Rong-Huei Yi

Organic molecular engineering exhibits miscellaneous multifunctional properties including aggregation-induced emission (AIE), multi-responsive polymorphs, mechanoluminescence, and organic light-emitting diodes (OLEDs) in a single anthracene-based-molecule.

Abstract

The design of multifunctional materials is a challenging and important objective for a wide array of multidisciplinary applications. However, a multifunctional organic emitter exhibiting simultaneous aggregation-induced emission (AIE), multi-responsive polymorphs, mechanoluminescence and electroluminescence have been scarce. In this study, two anthracene based compounds, namely 10-(4-(9H-carbazol-9-yl)phenyl)anthracene-9-carbonitrile (CzPACN) and 10-(4-(di-p-tolylamino)phenyl)anthracene-9-carbonitrile (DTPACN) was designed and synthesized with rigid and flexible donors, respectively. The CzPACN shows the bright blue emission and DTPACN shows the bright green emission in solution. We have demonstrated an effective strategy to achieve three polymorphic phases such as DTPACN-α, DTPACN-β and DTPACN-γ from DTPACN by controlling the temperature. Under mechanical stimuli, highly restricted and non-planar crystals of the structurally tuned polymorphs DTPACN-α, and DTPACN-β exhibited red shifted emission and DTPACN-γ showed blue shifted emission. Conversely, CzPACN is not showing polymorphism and is not sensitive to external stimuli. In addition, blue and green OLEDs were fabricated using CzPACN and DTPACN, respectively, as an emitter and achieved a maximum external quantum efficiency (EQE_max) of 5.5% and 5.7%, respectively, for blue and green OLEDs. Further, this study suggests designing multi-responsive smart materials via a simple modification by introducing a non-planar unit with a large twist.

Room temperature phosphorescence and magnetic circularly polarized luminescence have been achieved from achiral phosphoramides having Donor–Acceptor architecture. The PL quantum yields and thephosphorescence emission wavelengths depend on the number of donor moieties and the nature of the heteroatom attached to the “P” center. The D–A interactions in phosphoramides are mainly controlled by steric crowding around the “P” center.

Abstract

Herein, the synthesis, structure, room temperature phosphorescence, and magnetic chiroptical properties of phosphoramides 1- 6 with donor–acceptor architecture are reported.₅). The electronic interactions between donor phenothiazine (PTZ) and acceptor (C₆H₅)_n-P = X (n = 1, 2; X = O, S, and Se)moieties in 1–6 are modulated by varying the number of donor units. The Lewis acidity of the phosphorus center is controlled by the nature of heteroatoms and the number of phenyl moieties attached to it. All compounds exhibit phosphorescence in the solid state under ambient conditions with a photoluminescence lifetime in the millisecond range (≈8–40 ms). The energy of the phosphorescence bands and their excited state lifetimes are governed by the P=X units. The phosphorescence features observed for 1–6 are in complete contrast to the observations noted for (PTZ)₃P = X (X = O, S, and Se), where the phosphorescence process is completely controlled by the PTZ moiety, and there is a negligible contribution from the P = X moiety. Thus, the present and earlier studies together reveal that the steric crowding and the number of C₆H₅ moieties around the P = X unit play a crucial role in controlling the electronic coupling between the donor and acceptor moieties in phosphoramides containing non-planar donor moieties. Furthermore, for the first time, the magnetic chiroptical properties of achiral phosphoramides 1–6, exhibiting a luminescence dissymmetry factor (g _MCPL) in the order of 10⁻³ (≈4.0 × 10⁻³–7.4 × 10⁻³) are demonstrated.

A radiative singlet exciton ratio of over 60% and external quantum efficiency of over 11% are achieved for the organic light-emitting diodes based on two deep-blue triplet–triplet annihilation emitters consisting of isomeric naphtho[1,2-d]imidazole, owing to the efficient conversion of triplet intermediate state (³(TT)) into the lowest singlet excited state despite the energy of ³(TT) being higher than that of the second triplet excited state.

Abstract

The utilization of triplet excitons is of great importance for organic light-emitting diodes (OLEDs). Triplet–triplet annihilation (TTA) is one of the effective tactics to achieve high efficiency deep-blue organic electroluminescence emitters by converting two triplet excitons into one singlet exciton. Whereas, in addition to the 25% electrogenerated singlet excitons, the proportion of radiative singlet excitons (RSE) produced by the TTA process is usually only 15%; thus the total radiative excitons are 40%. In this study, ≈35% of RSE is achieved by the TTA process (total 60%) with two deep-blue emitters based on the isomeric naphthoimidazole (NI) unit and anthracene bridge. As a result, non-doped OLEDs based on the two NI derivatives as emitting layers achieve maximum external quantum efficiencies of 10.9% and 11.2% with an identical deep-blue emission peak of 452 nm, which are the best TTA OLEDs with a Commission Internationale de l'Eclairage chromaticity Y coordinate below 0.15. Theoretical and experimental results demonstrate that the TTA process can be improved owing to the efficient spin–orbit interactions, even though the energy levels of the triplet pairs are higher than the calculated second triplet excited states.

Efficient blue thermally activated delayed fluorescence materials with intramolecular hydrogen bonds are developed. They exhibit high thermal stability, excellent photoluminescence quantum yields, and large horizontal dipole ratios and can provide outstanding electroluminescence efficiencies with long device operational lifetimes.

Abstract

Efficient blue organic luminescent materials are highly desired for full-color displays and white lighting based on the organic light-emitting diode (OLED) technique, but the exploration of robust blue emitters remains challenging. In this work, a series of efficient blue thermally activated delayed fluorescence materials comprised of a benzonitrile acceptor, pyridine bridge, and carbazole-based donors is designed and synthesized. Intramolecular hydrogen bonds are formed in these molecules, which improve molecular planarity and rigidity. These molecules exhibit excellent thermal stability, high photoluminescence quantum yields, and large horizontal dipole ratios. Highly efficient non-doped and doped OLEDs are fabricated using these molecules as emitters, providing deep-blue to sky-blue lights with outstanding maximum external quantum efficiencies and good operational device lifetime. These results indicate that building intramolecular hydrogen bonds can be an effective strategy for the construction of efficient and robust blue emitters for OLED applications.

Publication date: 15 July 2023

Source: Chemical Engineering Journal, Volume 468

Author(s): Adane Desta Fenta, Chiao-Wen Lin, Syuan-Wei Li, Chao-Tsen Chen, Chin-Ti Chen

Publication date: 1 August 2023

Source: Chemical Engineering Journal, Volume 469

Author(s): Liuqi Kong, Yan Zhu, Shaochen Sun, Hongye Li, Shuo Dong, Fei Li, Farong Tao, Liping Wang, Guang Li

Publication date: 1 August 2023

Source: Chemical Engineering Journal, Volume 469

Author(s): Yan-Ding Lin, Pei-Wan Hsiao, Wun-Yu Chen, Sih-Yu Wu, Wei-Min Zhang, Chin-Wei Lu, Hai-Ching Su

Publication date: 15 August 2023

Source: Chemical Engineering Journal, Volume 470

Author(s): Jie Li, Yan Xia, Geng Li, Mingxing Chen, Jinhao Zhou, Wenjun Yan, Bo Zhao, Kunpeng Guo, Hua Wang

The effect of relative orientation on the Förster resonance energy transfer rate between the sensitizer and emitting dopant is investigated in phosphor-sensitized fluorescence-based organic light-emitting diodes. A geometrical model for the relative orientation is established in which the prediction map is based on the horizontal emitting dipole orientations of the donor and acceptor.

Abstract

Förster resonance energy transfer (FRET) in sensitized fluorescent (SF) organic light-emitting diodes (OLEDs) is an important process for suppressing triplet exciton loss during energy transfer toward the fluorescent dopant. Herein, the contribution of the relative orientation between the sensitizer and emitting dopant to the FRET in state-of-the-art SF OLEDs is explained using experimental and theoretical approaches. The enhanced relative orientation factor (κ ²) from 0.375 to 1.250 is theoretically demonstrated in the FRET theory depending on the orientation of the sensitizer and emitting dopant. On comparing two SF OLED systems with different sensitizers, the sensitizer with a higher horizontal dipole orientation exhibits a higher FRET rate, resulting in the enhanced κ ². The exciton dynamics under device operation are explored to quantitatively verify the contribution of the enhanced FRET rate to the exciton transfer processes; the triplet consumption rate of the sensitizer improves by 2.2 times, demonstrating an efficient exciton transfer.

Two novel rod-like acceptor-donor-acceptor-configured thermally activated delayed fluorescence emitters with disk boron, nitrogen-contained polycyclic aromatic hydrocarbons (B,N-PAHs) fragments have been designed and synthesized. The orange-red and single-emission-layer white organic light-emitting diodes employing these dopants exhibit external quantum efficiency over 30% and low roll-off.

Abstract

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) materials are promising for the realization of highly efficient emitters. However, severe efficiency roll-off at high brightness still remains as a huge challenge for TADF-based OLEDs. Herein, rod-like orange-red TADF emitters of 2BNCz-PZ and 2BNtCz-PZ with acceptor-donor-acceptor (A-D-A) configuration are developed by bearing dihydrophenazine donor and discoidal rigid boron, nitrogen-contained polycyclic aromatic hydrocarbons acceptors. Both emitters exhibit hybrid long-range/short-range charge-transfer excitation for small singlet-triplet energy splitting, short delayed lifetime, and high photoluminescence quantum yield, leading to fast singlet radiation rate over 10⁷ s⁻¹ and fast reverse intersystem crossing rate over 10⁶ s⁻¹. Furthermore, a horizontal emitting dipole orientation factor over 90% is realized. The optimized orange-red OLED based on 2BNtCz-PZ presents a maximum external quantum efficiency (EQE) of 31.0% and a slight EQE roll-off to 22.2% at 1 000 cd m⁻² with emission peak over 600 nm. In addition, the single-emitting layer white OLEDs achieve a maximum EQE of 30.6% due to the use of these orange-red dopants with intense charge-transfer absorption band. This work reveals the potential of the rod-like A-D-A configuration for constructing highly efficient orange-red TADF emitters with low-efficiency roll-off.

State-of-the-art deep-red/near-infrared (NIR)-organic light-emitting diodes were achieved through strategically manipulating intermolecular interactions of a thermally activated delayed fluorescence emitter. Its doped devices afford record-high external quantum efficiencies (EQEs) of 36.1, 29.3, 28.2, and 24.0% peaked at 656, 688, 696, 716 nm, respectively. Moreover, its nondoped NIR device also retains a champion EQE of 2.61% at 800 nm.

Abstract

Deep-red/near-infrared (DR/NIR) organic light-emitting diodes (OLEDs) are promising for applications such as night-vision readable marking, bioimaging, and photodynamic therapy. To tune emission spectra into the DR/NIR region, red emitters generally require assistance from intermolecular interactions. But such interactions generally lead to sharp efficiency declines resulting from unwanted quenching events. To overcome this challenge, herein, an advanced method via strategically managing the intermolecular interactions of thermally activated delayed fluorescence (TADF) emitters is proposed. The proof-of-concept molecule called DCN-SPTPA exhibits impressive resistance to quenching while delivering controllable aggregation behavior for redshifting the emission by installing an end-spiro group. Consequently, two emitters demonstrate similar photophysical properties and device performance at very low doping levels; while DCN-SPTPA-based OLEDs demonstrate a 1.3–1.4-fold enhancement of the external quantum efficiencies (EQEs) with respect to the control molecule at 5–20 wt.% doping ratios, affording DR/NIR emission at 656, 688, 696, and 716 nm with record-breaking EQEs of 36.1%, 29.3%, 28.2%, and 24.0%, respectively. Moreover, DCN-SPTPA-based nondoped NIR device also retains a state-of-the-art EQE of 2.61% peaked at 800 nm. This work first demonstrates instructive guidance for accurately manipulating the intermolecular interactions of red TADF emitters, which will spur future developments in high-performance DR/NIR OLEDs.

The new strategy for using two triplet–triplet annihilation (TTA) up-conversion materials to improve the efficiency of Dexter energy transfer and manage high-lying triplet excitons of aggregation-induced emission (AIE) emitters. Finally, the maximum external quantum efficiency of the device reaches as high as 14.8%, realizing the breakthrough in efficiency of blue fluorescence OLEDs based on AIE emitter.

Abstract

Aggregation-induced emission (AIE) materials are attractive for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) owing to the “hot exciton” process by reverse intersystem crossing (hRISC) and high photoluminescence quantum yields (PLQY). However, the internal conversion (IC) from the high-lying triplet excitation states (T_n, n≥2) to the lower excited triplet state (T_n-1) is inevitable, resulting in severe exciton losses. Herein, an effective device structure is designed that reuses the lost triplet excitons caused by IC and realizes the breakthrough in the efficiency of blue fluorescence OLEDs based on AIE molecule as an emitter. The maximum external quantum efficiency reached as high as 14.8% and is kept at 14.4% at the luminance of 1000 cd m⁻². In the designed device, a triplet–triplet annihilation (TTA) up-conversion material 1-[2,5-dimethyl-4-(1-pyrenyl)phenyl]pyrene (DMPPP) is introduced into the AIE emitter as a triplet sensitizer to receive the lost triplet excitons, and a thin TTA up-conversion layer 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (CzPA) is introduced in the middle of the doped layer to form the emissive layer (EML). It is found that the hRISC process of iTPB-2AC greatly enhances the utilization efficiency of TTA intermediate state ([TT]*) excitons on CzPA to iTPB-2AC so that the utilization of the lost excitons is maximized. This work establishes physical insights into the AIE emission materials and device fabrication of high-efficiency blue fluorescence OLEDs.

Publication date: 1 July 2023

Source: Chemical Engineering Journal, Volume 467

Author(s): Xiangan Song, Shaogang Shen, Shengnan Zou, Fengyun Guo, Ying Wang, Shiyong Gao, Yong Zhang

Publication date: 15 July 2023

Source: Chemical Engineering Journal, Volume 468

Author(s): Shi-Peng Wan, Wen-Long Zhao, Ke-Ke Tan, Hai-Yan Lu, Meng Li, Chuan-Feng Chen

Publication date: 15 July 2023

Source: Chemical Engineering Journal, Volume 468

Author(s): Hao-Yu Yang, Heng-Yuan Zhang, Ming Zhang, Hao Zhuo, Hui Wang, Hui Lin, Si-Lu Tao, Cai-Jun Zheng, Xiao-Hong Zhang

A series of orange-red thermally activated delayed fluorescence polymers with dibenzothiophene and carbazole units as joint backbones are synthesized. And the connection positions of dibenzothiophene units are regulated through backbone engineering. The OLEDs based on optimally designed polymer with best photophysical properties achieve an EQE_max of 20.16%, and maintain 10.61% at 500 cd m⁻², which is in first tier among orange-red polymers.

Abstract

Thermally activated delayed fluorescence (TADF) materials have attracted extensive attention because of their 100% theoretical exciton utilization. Solution-processable orange-red TADF polymers are one of indispensable participants. Herein, a series of orange-red TADF polymers with dibenzothiophene (DBT) and carbazole (Cz) units as joint backbones are synthesized. Their performance can be successfully optimized by regulating the connection positions of DBT units through backbone engineering. It is found that the pNAI37 series with DBT units embedded in the polymeric backbones at the 3, 7 sites display a better performance than those connected at the 2, 8 sites. The optimal polymer, pNAI3705, exhibits a better excited state nature, leading to the photoluminescence quantum yield of 60%. Consequently, pNAI3705 based organic light-emitting diodes reach a maximum external quantum efficiency of 20.16%, and maintain 10.61% at 500 cd m⁻², which is in first tier among orange-red polymers. These results unambiguously suggest the potential application of the combined DBT and Cz backbones in TADF polymers. This design strategy may provide a versatile approach for optimizing the properties of TADF polymers through backbone engineering.

A spiro unit is first implemented as the bridging group instead of a conventional phenyl bridge to construct a donor-π-acceptor-type red thermally activated delayed fluorescence emitter, 3-(2-(diphenylamino)-9,9′-spirobi[fluoren]-7-yl)dibenzo[a,c]phenazine-11,12-dicarbonitrile (DCN-SP-DPA). The spiro bridge enables enhanced rigidity, sufficient steric hindrance, elongated molecule length, and obvious aggregation-induced emission characteristics. Thus, organic light-emitting diodes based on DCN-SP-DPA render a maximum external quantum efficiency of 36.9%.

Abstract

Constructing donor-π-acceptor (D-π-A)-type molecular structures by employing a phenyl as the π-bridge to link donor (D) and acceptor (A) units has been recognized as an effective way to develop highly efficient red thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). However, flexible and relatively planar structures would open potential energy loss channels, such as nonradiative inactivation and aggregation-induced triplet quenching processes. Here, a bulky spiro-9,9′-bifluorene unit is first implemented to serve as a bridging group to construct a D-π-A molecule, enabling it to have higher overall rigidity, more sufficient steric hindrance, prolonged molecule length, and obvious aggregation-induced emission characteristics compared with a common phenyl bridge. As a result, energy dissipation routes are effectively relieved at the unimolecular level, together with mitigated interchromophore quenching, rendering a 100% photoluminescence quantum yield and a larger horizontal dipole ratio of 89%. The OLED based on 3-(2-(diphenylamino)-9,9′-spirobi[fluoren]-7-yl)dibenzo[a,c]phenazine-11,12-dicarbonitrile exhibits an excellent external quantum efficiency of nearly 37% at 612 nm, which is over 1.38-fold enhancement compared with the phenyl bridge-based control molecule. This work provides an instructive solution to design highly efficient red TADF emitters exploiting D-π-A-type molecular structures.