Exceptionally efficient deep blue anthracene-based luminogens: design, synthesis, photophysical, and electroluminescent mechanisms

Regulation on the excited state of anthracene-bridged fluorophores for highly efficient blue non-doped OLEDs with ultra-low efficiency roll-off

Blue hot exciton fluorophores are deemed as promising candidates towards high-efficiency organic light emitting diodes (OLEDs) with negligible efficiency roll-off, which can break through the bottleneck on the accumulation of the lowest triplet state excitons rooting in OLEDs with phosphorescent/thermally activated delayed fluorescence light-emitting materials. Herein, two novel blue D-π-A fluorophores 4-(10-(4-(phenanthro[9,10-d]oxazol-2-yl)phenyl)anthracen-9-yl)-N,N- diphenylaniline (POANTP) and 2-(4-(10-(4-(9H-carbazol-9-yl)phenyl)anthracen-9-yl)phenyl)phenanthro[9,10-d]oxazole (POANCZ) were designed and synthesized by changing the electron-donating capacity to establish a molecular design strategy for manipulation on excited state from charge transfer (CT) state to local excitation (LE) state. By integrating the photophysical experiments and theoretical calculations of POANTP and POANCZ, it was validated that both POANTP and POANCZ exhibited high fluorescence luminous efficiency (74 ∼ 79 %) and possessed the "hot exciton" channels with T2 → S1, respectively. It was demonstrated that the excited states of two fluorophores were a HLCT excited state. The cyclic voltammetry experiment and ground-state calculation showed that the design strategy achieved fine-modulation of the highest occupied molecular orbitals energy levels. Furthermore, both non-doped OLEDs exhibited high efficiencies, especially the POANCZ-based blue non-doped OLED reached the maximum external quantum efficiency of 8.72 % associated with negligible efficiency roll-off of only 0.34 % at 1000 cd m-2. These results provide a simple and effective method for designing hot exciton fluorophores towards high-performance blue non-doped OLEDs.

Deep-blue emitting 9,10-bis(perfluorobenzyl)anthracene

A new deep-blue emitting and highly fluorescent anthracene (ANTH) derivative containing perfluorobenzyl (BnF) groups, 9,10-ANTH(BnF)2, was synthesized in a single step reaction of ANTH or ANTH(Br)2 with BnFI, using either a high-temperature Cu-/Na2S2O3-promoted reaction or via a room-temperature photochemical reaction. Its structure was elucidated by NMR spectroscopy and single crystal X-ray diffractometry. The latter revealed no π-π interaction between neighboring ANTH cores. A combination of high photoluminescence quantum yield (PLQY) of 0.85 for 9,10-ANTH(BnF)2, its significantly improved photostability compared to ANTH and 9,10-ANTH derivatives, and a simple synthetic access makes it an attractive material as a deep-blue OLED emitter and an efficient fluorescent probe.

A comparative investigation on excimer fluorescence toward its bright future

Must excimers quench fluorescence? This study aims to clarify the misconception that excimers are defective species with weak fluorescence. For this purpose, we utilized a rigid xanthene template to connect anthracene units for constructing an inter-excimer and an intra-excimer. Their photophysical properties were systematically investigated in solution and crystal forms, representing dynamic and static environments, respectively. In solutions, the inter- and intra-excimers exhibited low fluorescence efficiencies due to the limited formation and ease of dissociation of the excimers. In crystals, the inter- and intra-excimers both demonstrated a significant increase in fluorescence efficiency, which was ascribed to the greatly suppressed non-radiation for the static excimer in a rigid environment. Furthermore, the efficiency of the inter-excimer was higher than that of the intra-excimer, which arose from the more stable excited state for more effective non-radiative suppression. Therefore, it was concluded that the probability and stability of excimer formation are the key factors for improving excimer fluorescence efficiency. Overall, their fluorescence efficiencies can be ranked as follows: dynamic inter-excimer < dynamic intra-excimer < static intra-excimer < static inter-excimer, which is subjected to environmental rigidity and excimer stability. This work will provide a comprehensive understanding of excimers and propose a novel design strategy to achieve high-efficiency fluorescent materials for innovative organic photo-functional applications.

Asymmetric Structural Engineering of Hot-Exciton Emitters Achieving a Breakthrough in Non-Doped BT.2020 Blue OLEDs with a Record 9.5% External Quantum Efficiency

High-efficiency non-doped deep-blue organic light-emitting diodes (OLEDs) meeting the standard of BT.2020 color gamut is desired but rarely reported. Herein, an asymmetric structural engineering based on crossed long-short axis (CLSA) strategy is developed to obtain three new deep-blue emitters (BICZ, PHDPYCZ, and PHPYCZ) with a hot-exciton characteristic. Compared to 2BuCz-CNCz featuring a symmetric single hole-transport framework, these asymmetric emitters with the introduction of different electron-transport units show the enhancement of photoluminescence efficiency and improvement of bipolar charge transport capacity. Further combined with high radiative exciton utilization efficiency and light outcoupling efficiency, the non-doped OLED based on PHPYCZ exhibits the best performance with an excellent current efficiency of 3.49%, a record-high maximum external quantum efficiency of 9.5%, and a CIE y coordinate of 0.049 approaching the BT.2020 blue point. The breakthrough obtained in this work can inspire the molecular design of deep-blue emitters for high-performance non-doped BT.2020 blue OLEDs.

A record-high EQE of 7.65%@3300 cd m-2 achieved in non-doped near-ultraviolet OLEDs based on novel D'-D-A type bipolar fluorophores upon molecular configuration engineering

Developing a high-performance near-ultraviolet (NUV) material and its simple non-doped device with a small efficiency roll-off and good color purity is a promising but challenging task. Here, we proposed a novel donor'-donor-acceptor (D'-D-A) type molecular strategy to largely solve the intrinsic contradictions among wide-bandgap NUV emission, fluorescence efficiency, carrier injection and transport. An efficient NUV fluorophore, 3,6-mPPICNC3, exhibiting a hybridized local and charge-transfer state, is achieved through precise molecular configuration engineering, realizing similar hole and electron mobilities at both low and high electric fields. Moreover, the planarized intramolecular charge transfer excited state and steric hindrance effect endow 3,6-mPPICNC3 with a considerable luminous efficiency and good color purity in the aggregation state. Consequently, the non-doped device emitting stable NUV light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.160, 0.032) and a narrow full width at half maximum of 44 nm exhibits a state-of-the-art external quantum efficiency (EQE) of 7.67% and negligible efficiency roll-off over a luminance range from 0 to 3300 cd m-2. This is a record-high efficiency among all the reported non-doped NUV devices. Amazingly, an EQE of 7.85% and CIE coordinates of (0.161, 0.025) are achieved in the doped device. This demonstrates that the D'-D-A-type molecular structure has great potential for developing high-performance organic light-emitting materials and their optoelectronic applications.

Creation of High-Quality Deep-Blue AIE Emitter with a Crossed Long-Short Axis Structure for Efficient and Versatile OLEDs

Developing deep-blue emitters for organic light-emitting diodes (OLEDs) is critical but challenging, which requires a good balance between light color, exciton utilization, and photoluminescence quantum yield (PLQY) of solid film. Herein, a high-quality deep-blue emitter, abbreviated 2TriPE-CzMCN, is designed by introducing an aggregation-induced emission (AIE) group into a crossed long-short axis (CLSA) skeleton. Theoretical and experimental investigations reveal that the CLSA molecular design can achieve a balance between deep-blue emission and triplet-excitons utilization, while the high PLQY of the solid film resulting from the AIE feature helps to improve the performance of OLEDs. Consequently, when 2TriPE-CzMCN is used as the emitting dopant, the OLED exhibits a deep-blue emission at 430 nm with a record-high maximum external quantum efficiency (EQE) of 8.84%. When 2TriPE-CzMCN serves as the host material, the sensitized monochrome orange and two-color white OLEDs (WOLEDs) realize high EL performances that exceed the efficiency limit of conventional fluorescent OLEDs. Moreover, high-performance three-color WOLEDs with a color rendering index (CRI) exceeding 90 and EQE up to 18.08% are achieved by using 2TriPE-CzMCN as the blue-emitting source. This work demonstrates that endowing CLSA molecule with AIE feature is an effective strategy for developing high-quality deep-blue emitters, and high-performance versatile OLEDs can be realized through rational device engineering.

Triphenylene-Based Emitters with Hybridized Local and Charge-Transfer Characteristics for Efficient Nondoped Blue OLEDs with a Narrowband Emission and a Small Efficiency Roll-Off

Developing high-efficiency nondoped blue organic light-emitting diodes (OLEDs) with high color purity and low-efficiency roll-off is vital for display and lighting applications. Herein, we developed two asymmetric D-π-A blue emitters, PIAnTP and PyIAnTP, in which triphenylene is first utilized as a functional acceptor. The relatively weak charge transfer (CT) properties, rigid molecular structures, and multiple supramolecular interactions in PIAnTP and PyIAnTP can significantly enhance the fluorescence efficiency and suppress the structural relaxations to obtain a narrowband blue emission. The photophysical experiments and theoretical simulations reveal that they both exhibit a typical hybridized local and charge-transfer (HLCT) excited state and achieve high external quantum efficiency (EQE) via a "hot exciton" channel. As a result, PIAnTP- and PyIAnTP-based nondoped devices realize blue emission at 456 and 464 nm, corresponding to CIE coordinates of (0.16, 0.14) and (0.16, 0.19), narrow full width at half-maximums of 52 and 60 nm, and the high EQEs of 8.36 and 8.69%, respectively. More importantly, the PIAnTP- and PyIAnTP-based nondoped devices show small EQE roll-offs of only 5.9 and 2.4% at 1000 cd m-2, respectively. These results signify an advance in designing a highly efficient blue emitter for nondoped OLEDs.

Highly efficient and stable deep-blue OLEDs based on narrowband emitters featuring an orthogonal spiro-configured indolo[3,2,1-de]acridine structure

High-efficiency and stable deep-blue bottom-emitting organic light-emitting diodes with Commission Internationale de l'Eclairage y coordinates (CIE y s) < 0.08 remain exclusive in the literature owing to the high excited-state energy of the emitters. Here, we propose the utilization of narrowband emitters to lower the excited-state energy for stable deep-blue devices by taking advantage of their high color purity. Two proof-of-concept deep-blue emitters with nitrogen-containing spiro-configured polycyclic frameworks are thereafter developed to introduce a multi-resonance effect for narrow emissions and sterically orthogonal configurations for alleviated molecular interactions. Both emitters show bright ultrapure deep-blue emissions with an extremely small full-width-at-half-maxima of only 18-19 nm, which can be maintained even in heavily doped films. Small CIE y s of 0.054 and 0.066 are therefore measured from the corresponding electroluminescence devices with peak energies of only 2.77 eV (448 nm) and 2.74 eV (453 nm), accounting for the remarkably long LT80s (lifetime to 80% of the initial luminance) of 18 900 and 43 470 hours at 100 cd m-2, respectively. Furthermore, by adopting a thermally activated delayed fluorescence sensitizer, impressive maximum external quantum efficiencies of 25% and 31% are recorded respectively, representing state-of-the-art performances for deep-blue devices.