Rong-Huei Yi

A new strategy, namely CP-MR-TADF, is proposed to generate organic emitters with circularly polarized luminescence (CPL) activity, thermally activated delayed fluorescence (TADF), and narrowband emission, by merging the CPL optical property and multiple resonance (MR) induced TADF characteristic into one molecule. This approach is propitious to the direct realization of 3D and wide color gamut display applications in the future.

Abstract

Circularly polarized luminescence (CPL) materials that concurrently exhibit high efficiency and narrowband emission are extremely promising applications in 3D and wide color gamut display. By merging the CPL optical property and multiple resonance (MR) induced thermally activated delayed fluorescence (TADF) characteristic into one molecule, a new strategy, namely CP-MR-TADF, is proposed to generate organic emitters with CPL activity, TADF and narrowband emission. High-performance red, green and blue CP-MR-TADF emitters have been developed following this strategy. Herein, the present status and progress of CP-MR-TADF materials in the field of organic light-emitting diodes (OLEDs) is summarized. Finally, for this rapidly growing new research field, the future opportunities are forecasted and the present challenges are discussed.

A triarylamine-based macrocyclic donor is adopted to design new blue thermally activated delayed fluorescence (TADF) emitter. The restricted conformation of macrocyclic donor twisting against the dimethyl substituted phenylene bridge leads to the reduced singlet–triplet energy difference (ΔE _ST) as well as the enhanced horizontal ratio of emission dipole. These beneficial effects contribute to a highly efficient blue TADF organic light-emitting diode.

Abstract

This work reports the incorporation of a triphenylamine-based macrocyclic donor to design new donor-π-acceptor-configured blue thermally activated delayed fluorescence (TADF) emitters. The X-ray structure analyses manifest the degree of twisted conformations that can be modulated by methyl substituents of the π-bridge and macrocyclic donor, leading to well-separated highest occupied natural transition orbital and lowest unoccupied natural transition orbital frontier orbitals, thus sufficiently small singlet–triplet energy difference (ΔE _ST) for TADF. The theoretical analyses elucidate the structure–property relationship and reveal the beneficial effect of macrocyclic donor on increasing reverse intersystem crossing (RISC) process that can contribute to improved triplet-upconversion efficiency. The blue device employing c-NN-TRZ as emitter gave a maximum external quantum efficiecny (EQE_max) of 26.3% as compared to that (19.1%) of the device using the model compound DPA-MeTRZ without the macrocyclic donor, suggesting the contribution of macrocyclic donor to enhance device performance. Benefiting from the combined advantages of macrocyclic donor and methyl substituents, the device incorporating c-NN-MeTRZ as emitter achieves an outstanding EQE_max of 32.2%, which is attributed to the more horizontally oriented emission dipoles as well as the significantly accelerated RISC rate constant (k _RISC) resulting from reduced ΔE _ST. This work represents a new strategy of designing twisted TADF emitter incorporating macrocyclic donor to achieve highly efficient blue device.

Bridging of 1,12-dihydroxy and 1,6,7,12-tetrahydroxy bay-positions by silicon affords non-aggregating perylene bisimide (PBI) chromophores with well-resolved absorption and emission spectra. These lightweight dyes exhibit high fluorescence quantum yields even in the solid state and are sublimable in vacuo to afford organic light-emitting diodes with external quantum efficiencies of up to 3.1%.

Abstract

Perylene bisimides (PBIs) are among the best fluorophores but have to be enwrapped for optoelectronic applications by large and heavy substituents to prevent their ππ-stacking, which is known to accelerate non-radiative decay processes in the solid state. Here, light-weight di-tert-butylsilyl groups are introduced to bridge 1,12-dihydroxy and 1,6,7,12-tetrahydroxy PBIs to afford sublimable dyes for vacuum-processed optoelectronic devices. For both new compounds, this substitution provides a twisted and shielded perylene π-core whose, via OSiObridges, rigid structure affords well-resolved absorption and emission spectra with strong fluorescence in solution, as well as in the solid state. The usefulness of these dyes for vacuum-processed optoelectronic devices is demonstrated in organic light-emitting diodes (OLEDs) that show monomer-like emission spectra and high maximum external quantum efficiency (EQE_max) values of up to 3.1% for the doubly silicon-bridged PBI.

Developing new all-visible-light driven fluorescent molecular switches has attracted great interests in materials science and life science. Herein, three novel all-visible-light triggered fluorescent molecular photoswitches (2P3, 4F, and 4Cl) have been designed by prolonging π-conjugation length with triphenylethylene units. More interestingly, two distinct conformations with different photochromic and fluorescent abilities can be altered by solvents polarity.

Abstract

Developing new all-visible-light driven fluorescent molecular switches has attracted much interests due to their important application prospects in materials science and life science. Herein, three novel all-visible-light triggered fluorescent molecular photoswitches (2P3, 4F, and 4Cl) are designed and prepared, which are realized by prolonging π-conjugation length with aggregated-induced emission units. These photoswitches show reversible photochromic and fluorescent switching in dilute solutions and films under 420 nm and 560 nm irradiations. Meanwhile, upon alternating irradiations with 450 nm/560 nm light, effective photochromic and fluorescent switching in powder states are also achieved. More interestingly, two distinct conformations with different photochromic abilities can be altered by solvents polarity. The size difference of the two conformations is over 5 Å in the vertical direction which is one of the largest changes among photochromic molecules based on dithienylethene. The accompanying conformational changes are analysed to understand the relationship between solvent characteristics and crystal structure, as well as the intermolecular interaction in solid state for all-visible-light triggered photoswitching molecules.

Based on the “exciton recovery” strategy, a novel deep-blue emitter named CAT is successfully designed, which can achieve the multi-channeled pathways of exciton utilization via triplet-triplet annihilation and hot-exciton processes. Given this fact, the emitter exhibits high exciton utilization and successfully achieves high-performance non-doped deep-blue and host-sensitized OLEDs.

Abstract

The “hot exciton” mechanism based on high-lying reverse intersystem crossing process has great advantages in achieving high-performance deep-blue organic light-emitting diodes (OLEDs). Nevertheless, how to solve the loss of high-lying excitons to improve device performance further is a pressing and challenging issue. To break through this shackle, a novel deep-blue emitter based on “exciton recovery” strategy is successfully design, namely CAT. By combining the transient absorption spectrum, theoretical calculation, magneto-electroluminescence, and transient-electroluminescence measurements, the multi-channeled pathways of excitons utilization via hot exciton and triplet-triplet annihilation processes is comprehensively demonstrated, and the proportion of singlet excitons by each channel. Finally, a high exciton utilization efficiency is successfully achieved, and the non-doped OLED based on CAT exhibit an excellent external quantum efficiency of 10.39% with the CIE coordinates of (0.15, 0.087). Furthermore, the sensitized blue fluorescent OLED by CAT as host also achieves excellent performance. More importantly, the operational lifetime of the “multi-channel” sensitized device is evaluated for the first time, performing the remarkable LT₅₀ (lifetime to 50% of the initial luminance) of 320 h at 540 cd m⁻². These results fully reveal the significance of the “exciton recovery” strategy and give new inspiration for exploring high-performance blue OLEDs.

Two triphenylamine-functionalized multiple-resonance thermally activated delayed fluorescence materials are developed and simultaneously show accelerated reverse intersystem crossing and aggregation-induced emission enhancement characteristics for aggregation-caused quenching suppression. Highly efficient narrowband organic light-emitting devices with external quantum efficiencies as high as 27.6% with reduced efficiency roll-offs are demonstrated.

Abstract

Multiple-resonance (MR) thermal activated delayed fluorescence (TADF) emitters have attracted increasing attention in organic electroluminescence devices, owing to their superior quantum efficiency and narrowband emission for high color purity. However, MR-TADF materials often suffer from severe aggregation-caused quenching (ACQ) and efficiency roll-off problems due to their rigid planar structures and the lack of sufficient charge-transfer (CT) characters with inefficient reverse intersystem crossing (RISC). Herein, by attaching electron-rich triphenylamine (TPA) with twisted spatial conformation to the MR framework, two efficient narrowband MR-TADF emitters, namely BNCz-pTPA and BNCz-mTPA, are developed. The TPA substituent endows the new emitters with aggregation-induced emission enhancement (AIEE) for ACQ suppression. The unprecedented AIEE-MR-TADF emitters exhibit CT character in high-lying triplet excited states for faster RISC, while the locally-excited (LE) character of the first singlet excited state is retained for narrowband emission with high emission efficiency. An organic light-emitting diode (OLED) based on BNCz-pTPA exhibits a maximum external quantum efficiency of 27.3% with slow efficiency roll-off, demonstrating much higher performances than those of the BNCz-based OLED. This study may provide a simple but effective approach to constructing high-performance emitters for wide-color-gamut OLED displays.

Publication date: 1 February 2023

Source: Chemical Engineering Journal, Volume 457

Author(s): Chao Qu, Huiqin Wang, Yi Man, Zhe Li, Peng Ma, Peng Chang, Xin Li, Chunmiao Han, Yudong Pang, Hui Xu