New Light-Green Thermally Activated Delayed Fluorescence Polymer Based on Dimethylacridine-Triphenyltriazine Light-Emitting Unit and Tetraphenylsilane Moiety as Non-Conjugated Backbone

In the search for solution-processable TADF materials as a light emitting layer for OLED devices, polymers have attracted considerable attention due to their better thermal and morphological properties in the film state with respect to small molecules. In this work, a new polymer (p-TPS-DMAC-TRZ) with thermally activated delayed fluorescence (TADF) light-emitting characteristics was prepared from a conjugation-break unit (TPS) and a well-known TADF core (DAMC-TRZ). This material was designed to preserve the photophysical properties of DAMC-TRZ, while improving other properties, such as thermal stability, promoted by its polymerization with a TPS core. Along with excellent solubility in common organic solvents such as toluene, chloroform and THF, the polymer (M_n = 9500; M_w = 15200) showed high thermal stability (TDT_5% = 481 °C), and a T_g value of 265 °C, parameters higher than the reference small molecule DMAC-TRZ (TDT_5% = 305 °C; T_g = 91 °C). The photoluminescence maximum of the polymer was centered at 508 nm in the solid state, showing a low redshift compared to DMAC-TRZ (500 nm), while also showing a redshift in solution with solvents of increasing polarity. Time-resolved photoluminescence of p-TPS-DMAC-TRZ at 298 K, showed considerable delayed emission in solid state, with two relatively long lifetimes, 0.290 s (0.14) and 2.06 s (0.50), and a short lifetime of 23.6 ns, while at 77 K, the delayed emission was considerably quenched, and two lifetimes in total were observed, 24.6 ns (0.80) and 180 ns (0.20), which was expected from the slower RISC process at lower temperatures, decreasing the efficiency of the delayed emission and demonstrating that p-TPS-DMAC-TRZ has a TADF emission. This is in agreement with room temperature TRPL measurements in solution, where a decrease in both lifetime and delayed contribution to total photoluminescence was observed when oxygen was present. The PLQY of the mCP blend films with 1% p-TPS-DMAC-DMAC-TRZ as a dopant was determined to be equal to 0.62, while in the pure film, it was equal to 0.29, which is lower than that observed for DMAC-TRZ (0.81). Cyclic voltammetry experiments showed similarities between p-TPS-DMAC-TRZ and DAMC-TRZ with HOMO and LUMO energies of -5.14 eV and -2.76 eV, respectively, establishing an electrochemical bandgap value of 2.38 eV. The thin film morphology of p-TPS-DMAC-TRZ and DMAC-TRZ was compared by AFM and FE-SEM, and the results showed that p-TPS-DMAC-TRZ has a smoother surface with fewer defects, such as aggregations. These results show that the design strategy succeeded in improving the thermal and morphological properties in the polymeric material compared to the reference small molecule, while the photophysical properties were mostly maintained, except for the PLQY determined in the pure films. Still, these results show that p-TPS-DMAC-TRZ is a good candidate for use as a light-emitting layer in OLED devices, especially when used as a host-guest mixture in suitable materials such as mCP.

Regiochemistry of Donor Dendrons Controls the Performance of Thermally Activated Delayed Fluorescence Dendrimer Emitters for High Efficiency Solution-Processed Organic Light-Emitting Diodes

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 ∆EST , 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.

Thermally Activated Delayed Fluorescent Dendrimers that Underpin High-Efficiency Host-Free Solution-Processed Organic Light-Emitting Diodes

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 .

Rigid Polyimides with Thermally Activated Delayed Fluorescence for Polymer Light-Emitting Diodes with High External Quantum Efficiency up to 21

A series of rigid nonconjugated polyimide (PI)-based thermally activated delayed fluorescence (TADF) polymers were reported for the first time, based on a "TADF-Linker-Host" strategy. Among of which, the TADF unit contains a typical TADF luminous core structure, the "Host" unit exhibits effective conjugation length that endows polyimide with high triplet energy, and the "Linker" unit has an aliphatic ring structure to improve solubility and inhibits intramolecular charge transfer effect. All the TADF polymers exhibit high thermal stability (T_g >308.7 °C) and refractive index (1.76-1.79). Remarkably, highly-efficient polymer light-emitting diodes (PLEDs) based on the polymers are successfully realized, leading to a maximal external quantum efficiency of 21.0 % along with low efficiency roll-off. Such outstanding efficiency is amongst the state-of-the-art performance of nonconjugated PLEDs, confirming the effectiveness of structural design strategy, providing helpful and valuable guidance on the development of highly-efficient fluorescent polymer materials and PLEDs.

Simple-Structured Blue Thermally Activated Delayed Fluorescence Emitter for Solution-Processed Organic Light-Emitting Diodes with External Quantum Efficiency of over 20

Solution-processed organic light-emitting diodes (OLEDs) are much preferred for the manufacture of low-temperature, low-cost, large-area, and flexible lighting and displaying devices. However, these devices with high external quantum efficiency are still limited, especially for blue ones. In addition, the molecular configurations of emitters are usually complicated, indicative of high costs. In this study, two simple-structured thermally activated delayed fluorescent emitters M1 and its polymer P1 were synthesized with acridine as a donor and benzophenone as an acceptor. Solution-processed OLEDs were prepared based on M1 and P1 as doped light-emitting layer, and M1-based doped device could achieve maximum external quantum efficiency of up to 20.6% with blue-light emission.

Recent Advances in Thermally Activated Delayed Fluorescent Polymer-Molecular Designing Strategies

Thermally activated delayed fluorescent (TADF) materials, as the third generation of organic electroluminescent materials, have many advantages over other organic light-emitting diodes (OLEDs) materials, such as 100% internal quantum efficiency, no doping of heavy metals, and avoiding the shortages of ordinary fluorescent materials and phosphorescent materials. So it is considered to be the most competitive organic light-emitting materials, and has great application prospects in the field of OLEDs. So far, small-molecule TADF materials have achieved high quantum yield and full-color range of red, green, and blue. However, TADF polymers suitable for low-cost and easily scalable solution processing are less developed, which are confined by the preparation methods and polymers designing, and there are still challenges of increasing quantum efficiency and strengthening device performance. This review mainly summarizes different synthesis strategies of TADF polymers and the latest development in the field. Special attention is focused on illustrating the designing and structure-property relationship of TADF polymers, and finally, an outlook is given for the design and application prospect of TADF polymers in the future.

High-Performance, Solution-Processable Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes Realized via the Adjustment of the Composition of the Organoboron Acceptor Monomer in Copolymer Host Materials

Organic polymers that exhibit features pertinent to functioning as host materials for thermally activated delayed fluorescence (TADF) emitters have considerable potential in solution-processable organic light-emitting diodes (OLEDs), allowing simple, low-cost, and large-area applications. In particular, polymer hosts have superior characteristics, including facile functionality to introduce various electron donor and acceptor entities, the ability to uniformly disperse and contain small molecular dopants, and the ability to produce more smooth and homogeneous films, compared to those of their small-molecule counterparts. This manuscript describes the design and development of three new styrene-based copolymers (ABP91, ABP73, and ABP55) bearing diphenylacridine as the electron donor and 2,12-di-tert-butyl-7-phenyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene as the electron acceptor. In particular, ABP91, ABP73, and ABP55 were synthesized via variations in the ratio of donor to acceptor monomers to substantiate their influence in OLED applications. With the ability of the styrene backbone of interrupting the direct electronic coupling between the adjacent electron donor and acceptor entities through non-conjugated linkages, high triplet energy can be inherited by the resulting polymers (>2.70 eV). Furthermore, these materials manifest thermal robustness through high decomposition temperatures (between 348 and 366 °C) and high glass transition temperatures (between 234 and 277 °C). Consequently, solution-processable OLEDs fabricated using the newly synthesized copolymers as host materials and the familiar t4CzIPN as a green-emissive TADF dopant deliver state-of-the-art performance with maximum external quantum efficiencies of 21.8, 22.2, and 19.7% for ABP91, ABP73, and ABP55, respectively. To our knowledge, this is, to date, the best performance reported when organic polymers are used as host materials in solution-processable TADF OLEDs. The pragmatic outcomes obtained in this study can provide useful insights into the structure-property relationship to the OLED community for the further development of efficient polymer hosts for use in solution-processable TADF OLEDs.

Saturated Red Electroluminescence From Thermally Activated Delayed Fluorescence Conjugated Polymers

Two sets of conjugated polymers with anthraquinone groups as pendant acceptors were designed and synthesized. The acceptor is tethered to an diphenylamine group via a phenylene bridge, constructing a thermally activated delayed fluorescence (TADF) unit, which is embedded into the polymer backbone through its donor fragment, while the backbone is composed of dibenzothiophene-S, S-dioxide and 2, 7-fluorene or 2, 7-carbazole groups. The polymers show distinct TADF characteristics, confirmed by transient photoluminescence spectra and theoretical calculations. The carbazole-based polymers exhibit shorter delay lifetimes and lower energy emission relative to the fluorene-based polymers. The non-doped organic light-emitting diodes fabricated via solution processing approach produce efficient red emissions with the wavelengths of 625-646 nm. The carbazole containing polymer with 2% molar content of the TADF unit exhibits the best maximum external quantum efficiency of 13.6% and saturated red electroluminescence with the Commission Internationale de l'Eclairage coordinates of (0.62, 0.37).

Fluorescent chemosensors based on conjugated polymers with N-heterocyclic moieties: two decades of progress

A brief summary of representative fluorescent chemosensors based on conjugated polymers with N-heterocyclic moieties, followed by a discussion on the limitations and challenges of current systems, as well as possible future research directions.

Developing Through-Space Charge Transfer Polymers as a General Approach to Realize Full-Color and White Emission with Thermally Activated Delayed Fluorescence

Through-space charge transfer polymers (TSCT polymers) that contain a non-conjugated polystyrene backbone and spatially separated donor and acceptor units for solution-processed OLEDs with full-color and white emission is reported. By tuning the charge transfer strength between donor and acceptors with different electron-accepting ability, emission color spanning from deep blue to red can be achieved. By incorporating two kinds of donor/acceptor pairs in one polymer to create duplex through-space charge-transfer channels, blue and yellow emission can be simultaneously obtained to realize white electroluminescence from a single polymer. The TSCT polymers exhibit thermally activated delayed fluorescence effect with delayed-component lifetimes in range of 0.36-1.98 μs, and unexpected aggregation-induced emission (emission intensity enhancement of up to 117 from solution to aggregation state).

Through-space charge transfer hexaarylbenzene dendrimers with thermally activated delayed fluorescence and aggregation-induced emission for efficient solution-processed OLEDs

Through-space electron interaction plays a critical role in determining the optical and charge transport properties of functional materials featuring π-stacked architectures. However, developing efficient organic luminescent materials with such interactions has been a challenge because of the lack of well-established prototypical molecules. Here we report the design of through-space charge transfer hexaarylbenzenes (TSCT-HABs) containing circularly-arrayed electron donors (acridan/dendritic triacridan) and acceptors (triazine), which exhibit both thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) effects for high-efficiency solution-processed organic light-emitting diodes (OLEDs). Spatial separation of donors and acceptors in the TSCT-HABs induces a small singlet-triplet energy splitting of 0.04-0.08 eV, leading to delayed fluorescence with microsecond-scale lifetimes. Meanwhile, the TSCT-HABs display the AIE effect with emission intensity enhanced by 6-17 fold from solution to the aggregation state owing to their propeller-shaped configuration. Solution-processed OLEDs based on the TSCT-HABs show maximum external quantum efficiency up to 14.2%, making them among the most efficient emitters for solution-processed TADF OLEDs.

Effect of a Pendant Acceptor on Thermally Activated Delayed Fluorescence Properties of Conjugated Polymers with Backbone-Donor/Pendant-Acceptor Architecture

Three sets of conjugated polymers with backbone-donor/pendant-acceptor architectures, named PCzA3PyB, PCzAB2Py, and PCzAB3Py, are designed and synthesized. The three isomeric benzoylpyridine-based pendant acceptor groups are 6-benzoylpyridin-3-yl (3PyB), 4-((pyridin-2-yl)carbonyl)phenyl (B2Py) and 4-((pyridin-3-yl)carbonyl)phenyl (B3Py), whereas the identical backbone consists of 3,6-carbazolyl and 2,7-acridinyl rings. One acridine ring and each acceptor group constitute a definite thermally activated delayed fluorescence (TADF) unit, incorporated into the main chain of the polymers through the 2,7-position of the acridine ring with the varied content. All of the polymers display legible TADF features with a short microsecond-scale delayed lifetime (0.56-1.62 μs) and a small singlet/triplet energy gap (0.10-0.19 eV). Progressively redshifted emissions are observed in the order PCzAB3Py, PCzA3PyB, and PCzAB2Py owing to the different substitution patterns of the pyridyl group. Photoluminescence quantum yields can be improved by regulating the molar content of the TADF unit in the range 0.5-50 %. The non-doped organic light-emitting devices (OLEDs) fabricated by solution-processing technology emit yellow-green to orange light. The polymers with 5 mol % of the TADF unit exhibit excellent comprehensive electroluminescence performance, in which PCzAB2Py5 achieves a maximum external quantum efficiency (EQE) of 11.9 %, low turn-on voltage of 3.0 V, yellow emission with a wavelength of 573 nm and slow roll-off with EQE of 11.6 % at a luminance of 1000 cd m^-2 and driving voltage of 5.5 V.

Phthalimide-based “D–N–A” emitters with thermally activated delayed fluorescence and isomer-dependent room-temperature phosphorescence properties

Phthalimide-based “D–N–A” emitters showed TADF and ultralong RTP properties, and could be applied as a security ink in anti-counterfeiting materials.

Resemblances of experiment and theory on aryl substituted luminogenic polypyrazolines

Polyarylpyrazolines (PPB, PPA, PPT, PPBt) containing various aryl substituents emit light in a broad color range from orange to blue, making them suitable for optoelectronics.

Synthesis of delayed-emissive poly(2,7-carbazole)s having an anchored triazine pendant at the N-position

The design, synthesis, and characterization of delayed-emissive poly(2,7-carbazole)s having an anchored triazine pendant at the N-position.

Solution-processed thermally activated delayed fluorescence organic light-emitting diodes using a new polymeric emitter containing non-conjugated cyclohexane units

A new solution-processable polymeric emitter containing non-conjugated cyclohexane units was developed for high-performing TADF-OLEDs.

Blue Thermally Activated Delayed Fluorescence Polymers with Nonconjugated Backbone and Through-Space Charge Transfer Effect

We demonstrate novel molecular design for thermally activated delayed fluorescence (TADF) polymers based on a nonconjugated polyethylene backbone with through-space charge transfer effect between pendant electron donor (D) and acceptor (A) units. Different from conventional conjugated D-A polymers with through-bond charge transfer effect, the nonconjugated architecture avoids direct conjugation between D and A units, enabling blue emission. Meanwhile, spatial π-π interaction between the physically separated D and A units results in both small singlet-triplet energy splitting (0.019 eV) and high photoluminescence quantum yield (up to 60% in film state). The resulting polymer with 5 mol % acceptor unit gives efficient blue electroluminescence with Commission Internationale de l'Eclairage coordinates of (0.176, 0.269), together with a high external quantum efficiency of 12.1% and low efficiency roll-off of 4.9% (at 1000 cd m^-2), which represents the first example of blue TADF nonconjugated polymer.