL) and 812872 (TADFlife). We thank Umicore for their generous supply of catalysts. S.S. acknowledges support from the Marie Skłodowska-Curie Individual Fellowship. SB acknowledges support from the Bayrisches Staatsministerium für Wissenschaft und Kunst (Stmwk) in the framework of the initiative "SolTech", as well as from the German Science foundation (DFG) (No. 392306670). Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n111754. An easy-to-access, near-UV-emitting linearly extended B,N-doped heptacene with high thermal stability is designed and synthesized in good yields. This compound exhibits thermally activated delayed fluorescence (TADF) at ambient temperature from a multiresonant (MR) state and represents a rare example of a non-triangulene-based MR-TADF emitter. At lower temperatures triplet–triplet annihilation dominates. The compound simultaneously possesses narrow, deep-blue emission with CIE coordinates of (0.17, 0.01). While delayed fluorescence results mainly from triplet–triplet annihilation at lower temperatures in THF solution, where aggregates form upon cooling, the TADF mechanism takes over around room temperature in solution when the aggregates dissolve or when the compound is well dispersed in a solid matrix. The potential of our molecular design to trigger TADF in larger acenes is demonstrated through the accurate prediction of ΔEST using correlated wave-function-based calculations. On the basis of these calculations, we predicted dramatically different optoelectronic behavior in terms of both ΔEST and the optical energy gap of two constitutional isomers where only the boron and nitrogen positions change. A comprehensive structural, optoelectronic, and theoretical investigation is presented. In addition, the ability of the achiral molecule to assemble on a Au(111) surface to a highly ordered layer composed of enantiomorphic domains of racemic entities is demonstrated by scanning tunneling microscopy. Postprint

International audience; We have investigated the potential as an OLED emitter of a spiro compound with phosphine substituents initially designed as a host layer for triplet emitters. This is motivated by the fact that the spiro architecture is expected to reduce the detrimental impact of intermolecular interactions, thus allowing for the use of a single-component layer. Moreover, theoretical calculations suggest that this compound displays all features required for a thermally-assisted delayed fluorescence (TADF) behavior. The photophysical properties of the compound in solution are extremely unusual, as illustrated in particular by a dual emission band with relative intensities strongly depending on the experimental conditions. Although no TADF behavior is evidenced in OLED devices, the compound unexpectedly generates a white light signal, which is interpreted as the result of the formation of both intramolecular and intermolecular (exciplex) charge-transfer states.

The development of multiple resonance thermally activated delayed fluorescence (MR-TADF) materials possessing narrow emission spectra has attracted significant attention as emitters for high colour purity organic light emitting diodes (OLEDs). In this work, a simple design strategy is introduced to construct an MR-TADF emitter, DDiKTa, through dimerization of the known MR-TADF emitter DiKTa. This design permits concentration quenching to be largely suppressed, which is a known weakness of previously reported MR-TADF emitters. OLEDs based on DDiKTa show an EQEmax of 19% at a doping concentration of 9 wt%. The electroluminescence spectrum is red-shifted into the green, producing a rare example of a green-emitting MR-TADF OLED.

Examples of multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters that emit at longer wavelengths remain rare. To reach that goal, we decorate different numbers and types of electron-donors about a central MR-TADF core, DiKTa. Depending on the identity and number of donor groups, the excited state either remains short range charge transfer (SRCT) and thus characteristic of an MR-TADF emitter or becomes a long-range charge transfer (CT) that is typically observed in donor-acceptor TADF emitters. The impact is that in three examples, Cz-DiKTa, Cz-Ph-DiKTa and 3Cz -DiKTa, which emit from a SRCT state, the emission remains narrow, while in four examples, TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa and 3DMAC-DiKTa, which emit via a CT state, the emission broadens significantly. Through this strategy, the organic light-emitting diodes fabricated with the three MR-TADF emitters show maximum electroluminescence emission wavelengths, EL, of 511, 492 and 547 nm with moderate full-width at half maxima (FWHM) of 62, 61 and 54 nm, respectively. Importantly, each of these devices show high maximum external quantum efficiencies (EQEmax) of 24.4%, 23.0% and 24.4%, which are amongst the highest reported with ketone-based MR-TADF emitters. OLEDs with D-A type emitters, DMAC-DiKTa and TMCz-DiKTa, also show high efficiencies, with EQEmax of 23.8% and 20.2%, but accompanied by broad emission at EL of 549 and 527 nm, respectively. Noteworthy is that the DMAC-DiKTa-based OLED shows very small efficiency roll-off, and its EQE remains 18.5% at 1000 cd m-2. Therefore, this work demonstrates that manipulating the nature and numbers of donor groups decorating a central MR-TADF core is a promising strategy for both red-shifting the emission and improving the performance of the OLEDs.

The use of thermally activated delayed fluorescence emitters and emitters that show preferential horizontal orientation of their transition dipole are two emerging strategies to enhance the efficiency of organic light-emitting diodes. We present the first example of a liquid crystalline multi-resonance thermally activated delayed fluorescent emitter, DiKTaLC. The neat film of DiKTaLC shows a photoluminescence quantum yield of 41%, a singlet-triplet energy gap, ΔEST, of 0.20 eV, and a delayed lifetime, τd, of 70.2 µs. The compound possesses a nematic discotic liquid crystalline phase between 80 °C and 110 °C. More importantly, the transition dipole moment of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.26. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed organic light-emitting diodes.

Developing high-efficiency purely organic blue organic light-emitting diodes (OLEDs) that meet the stringent industry standards is a major current research challenge. Hyperfluorescent device approaches achieve in large measure the desired high performance by combining the advantages of a high-efficiency thermally activated delayed fluorescence (TADF) assistant dopant with a narrowband deep-blue multi-resonant TADF (MR-TADF) terminal emitter. However, this ap-proach requires suitable spectral overlap to support Förster resonance energy transfer (FRET) between the two. Here we demonstrate colour tuning of a recently reported MR-TADF B,N-heptacene core through control of the boron substituents. While there is little impact on the intrinsic TADF properties - as both singlet and triplet energies decrease in tandem - this approach improves the emission colour coordinate as well as the spectral overlap for blue hyperfluorescence OLEDs (HF OLEDs). Crucially, the red-shifted and more intense absorption allows us to pair this MR-TADF emitter with a high-performance TADF assistant dopant and achieve maximum external quantum efficiency (EQEmax) of 15% at colour coordinates of (0.15, 0.10). The efficiency values recorded for our device at a practical luminance of 100 cd m-2 are among the highest reported for HF TADF OLEDs with CIEy ≤ 0.1.

The St Andrews team would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F. R. S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. We acknowledge support from the European Union's Horizon 2020 research and innovation programme under the ITN TADFlife (GA 812872). Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant no. F.4534.21 (MIS-IMAGINE). D. B. is a FNRS Research Director. EZ-C is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). In this work we present a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter paradigm, demonstrating that the structure need not require the presence of acceptor atoms. Based on an in silico design, the compound DiICzMes4 possesses a red-shifted emission, enhanced photoluminescence quantum yield, and smaller singlet-triplet energy gap, ΔEST, than the parent indolocarbazole that induces MR-TADF properties. Coupled cluster calculations accurately predict the magnitude of the ΔEST when the optimized singlet and triplet geometries are used. Slow yet optically detectable reverse intersystem crossing contributes to low efficiency in organic light-emitting diodes using DiICzMes4 as the emitter. However, when used as a terminal emitter in combination with a TADF assistant dopant within a hyperfluorescence device architecture, maximum external quantum efficiencies of up to 16.5% were achieved at CIE (0.15, 0.11). This represents one of the bluest hyperfluorescent devices reported to date. Simultaneously, recognising that MR-TADF emitters do not require acceptor atoms reveals an unexplored frontier in materials design, where yet greater performance may yet be discovered. Publisher PDF

Funding: Scottish Funding Council (SFC/AN/08/020), Leverhulme Trust (RPG-2016-047), H2020 Marie Skłodowska-Curie Actions (838009, 859752), UK Engineering and Physical Sciences Research Council (EP/P010482/1, EP/R00188X/1), Royal Society (SRF\R1\201089), Fonds De La Recherche Scientifique - FNRS (2.5020.11, F.4534.21), Università di Pisa (PRA 2020_21). We present the first example of a multi-resonant thermally activated delayed fluorescent (MR-TADF) extended helicene, Hel-DiDiKTa. This S-shaped double helicene exhibits sky-blue emission, a singlet-triplet energy gap, ΔEST, of 0.15 eV and narrow emission at a peak maximum of 473 nm with a full-width at half-maximum of 44 nm in toluene. The MR-TADF character is confirmed by the small degree of positive solvatochromism and temperature-dependent increase in intensity of the delayed emission. The chiroptical properties of the separated enantiomers are similar to other large helicenes with comparable dissymmetry values, but with the added benefit of MR-TADF. (P)-Hel-DiDiKTa is stable towards enantiomerization, with a Gibbs free energy of activation for enantiomerization (ΔGe‡) of 31± 2 kcal mol-1 at 298 K, a value similar to other reported double helicenes. (P)-Hel-DiDiKTa is also thermally stable, with a 5% weight loss at 399 oC revealed by thermogravimetric analysis (TGA). Thus, this study further strengthens the burgeoning area of chiral TADF emitters for use in cutting-edge optoelectronic and photocatalytic molecules and materials. Publisher PDF

The St Andrews team would like to thank the Leverhulme Trust (RPG-2016-047) and EPSRC (EP/P010482/1) for financial support. E. Z.-C. is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). Multi-resonant thermally activated delayed fluorescence (MR-TADF) materials have attracted considerable attention recently. The molecular design frequently incorporates cycloboration. However, to the best of our knowledge MR-TADF compounds containing nitrogen chelated to boron are still unknown. Reported herein is a new class of tetracoordinate boron-containing MR-TADF emitters bearing C^N^C- and N^N^N-chelating ligands. We demonstrate that the replacement of the B–C covalent bond in the C^N^C-chelating ligand by the B–N covalent bond affords an isomer, which dramatically influences the optoelectronic properties of the molecule. The resulting N^N^N-chelating compounds show bathochromically shifted absorption and emission spectra relative to C^N^C-chelating compounds. The incorporation of a tert-butylcarbazole group at the 4-position of the pyridine significantly enhances both the thermal stability and the reverse intersystem crossing rate, yet has a negligible effect on emission properties. Consequently, high-performance hyperfluorescent organic light-emitting diodes (HF-OLEDs) that utilize these molecules as green and yellow-green emitters show a maximum external quantum efficiency (ηext) of 11.5% and 25.1%, and a suppressed efficiency roll-off with an ηext of 10.2% and 18.7% at a luminance of 1000 cd m−2, respectively. Publisher PDF

Thermally-activated delayed fluorescence (TADF) enables organic semiconductors with charge transfer (CT)-type excitons to convert dark triplet states into bright singlets via a reverse intersystem crossing (rISC) process. Here, we consider the role of the dielectric environment in a range of TADF materials with varying changes in dipole moment upon optical excitation. In a dipolar reference emitter, TXO-TPA, environmental reorganisation after excitation in both solution and doped films triggers the formation of the full CT product state. This lowers the singlet excitation energy by 0.3 eV and minimises the singlet-triplet energy gap ({\Delta}EST). Using impulsive Raman measurements, we observe the emergence of two (reactant-inactive) modes at 412 and 813 cm-1 as a vibrational fingerprint of the CT product. In contrast, the dielectric environment plays a smaller role in the electronic excitations of a less dipolar material, 4CzIPN. Quantum-chemical calculations corroborate the appearance of these new product modes in TXO-TPA and show that the dynamic environment fluctuations are large compared to {\Delta}EST. The analysis of the energy-time trajectories and the corresponding free energy functions reveals that the dielectric environment significantly reduces the activation energy for rISC, thus increasing the rISC rate by up to three orders of magnitude when compared to a vacuum environment., Comment: 67 pages, 7 main-text figures

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm−1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters., A low singlet-triplet energy gap, necessary for delayed fluorescence organic semiconductors, results in a small radiative rate that limits performance in OLEDs. Here, the authors show that it is possible to reconcile these conflicting requirements in materials that can access both high oscillator strength intramolecular excitations and intermolecular charge transfer states.

With the surge of interest in multi-resonant thermally activated delayed fluorescent (MR-TADF) materials it is important that there exist computational methods to accurately model their excited states. Here, building on our previous work, we demonstrate how the Spin-Component Scaling second-order approximate Coupled-Cluster (SCS-CC2), a wavefunction-based method, is robust at predicting the ΔEST (i.e., the energy difference between the lowest singlet and triplet excited states) of a large number of MR-TADF materials, with a mean average deviation (MAD) of 0.04 eV compared to experimental data. Time-Dependent Density Functional Theory calculations with the most common DFT functionals as well as the consideration of the Tamm-Dancoff approximation (TDA) consistently predict a much larger ΔEST owing to the absence of an explicit account of double (or higher order) excitations. This contribution is key in order to describe more precisely the Coulomb correlation that results in a stabilization of the S1 state. We also employed SCS-CC2 to evaluate donor-acceptor systems that contain a MR-TADF moiety acting as the acceptor and show that the broad emission observed for some of these compounds arises from the solvent-promoted stabilization of a higher-lying charge transfer (CT) singlet state (S2). This work highlights the importance of using wavefunction methods in relation MR-TADF emitter design and associated photophysics.

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet-triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy-time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule.

E.Z.-C. thanks the Leverhulme Trust (No. RPG-2016-047) and the University of St Andrews for support. The authors are grateful to the EPSRC for financial support (grants EP/ P007805/1, EP/P010482/1, EP/L017008/1, EP/J01771X, and EP/J00916). M.M. thanks the Innovation Programme H2020-MSCA-IF-2014-659237 for financial support. W.L. thanks the China Scholarship Council (grant number 201708060003). V.L. thanks the F.R.S.-FNRS for his Research Associate position. Computational resources have been provided by the Consortium des É quipements de Calcul Intensif (CÉ CI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.FNRS) under Grant No. 2.5020.11, GEQ U.G006.15, 1610468, and RW/GEQ. (2016). D.B. is an FNRS Research Director. Herein, we report the use of spiro-configured fluorene-xanthene scaffolds as a novel, promising, and effective strategy in thermally activated delayed fluorescence (TADF) emitter design to attain high photoluminescence quantum yields (ΦPL), short delayed luminescence lifetime, high external quantum efficiency (EQE), and minimum efficiency roll-off characteristics in organic light-emitting diodes (OLEDs). The optoelectronic and electroluminescence properties of SFX (spiro-(fluorene-9,9′-xanthene))-based emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally. All three emitters exhibited sky blue to green emission enabled by a Herzberg–Teller mechanism in the excited state. They possess short excited-state delayed lifetimes (