This review is aimed at updating the recent development on the metal complexes bearing azolate‐containing chelates that have received a growing attention from both the industrial and academic ...sectors. Particular emphasis is given to the luminescent metal complexes, for which tridentate and multidentate bonding interactions give rise to both higher ligand field strength and better rigidity versus their bidentate counterparts—consequently, this is beneficial to the chemical stability and emission efficiency needed for applications such as organic light‐emitting diodes and bio‐imaging. Their basic designs involve chelates, such as monoanionic 6‐azolyl 2,2′‐bipyridine, dianionic 2,6‐diazolylpyridine, and 2‐azolyl‐6‐phenylpyridine, and the core metal ion spanning from main group elements, such as GaIII and InIII, to the late transition metal ions such as RuII, OsII, IrIII, and PtII and even the lanthanides. Furthermore, the great versatility of these azolate chelates for assembling the robust and emissive metal complexes, provides bright prospect in future optoelectronic investigations.
Di, tri, or tetra? That is the question! Multidentate, particularly the tridentate, azolate‐containing chelates and associated metal complexes are reviewed, with an emphasis on applications such as organic light‐emitting diodes and bio‐imaging.
Long‐wavelength light‐emitting electrochemical cells (LECs) are potential deep‐red and near infrared light sources with solution‐processable simple device architecture, low‐voltage operation, and ...compatibility with inert metal electrodes. Many scientific efforts have been made to material design and device engineering of the long‐wavelength LECs over the past two decades. The materials designed the for long‐wavelength LECs cover ionic transition metal complexes, small molecules, conjugated polymers, and perovskites. On the other hand, device engineering techniques, including spectral modification by adjusting microcavity effect, light outcoupling enhancement, energy down‐conversion from color conversion layers, and adjusting intermolecular interactions, are also helpful in improving the device performance of long‐wavelength LECs. In this review, recent advances in the long‐wavelength LECs are reviewed from the viewpoints of materials and device engineering. Finally, discussions on conclusion and outlook indicate possible directions for future developments of the long‐wavelength LECs. This review would like to pave the way for the researchers to design materials and device engineering techniques for the long‐wavelength LECs in the applications of displays, bio‐imaging, telecommunication, and night‐vision displays.
Recent advances in long‐wavelength light‐emitting electrochemical cells (LECs) are reviewed from the perspectives of materials, including ionic transition metal complexes, small molecules, conjugated polymers, and perovskites, as well as device engineering. Device engineering techniques, including spectral modification by adjusting the microcavity effect, and light outcoupling enhancement, are also discussed. This review aims to pave the way for researchers to design materials and device engineering techniques for long‐wavelength LECs in the applications of displays, bio‐imaging, telecommunication, and night‐vision displays.
Recently, the fields of organic light‐emitting diodes (OLEDs) and light‐emitting electrochemical cells (LECs) have improved tremendously with regard to tunable emission, efficiency, brightness, and ...thermal stability. Imidazole derivatives are excellent deep blue‐green light‐emitting layers in the OLED or LEC devices. This Review summarizes the major breakthroughs of various electroluminescence (EL) layers with imidazole‐containing organic or organometallic derivatives, the molecular design principles, and their light‐emitting performances as effective EL materials. The highly tunable chemical structures and flexible molecular design strategies of imidazole‐based compounds are advantages that provide great opportunities for researchers. They can provide a good basis for the design and development of new EL materials with narrower emission and higher efficiency. Moreover, imidazole compounds have demonstrated breakthrough performances in thermally activated delayed fluorescence (TADF) properties where triplet excitons are utilized to inhibit anti‐intersystem quenching, showing great promise in breaking the theoretical external quantum efficiencies (EQE) limits in traditional fluorescent devices.
Imidazole emitters: In recent years, electroluminescent devices have flourished, and maximizing the efficiency of organic light‐emitting diodes (OLEDs) and light‐emitting electrochemical cells (LECs) has become the driving force and goal of a great deal of research. Imidazole derivatives have developed many excellent electroluminescent devices due to their structural diversity. This Review article discusses imidazole derivatives for fluorescent emitters and ligands as phosphorescent emitters and demonstrates their properties.
The electron positive boron atom usually does not contribute to the frontier orbitals for several lower‐lying electronic transitions, and thus is ideal to serve as a hub for the spiro linker of ...light‐emitting molecules, such that the electron donor (HOMO) and acceptor (LUMO) moieties can be spatially separated with orthogonal orientation. On this basis, we prepared a series of novel boron complexes bearing electron deficient pyridyl pyrrolide and electron donating phenylcarbazolyl fragments or triphenylamine. The new boron complexes show strong solvent‐polarity dependent charge‐transfer emission accompanied by a small, non‐negligible normal emission. The slim orbital overlap between HOMO and LUMO and hence the lack of electron correlation lead to a significant reduction of the energy gap between the lowest lying singlet and triplet excited states (ΔET‐S) and thereby the generation of thermally activated delay fluorescence (TADF).
Reducing the gap: Using a boron atom as the spiro linker between an electron‐deficient pyridyl pyrrolide and an electron‐donating phenylcarbazolyl or triphenylamine fragment, boron complexes with a narrow HOMO–LUMO orbital overlap, small singlet–triplet energy gap (down to 38 meV), and strong thermally activated delayed fluorescence (TADF) were prepared. For the first time boron‐complex‐based OLEDs show a significant TADF contribution.
Two binuclear heteroleptic Cu
complexes, namely Cu-NIR1 and Cu-NIR2, bearing rigid chelating diphosphines and π-conjugated 2,5-di(pyridin-2-yl)thiazolo5,4-dthiazole as the bis-bidentate ligand are ...presented. The proposed dinuclearization strategy yields a large bathochromic shift of the emission when compared to the mononuclear counterparts (M1-M2) and enables shifting luminescence into the near-infrared (NIR) region in both solution and solid state, showing emission maximum at ca. 750 and 712 nm, respectively. The radiative process is assigned to an excited state with triplet metal-to-ligand charge transfer (
MLCT) character as demonstrated by in-depth photophysical and computational investigation. Noteworthy, X-ray analysis of the binuclear complexes unravels two interligand π-π-stacking interactions yielding a doubly locked structure that disfavours flattening of the tetrahedral coordination around the Cu
centre in the excited state and maintain enhanced NIR luminescence. No such interaction is present in M1-M2. These findings prompt the successful use of Cu-NIR1 and Cu-NIR2 in NIR light-emitting electrochemical cells (LECs), which display electroluminescence maximum up to 756 nm and peak external quantum efficiency (EQE) of 0.43 %. Their suitability for the fabrication of white-emitting LECs is also demonstrated. To the best of our knowledge, these are the first examples of NIR electroluminescent devices based on earth-abundant Cu
emitters.
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.
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.
A series of dicyano‐imidazole‐based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain pure blue‐emitting organic light‐emitting diodes (OLEDs). The ...targeted molecules used dicyano‐imidazole with a short‐conjugated system as the electron acceptor to strong intermolecular π‐π interactions, and provide a relatively shallow energy level of the lowest unoccupied molecular orbital (LUMO). The cyano group was selected to improve imidazole as an electron acceptor due to its prominent electron‐transporting characteristics. Four different electron donors, that is, 9,9‐dimethyl‐9,10‐dihydroacridine (DMAC), 10H‐spiro(acridine‐9,9’‐fluoren) (SPAC), and 9,9‐diphenyl‐9,10‐dihydroacridine (DPAC), were used to alternate the highest occupied molecular orbital (HOMO) energy level to tune the emission color further. The crowded molecular structure in space makes the electron donor and acceptor almost orthogonal, reducing the energy gap (ΔEST) between the first excited singlet (S1) and the triplet (T1) states and introducing significant TADF property. The efficiencies of the blue‐emissive devices with imM‐SPAC and imM‐DMAC obtained in this work are the highest among the reported imidazole‐based TADF‐OLEDs, which are 13.8 % and 13.4 %, respectively. Both of Commission Internationale de l′Eclairage (CIE) coordinates are close to the saturated blue region at (0.17, 0.18) and (0.16, 0.19), respectively. Combining these tailor‐made TADF compounds with specific device architectures, electroluminescent (EL) emission from sky‐blue to deep‐blue could be achieved, proving their great potential in EL applications.
The efficiencies of the blue‐emissive devices with imM‐SPAC and imM‐DMAC obtained in this work are the highest among the reported imidazole‐based TADF‐OLEDs, which are 13.8 % and 13.4 %, respectively. Both of Commission Internationale de l′Eclairage (CIE) coordinates are close to the saturated blue region at (0.17, 0.18) and (0.16, 0.19), respectively. Combining these tailor‐made TADF compounds with specific device architectures, electroluminescent (EL) emission from sky‐blue to deep‐blue could be achieved, proving their great potential in EL applications.
Solid‐state light‐emitting electrochemical cells (LECs) have several advantages, such as low‐voltage operation, compatibility with inert metal electrodes, large‐area flexible substrates, and simple ...solution‐processable device architectures. However, most of the studies on saturated red LECs show low or moderate device efficiencies (external quantum efficiency (EQE) <3.3 %). In this work, we demonstrate a series of five red‐emitting cationic iridium complexes (RED1‐‐RED5) with 2,2′‐biquinoline ligands and test their electroluminescence (EL) characteristics in LECs. The Commission Internationale de l′Eclairage (CIE) 1931 coordinates for the LECs based on these complexes are all beyond the National Television System Committee (NTSC) red standard point (0.67, 0.33). The maximal EQE of the neat‐film RED1‐based LECs reaches 7.4 %. The reddest complex, RED3, is doped in the blue‐emitting host complex, BG, to fabricate host–guest LECs. The maximal EQE of the host–guest LECs (1 wt % complex RED3) reaches 9.4 %, which is among the highest reported for the saturated red LECs.
In the red: Solid‐state light‐emitting electrochemical cells (LECs) have many advantages; however, most of the studies on saturated red LECs show low or moderate device efficiencies (external quantum efficiency (EQE) <3.3 %). A series of five red‐emitting cationic iridium complexes (RED1‐‐RED5) with 2,2′‐biquinoline ligands is demonstrated and their electroluminescence characteristics in LECs tested.
Solid‐state white light‐emitting electrochemical cells (LECs) show promising advantages of simple solution fabrication processes, low operation voltage, and compatibility with air‐stable cathode ...metals, which are required for lighting applications. To date, white LECs based on ionic transition metal complexes (iTMCs) have shown higher device efficiencies than white LECs employing other types of materials. However, lower emission efficiencies of red iTMCs limit further improvement in device performance. As an alternative, efficient red CdZnSeS/ZnS core/shell quantum dots were integrated with a blue iTMC to form a hybrid white LEC in this work. By achieving good carrier balance in an appropriate device architecture, a peak external quantum efficiency and power efficiency of 11.2 % and 15.1 lm W−1, respectively, were reached. Such device efficiency is indeed higher than those of the reported white LECs based on host–guest iTMCs. Time‐ and voltage‐dependent electroluminescence (EL) characteristics of the hybrid white LECs were studied by means of the temporal evolution of the emission‐zone position extracted by fitting the simulated and measured EL spectra. The working principle of the hybrid white LECs was clarified, and the high device efficiency makes potential new white‐emitting devices suitable for solid‐state lighting technology possible.
White LECs: Efficient hybrid white light‐emitting electrochemical cells (LECs) based on an emissive bilayer of a blue ionic transition metal complex (iTMC) and red quantum dots (QDs) were fabricated. The judiciously arranged device structure, which avoids a high carrier‐injection barrier at the QDs/electrode interface, results in superior carrier balance, and the maximal external quantum efficiency (EQE) and power efficiency (P.E.) reach 11.2 % and 15.1 lm W−1, respectively.
A series of donor–acceptor–donor triazine‐based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain highly efficient blue‐emitting OLEDs with ...non‐doped emitting layers (EMLs). The targeted molecules use a triazine core as the electron acceptor, and a benzene ring as the conjugated linker with different electron donors to alternate the energy level of the HOMO to further tune the emission color. The introduction of long alkyl chains on the triazine core inhibits the unwanted intermolecular D–D/A–A‐type π–π interactions, resulting in the intermolecular D–A charge transfer. The weak aggregation‐caused quenching (ACQ) effect caused by the suppressed intermolecular D–D/A–A‐type π–π interaction further enhances the emission. The crowded molecular structure allows the electron donor and acceptor to be nearly orthogonal, thereby reducing the energy gap between triplet and singlet excited states (ΔEST). As a result, blue‐emitting devices with TH‐2DMAC and TH‐2DPAC non‐doped EMLs showed satisfactory efficiencies of 12.8 % and 15.8 %, respectively, which is one of the highest external quantum efficiency (EQEs) reported for blue TADF emitters (λpeak<475 nm), demonstrating that our tailored molecular designs are promising strategies to endow OLEDs with excellent electroluminescent performances.
Getting the blues: A series of donor–acceptor–donor triazine‐based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain highly efficient blue‐emitting OLEDs with non‐doped emitting layers (EMLs). The targeted molecules use a triazine core as the electron acceptor, and a benzene ring as the conjugated linker with different electron donors to alternate the energy level of the HOMO to further tune the emission color.
