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New materials for polymer organic light-emitting diodes based on a polymer matrix doped with phosphorescent dyes are presented. The matrix system is based on a polystyrene backbone bearing either electron or hole transporting units at the 4-position of each repeat unit. Random copolymers and polymer blend systems of the homopolymers are prepared, both with 62 wt.% electron transporting and 38 wt.% hole transporting moieties. Adding a green electrophosphorescent dye to the polymer matrix leads to efficient electroluminescence with a maximum current efficiency of 35 cd/A and a maximum external quantum efficiency of up to 10%. The mobilities of electrons and holes in the dye-doped copolymer, as measured by transient electroluminescence, are around 5 × 10-5 and 5 × 10-6 cm2/Vs, respectively, while the blend of the two homopolymers exhibits slightly lower mobilities of both types of carriers. Despite the pronounced imbalance of charge transport, the device performance is almost entirely limited by the phosphorescence efficiency of the dye, implying balanced flow of holes and electrons into the active region. Also, devices made with either the copolymer or the blend yielded very similar device efficiencies, despite the noticeable difference in electron and hole mobility. It is proposed that electrons are efficiently blocked at the interlayer and that the so-formed space charge assists the balanced injection of holes. [ABSTRACT FROM AUTHOR]

We investigated the magnetic field effect (MFE) on current and electroluminescence in organic light emitting diodes based on poly(paraphenylene vinylene). The MFE was strictly positive in the full range of device operation and showed nonmonotonic dependencies on applied voltage and temperature. Furthermore, the MFE on current obtained in bipolar devices was significantly larger than in hole-dominated devices. We discuss our results in the framework of an electron-hole pair model and show that the model can explain all functional dependencies observed in our devices. [ABSTRACT FROM AUTHOR]

Recently, it has been discovered that the magnetic field effect (MFE) in organic light emitting diodes (OLEDs) based on poly(para-phenylene vinylene) can be enhanced by exposing the diode to moderate electrical stress. Here, we disclose the mechanism behind this way of improving the MFE. We first show that electronic traps in general play an important role for the MFE. Optical depletion of available trap states by infrared illumination leads to a decrease in the MFE. Furthermore, we demonstrate that annealing of the OLED at high temperatures eliminates the MFE improvement of the previously performed electrical conditioning. However, the improvement can be restored by subsequent conditioning at higher current or voltage. Thus it is likely that electrical stress is accompanied by a transformation of the polymer morphology or conformation resulting in a formation of energetic traps for charge carriers. [ABSTRACT FROM AUTHOR]

The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The properties of light-emitting diodes based on the electrophorescent platinum-porphyrin dye PtOEP blended into a ladder-type poly(para-phenylene) (LPPP) polymer host are presented. Due to the small difference between the highest occupied molecular orbital (HOMO) levels of the guest and the host, the operating voltage of single layer devices is almost independent of the dopant concentration in clear contrast to what has been observed in the case of wide-band gap host polymers. However, the efficiency and the color purity of these single-layer devices is quite poor, which can be attributed to the weak trapping of carriers on the phosphorescent dye. Incorporating an electron-transporting/hole-blocking layer greatly increases the efficiency of the devices but at the same time emission from the host becomes more significant. Adding a hole-transporting/ electron-blocking layer further increases the efficiency of the devices. Pure red emission is, however, only obtained if the HOMO of the hole-transporting layer is well above that of the LPPP host. This effect is interpreted in terms of a direct injection of holes from the hole-transporting layer into the HOMO level of the dopant. Further, prompt fluorescence and phosphorescence studies indicate that energy transfer from LPPP singlet excitons contributes to the emission from the dopant, but that the efficiency of singlet exciton transfer is too small to explain the almost complete absence of host emission in the electroluminescence spectra of single- and three-layer devices. [ABSTRACT FROM AUTHOR]

We have studied the mechanisms for quenching of the fluorescence of conjugated poly(p-phenylene) polymers by benzil and dimethylaminobenzil molecules. We have shown that molecules in the diketone series are quenching agents for the fluorescence of the indicated polymers, and can serve as singlet-triplet converters capable of populating the triplet state of the polymer. We have observed that the efficiency of quenching of the fluorescence of the studied polymers depends considerably on the presence of bulky side groups in the polymer or in the activator molecules. Based on analysis of the data obtained, we conclude that in the case of a rigid planar structure for the polymer, a significant contribution to quenching of its fluorescence comes from not only singlet-singlet energy transfer but also charge transfer, leading to formation of intermolecular complexes (exciplexes). [ABSTRACT FROM AUTHOR]

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 (T 1 ) and singlet (S 1 ) 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 an approach to enhance the efficiency of TADF-related processes without changing the blue photoluminescence color., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The potential of dendrimers exhibiting thermally activated delayed fluorescence (TADF) as emitters in solution-processed organic light-emitting diodes (OLEDs) has to date not yet been realized. This in part is due to a poor understanding of the structure-property relationship in dendrimers where reports of detailed photophysical characterization and mechanism studies are lacking. In this report, using absorption and solvatochromic photoluminescence studies in solution, the origin and character of the lowest excited electronic states in dendrimers with multiple dendritic electron-donating moieties connected to a central electron-withdrawing core via a para- or a meta-phenylene bridge is probed. Characterization of host-free OLEDs reveals the superiority of meta-linked dendrimers as compared to the already reported para-analogue. Comparative temperature-dependent time-resolved solid-state photoluminescence measurements and quantum chemical studies explore the effect of the substitution mode on the TADF properties and the reverse intersystem crossing (RISC) mechanism, respectively. For TADF dendrimers with similarly small ∆E ST , it is observed that RISC can be enhanced by the regiochemistry of the donor dendrons due to control of the reorganization energies, which is a heretofore unexploited strategy that is distinct from the involvement of intermediate triplet states through a nonadiabatic (vibronic) coupling with the lowest singlet charge transfer state., (© 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.)

The development of high-performance solution-processed organic light-emitting diodes (OLEDs) remains a challenge. An effective solution, highlighted in this work, is to use highly efficient thermally activated delayed fluorescence (TADF) dendrimers as emitters. Here, the design, synthesis, density functional theory (DFT) modeling, and photophysics of three triazine-based dendrimers, tBuCz2pTRZ, tBuCz2mTRZ, and tBuCz2m2pTRZ, is reported, which resolve the conflicting requirements of achieving simultaneously a small ΔE ST and a large oscillator strength by incorporating both meta- and para-connected donor dendrons about a central triazine acceptor. The solution-processed OLED containing a host-free emitting layer exhibits an excellent maximum external quantum efficiency (EQE max ) of 28.7%, a current efficiency of 98.8 cd A -1 , and a power efficiency of 91.3 lm W -1 . The device emits with an electroluminescence maximum, λ EL , of 540 nm and Commission International de l'Éclairage (CIE) color coordinates of (0.37, 0.57). This represents the most efficient host-free solution-processed OLED reported to date. Further optimization directed at improving the charge balance within the device results in an emissive layer containing 30 wt% OXD-7, which leads to an OLED with the similar EQE max of 28.4% but showing a significantly improved efficiency rolloff where the EQE remains high at 22.7% at a luminance of 500 cd m -2 ., (© 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

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, Δ E ST , than the parent indolocarbazole that induces MR-TADF properties. Coupled cluster calculations accurately predict the magnitude of the Δ E ST 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.

Thermally activated delayed fluorescence (TADF) relies on a small energy gap between the emissive singlet and the nonemissive triplet state, obtained by reducing the wave function overlap between donor and acceptor moieties. Efficient emission, however, requires maintaining a good oscillator strength, which is itself based on sufficient overlap of the wave functions between donor and acceptor moieties. We demonstrate an approach to subtly fine-tune the required wave function overlap by employing donor dendrons of changing functionality. We use a carbazolyl-phthalonitrile based donor-acceptor core (2CzPN) as a reference emitter and progressively localize the hole density through substitution at the 3,6-positions of the carbazole donors (Cz) with further carbazole, (4- tert -butylphenyl)amine ( t BuDPA), and phenoxazine (PXZ). Using detailed photoluminescence studies, complemented with density functional theory (DFT) calculations, we show that this approach permits a gradual decrease of the singlet-triplet gap, Δ E ST , from 300 to around 10 meV in toluene, yet we also demonstrate why a small Δ E ST alone is not enough. While sufficient oscillator strength is maintained with the Cz- and t BuDPA-based donor dendrons, this is not the case for the PXZ-based donor dendron, where the wave function overlap is reduced too strongly. Overall, we find the donor dendron extension approach allows successful fine-tuning of the emitter photoluminescence properties.

Six luminophores bearing an OBO-fused benzo[fg]tetracene core as an electron acceptor were designed and synthesized. The molecular structures of three molecules (PXZ-OBO, 5PXZ-OBO, 5DMAC-OBO) were determined by single crystal X-ray diffraction studies and revealed significant torsion between the donor moieties and the OBO acceptor with dihedral angles between 75.5 and 86.2°. Photophysical studies demonstrate that blue and deep blue emission can be realized with photoluminescence maxima (λ PL ) ranging from 415 to 480 nm in mCP films. The emission energy is modulated by simply varying the strength of the donor heterocycle, the number of donors, and their position relative to the acceptor. Although the DMAC derivatives show negligible delayed emission because of their large singlet-triplet excited state energy difference, Δ E ST , PXZ-based molecules, especially PXZ-OBO with an experimental Δ E ST of 0.25 eV, demonstrate delayed emission in blend mCP films at room temperature, which suggests triplet exciton harvesting occurs in these samples, potentially by thermally activated delayed fluorescence., (Copyright © 2020 Zhang, Kumar, Bagnich, Spuling, Hundemer, Nieger, Hassan, Köhler, Zysman-Colman and Bräse.)

The synthesis of stable blue TADF emitters and the corresponding matrix materials is one of the biggest challenges in the development of novel OLED materials. We present six bipolar host materials based on triazine as an acceptor and two types of donors, namely, carbazole, and acridine. Using a tool box approach, the chemical structure of the materials is changed in a systematic way. Both the carbazole and acridine donor are connected to the triazine acceptor via a para- or a meta-linked phenyl ring or are linked directly to each other. The photophysics of the materials has been investigated in detail by absorption-, fluorescence-, and phosphorescence spectroscopy in solution. In addition, a number of DFT calculations have been made which result in a deeper understanding of the photophysics. The presence of a phenyl bridge between donor and acceptor cores leads to a considerable decrease of the triplet energy due to extension of the overlap electron and hole orbitals over the triazine-phenyl core of the molecule. This decrease is more pronounced for the para-phenylene than for the meta-phenylene linker. Only direct connection of the donor group to the triazine core provides a high energy of the triplet state of 2.97 eV for the carbazole derivative CTRZ and 3.07 eV for the acridine ATRZ . This is a major requirement for the use of the materials as a host for blue TADF emitters., (Copyright © 2020 Rodella, Bagnich, Duda, Meier, Kahle, Athanasopoulos, Köhler and Strohriegl.)

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 Δ E ST using correlated wave-function-based calculations. On the basis of these calculations, we predicted dramatically different optoelectronic behavior in terms of both Δ E ST 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.

We present a combined detailed spectroscopic and quantum chemical study on the bipolar host materials BPTRZ and MBPTRZ in solution and in neat film. In the two compounds, the hole transporting carbazole is separated from the electron transporting triazine moiety by a fully aromatic but non-conjugated meta-linked biphenyl unit. The two materials differ by an additional steric twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2- and 2'-position of the biphenyl. We find that while the twist shifts the triplet state in MBPTRZ to higher energies (3.0 eV in solution) compared to BPTRZ (2.8 eV in solution), this also localizes electron density on the carbazole moiety, leading to excimer formation in neat films.

Using optical spectroscopy in solution and thin film, and supported by quantum chemical calculations, we investigated the aggregation process of the donor-acceptor type molecule p-DTS(FBTTH2)2. We demonstrate that cooling a solution induces a disorder-order phase transition that proceeds in three stages analogous to the steps observed in semi-rigid conjugated polymers. By analyzing the spectra, we are able to identify the spectral signature of monomer and aggregate in absorption and emission. From this we find that in films, the fraction of aggregates is near 100% which is in contrast to films made from semi-rigid conjugated polymers.

We have investigated the electron absorption, fluorescence and phosphorescence spectra of 8-azasteroids and model compounds containing enaminodicarbonyl fragment. It has been shown that the investigated compounds have absorption and phosphorescence spectra similar in form and position. The results obtained permit the conclusion that the main deactivation channel of the excited singlet state is the intersystem crossing. It is suggested that the investigated 8-azasteroids of enaminodicarbonyl series under light irradiation undergo phototransformations presumably into derivatives with a gamma-pyridone fragment.