Rong-Huei Yi

Light: Science & Applications, Published online: 31 January 2023; doi:10.1038/s41377-022-01062-6

A series of hitherto unknown 5,14-diphenylbenzo[j]naphtho[2,1,8-def][2,7]phenanthrolines were prepared by combination of Pd-catalyzed cross-coupling reactions with a one-pot Povarov/cycloisomerization reaction. Four new bonds are formed in one step in the final key step. The optical and electrochemical properties were studied experimentally and supported by DFT/TD-DFT and NICS calculations.

Abstract

A series of hitherto unknown 5,14-diphenylbenzo[j]naphtho[2,1,8-def][2,7]phenanthrolines, containing a 5-azatetracene and a 2-azapyrene subunit, were prepared by combination of Pd-catalyzed cross-coupling reactions with a one-pot Povarov/cycloisomerization reaction. In the final key step four new bonds are formed in one step. The synthetic approach allows for a high degree of diversification of the heterocyclic core structure. The optical and electrochemical properties were studied experimentally and by DFT/TD-DFT and NICS calculations. Due to the presence of the 2-azapyrene subunit, the typical electronic nature and characteristics of the 5-azatetracene moiety are lost and the compounds are electronically and optically more related to 2-azapyrenes.

Publication date: 15 February 2023

Source: Chemical Engineering Journal, Volume 458

Author(s): Deli Li, Mengke Li, Wei Li, Wenqi Li, Zijian Chen, Xiaomei Peng, Denghui Liu, Guo-Xi Yang, Simin Jiang, Yiyang Gan, Zhihai Yang, Kunkun Liu, Shi-Jian Su

White light-emitting electrochemical cells (LECs) based on phosphor-sensitized thermally activated delayed fluorescence show a high external quantum efficiency of 9.6 %, which is comparable with that obtained from all-phosphorescent white LECs.

Abstract

Solid-state light-emitting electrochemical cells (LECs) show promising advantages of simple device architecture, low operation voltage, and insensitivity to the electrode work functions such that they have high potential in low-cost display and lighting applications. In this work, novel white LECs based on phosphor-sensitized thermally activated delayed fluorescence (TADF) are proposed. The emissive layer of these white LECs is composed of a blue-green phosphorescent host doped with a deep-red TADF guest. Efficient singlet-to-triplet intersystem crossing (ISC) on the phosphorescent host and the subsequent Förster energy transfer from the host triplet excitons to guest singlet excitons can make use of both singlet and triplet excitons on the host. With the good spectral overlap between the host emission and the guest absorption, 0.075 wt.% guest doping is sufficient to cause substantial energy transfer efficiency (ca. 40 %). In addition, such a low guest concentration also reduces the self-quenching effect and a high photoluminescence quantum yield of up to 84 % ensures high device efficiency. The phosphor-sensitized TADF white LECs indeed show a high external quantum efficiency of 9.6 %, which is comparable with all-phosphorescent white LECs. By employing diffusive substrates to extract the light trapped in the substrate, the device efficiency can be further improved by ca. 50 %. In the meantime, the intrinsic EL spectrum and device lifetime of the white LECs recover since the microcavity effect is destroyed. This work successfully demonstrates that the phosphor-sensitized TADF white LECs are potential candidates for efficient white light-emitting devices.

Steric protection of the Zn-Carbene bond by a bridging coordination mode of 1,2-dithiolate ligands leads to air- and moisture-stable complexes. Efficient TADF with high radiative rate constants at room temperature is observed from ^1/3LL/LMCT states according to variable temperature, transient absorption and DFT/MRCI studies. Application in Dexter energy transfer catalysis is demonstrated, exemplifying the potential of this new class of abundant 3d photoactive systems.

Abstract

A dimeric Zn^II carbene complex featuring bridging and chelating benzene-1,2-dithiolate ligands is highly stable towards air and water. The donor-Zn-acceptor structure leads to visible light emission in the solid state, solution and polymer matrices with λ _max between 577–657 nm and, for zinc(II) complexes, unusually high radiative rate constants for triplet exciton decay of up to k _r=1.5×10⁵ s⁻¹ at room temperature. Variable temperature and DFT/MRCI studies show that a small energy gap between the ^1/3LL/LMCT states of only 79 meV is responsible for efficient thermally activated delayed fluorescence (TADF). Time-resolved luminescence and transient absorption studies confirm the occurrence of long-lived, dominantly ligand-to-ligand charge transfer excited states in solution, allowing for application in Dexter energy transfer photocatalysis.

A yellow-orange emission exciplex-forming blend composed of a 2,3 dicyanopyrazinophenanthrene-based acceptor and a carbazole-based donor is employed as a thermally activated delayed fluorescence (TADF) cohost system for a donor-acceptor-donor-configured organic near-infrared emitter to achieve an organic light-emitting device with electroluminescence centered at 743 nm and a maximum external quantum efficiency of 4.79 % by virtue of the effective energy transfer mechanism.

Abstract

Two new 2,3-dicyanopyrazinophenanthrene-based acceptors (A) p-QCN and m-QCN were synthesized to blend with a donor (D) CPTBF for the exciplex formation. The energy levels of p-QCN and m-QCN are modulated by the peripheral substituents 4- and 3-benzonitrile, respectively. Exciplex-forming blends were identified by the observation of the red-shifted emissions from various D : A blends with higher ratios of donor for suppressing the aggregation of acceptor. The two-component relaxation processes observed by time-resolved photoluminescence support the thermally activated delayed fluorescence (TADF) character of the exciplex-forming blends. The device employing CPTBF : p-QCN and (2 : 1) and CPTBF : m-QCN (2 : 1) blend as the emitting layer (EML) gave EQE_max of 1.76 % and 5.12 %, and electroluminescence (EL) λ_max of 629 nm and 618 nm, respectively. The device efficiency can be further improved to 4.32 % and 5.57 % with CPTBF : p-QCN and (4 : 1) and CPTBF : m-QCN (4 : 1) as the EML, which is consistent with their improved photoluminescence quantum yields (PLQYs). A new fluorescent emitter BPBBT with photoluminescence (PL) λ_max of 726 nm and a high PLQY of 67 % was synthesized and utilized as the dopant of CPTBF : m-QCN (4 : 1) cohost system. The device employing CPTBF : m-QCN (4 : 1): 5 wt.% BPBBT as the EML gave an EQE_max of 5.02 % and EL λ_max centered at 735 nm, however, the weak residual exciplex emission remains. By reducing the donor ratio, the exciplex emission can be completely transferred to BPBBT and the corresponding device with CPTBF : m-QCN (2 : 1): 5 wt.% BPBBT as the EML can achieve EL λ_max of 743 nm and EQE_max of 4.79 %. This work manifests the high efficiency near infrared (NIR) OLED can be realized by triplet excitons harvesting of exciplex-forming cohost system, followed by the effective energy transfer to an NIR fluorescent dopant.

A class of macrocycles emitting room-temperature phosphorescence or thermally activated delayed fluorescence are established based on hydrogen-bonded cyclo[6]aramide by introducing various donors, which exhibit tunable colors including white-light emission through host–guest complexation. The results highlight the potential of such rigid and planar macrocycle for developing novel luminescent materials.

Abstract

Despite vast applications of macrocycles in supramolecular chemistry, achieving long-lived emissions including room-temperature phosphorescence (RTP) or thermally activated delayed fluorescence (TADF) for potential use still presents a great challenge. This work first reports hydrogen-bonded (H-bonded) macrocycles emitting RTP and TADF by introducing various donors onto the same aramide skeleton containing a rigid acceptor. The formation of charge transfer effectively enhances the photoluminescence efficiency. Aromatic carbonyl groups promote the intersystem crossing. The drastically reduced flexibility of chromophores fixed by the H-bonded macrocyclic framework contributes to suppress the nonradiative decay to stabilize triplet excitons. Therefore, RTP and TADF are acquired by altering donors, and are systematically revealed by comparisons with control compounds and theoretical calculations. Finally, near white-light emission (CIE, 0.30, 0.33) is realized via host–guest interactions.

Mixing a sterically bulky, electron-transporting host material into a conventional single host–guest emissive layer is demonstrated to suppress phase separation of the host matrix while increasing the efficiency by 120% and the operational lifetime of deep-blue phosphorescent organic light-emitting diodes with chromaticity coordinates of (0.14, 0.15).

Abstract

Mixing a sterically bulky, electron-transporting host material into a conventional single host–guest emissive layer is demonstrated to suppress phase separation of the host matrix while increasing the efficiency and operational lifetime of deep-blue phosphorescent organic light-emitting diodes (PHOLEDs) with chromaticity coordinates of (0.14, 0.15). The bulky host enables homogenous mixing of the molecules comprising the emissive layer while suppressing single host aggregation; a significant loss channel of nonradiative recombination. By controlling the amorphous phase of the host-matrix morphology, the mixed-host device achieves a significant reduction in nonradiative exciton decay, resulting in 120 ± 6% increase in external quantum efficiency relative to an analogous, single-host device. In contrast to single host PHOLEDs where electrons are transported by the host and holes by the dopants, both charge carriers are conducted by the mixed host, reducing the probability of exciton annihilation, thereby doubling of the deep-blue PHOLED operational lifetime. These findings demonstrate that the host matrix morphology affects almost every aspect of PHOLED performance.

A TADF emitter tBuTPA-CNQx is designed and synthesized to enable highly efficient pure red emitting solution-processed OLEDs. Relying on its small ΔE _ST of 0.07 eV, high PLQY of 92%, and high K _RISC of 3.98 × 10⁵ s⁻¹, solution-processed OLED doped with tBuTPA-CNQx exhibit a high EQE of 16.7% at 662 nm with CIE coordinates of (0.67, 0.32).

Abstract

In spite of recent research progress in red thermally activated delayed fluorescence (TADF) emitters, highly efficient solution-processable pure red TADF emitters are rarely reported. Most of the red TADF emitters reported to date are designed using a rigid acceptor unit which renders them insoluble and unsuitable for solution-processed organic light-emitting diodes (OLEDs). To resolve this issue, a novel TADF emitter, 6,7-bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-5,8-dicarbonitrile (tBuTPA-CNQx) is designed and synthesized. The highly twisted donor-acceptor architecture and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital distribution lead to a very small singlet-triplet energy gap of 0.07 eV, high photoluminescence quantum yield of 92%, and short delayed fluorescence lifetime of 52.4 µs. The peripheral t-butyl phenyl decorated quinoxaline acceptor unit and t-butyl protected triphenylamine donor unit are proven to be useful building blocks to improve solubility and minimize the intermolecular interaction. The solution-processed OLED based on tBuTPA-CNQx achieves a high external quantum efficiency (EQE) of 16.7% with a pure red emission peak at 662 nm, which is one of the highest EQE values reported till date in the solution-processed pure red TADF OLEDs. Additionally, vacuum-processable OLED based on tBuTPA-CNQx exhibits a high EQE of 22.2% and negligible efficiency roll-off.

Donor extension provides delicate control on photophysical processes. Conjugation attached on acridine ensures an enhanced reversed intersystem crossing (RISC) in IA-TRZ, while substitution via spiro-junction induces an accelerated radiative decay in IT-TRZ and 2S-TRZ. IT-TRZ with a moderate radiative decay and RISC achieves a high external quantum efficiency of 36.1% and a much-relieved efficiency roll-off of 0.1%/11% under 100/1000 cd m⁻².

Abstract

Organic light emitting diodes based on thermally activated delayed fluorescent (TADF) emitters with both high external quantum efficiency (EQE) and low efficiency roll-off are under urgent pursuing. Three TADF emitters based on fluorene-spiro-acridine derivatives as donors are developed. Composing of rigid donor and acceptor, all emitters exhibit high PLQYs of 96–99%. Direct conjugation attached on acridine offers an enhanced reverse intersystem crossing (k _RISC) in IA-TRZ, while the extension via spiro-junction accelerates the radiative decay rate (k _r) in 2S-TRZ and IT-TRZ. Remarkably, all emitters enable high electroluminescent performance with EQE of 35.6% for 2S-TRZ, 36.1% for IT-TRZ and 32.0% for IA-TRZ, respectively, and very high luminance of up to 100 000 cd m⁻². Surprisingly, 2S-TRZ and IT-TRZ that hold larger k _r and smaller k _RISC than that of IA-TRZ exhibit more relieved efficiency roll-off, indicating the significance of k _r. More importantly, IT-TRZ with a moderate k _r and k _RISC among these emitters shows the lowest roll-off which demonstrates that the comprehensive consideration of all photophysical processes is important to attain excellent devices. It also emphasizes the molecular modulation method through side group that can induce elaborate control on decay channels for finely optimizing emitters. This study can provide a useful perspective in designing practical emitters.

Nature Photonics, Published online: 09 January 2023; doi:10.1038/s41566-022-01106-8