Efficient Deep-Blue Multiple-Resonance Emitters Based on Azepine-Decorated ν-DABNA for CIEy below 0.06

Ultrapure deep-blue emitters are in high demand for organic light-emitting diodes (OLEDs). Although color coordinates serve as straightforward parameters for assessing color purity, precise control over the maximum wavelength and full-width at half-maximum is necessary to optimize OLED performance, including luminance efficiency and luminous efficacy. Multiple-resonance (MR) emitters are promising candidates for achieving ideal luminescence properties; consequently, a wide variety of MR frameworks have been developed. However, most of these emitters experience a wavelength displacement from the ideal color, which limits their practical applicability. Therefore, a molecular design that is compatible with MR emitters for modulating their energy levels and color output is particularly valuable. Here, it is demonstrated that the azepine donor unit induces an appropriate blue-shift in the emission maximum while maintaining efficient MR characteristics, including high photoluminescence quantum yield, narrow emission, and a fast reverse intersystem crossing rate. OLEDs using newly developed MR emitters based on the ν-DABNA framework simultaneously exhibit a high quantum efficiency of ≈30%, luminous efficacy of ≈20 lm W-1, exceptional color purity with Commission Internationale de l'Éclairage coordinates as low as (0.14, 0.06), and notably high operational stability. These results demonstrate unprecedentedly high levels compared with those observed in previously reported deep-blue emitters.

Enhancing OLED emitter efficiency through increased rigidity

Three new blue materials, TPI-InCz, PAI-InCz, and CN-PAI-InCz, have been developed. In the film state, TPI-InCz and PAI-InCz exhibited emission peaks at 411 and 431 nm indicating deep blue emission. CN-PAI-InCz showed a peak emission at 452 nm, within the real blue region. When these three materials were used as the emissive layer to fabricate non-doped devices, CN-PAI-InCz showed the highest current efficiency of 2.91 cd A-1, power efficiency of 1.93 lm W-1, and external quantum efficiency of 3.31%. Among the synthesized materials, CN-PAI-InCz exhibited superior charge balance due to the introduction of CN groups, as confirmed by hole-only devices and electron-only devices. PAI-InCz demonstrated fast hole mobility with a value of 1.50 × 10-3 cm2 V-1 s-1, attributed to its planar and highly rigid structure. In the resulting devices, the Commission Internationale de l'Eclairage coordinates for TPI-InCz, PAI-InCz, and CN-PAI-InCz were (0.162, 0.048), (0.0161, 0.067), and (0.155, 0.099), all indicating emission in the blue region.

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.

Electroluminescent materials toward near ultraviolet region

Near ultraviolet (NUV) light-emitting materials and devices are significant due to unique applications in anti-counterfeit, manufacturing industries, and hygienic treatments. However, the development of high-efficiency NUV electroluminescent devices encounters great challenges and is far behind their RGB emitter counterparts. Besides the photoluminescence quantum yields (PLQYs) of NUV materials being higher than 40%, charge injection and lopsided carrier transport also determine the device performance, leading to great efforts in optimizing the frontier molecular orbitals to fit the adjacent function layer. In the exploration of NUV materials, organic molecules are one of the primary candidates, given their preparative facility and structural variability. Recently, all-inorganic quantum-dot light-emitting diodes (QLEDs) of Cd-based, ZnSe, graphene and inorganic perovskite emitters and organic-inorganic hybrid lead halide perovskite nanocrystals (NCs) were demonstrated for achieving NUV electroluminescence. Owing to the great efforts devoted to NUV material engineering and device configuration, NUV materials and devices have achieved great advances over the last two decades. In this review, we retrospect the development of NUV materials and devices covering all promising systems, which may inspire the enthusiasm of researchers to explore the huge potential in the NUV region.

Regulation of Singlet and Triplet Excitons in a Single Emission Layer: Efficient Fluorescent/Phosphorescent Hybrid White Organic Light-Emitting Diodes

Two efficient fluorescent molecules, viz., (E)-2-(2-4-(1-2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-4,5-diphenyl-H-imidazole-2-yl)-[1,1-biphenyl]-4-yl)vinyl-1-yl(naphthalene-1-yl)-1H-phenanthro[9,10-d]imidazole (DDIBNPPI) and (E)-4-(2-(2-(-4'-1(2,3-dihyderobenzo[b][1,4]dioxin-5-yl)-4,5-diphenyl-1H-imidazole-2-yl)-[1,1'-biphenyl]-4-yl)vinyl)-1H-phenanthr[9,10-d]imidazole-1-yl)-1-napthronitrile (DDIBPPIN), were designed and synthesized. DDIBNPPI and DDIBPPIN were obtained by rupturing the covalent bond of phenanthrimidazole core to prevent aggregation-induced quenching. In DDIBPPIN, naphthonitrile group was incorporated into azomethine nitrogen of phenanthrimidazole to enhance charge-transfer ability. The DDIBPPIN/CBP:DDIBPPIN-based device shows blue emission with η_c (current efficiency) of 4.91/4.10 cd/A, η_p (power efficiency) of 4.56/3.84 lm/W, and η_ex (external quantum efficiency) of 5.11/5.96%. The η_s (exciton utilization efficiency) values of DDIPNPPI and DDIBPPIN are of 27.0 and 30.3%, respectively. The DDIPNPPI and DDIBPPIN materials employed as a host to fabricate green and red phosphorescent organic light-emitting diodes. The red/white devices (with 0.4% dopant concentration) with DDIBPPIN:Ir(MDQ)₂(acac) exhibit maximum L of 69889/26319 cd/m², η_ex of 19.6/16.6%, η_c of 34.6/35.6 cd/A, and η_p of 35.8/36.6 lm/W. The device with DDIBPPIN:Ir(ppy)₃/DDIPNPPI:Ir(ppy)₃ exhibits green emission [Commission Internationale de l'Eclairage (0.30,0.60)/(0.30,0.60)] with maximum L of 69906/69482 cd/m², η_ex of 17.9/17.0%, η_c of 59.8/58.6 cd/A, and η_p of 63.6/61.3 lm/W. The white device using DDIBPPI:Ir(ppy)₃ (with 0.4% dopant concentration) exhibits maximum L of 22152 cd/m², η_ex of 15.8%, η_c of 31.4 cd/A, and η_p of 36.1 lm/W.

Derivatives of Cyanonaphthyl-Substituted Phenanthroimidazole as Blue Emitters for Nondoped Organic Light-Emitting Diodes

New multifunctional blue-emissive materials with superior thermal properties, viz., 4,4'-bis(1-(4-naphthyl)-1H-phenanthro[9,10-d]imidazol-2-yl)binaphthyl (NPIBN), 4,4'-bis(1-(4-cyanonaphthyl)-1H-phenanthro[9,10-d]imidazol-2-yl)biphenyl (CNPIBP), and 4,4'-bis(1-(4-cyanonaphthyl)-1H-phenanthro[9,10-d]imidazol-2-yl)binaphthyl (CNPIBN) have been synthesized. The said molecules show high photoluminescence quantum yield (Φs/f: NPIBN-0.75/0.68, CNPIBP-0.85/0.76, and CNPIBN-0.90/0.88). The fabricated nondoped/doped device with CNPIBN/4,4'-bis(carbazol-9-yl)biphenyl: CNPIBN display maximum efficiencies (ηex 4.96/5.4%; ηc 7.46/7.56 cd A-1; ηp 6.85/6.91 lm W-1) at low turn-on voltage (3.5/3.8 V). Nondoped devices based on D-π-A architecture, 4-(2-(5-(9H-carbazol-9-yl)thiophen-2-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile exhibit maximum efficiencies (ηex 2.32%; ηc 4.00 cd A-1; ηp 3.42 lm W-1) compared to 4-(2-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (ηex 2.01%; ηc 3.89 cd A-1; ηp 3.15 lm W-1).

Deep-Blue and Hybrid-White Organic Light Emitting Diodes Based on a Twisting Carbazole-Benzofuro[2,3-b]Pyrazine Fluorescent Emitter

A novel deep-blue fluorescent emitter was designed and synthesized. The external quantum efficiency (η_EQE) of the blue-emitting, doped, organic light-emitting diode (OLED) was as high as 4.34%. The device also exhibited an excellent color purity with Commission Internationale de l’Eclairage (CIE) coordinates of x = 0.15 and y = 0.05. In addition, the triplet energy had a value of 2.7 eV, which is rare for an emitter with deep-blue emission, which makes it a preferred choice for high-performance white OLEDs. By optimizing the device architectures, the color of hybrid-white OLEDs could be tunable from warm white to cool white using the aforementioned material as a bifunctional material. That is, the η_EQE of the hybrid warm-white OLED is 20.1% with a CIE x and y of 0.46 and 0.48 and the η_EQE of the hybrid cool-white OLED is 9% with a CIE x and y of 0.34 and 0.33.

Hot exciton transition for organic light-emitting diodes: tailoring excited-state properties and electroluminescence performances of donor–spacer–acceptor molecules

The photophysical, electrochemical and electroluminescent properties of newly synthesized blue emitters with donor–π–acceptor geometry, namely, 4′-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1′-biphenyl]vinyl)-4-amine (NSPI-TPA), 4′-(1-(2-methylnaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1′-biphenyl]vinyl)-4-amine (MNSPI-TPA), 4-(2-(4′-(diphenylamino)-(2-[1,1′-biphenyl]vinyl)-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)-1-naphthalene-1-carbonitrile (SPNCN-TPA) and 4-(2-(4-(9H-carbazol-9-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (SPNCN-Cz) were analyzed. The conjugation length in the emitters is not conducive to pure emission and hence, a molecular twisting strategy was adopted in NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz to enhance pure emission. The emissive state (HLCT) of twisted D–π–A molecules containing both LE and CT (HLCT) states was tuned for high PL (η_PL) (LE) and high exciton utilization (η_s) (CT) efficiencies by replacing triphenylamine (strong donor) with carbazole (weak donor). Among strong donor compounds, namely, NSPI-TPA, MNSPI-TPA and SPNCN-TPA, the SPNCN-TPA-based device exhibited blue emission (451 nm) with CIE coordinates (0.15, 0.08), maximum current efficiency (η_c) of 2.32 cd A⁻¹, power efficiency (η_p) of 2.01 lm W⁻¹ and external quantum efficiency (η_ex) of 3.02%. The device with SPNCN-Cz emitter exhibited higher electroluminescence efficiencies than the SPNCN-TPA-based device, with pure blue emission (443 nm, CIE: 0.15,0.07), η_ex of 3.15%, η_c of 2.56 cd A⁻¹ and η_p of 2.45 lm W⁻¹.

SPNCN-Cz device exhibits η_ex (3.15%), η_c (2.56 cd A⁻¹), η_p (2.45 lm W⁻¹) with CIE (0.15, 0.07).

Tailoring the molecular design of twisted dihydrobenzodioxin phenanthroimidazole derivatives for non-doped blue organic light-emitting devices

Three fused polycyclic aryl fragments, namely, naphthyl, methoxynaphthyl, and pyrenyl have been used to construct blue-emissive phenanthroimidazole-functionalized target molecules, i.e., 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazole (1), 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2-(1-methoxynaphthalen-4-yl)-1H-phenanthro[9,10-d]imidazole (2), and 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2-(pyren-10-yl)-1H-phenanthro[9,10-d]imidazole (3). The up-conversion of triplets to singlets via a triplet–triplet annihilation (TTA) process is dominant in these compounds due to 2E_T1 > E_S1. The pyrenyl dihydrobenzodioxin phenanthroimidazole (3)-based nondoped OLED exhibits blue emission (450 nm) with CIE (0.15, 0.14), a luminance of 53 890 cd m⁻², power efficiency of 5.86 lm W⁻¹, external quantum efficiency of 5.30%, and current efficiency of 6.90 cd A⁻¹. The efficient device performance of pyrenyl dihydrobenzodioxin phenanthroimidazole is due to the TTA contribution to the electroluminescent process.

Efficient blue emitters, 1-(2,3-dihydrobenzodioxinyl)-2-naphthylphenanthroimidazole, 1-(2,3-dihydrobenzodioxinyl)-2-methoxynaphthylphenanthroimidazole and 1-(2,3-dihydrobenzodioxinyl)-2-pyrenylphenanthroimidazole have been reported.

Synthesis and characterization of high quantum yield and oscillator strength 6-chloro-2-(4-cynophenyl)-4-phenyl quinoline (cl-CN-DPQ) organic phosphor for solid-state lighting

A novel blue luminescent 6-chloro-2-(4-cynophenyl) substituted diphenyl quinoline (Cl-CN DPQ) organic phosphor has been synthesized by the acid-catalyzed Friedlander reaction and then characterized to confirm structural, optical and thermal properties. Structural properties of Cl-CN-DPQ were analyzed by Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX) spectroscopy. FTIR spectra confirmed the presence of different functional groups and bond stretching. ¹ H-NMR and ¹³ C-NMR confirmed the formation of an organic Cl-CN-DPQ compound. X-ray diffraction study provided its crystalline nature. The surface morphology of Cl-CN-DPQ was analyzed by SEM, while EDAX spectroscopy revealed the elemental analysis. Differential thermal analysis (TGA/DTA) disclosed its thermal stability up to 250°C. The optical properties of Cl-CN-DPQ were investigated by UV-vis absorption and photoluminescence (PL) measurements. Cl-CN-DPQ exhibits intense blue emission at 434 nm in a solid-state crystalline powder with CIE co-ordinates (0.157, 0.027), when excited at 373 nm. Cl-CN-DPQ shows remarkable Stokes shift in the range 14800-5100 cm^-1 , which is the characteristic feature of intense light emission. A narrow full width at half-maximum (FWHM) value of PL spectra in the range 42-48 nm was observed. Oscillator strength, energy band gap, quantum yield, and fluorescence energy yield were also examined using UV-vis absorption and photoluminescence spectra. These results prove its applications towards developing organic luminescence devices and displays, organic phosphor-based solar cells and displays, organic lasers, chemical sensors and many more.

An ambipolar 3,3′-dimethyl-9,9′-bianthracene derivative as a blue host material for high-performance OLEDs

A new ambipolar 3,3′-dimethyl-9,9′-bianthracene derivative (MBAn-(4)-tBu) was designed as a deep blue fluorophore and blue host material for high-performance OLEDs.

Novel electroluminescent donor–acceptors based on dibenzo[a,c]phenazine as hole-transporting materials for organic electronics

Novel yellow-green fluorescent 3,6,11-trisubstitued-dibenzo[a,c]phenazine derivatives were synthesized via a Buchwald–Hartwig palladium-catalyzed C–N amination reaction for the hole-transporting materials.

A novel electron-acceptor moiety as a building block for efficient donor-acceptor based fluorescent organic lighting-emitting diodes

A new electron-withdrawing moiety (BFPz) has been used for the first time as an acceptor in OLEDs and its corresponding core unit (2-Br-BFPz) was synthesized. Combined with an electron-donating moiety triphenylamine, a novel fluorescent material with a D-A structure named TPA-BFPz was synthesized. Encouragingly, the EQE values of non-doped and doped blue OLEDs reach 3.68% and 4.42%, respectively.

Highly Efficient Deep Blue Organic Light-Emitting Diodes Based on Imidazole: Significantly Enhanced Performance by Effective Energy Transfer with Negligible Efficiency Roll-off

Great efforts have been devoted to develop efficient deep blue organic light-emitting diodes (OLEDs) materials meeting the standards of European Broadcasting Union (EBU) standard with Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.06) for flat-panel displays and solid-state lightings. However, high-performance deep blue OLEDs are still rare for applications. Herein, two efficient deep blue emitters, PIMNA and PyINA, are designed and synthesized by coupling naphthalene with phenanthreneimidazole and pyreneimidazole, respectively. The balanced ambipolar transporting natures of them are demonstrated by single-carrier devices. Their nondoped OLEDs show deep blue emissions with extremely small CIE_y of 0.034 for PIMNA and 0.084 for PyINA, with negligible efficiency roll-off. To take advantage of high photoluminescence quantum efficiency of PIMNA and large fraction of singlet exciton formation of PyINA, doped devices are fabricated by dispersing PyINA into PIMNA. A significantly improved maximum external quantum efficiency (EQE) of 5.05% is obtained through very effective energy transfer with CIE coordinates of (0.156, 0.060), and the EQE remains 4.67% at 1000 cd m^-2, which is among the best of deep blue OLEDs reported matching stringent EBU standard well.

A Significantly Twisted Spirocyclic Phosphine Oxide as a Universal Host for High-Efficiency Full-Color Thermally Activated Delayed Fluorescence Diodes

A universal thermally activated delayed fluorescence (TADF) host, 4'-diphenylphosphinoylspiro[fluorene-9,9'-xanthene] (SFXSPO), is constructed with a highly distorted and asymmetric configuration and disordered molecular packing in its solid state. SFXSPO successfully endows its full-color TADF diodes with state-of-the-art performance, e.g., the record external quantum efficiency of 22.5% and 19.1% and internal quantum efficiency of ≈100% for its yellow TADF diodes and single-host full-TADF nearly-white-emitting devices, respectively.

Highly efficient non-doped deep blue fluorescent emitters with horizontal emitting dipoles using interconnecting units between chromophores

New deep blue fluorescent emitters composed of anthracene as an electron rich unit, a diphenyltriazine as a strong electron acceptor unit, and phenyl or xylene as interconnecting units were synthesised.

Linkage engineering in hosts for dramatic efficiency enhancement of blue phosphorescent organic light-emitting diodes

Higher triplet energy and balanced charge mobility is two key factors for high-efficiency host materials of phosphorescent organic light-emitting diodes (PhOLED), which are integrated in a carbazole-diphenylene-fluorene hybrid FDPCz2 (9,9'-(4',4"-(9H-fluorene-9,9-diyl)bis(biphenyl-4',4-diyl))bis(9H-carbazole)) on the basis of indirect linkage strategy. Owing to rationally spatial allocation of conjugation blocking and extension for diphenylene linkages, FDPCz2 achieves both high triplet energy of 2.97 eV and favorable charge mobility by order of 6.3 × 10(-6) cm(2) V(-1) s(-1). Compared to conventional hosts and a short-conjugated analogue FPCz2 (9,9'-(4,4'-(9H-fluorene-9,9-diyl)bis(4,1-phenylene)) bis(9H-carbazole)), FDPCz2 dramatically elevated device efficiencies with peak values of 40.6 cd A(-1) and 20.2% for blue PhOLEDs.

Carbazole/fluorene based conjugated small molecules: synthesis and comparative studies on the optical, thermal and electrochemical properties

A series of new molecular materials with varying carbazole and fluorene contents were prepared employing Suzuki coupling and their properties were compared.

Deep blue organic light-emitting devices enabled by bipolar phenanthro[9,10-d]imidazole derivatives

Novel phenanthroimidazole derivatives with D–π–A structures have been successfully designed and prepared. Non-doped organic light emitting diodes (OLEDs) were fabricated by employing the compounds, which display deep blue emission.