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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

AKG is grateful to the Royal Society for Newton International Fellowship NF171163. We acknowledge support from the Engineering and Physical Sciences Research Council of the UK (grants EP/P010482/1 and EP/L017008/1). We are also grateful for financial support from the University of St Andrews Restarting Research and Restarting Interdisciplinary Research Funding Schemes (SARRF and SARIRF) which are funded through the Scottish Funding Council grant reference SFC/AN/08/020. We have designed a highly twisted small TADF emitter PXZ-α-DK based on an a-diketone (α-DK) as a strong acceptor and phenoxazine (PXZ) as a strong donor to obtain red-shifted emission in comparison to the equivalent a-diketone linked to 9,9-dimethyl-9,10-dihydroacridine (DMAC). The PXZ-α-DK shows emission at 586 nm and DMAC-α-DK shows emission at 548 nm in 1,3-bis(N-carbazolyl)benzene (mCP) host at 1.5 wt% doping of the emitter, with short-delayed lifetimes of 6.9 μs for PXZ-α-DK and 7.6 μs for DMAC-α-DK. OLEDs fabricated using these emitters show green electroluminescence at 555 nm for DMAC-α-DK, with a maximum external quantum efficiency, EQEmax, of 6.3%, and orange electroluminescence at 585 nm for PXZ-α-DK, with an EQEmax of 0.8%. We corroborate the optoelectronic properties of these emitters with DFT calculations. Postprint

Authors thank EU Horizon 2020 Grant Agreement No. 812872 (TADFlife) for funding. The St Andrews team acknowledges support from the Engineering and Physical Sciences Research Council of the UK (grant EP/P010482/1). The authors from the University of Parma acknowledge the support from the HPC (High Performance Computing) facility of the University of Parma, Italy. Moreover, authors from University of Parma benefited from the equipment and support of the COMP-HUB Initiative, funded by the “Departments of Excellence” program of the Italian Ministry for Education, University and Research (MIUR, 2018-2022). We present the synthesis, optoelectronic characterization, and a detailed theoretical study of DMAC-py-TRZ, a novel, efficient TADF emitter. This compound is a structural relative of the well-known TADF compound DMAC-TRZ, substituting the bridging phenylene for a pyridyl group. This marginal change has an enormous impact on the structure and hence on the photophysics as the steric interactions between the DMAC donor and the bridge that force DMAC-TRZ into an orthogonal conformation are attenuated and permit DMAC-py-TRZ to adopt a planar and slightly bent structure in the ground state. The large degree of conjugation in the bent DMAC-py-TRZ structure, demonstrated by the strong intensity of the lowest excitation with CT character, is responsible for a large singlet triplet gap, hence hindering TADF of this bent conformer. The computational analysis predicts that emission occurs, however, from a relaxed orthogonal excited-state geometry, as confirmed by the huge Stokes shift observed in non-polar solvents. In this relaxed orthogonal geometry TADF is indeed observed. Emission from the unrelaxed state is recovered in glassy frozen solvents, where the emission band is largely blue-shifted compared with measurements in liquid solvent, and TADF is quenched. In amorphous matrices, structural disorder leads to the coexistence of both conformers, even if, depending on the emitter concentration, dual fluorescence may disappear due to a fast energy transfer from the bent to the orthogonal conformers. We maintain that this efficient energy transfer is responsible for the good efficiency of DMAC-py-TRZ devices, because of the presence in the matrix of a sizable proportion of compounds that adopt the bent structure, favorable to act as the host for the orthogonal TADF conformer of DMAC-py-TRZ. Publisher PDF

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

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

Funding: The St Andrews team is grateful to the Engineering and Physical Sciences Research Council (EPSRC) for support from grants EP/P010482/1, EP/R035164/1 and EP/L017008/1. Kyushu team would like to acknowledge Kyulux inc and JSPS Core-to-Core Program (grant number: JPJSCCA20180005) for the support of this project. Three new 1,2,4,5-tetraphenylimidazole derivatives, 9,9-dimethyl-10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-9,10-dihydroacridine ( DMAC - TPI ), 10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-10H-phenoxazine ( PXZ-TPI ), and 10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-10H-phenothiazine ( PTZ - TPI ), bearing different electron donors at the N1 position of the imidazole were synthesised and characterised. DMAC - TPI and PXZ - TPI showed narrow emission at λPL of 388 and 418 nm in toluene, and in doped films in Zeonex polymer (1 wt.%) at λPL 381 and 407 nm, respectively, with a full width at half maximum (FWHM) ranging 0.42-0.44 eV. DMAC - TPI and PXZ - TPI are predicted to show very low oscillator strength for the low-energy transitions, which aligns to the observed low photoluminescence quantum yields. Both molecules showed a singlet-triplet energy gap (ΔEST of around 1.2 eV) that is much too large to enable reverse intersystem crossing and thermally activated delayed fluorescence. Connecting a donor group to TPI at the N1 position can lead to room temperature phosphorescence (RTP), as the example of PTZ - TPI showed. Publisher PDF

We present the synthesis, optoelectronic characterization, and a detailed theoretical study of DMAC-py-TRZ, a novel, efficient TADF emitter. This compound is a structural relative of the well-known TADF compound DMAC-TRZ, substituting the bridging phenylene for a pyridyl group. This marginal change has an enormous impact on the structure and hence on the photophysics as the steric interactions between the DMAC donor and the bridge that force DMAC-TRZ into an orthogonal conformation are attenuated and permit DMAC-py-TRZ to adopt a planar and slightly bent structure in the ground state. The large degree of conjugation in the bent DMAC-py-TRZ structure, demonstrated by the strong intensity of the lowest excitation with CT character, is responsible for a large singlet triplet gap, hence hindering TADF of this bent conformer. The computational analysis predicts that emission occurs, however, from a relaxed orthogonal excited-state geometry, as confirmed by the huge Stokes shift observed in non-polar solvents. In this relaxed orthogonal geometry TADF is indeed observed. Emission from the unrelaxed state is recovered in glassy frozen solvents, where the emission band is largely blue-shifted compared with measurements in liquid solvent, and TADF is quenched. In amorphous matrices, structural disorder leads to the coexistence of both conformers, even if, depending on the emitter concentration, dual fluorescence may disappear due to a fast energy transfer from the bent to the orthogonal conformers. We maintain that this efficient energy transfer is responsible for the good efficiency of DMAC-py-TRZ devices, because of the presence in the matrix of a sizable proportion of compounds that adopt the bent structure, favorable to act as the host for the orthogonal TADF conformer of DMAC-py-TRZ.

Three new 1,2,4,5-tetraphenylimidazole derivatives, 9,9-dimethyl-10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-9,10-dihydroacridine (DMAC-TPI), 10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-10H-phenoxazine (PXZ-TPI), and 10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-10H-phenothiazine (PTZ-TPI), bearing different electron donors at the N1 position of the imidazole were synthesised and characterised. DMAC-TPI and PXZ-TPI showed narrow, deep blue emission at λPL of 388 and 418 nm in toluene, and in doped films in Zeonex polymer (1 wt.%) at λPL 381 and 407 nm, respectively, with a full width at half maximum (FWHM) ranging 0.42-0.44 eV. DMAC-TPI and PXZ-TPI are predicted to show very low oscillator strength for the low-energy transitions, which aligns to the observed low photoluminescence quantum yields. Both molecules showed a singlet-triplet energy gap (EST of around 1.2 eV) that is much too large to enable reverse intersystem crossing and thermally activated delayed fluorescence. Connecting a donor group to TPI at the N1 position can lead to room temperature phosphorescence (RTP), as the example of PTZ-TPI showed.

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%.

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

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

We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound, DiKTa. The emission of the charged derivatives is red-shifted compared to the parent com-pound. For instance, DiKTa-OBuIm emits in the green (λPL = 499 nm, 1 wt% in mCP) while DiKTa-DPA-OBuIm emits in the red (λPL = 577 nm, 1 wt% in mCP). In 1 wt% mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%, and delayed life-times of 316.6 μs and 241.7 μs, respectively, for DiKTa-OBuIm and DiKTa-DPA-OBuIm, leading to reverse intersystem crossing rates of 2.85 × 103 s-1 and 3.04 × 103 s-1. Light-emitting electrochemical cells were prepared using both DiKTa-OBuIm and DiKTa-DPA-OBuIm as ac-tive emitters showing green (λmax = 534 nm) and red (λmax = 656 nm) emission, respectively.

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.

G. P. thanks the SCBM, the “PTC du CEA” (POLEM) and the ANR (iChiralight, ANR-19-CE07-0040) for funding and David Buisson, Amélie Goudet and Sabrina Lebrequier. J. C. and L. Fa. acknowledge the Ministère de l’Education Nationale, de la Recherche et de la Technologie, the CNRS and the Spectroscopies-CDTP core facility is also acknowledged. The St. Andrews team thanks the China Scholarship Council, 201906250199 to W. S. and 202006250026 to J. W., E. Z.-C. is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). We thank the EPSRC (EP/R035164/1) for funding. We describe herein a molecular design to generate circularly polarized thermally activated delayed fluorescence emitters in which chiral bicarbazole donors are connected to acceptor units via a rigid 8-membered cycle and how the nature of the donor and acceptor units affect the photophysical and chiroptical properties. Publisher PDF

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.

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

We have designed a highly twisted small TADF emitter PXZ-α-DK based on an -diketone (α-DK) as a strong acceptor and phenoxazine (PXZ) as a strong donor to obtain red-shifted emission in comparison to the equivalent -diketone linked to 9,9-dimethyl-9,10-dihydroacridine (DMAC). The PXZ-α-DK shows emission at 586 nm and DMAC-α-DK shows emission at 548 nm in 1,3-bis(N-carbazolyl)benzene (mCP) host at 1.5 wt% doping of the emitter, with short-delayed lifetimes of 6.9 μs for PXZ-α-DK and 7.6 μs for DMAC-α-DK. OLEDs fabricated using these emitters show green electroluminescence at 555 nm for DMAC-α-DK, with a maximum external quantum efficiency, EQEmax, of 6.3%, and orange electroluminescence at 585 nm for PXZ-α-DK, with an EQEmax of 0.8%. We corroborate the optoelectronic properties of these emitters with DFT calculations.

Authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for support from grants EP/P010482/1 and EP/R035164/1. P. Rajamalli acknowledges support from a Marie Skłodowska-Curie Individual Fellowship (MCIF; No. 749557). Dongyang Chen thanks the China Scholarship Council (grant numbers 201603780001). We present a study of two isomeric thermally activated delayed fluorescence (TADF) emitters 9,9'- (sulfonylbis(pyrimidine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) ( pDTCz-DPmS ) and 9,9'- (sulfonylbis(pyrazine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) ( pDTCz-DPzS ). The use of pyrimidine and pyrazine as bridging units between the electron donor and acceptor moieties is found to be advantageous compared to the phenyl- (pDTCz-DPS) and pyridine-based analogues ( pDTCz-3DPyS and pDTCz-2DPyS ). Conformational modulation of the donor groups as a function of the bridge results in high photoluminescence quantum yields (ΦPL > 68%) and small energy gaps between singlet and triplet excited states (ΔEST < 160 meV). OLEDs using pDTCz-DPmS and pDTCz-DPzS as emitters exhibit blue and green electroluminescence, respectively, with higher maximum external quantum efficiencies (EQEmax of 14% and 18%, respectively) and reduced efficiency roll-off as compared to the reference devices using pDTCz-DPS , pDTCz-3DPyS , and pDTCz-2DPyS as the emitters. Our results provide a more complete understanding on the impact of the bridge structure in D-A-D TADF systems on the optoelectronic properties of the emitter, and how the balance between color purity and EQE in the devices can be controlled, advancing the design strategies for TADF emitters. Postprint

Two new asymetric D–A–D′ type compounds containing benzophenone and diphenylsulfone as electron accepting core, and acridan and thianthrene as electron donating moieties were design as multifunctional materials. The influence of different acceptor and donor units on thermally activated delayed fluorescence and room temperature phosphorescence was investigated and discussed. The combination of different donors with benzophenone unit resulted in the compound with effective frontier orbital separation, as well as the sustainment of high radiative rate. These statements are both experimentally and theoretically supported using time-resolved photoluminescence spectroscopy and DFT calculations. Deoxygenation experiment revealed higher triplet exciton contribution in the emission of benzophenone derivative, as compared to that of diphenylsulfone-based compound. The films of molecular mixtures of these emitters with 1,3-bis(N-carbazolyl)benzene demonstrated temperature-dependent photoluminescence. The estimated energy gaps between the lowest singlet and triplet states were found to be 0.06 eV and 0.27 eV for benzophenone and diphenylsulfone derivatives, respectively. Due to the relatively high singlet-triplet energy splitting of the diphenylsulfone-based compound, triplet harvesting was detected not only via temperature-stimulated fluorescence but also via room temperature phosphorescence. Displaying effect of triplet harvesting on efficiency of all-organic blue and green OLEDs based on thianthrene and acridan-substituted benzophenone or diphenylsulfone derivatives demonstrated very different maximum external quantum efficiencies of 1.4 and 22.2% respectively.

Excited state dynamics of trinary star-shaped dendritic compounds with triphenylamine arms and different cores were studied by means of time-resolved fluorescence and transient absorption. Under optical excitation, nonpolar C3 symmetry molecules form polar excited states localized on one of the molecular substituents. Conformational excited state stabilization of molecules with an electron-accepting core causes a formation of twisted internal charge transfer (TICT) states in polar solvents. A low transition dipole moment from TICT state to the ground state causes very weak fluorescence of those compounds and strong dependence on the solvent polarity. The compound formed from the triphenylamine central core and identical arms also experiences excited state twisting, however, weakly sensitive to the solvent polarity.

Elaboration of the appropriate host materials proved to be not less important for the fabrication of a highly efficient OLED than the design of emitters. In the present work, we show how by simple variation of molecular structure both blue emitters exhibiting delayed fluorescence and ambipolar high triplet energy hosts can be obtained. The compounds with a para-junction revealed higher thermal stability (TID up to 480 °C), lower ionization potentials (5.51–5.60 eV), exclusively hole transport, and higher photoluminescence quantum efficiencies (0.90–0.97). Meta-linkage leads to ambipolar charge transport and higher triplet energies (2.82 eV). Introduction of the accepting nitrile groups in the para-position induces intensive delayed fluorescence via a triplet–triplet annihilation up-conversion mechanism. By utilization of the para-substituted derivative as an emitter and the meta-substituted isomer as the host, a deep-blue OLED with the external quantum efficiency of 14.1% was fabricated.

In this work, we report on the synthesis and photophysical investigation of a new star-shaped triazine-carbazole derivative 2,4,6-tris(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,3,5-triazine. Comparative study of the photophysical properties of the newly synthesized emitter along with its para-substituted isomer 2,4,6-tris(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,3,5-triazine was performed. While para-linkage caused higher oscillator strength of the lowest energy absorption band and high fluorescence quantum yield, the meta-linkage resulted in stronger charge transfer character as well as higher triplet energy. Delayed emission of meta-isomer was found to be 3 orders of magnitude more intense than that of para-isomer. Temperature dependent measurements of meta-isomer confirmed the thermally activated delayed fluorescence origin of its delayed fluorescence with the activation energy of 0.07 eV. Organic light emitting diode containing this emitter dispersed in bis[2-(diphenylphosphino)phenyl] et...

Despite the commercial success of organic emitters based on thermally activated delayed fluorescence (TADF), a general understanding of emitter-matrix interactions is still lacking. How does the dielectric constant around an emitter impact its delayed fluorescence? The authors study the photophysical properties of three emitters in solution, showing that a higher excited triplet state at low dielectric constants can boost the efficiency of blue TADF emitters. Experiments are supported by state-of-the-art quantum simulations. The findings may be generalized to solid-state systems, pointing to tuning of host-guest architectures to improve the efficiency of TADF-based blue OLEDs.

Fluorine-containing donor molecules TFTF, CNTF and PRTF are designed and isomer selectively synthesized for application in vacuum-deposited organic solar cells. These molecules comprise a donor acceptor molecular architecture incorporating thiophene and benzothiadiazole derivatives as the electron-donating and electron-withdrawing moieties, respectively. As opposed to previously reported materials from this class, PRTF can be purified by vacuum sublimation at moderate to high yields because of its higher volatility and better stabilization due to a stronger intramolecular hydrogen bond, as compared to TFTF and CNTF. The UV-vis absorption spectra of the three donors show an intense broadband absorption between 500 nm and 800 nm with, similar positions of their frontier energy levels. The photophysical properties of the three donor molecules are thoroughly tested and optimized in bulk heterojunction solar cells with C-60 as acceptor. PRTF shows the best performance, yielding power conversion efficiencies of up to 3.8%. Moreover, the voltage loss for the PRTF device due to the non radiative recombination of free charge carriers is exceptionally low (0.26 V) as compared to typical values for organic solar cells (>0.34V). (C) 2016 Published by Elsevier B.V.

Three new bipolar star-shaped derivatives of 2,4,6-triphenyl-1,3,5-triazine containing carbazolyl groups were designed, synthesized and characterized. All the materials possess high thermal stability and high glass transition temperatures ranging 97–226 °C. Photophysical study of the dilute solutions and neat films of the synthesized compounds was performed. Lippert–Mataga plots revealed linear dependence of Stokes shifts on the orientation polarizability for all the compounds. The dilute solutions of the triazine derivatives exhibited high photoluminescence quantum yields reaching 0.85, while for the neat films photoluminescence efficiency of 0.20–0.33 was observed. Ionization potentials of the solid layers of carbazole-triazine adducts estimated by photoelectron spectroscopy were found to be in the range of 5.49–5.97 eV. Hole drift mobility of the materials well exceeded the magnitude of 10−3 cm2 V−1 s−1 at an electric field of 6.4 · 105 V/cm. The selected compounds were tested as light emitting materials in organic light emitting diodes based on host-guest systems.

Materials with high triplet energies have attracted great attention for their application in exciplex-based or thermally activated delayed fluorescence-based organic light-emitting diodes. To study the correlation between triplet energy and chemical structure, six compounds with typical donor-acceptor-donor structures were designed, synthesized, and characterized. Through the adjustment of molecular configuration by weakening the strengths of donors and increasing the angles between donors and acceptors, the triplet energies of the materials were gradually increased from 2.5 to 3.0 eV. Three exciplex-based organic light-emitting diodes containing one of the synthesized compounds were fabricated, which exhibited efficient blue, green-yellow, and white thermally activated delayed fluorescence. Blue and green-yellow organic light-emitting diodes exhibited the maximum external quantum efficiencies of 9.1% and 8.3%, respectively. White organic light-emitting diodes showed turn-on voltage of 4.8 V, high maximum luminance of 18474 cdm-2, and maximum external quantum efficiency of 10.6%. Maximum external quantum efficiencies of blue and white organic light-emitting diodes were close to the record values observed for all-exciplex-based blue and white organic light-emitting diodes.

Triphenylamine derivatives, comprised of two methoxy-substituted triphenylamine moieties linked through 9,9-fluorenylidene unit, were synthesized via Ullmann-type coupling reaction from primary aromatic diamine, 9,9-bis(4-aminophenyl)fluorene, and respective aryl iodides. Thermal behaviour, photophysical, electrochemical and charge-transporting properties of the materials were investigated. Ortho - and meta -methoxy-substituted derivatives readily form molecular glasses which exhibit glass-transition temperatures of 114 °C and 86 °C, respectively. Solid state ionization potentials of methoxy-substituted derivatives are set in the range of 5.35–5.58 eV. The materials exhibit fluorescence in UV-violet light region with the maxima situated in the range of 360–389 nm. The phosphorescence spectra of all the compounds are almost identical and not influenced by methoxy-substitution. The materials exhibit high triplet-energy levels of 2.88 eV. Hole mobilities in solid amorphous films of the neat compounds exceed 10 −3 cm 2 V −1 s −1 at the electric field of 1 MV cm −1 .