Efficient Deep-Blue Fluorescent OLEDs with a High Exciton Utilization Efficiency from a Fully Twisted Phenanthroimidazole-Anthracene Emitter

Overview of imidazole-based fluorescent materials with hybridized local and charge transfer and hot-exciton pathway characteristics in excited states

Herein, we discuss an imidazole-based molecular framework, which can successfully transform triplet excitons present in high triplet levels into singlet states. We explain the working mechanisms of different methods for collecting triplet excitons, including hot excitons or HLCT states. After the development of an hot exciton material by Ma and Yang, many studies have demonstrated that the organic conjugated molecules having imidazole core have possibilities to show high efficiencies via hot exciton pathways. Finally, we provide a detailed investigation of recently published hot exciton luminogens based on imidazole molecular frameworks. This review provides an overview of the molecular structures, frontier molecular orbital information, and glass transition temperature of developed luminogens as well as the efficiency of organic light-emitting diodes (OLED) devices.

Doubly Bridged Anthracenes: Blue Emitters for OLEDs

The photooxidative stability of a series of doubly bridged anthracenes was evaluated after their preparation via twofold macrocyclization of a bis(resorcinyl)anthracene. Lightfastness correlates with the energy levels of the highest occupied molecular orbital (HOMO), resulting in superior stability of the tetraesters compared to the tetraethers. The lengths and steric demand of the linker only plays a minor role for the ester-based compounds, which can be prepared in reasonable yields and thus tested in proof-of-concept organic light-emitting diodes. Double ester-bridging allows deep blue electro-luminescence, highlighting the importance of the choice of the functional groups used for macrocyclization.

Aggregation behaviour of pyrene-based luminescent materials, from molecular design and optical properties to application

Molecular aggregates are self-assembled from multiple molecules via weak intermolecular interactions, and new chemical and physical properties can emerge compared to their individual molecule. With the development of aggregate science, much research has focused on the study of the luminescence behaviour of aggregates rather than single molecules. Pyrene as a classical fluorophore has attracted great attention due to its diverse luminescence behavior depending on the solution state, molecular packing pattern as well as morphology, resulting in wide potential applications. For example, pyrene prefers to emit monomer emission in dilute solution but tends to form a dimer via π-π stacking in the aggregation state, resulting in red-shifted emission with quenched fluorescence and quantum yield. Over the past two decades, much effort has been devoted to developing novel pyrene-based fluorescent molecules and determining the luminescence mechanism for potential applications. Since the concept of "aggregation-induced emission (AIE)" was proposed by Tang et al. in 2001, aggregate science has been established, and the aggregated luminescence behaviour of pyrene-based materials has been extensively investigated. New pyrene-based emitters have been designed and synthesized not only to investigate the relationships between the molecular structure and properties and advanced applications but also to examine the effect of the aggregate morphology on their optical and electronic properties. Indeed, new aggregated pyrene-based molecules have emerged with unique properties, such as circularly polarized luminescence, excellent fluorescence and phosphorescence and electroluminescence, ultra-high mobility, etc. These properties are independent of their molecular constituents and allow for a number of cutting-edge technological applications, such as chemosensors, organic light-emitting diodes, organic field effect transistors, organic solar cells, Li-batteries, etc. Reviews published to-date have mainly concentrated on summarizing the molecular design and multi-functional applications of pyrene-based fluorophores, whereas the aggregation behaviour of pyrene-based luminescent materials has received very little attention. The majority of the multi-functional applications of pyrene molecules are not only closely related to their molecular structures, but also to the packing model they adopt in the aggregated state. In this review, we will summarize the intriguing optoelectronic properties of pyrene-based luminescent materials boosted by aggregation behaviour, and systematically establish the relationship between the molecular structure, aggregation states, and optoelectronic properties. This review will provide a new perspective for understanding the luminescence and electronic transition mechanism of pyrene-based materials and will facilitate further development of pyrene chemistry.

Overcoming the Limitation of Spin Statistics in Organic Light Emitting Diodes (OLEDs): Hot Exciton Mechanism and Its Characterization

Triplet harvesting processes are essential for enhancing efficiencies of fluorescent organic light-emitting diodes. Besides more conventional thermally activated delayed fluorescence and triplet-triplet annihilation, the hot exciton mechanism has been recently noticed because it helps reduce the efficiency roll-off and improve device stability. Hot exciton materials enable the conversion of triplet excitons to singlet ones via reverse inter-system crossing from high-lying triplet states and thereby the depopulation of long-lived triplet excitons that are prone to chemical and/or efficiency degradation. Although their anti-Kasha characteristics have not been clearly explained, numerous molecules with behaviors assigned to the hot exciton mechanism have been reported. Indeed, the related developments appear to have just passed the stage of infancy now, and there will likely be more roles that computational elucidations can play. With this perspective in mind, we review some selected experimental studies on the mechanism and the related designs and then on computational studies. On the computational side, we examine what has been found and what is still missing with regard to properly understanding this interesting mechanism. We further discuss potential future points of computational interests toward aiming for eventually presenting in silico design guides.

Luminescent columnar discotics as highly efficient emitters in pure deep-blue OLEDs with an external quantum efficiency of 4.7

Development of materials that serve as efficient blue emitters in solution-processable OLEDs is challenging. In this study, we report three derivatives of C₃-symmetric 1,3,5-tris(thien-2-yl)benzene-based highly luminescent room temperature columnar discotic liquid crystals (DLCs) suitable as solid-state emitters in OLED devices. When employed in solution-processed OLEDs, one of the derivatives having the highest photoluminescence quantum yield exhibited a maximum EQE of 4.7% and CIE chromaticity of (0.16, 0.05) corresponding to the ultra deep-blue emission. The finding is sufficiently significant in the field of DLC-based deep blue emitters.

Turning conventional non-TADF units into high-lying reverse intersystem crossing TADF emitters: different symmetric D–A–D-type modified donor units

DFT and TD-DFT calculations were performed to turn conventional non-TADF units into high-lying reverse intersystem crossing D–A–D-type TADF emitters.

Kinetic Stabilization of Blue-Emissive Anthracenes: Phenylene Bridging Works Best

In attempts at kinetically stabilizing blue-emissive anthracenes, a series of 9,10-diaryl substituted derivatives were tested for their photochemical and photooxidative persistence. A major breakthrough in light fastness comes from a new bis-meta-terphenylyl substituted anthracene which is much superior to industrially relevant 9,10-biarylated anthracenes. The key issue is the steric shielding of the anthracene core. Further, intramolecular ring closure via Yamamoto coupling furnished a doubly bridged anthracene as a "self-encapsulated" sky-blue emitter which is most resistant to photodegradation. The improved stabilization was corroborated by time-resolved irradiation experiments and rationalized by X-ray crystallography.

Constructing Highly Efficient Blue OLEDs with External Quantum Efficiencies up to 7.5 % Based on Anthracene Derivatives

Acquiring desirable device performance with deep-blue color purity that fulfills practical application requirements is still a challenge. Bipolar fluorescent emitters with hybrid local and charge transfer (HLCT) state may serve to address this issue. Herein, by inserting anthracene core in the deep-blue building blocks, the authors successfully developed two highly twisted D-π-A fluorescent emitters, ICz-An-PPI and IP-An-PPI, featuring different acceptor groups. Both exhibited superb thermal stabilities, high photo luminescent quantum yields and excellent bipolar transport capabilities. The non-doped OLEDs using ICz-An-PPI and IP-An-PPI as the emitting layers showed efficient blue emission with an external quantum efficiency (EQE_max ) of 4.32 % and 5.41 %, and the CIE coordinates of (0.147, 0.180) and (0.149, 0.150), respectively. In addition, the deep blue doped device based on ICz-An-PPI was achieved with an excellent CE_max of 5.83 cd A^-1 , EQE_max of 4.6 % and the CIE coordinate of (0.148, 0.078), which is extremely close to the National Television Standards Committee (NTSC) standard. Particularly, IP-An-PPI-based doped device had better performance, with an EQE_max of 7.51 % and the CIE coordinate of (0.150, 0.118), which was very impressive among the recently reported deep-blue OLEDs with the CIE_y <0.12. Such high performance may be attributed to the hot exciton HLCT mechanism via T₇ to S₂ . Our work may provide a new approach for designing high-efficiency deep-blue materials.

Rational Design of Chrysene-Based Hybridized Local and Charge-Transfer Molecules as Efficient Non-Doped Deep-Blue Emitters for Simple-Structured Electroluminescent Devices

Herein, we present a molecular design of chrysene-based deep-blue emissive materials (TC, TpPC, TpXC, and TmPC), in which chrysene as a core is functionalized with different triphenylamine moieties to realize a fine-tuning deep-blue fluorescence with superior electroluminescent (EL) performance. The photophysical analyses and density functional theory (DFT) calculations disclose that TC, TpPC, and TpXC possess HLCT characteristics with intense deep-blue emission in the solid-state, good hole-transporting ability, and high thermal and electrochemical stabilities. They are successfully employed as non-doped emitters in simple structured OLEDs (ITO/PEDOT : PSS : NF/emitter/TPBi/LiF : Al). In particular, TC-based device emits a deep-blue light with an emission peak at 446 nm and CIE color coordinates of (0.148, 0.096), a maximum external quantum efficiency (EQE_max ) of 4.31%, and a low turn-on voltage of 2.8 V.

Hot exciplexes in U-shaped TADF molecules with emission from locally excited states

Fast emission and high color purity are essential characteristics of modern opto-electronic devices, such as organic light emitting diodes (OLEDs). These properties are currently not met by the latest generation of thermally activated delayed fluorescence (TADF) emitters. Here, we present an approach, called "hot exciplexes" that enables access to both attributes at the same time. Hot exciplexes are produced by coupling facing donor and acceptor moieties to an anthracene bridge, yielding an exciplex with large T1 to T2 spacing. The hot exciplex model is investigated using optical spectroscopy and quantum chemical simulations. Reverse intersystem crossing is found to occur preferentially from the T3 to the S1 state within only a few nanoseconds. Application and practicality of the model are shown by fabrication of organic light-emitting diodes with up to 32 % hot exciplex contribution and low efficiency roll-off.

Highly sensitive detecting system to precisely evaluate the emission spectra and quantum efficiency of organic crystal light-emitting transistors

The lack of a reliable and sensitive measure system has strongly hindered the progress of organic single-crystal light-emitting transistors, since their emission spectra and absolute intensity were rather difficult to be measured as a consequence of small area and weak emission, as well as their edge emission feature. Hence, a tailor-made detecting system was set up by combining a semiconductor analyzer and a rotatable microscope coupling with a highly sensitive grating spectrometer. The detecting system solved the important issue of quantitatively characterizing the optoelectronic properties of organic transistors, especially those based on single crystals. With organic lamellar single crystals, we confirmed that the measurement system was reliable and had enough sensitivity to precisely evaluate the performance of light-emitting transistors corresponding to the emission spectra.

Synthesis of some new distyrylbenzene derivatives using immobilized Pd on an NHC-functionalized MIL-101(Cr) catalyst: photophysical property evaluation, DFT and TD-DFT calculations

In this study the catalytic application of a heterogeneous Pd-catalyst system based on metal organic framework [Pd-NHC-MIL-101(Cr)] was investigated in the synthesis of distyrylbenzene derivatives using the Heck reaction. The Pd-NHC-MIL-101(Cr) catalyst showed high efficiency in the synthesis of these π-conjugated materials and products were obtained in high yields with low Pd-contamination based on ICP analysis. The photophysical behaviors for some of the synthesized distyrylbenzene derivatives were evaluated. The DFT and TD-DFT methods were employed to determine the optimized molecular geometry, band gap energy, and the electronic absorption and emission wavelengths of the new synthesized donor-π-acceptor (D-π-A) molecules in the gas phase and in various solvents using the chemical model B3LYP/6-31+G(d,p) level of theory.

Recent progress in hot exciton materials for organic light-emitting diodes

According to Kasha's rule, high-lying excited states usually have little effect on fluorescence. However, in some molecular systems, the high-lying excited states partly or even mainly contribute to the photophysical properties, especially in the process of harvesting triplet excitons in organic electroluminescent devices. In the current review, we focus on a type of organic light-emitting diode (OLED) materials called "hot exciton" materials, which can effectively harness the non-radiative triplet excitons via reverse intersystem crossing (RISC) from high-lying triplet states to singlet states (T_n→ S_m; n≥ 2, m≥ 1). Since Ma and Yang proposed the hot exciton mechanism for OLED material design in 2012, there have been many reports aiming at the design and synthesis of novel hot exciton luminogens. Herein, we present a comprehensive review of the recent progress in hot exciton materials. The developments of the hot exciton mechanism are reviewed, the fundamental principles regarding molecular design are discussed, and representative reported hot exciton luminogens are summarized and analyzed, along with their structure-property relationships and OLED applications.

Imidazo[1,2-a]pyridine as an Electron Acceptor to Construct High-Performance Deep-Blue Organic Light-Emitting Diodes with Negligible Efficiency Roll-Off

Two novel bipolar deep-blue fluorescent emitters, IP-PPI and IP-DPPI, featuring different lengths of the phenyl bridge, were designed and synthesized, in which imidazo[1,2-a]pyridine (IP) and phenanthroimidazole (PI) were proposed as an electron acceptor and an electron donor, respectively. Both of them exhibit outstanding thermal stability and high emission quantum yields. All the devices based on these two materials showed negligible efficiency roll-off with increasing current density. Impressively, non-doped organic light-emitting diodes (OLEDs) based on IP-PPI and IP-DPPI exhibited external quantum efficiencies (EQEs) of 4.85 % and 4.74 % with CIE coordinates of (0.153, 0.097) and (0.154, 0.114) at 10000 cd m^-2 , respectively. In addition, the 40 wt % IP-PPI doped device maintained a high EQE of 5.23 % with CIE coordinates of (0.154, 0.077) at 10000 cd m^-2 . The doped device based on 20 wt % IP-DPPI exhibited a higher deep-blue electroluminescence (EL) performance with a maximum EQE of up to 6.13 % at CIE of (0.153, 0.078) and maintained an EQE of 5.07 % at 10000 cd m^-2 . To the best of our knowledge, these performances are among the state-of-the art devices with CIE_y ≤0.08 at a high brightness of 10000 cd m^-2 . Furthermore, by doping a red phosphorescent dye Ir(MDQ)₂ (MDQ=2-methyldibenzo[f,h]quinoxaline) into the IP-PPI and IP-DPPI hosts, high-performance red phosphorescent OLEDs with EQEs of 20.8 % and 19.1 % were achieved, respectively. This work may provide a new approach for designing highly efficient deep-blue emitters with negligible roll-off for OLED applications.

Pyrene[4,5-d]imidazole-Based Derivatives with Hybridized Local and Charge-Transfer State for Highly Efficient Blue and White Organic Light-Emitting Diodes with Low Efficiency Roll-Off

A family of pyrene[4,5-d]imidazole derivatives, PyPA, PyPPA, PyPPAC, and PyPAC, with different excited states are successfully developed. Among them, PyPPA and PyPPAC possess hybridized local and charge-transfer (HLCT) state, endowing them with pure blue fluorescence as well as high quantum yields. The nondoped organic light-emitting diode (OLED) based on PyPPA displays Commission Internationale de L'Eclairage coordinates of (0.14, 0.13) and achieves a maximum external quantum efficiency (EQE) of 8.47%, which are among the highest value reported to date for nondoped blue HLCT OLEDs. The nondoped OLED based on PyPPAC exhibits a maximum luminance of 50,046 cd m^-2 located in the blue region with CIE coordinates of (0.15, 0.21) and an EQE of 6.74% even when the luminance reached over 10,000 cd m^-2. In addition, they both reveal ultimate exciton utilizing efficiencies of nearly 100%. The potential of a blue emitter of PyPPA with an HLCT character for application in white OLED (WOLED) is further tested. The efficient two-color hybrid warm WOLED is successfully achieved, which provides the total EQE, power efficiency, and current efficiency of up to 21.19%, 61.46 lm W^-1, and 62.13 cd A^-1, respectively. The nondoped blue OLEDs and hybrid WOLEDs present good color stabilities with low efficiency roll-offs. Our results prove that taking advantage of the HLCT state, nondoped blue OLEDs as well as hybrid WOLEDs with high performance could be realized, which have a promising prospect for the displays and lightings in the future.

Highly Efficient Microcavity Organic Light-Emitting Devices with Narrow-Band Pure UV Emission

Ultraviolet organic light-emitting devices (UVOLEDs) may combine the tunability properties of organic materials through modifying the molecular structure and the advantages such as large-area, low-cost, and facile to realize high-performance UV sources. In the state-of-the-art UVOLEDs, the external quantum efficiencies (EQE) are more than 3%, but only a few have achieved pure UV emission and could not compromise the durability and irradiance at the same time. Portable compact UV sources with a narrow band made significant achievements in biomedical science and forensic appraisal. The microcavity effect is useful for achieving the desired narrow peak emission. In this study, asymmetric structural design with a specific distributed Bragg reflector (DBR) structure was employed to achieve narrow-band pure UV emission microcavity UVOLEDs (μC UVOLEDs). These μC UVOLEDs can realize tunable wavelength from 366 to 400 nm, with a full width at half maximum (FWHM) of 9.95-15.2 nm and a maximum irradiance of 2.79-5.63 mW/cm². Also, the durability of the μC UVOLED has been considered, which presents a lifetime of 63.2 h under an irradiance of 0.016 mW/cm². Moreover, the ability to identify 100 RMB with an efficient μC UVOLED has also been demonstrated. This investigation not only demonstrates the encouraging potential of narrow-band pure UVOLEDs but also provides a feasible strategy for the optimal design of μC UVOLEDs by utilizing the asymmetric structure.

Fine Modulation of the Higher-Order Excitonic States toward More Efficient Conversion from Upper-Level Triplet to Singlet

Hot exciton luminogens capable of harvesting nonemissive triplet excitons via reverse intersystem crossing from high-order triplet (hRISC) to singlet have great potential in high-efficiency fluorescent organic light-emitting diodes (OLEDs). Although spin-orbit coupling (SOC) is regarded as a key factor affecting the RISC process, its effects on hot exciton materials are poorly understood. Herein, we design and synthesize two blue-emitting hot exciton luminogens, PABP and PAIDO, to study this issue by modulating the excited-state properties. Theoretical and experimental research contributions demonstrate that a stronger SOC between energetically close S₁ (π-π*) and T_n (T₃, n-π*) of PAIDO gives rise to faster and more efficient hRISC in comparison to that of PABP, leading to a higher external quantum efficiency and a higher exciton utilization efficiency. Crucially, the experimentally measured hRISC rate (k_hRISC) of hot exciton materials is on the order of 10⁷ s^-1, which is much faster than that of the thermally activated delayed fluorescence materials.