Robust luminogens as cutting-edge tools for efficient light emission in recent decades

Blue luminogens play a vital role in white lighting and potential metal-free fluorescent materials and their high-lying excited states contribute to harvesting triplet excitons in devices. However, in TADF-OLEDs (ΔEST < 0.1 eV), although T1 excitons transfer to S1via RISC with 100% IQE, the longer lifetime of blue TADF suffers from efficiency roll-off (RO). In this case, hybridized local and charge transfer (HLCT) materials have attracted significant interest in lighting owing to their 100% hot exciton harvesting and enhanced efficiency. Both academics and industrialists widely use the HLCT strategy to improve the efficiency of fluorescent organic light-emitting diodes (FOLEDs) by harvesting dark triplet excitons through the RISC process. Aggregation-induced emissive materials (AIEgens) possess tight packing in the aggregation state, and twisted AIEgens with HLCT behaviour have a shortened conjugation length, inducing blue emission and making them suitable candidates for OLED applications. TTA-OLEDs are used in commercial BOLEDs because of their moderate efficiency and reasonable operation lifetime. In this review, we discuss the devices based on TTA fluorophores, TADF fluorophores, HLCT fluorophores, AIEgens and HLCT-sensitized fluorophores (HLCT-SF), which break through the statistical limitations.

Organic Binary and Ternary Cocrystal Engineering Based on Halogen Bonding Aimed at Room-Temperature Phosphorescence

Recently, there has been intense interest in pure organic room-temperature phosphorescence (ORTP) from cocrystals composed of 1,4-diiodotetrafluorobenzene (DITFB) and a variety of polycyclic aromatic hydrocarbons (PAHs) or their derivatives. To expand the possibility of halogen bonding-based cocrystals, the relationship between the crystal packing motifs and ORTP characteristics in binary cocrystals composed of DITFB and PAHs of phenanthrene (Phen), chrysene (Chry), and pyrene (Pyr), respectively, is investigated. The σ-hole···π and π-hole···π interactions determine not only the crystal packing motifs but also photoluminescence quantum yields (PLQYs). The Phen-DITFB and Chry-DITFB binary cocrystals with σ-hole···π interactions show higher PLQY compared with the Pyr-DITFB binary cocrystal with π-hole···π interaction. Further, to clarify the effect of crystal structures on PLQY, ternary cocrystals are prepared by partially doping Pyr into Phen-DITFB. The crystal packing motif of the ternary cocrystal originates from a Phen-DITFB cocrystal with σ-hole···π interaction, and some of the Phen sites are randomly replaced with Pyr molecules. The ORTP emission is derived from Pyr. The maximum PLQY is >20% due to suppressing nonradiative decay by changing the crystal packing motif.

Novel Star-Shaped Viologens Containing Phenyl and Triphenylamine Moieties for Electrochromic Applications

The two star-shaped viologens containing 1,3,5-substituted phenyl (1) and triphenylamine (2) central cores and n-hexyl chains were synthesized and characterized. Both compounds exhibited promising optoelectronic properties and underwent multiple oxidation/reduction processes resulting in various colors. Four possible redox states of tripyridium salt containing a phenyl or triphenylamine core can occur depending on the applied potentials. The wide color range, from colorless through blue, azure to green-gray, was observed during the electrochemical reduction of compound 1. In the case of compound 2, the color change observed during spectroelectrochemical measurements was from yellow to colorless during the cathodic process and from yellow to green during the anodic process. The observed color change for both viologens was reversible. The triphenylamine-cored viologen (2) also exhibited emission in visible range and solvatochromism. It also exhibited luminescence in the solid state when excited with a UV lamp. These studies provide insights into the design of advanced materials for applications in displays.

Switching thermally activated delayed fluorescence to room temperature phosphorescence for oxygen sensing: Effect of donor substituents of trifluoromethylphenyl

Two compounds consisting of electron-accepting trifluoromethylphenyl moiety and electron-donating phenoxazine and phenothiazine moieties were designed and synthesized via Buchwald-Hartwig coupling reaction. Thermal, photophysical, and electrochemical properties of the compounds are discussed. Only compound with phenothiazine form molecular glass, with glass transition temperatures of 90 °C. The geometry and electronic characteristics of the compounds were substantiated within density functional theory (DFT). 10,10'-(2-(Trifluoromethyl)-1,4-phenylene)bis(10H-phenoxazine) shows efficient thermally activated delayed fluorescence with high spin-orbit coupling values. 10,10'-(2-(Trifluoromethyl)-1,4-phenylene)bis(10H-phenothiazine) as efficient room-temperature phosphor shows high oxygen sensitivity.

Self-assembled porphyrin-based photosensitizer nanomicelles for enhanced photodynamic therapy

Preparation of a supermacromolecular photosensitizer that can stay in the tumor site and exhibits high photoconversion efficiency is useful for improving the efficacy of tumor photodynamic therapy (PDT). In this paper, we prepared tetratroxaminobenzene porphyrin (TAPP) loaded biodegradable silk nanospheres (NSs) and characterized their morphology, optical properties and the singlet oxygen-generating capacity. On this basis, the effect of in vitro photodynamic killing efficacy by as-prepared nanometer micelles was evaluated and the tumor retention ability and tumor killing effect of the nanometer micelles were verified by the co-culture of photosensitizer micelle and tumor cells. The results show that tumor cells were killed well under 660 nm laser irradiation even at a lower concentration of as-prepared TAPP NSs. In addition, due to the excellent safety of as-prepared nanomicelle, they exhibit great potential applications in improved tumor PDT.

Carboranes in drug discovery, chemical biology and molecular imaging

There exists a paucity of structural innovation and limited molecular diversity associated with molecular frameworks in drug discovery and biomolecular imaging/chemical probe design. The discovery and exploitation of new molecular entities for medical and biological applications will necessarily involve voyaging into previously unexplored regions of chemical space. Boron clusters, notably the carboranes, offer an alternative to conventional (poly)cyclic organic frameworks that may address some of the limitations associated with the use of novel molecular frameworks in chemical biology or medicine. The high thermal stability, unique 3D structure and aromaticity, kinetic inertness to metabolism and ability to engage in unusual types of intermolecular interactions, such as dihydrogen bonds, with biological receptors make carboranes exquisite frameworks in the design of probes for chemical biology, novel drug candidates and biomolecular imaging agents. This Review highlights the key developments of carborane derivatives made over the last decade as new design tools in medicinal chemistry and chemical biology, showcasing the versatility of this unique family of boron compounds.

Blue organic light-emitting diodes with hybridized local and charge-transfer excited state realizing high external quantum efficiency

Donor-spacer-acceptor (D-π-A) materials CAPI and CCAPI, with hybridized local and charge transfer (HLCT) emissive states, have been synthesized. The twisting D-π-A architecture promotes the partial separation of HOMO and LUMO, leading to an enhanced % CT component, and the anthracene moiety in CAPI and CCAPI increases the conjugation length, leading to an enhanced % LE component. The non-doped device with CCAPIb shows the blue emission (450 nm) with maximum current efficiency (η c), power efficiency (η p), and external quantum efficiency (η ex) of 16.83 cd A-1, 15.32 lm W-1, and 12.0%, respectively, as well as exciton utilization efficiency (EUE) of 95% with a luminance of 32 546 cd m-2 and a roll-off efficiency of 0.53%. The new design strategy has great potential for developing high-performance blue electroluminescent materials.

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.

Highly efficient non-doped blue fluorescent OLEDs with low efficiency roll-off based on hybridized local and charge transfer excited state emitters

Designing a donor–acceptor (D–A) molecule with a hybridized local and charge transfer (HLCT) excited state is a very effective strategy for producing an organic light-emitting diode (OLED) with a high exciton utilization efficiency and external quantum efficiency. Herein, a novel twisting D–π–A fluorescent molecule (triphenylamine–anthracene–phenanthroimidazole; TPAAnPI) is designed and synthesized. The excited state properties of the TPAAnPI investigated through photophysical experiments and density functional theory (DFT) analysis reveal that its fluorescence is due to the HLCT excited state. The optimized non-doped blue OLED using TPAAnPI as a light-emitting layer exhibits a novel blue emission with an electroluminescence (EL) peak at 470 nm, corresponding to the Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.22). A fabricated device termed Device II exhibits a maximum current efficiency of 18.09 cd A⁻¹, power efficiency of 12.35 lm W⁻¹, luminescence of ≈29 900 cd cm⁻², and external quantum efficiency (EQE) of 11.47%, corresponding to a high exciton utilization efficiency of 91%. Its EQE remains as high as 9.70% at a luminescence of 1000 cd m⁻² with a low efficiency roll-off of 15%. These results are among the best for HLCT blue-emitting materials involved in non-doped blue fluorescent OLEDs. The performance of Device II highlights a great industrial application potential for the TPAAnPI molecule.

A new pure fluorescent blue HLCT-emitter was designed and synthesized. Highly efficient non-doped blue OLEDs with low efficiency roll-off were achieved.

Efficient fluorescent OLEDS based on assistant acceptor modulated HLCT emissive state for enhancing singlet exciton utilization

Phenylamine phenanthroimidazole based bipolar compounds with donor-acceptor (D-A) architecture namely, 4-(1-(2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-6,9-di(pyren-4-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenylaniline (DDPPPA) and 4'-(1-(2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-6,9-di(pyren-4-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (DDPBA) have been synthesized with highly fluorescent pyrene moieties at C6- and C9-positions. The C6 and C9 modification enhanced the thermal, photochemical and electroluminescent properties. Both molecules were employed as blue emitters in non doped organic light emitting devices (OLEDs) and show high performances due to hybridized local and charge-transfer properties. An OLED with DDPPPA/DDPBA emissive layer shows deep-blue emission with maximum external quantum efficiency (η ex), current efficiency (η c) and power efficiency (η p) of 5.7/6.0%, 10.5/12.0 cd A-1 and 8.3/9.2 lm W-1, respectively. Both devices show high singlet exciton utilizing efficiency (η s) of DDPPPA-31.33% and DDPBA-35.29%. The doped device m-MTDATA:DDPPPA/m-MTDATA:DDPBA shows maximum efficiencies of η c -7.4/8.23 cd A-1; η p -5.8/6.13 lm W-1; η ex -4.72/5.63% (5 wt%):η c -8.36/9.15 cd A-1; η p -6.32/6.65 lm W-1; η ex -4.86/5.45% (10 wt%):η c -9.58/10.02 cd A-1; η p -7.8/8.25 lm W-1; η ex -5.96/6.25% (20 wt%). The doped device based on TAPC host TAPC:DDPPPA/TAPC:DDPBA exhibits maximum efficiencies of η c -9.60/10.03 cd A-1; η p -7.81/8.26 lm W-1; η ex -5.96/6.25% (20 wt%).

Molecularly engineered hole-transport material for low-cost perovskite solar cells

Triphenylamine-N-phenyl-4-(phenyldiazenyl)aniline (TPA-AZO) is synthesized via a facile CuI-catalyzed reaction and used as a hole transport material (HTM) in perovskite solar cells (PSCs), as an alternative to the expensive spiro-type molecular materials, including commercial 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD). Experimental and computational investigations reveal that the highest occupied molecular orbital (HOMO) level of TPA-AZO is deeper than that of spiro-OMeTAD, and optimally matches with the conduction band of the perovskite light absorber. The use of TPA-AZO as a HTM results in PSC prototypes with a power conversion efficiency (PCE) approaching that of the spiro-OMeTAD-based reference device (17.86% vs. 19.07%). Moreover, the use of inexpensive starting reagents for the synthesis of TPA-AZO makes the latter a new affordable HTM for PSCs. In particular, the cost of 1 g of TPA-AZO ($22.76) is significantly lower compared to that of spiro-OMeTAD ($170-475). Overall, TPA-AZO-based HTMs are promising candidates for the implementation of viable PSCs in large-scale production.

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.

Creation of Efficient Blue Aggregation-Induced Emission Luminogens for High-Performance Nondoped Blue OLEDs and Hybrid White OLEDs

Organic blue luminescent materials are essential for organic light-emitting diodes (OLEDs). However, high-quality blue materials that can fulfill the requirements of OLED commercialization are much rare. Herein, two novel blue luminogens, 9-(4-(2,6-di- tert-butyl-10-(4-(1,2,2-triphenylvinyl)phenyl)anthracen-9-yl)phenyl)-9 H-carbazole and 9-(4-(2,6-di- tert-butyl-10-(4-(1,2,2-triphenylvinyl)phenyl)anthracen-9-yl)1,3-di(9 H-carbazol-9-yl)benzene (TPE-TADC), consisting of anthracene, tetraphenylethene, and carbazole groups are successfully prepared, and their thermal, optical, electronic, and electrochemical properties are fully investigated. They exhibit prominent aggregation-induced emission property and strong blue fluorescence at ∼455 nm in neat films. Efficient nondoped OLEDs are fabricated with these blue luminogens, providing blue electroluminescence (EL) at 451 nm (CIE _{x, y} = 0.165, 0.141) and high EL efficiencies of 6.81 cd A^-1, 6.57 lm W^-1, and 5.71%. By utilizing TPE-TADC as a blue emissive layer, high-performance two-color hybrid white OLEDs are achieved, furnishing modulatable light color from pure white (CIE _{x, y} = 0.33, 0.33) to warm white (CIE _{x, y} = 0.44, 0.46) and excellent EL efficiencies of 56.7 cd A^-1, 55.2 lm W^-1, and 19.2%. More importantly, these blue and white OLEDs all display ultrahigh color and efficiency stabilities at high luminance, indicating the great potential of these blue luminogens for the application in OLED displays and white illumination.

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).

Efficient electroluminescent hybridized local and charge-transfer host materials with small singlet–triplet splitting to enhance exciton utilization efficiency: excited state transition configuration

A series of efficient electroluminescent materials with dual carrier transport properties shows enhanced singlet exciton utilization (η_s) due to small singlet–triplet splitting (ΔE_ST). The strong orbital-coupling transitions of N-(4-(1-(1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-4,5-diphenyl-1H-imidazol-2-yl)naphthalen-4-yl)phenyl)-N-phenyl benzenamine (DDPB) exhibit deep blue emission at 435 nm (CIEy, 0.07) with an external quantum efficiency of 2.01%. The electroluminescent efficiencies of 2-(1-(9H-carbazol-9-yl)naphthalen-4-yl)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazole (CDDPI) (L – 3992 cd m⁻²; η_ex – 3.01%; η_c – 2.56 cd A⁻¹; η_p – 2.12 lm W⁻¹) are higher than those of the N-(4-(1-(1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-H-phenanthro[9,10-d]imidazole-2-yl)naphthalen-4-yl)phenyl)-N-phenylbenzenamine (DBDPA) based device (L – 3015 cd m⁻²; η_ex – 2.85%; η_c – 2.01 cd A⁻¹; η_p – 1.92 lm W⁻¹). The blue emissive materials CDDPI and DBDPA are used as a host to construct green and red phosphorescent OLEDs: the green device based on CDDPI:Ir(ppy)₃ exhibits higher efficiencies (L – 8812 cd m⁻²; η_ex – 19.0%; η_c – 27.5 cd A⁻¹; η_p – 33.0 lm W⁻¹) at 2.7 V and the red device based on CDDPI:Ir(MQ)₂(acac) exhibits a maximum luminance of 39 661 cd m⁻² with excellent EL efficiencies [η_ex – 19.2%; η_c – 27.9 cd A⁻¹; η_p – 29.2 lm W⁻¹; CIE (0.64, 0.34)] compared with those of the DBDPA:Ir(MQ)₂(acac) based device [L – 37 621 cd m⁻²; η_ex – 18.5%; η_c – 25.2 cd A⁻¹; η_p – 25.8 lm W⁻¹; CIE (0.64, 0.34)].

CDDPI:Ir(ppy)₃ exhibits higher efficiencies: L = 8812 cd m⁻²; η_ex = 19.0%; η_c = 27.5 cd A⁻¹; η_p = 33.0 lm W⁻¹ at 2.7 V.

Strategic tuning of excited-state properties of electroluminescent materials with enhanced hot exciton mixing

Deep blue emitters with excellent stability, high quantum yield and multifunctionality are the major issues for full-color displays. In line with this, new multifunctional, thermally stable blue emitters viz., N-(4-(10-(1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)anthracen-9-yl)phenyl)-N-phenylbenzenamine (DPIAPPB) and 2-(10-(9H-carbazol-9-yl)anthracen-9-yl)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazole (CADPPI) with hybridized local charge transfer state (HLCT) and hot exciton properties have been synthesized. These molecules show high photoluminescence quantum yield (Φ_s/f): (DPIAPPB – 0.82/0.70 and CADPPI – 0.91/0.83). The CADPPI based device (EL – 467 nm) shows high efficiencies [η_c – 9.85 cd A⁻¹; η_p – 10.84 lm W⁻¹; η_ex – 4.78% at 2.8 V; CIE (0.15, 0.10)] compared to the DPIAPPB device (EL − 472 nm) [η_c – 6.56 cd A⁻¹; η_p – 6.16 lm W⁻¹; η_ex – 4.15% at 2.8 V with CIE (0.15, 0.12)]. The green device with CADPPI:Ir(ppy)₃ exhibits a maximum L – 59 012 cd m⁻²; η_ex – 16.8%; η_c – 37.3 cd A⁻¹; η_p – 39.8 lm W⁻¹ with CIE (0.30, 0.60) and the red device with CADPPI:Ir(MDQ)₂(acac) shows a maximum L – 43 456 cd m⁻²; η_ex – 21.9%; η_c – 36.0 cd A⁻¹; η_p – 39.6 lm W⁻¹ with CIE (0.64, 0.35).

The CADPPI:Ir(ppy)₃ device exhibits L – 90 12 cd m⁻²; η_ex – 18.8%; η_c − 27.3 cd A⁻¹; η_p – 29.8 lm W⁻¹; CIE (0.30, 0.60).

A bipolar triphenylamine-dibenzothiophene S,S-dioxide hybrid compound for solution-processable single-layer green OLEDs and as a host for red emitters

A bipolar triphenylamine-dibenzothiophene S,S-dioxide hybrid compound for solution-processable single-layer green OLEDs and as a host for red emitters.

Efficient full-colour organic light-emitting diodes based on donor–acceptor electroluminescent materials with a reduced singlet–triplet splitting energy gap

A series of efficient blue-emitting materials, namely, Cz-DPVI, Cz-DMPVI, Cz-DEPVI and TPA-DEPVI, possessing a donor–acceptor architecture with dual carrier transport properties and small singlet–triplet splitting is reported. These compounds exhibit excellent thermal properties with a very high glass-transition temperature (T_g), and thus, a stable uniform thin film was formed during device fabrication. Among the weak donor compounds, specifically, Cz-DPVI, Cz-DMPVI and Cz-DEPVI, the Cz-DEPVI-based device showed the maximum efficiencies (L: 13 955 cd m⁻², η_ex: 4.90%, η_c: 6.0 cd A⁻¹, and η_p: 5.4 lm W⁻¹) with CIE coordinates of (0.15, 0.06) at 2.8 V. The electroluminescent efficiencies of Cz-DEPVI were higher than that of the strong donor TPA-DEPVI-based device (L: 13 856 cd m⁻², η_ex: 4.70%, η_c: 5.7 cd A⁻¹, and η_p: 5.2 lm W⁻¹). Furthermore, these blue emissive materials were used as hosts to construct efficient green and red phosphorescent OLEDs. The green device based on Cz-DEPVI:Ir(ppy)₃ exhibited the maximum L of 8891 cd m⁻², η_ex of 19.3%, η_c of 27.9 cd A⁻¹ and η_p of 33.4 lm W⁻¹ with CIE coordinates of (0.31, 0.60) and the red device based on Cz-DEPVI:Ir(MQ)₂(acac) exhibited the maximum L of 40 565 cd m⁻², η_ex of 19.9%, η_c of 26.0 cd A⁻¹ and η_p of 27.0 lm W⁻¹ with CIE coordinates of (0.64, 0.37).

The Cz-DEPVI device showed high efficiencies of L: 13955 cd m⁻², η_ex: 4.90%, η_c: 6.0 cd A⁻¹, η_p: 5.4 lm W⁻¹ and CIE coordinates of (0.15, 0.06) at 2.8 V.

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.

Excimer formation and evolution of excited state properties in discrete dimeric stacking of an anthracene derivative: a computational investigation

Herein, density functional theory (DFT) computations were performed to investigate the discrete dimer of a mono-substituted anthracene derivative (2-TA-AN), which exhibited highly efficient pure excimer fluorescence in its crystal form. As a more practical model, its geometry, potential energy curve and excited state property were systematically calculated to better understand the excimer formation process and photophysical properties. The compressed excimer geometry is responsible for the highly efficient excimer emission, arising from the enhanced rigidity that greatly suppresses its non-radiative vibrations. Potential energy curves along three directions reveal the non-uniqueness of excimer formation along the long axis of anthracene, which is in a good agreement with the experimental findings. Upon decreasing the displacement, the intermonomer charge-transfer (CT) component gradually increased towards an approximately equivalent hybridization with the locally-emissive (LE) state of the monomer during the formation of the excimer. The excimer emission wavelength versus intermonomer CT content shows a similar trend along the three directions, revealing a turning point related to the essential transition of the excited state properties from the LE of the monomer to the HLCT of the excimer. The present results will contribute to the better understanding of the structure-property relationships in excimer formation and photophysical properties.

Two novel phenanthrene-based host materials in red and green organic light-emitting devices with low efficiency roll-off

Two novel phenanthrene-based host materials showed significant differences due to the different moieties at the same position of the phenanthrene backbone.

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.

Bright AIE Nanoparticles with F127 Encapsulation for Deep-Tissue Three-Photon Intravital Brain Angiography

Deep-tissue imaging is of great significance to biological applications. In this paper, a deep-red emissive luminogen 2,3-bis(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl) fumaronitrile (TPATCN) with aggregation-induced emission (AIE) feature is prepared. TPATCN molecules were then encapsulated within a polymeric matrix of Pluronic F-127 to form nanoparticles (NPs). TPATCN NPs exhibit bright three-photon fluorescence (3PF) in deep-red region, together with high chemical stability, good photostability, and biocompatibility. They are further utilized for in vivo 3PF imaging of the brain vasculature of mice, under the excitation of a 1550 nm femtosecond laser. A vivid 3D reconstruction of the brain vasculature is then built with a penetration depth of 875 µm, which is the largest in ever reported 3PF imaging based on AIE NPs. After that, by collecting both of the 3PF and third-harmonic generation signals, multichannel nonlinear optical imaging of the brain blood vessels is further realized. These results will be helpful to study the structures and functions of the brain in the future.

Highly efficient non-doped blue organic light emitting diodes based on a D–π–A chromophore with different donor moieties

The non-doped OLED based on MPPIS-Cz exhibits blue emission with CIE of (0.16, 0.08), maximum current and external quantum efficiency of 1.52 cd A⁻¹ and of 1.42%, respectively.

Theoretical investigation of high-efficiency organic electroluminescent material: HLCT state and hot exciton process

HLCT excited state with both high PL efficiency and a large fraction of singlet exciton generation in EL. We employed the DFT method to investigate twisting D–A molecules, and found that the twist angle had a significant effect on the CT component.

Efficient phenanthroimidazole-styryl-triphenylamine derivatives for blue OLEDs: a combined experimental and theoretical study

Blue emitting devices based on 2-(4′-9H-carbazol-9-yl)-[1,1′-styryl]-4-yl-1-benzylpiperidine-1H-phenanthro[9,10-d]imidazole (Cz-BPIS) exhibits blue emission with CIE coordinates of (0.16, 0.09), current density of 1.91 cd/A, power efficiency of 1.63 lm/W and external quantum efficiency of 2.61%.

Achieving a high-efficiency dual-core chromophore for emission of blue light by testing different side groups and substitution positions

The optical and electronic properties of hetero dual core derivatives can be controlled by simply substituting an electron-donating side group.

Computational investigation on the large energy gap between the triplet excited-states in acenes

Accurate description triplet excited states is a challenge of organic materials. In this work, we investigate excited state behavior of acenes. The results show that the symmetry of the transition orbital have a great influence on the energy gap.

High-Performance Blue OLEDs Based on Phenanthroimidazole Emitters via Substitutions at the C6- and C9-Positions for Improving Exciton Utilization

Donor-acceptor (D-A) molecular architecture has been shown to be an effective strategy for obtaining high-performance electroluminescent materials. In this work, two D-A molecules, Ph-BPA-BPI and Py-BPA-BPI, have been synthesized by attaching highly fluorescent phenanthrene or pyrene groups to the C6- and C9-positions of a locally excited-state emitting phenylamine-phenanthroimidazole moiety. Equipped with good physical and hybridized local and charge-transfer properties, both molecules show high performances as blue emitters in nondoped organic light-emitting devices (OLEDs). An OLED using Ph-BPA-BPI as the emitting layer exhibits deep-blue emission with CIE coordinates of (0.15, 0.08), and a maximum external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 4.56 %, 3.60 cd A(-1) , and 3.66 lm W(-1) , respectively. On the other hand, a Py-BPA-BPI-based, sky-blue OLED delivers the best results among nondoped OLEDs with CIEy values of < 0.3 reported so far, for which a very low turn-on voltage of 2.15 V, CIE coordinates of (0.17, 0.29), and maximum CE, PE, and EQE values of 10.9 cd A(-1) , 10.5 lm W(-1) , and 5.64 %, were achieved, respectively. More importantly, both devices show little or even no efficiency roll-off and high singlet exciton-utilizing efficiencies of 36.2 % for Ph-BPA-BPI and 39.2 % for Py-BPA-BPI.

Superior upconversion fluorescence dopants for highly efficient deep-blue electroluminescent devices

Highly efficient deep-blue electroluminescent devices are realized via an efficient triplet–triplet annihilation process of styrylpyrene-based emitters.

In this study, we revealed a new approach for the development of new triplet–triplet annihilation (TTA) materials with highly efficient deep-blue fluorescence via the incorporation of a styrylpyrene core and an electron-donating group. The resulting deep-blue emitters (PCzSP, DFASP, and DPASP) exhibit intramolecular charge transfer emissions with remarkably high emission quantum yields. The electroluminescent devices based on these three fluorophores as dopants using CBP as a host exhibit very high device efficiencies; in particular, the DPASP-doped device reveals an extremely high EQE of 12%, reaching the limit of a TTA-based device. The EL characteristics of DPASP-doped CBP-based devices at various doping concentrations (0–5%) suggest that the dopant DPASP is responsible for the TTA-type delayed fluorescence in the device; no delayed fluorescence was observed for the device using CBP as the host emitter. Moreover, when using DMPPP with ambipolar characteristics as the host, the deep-blue DPASP-doped device also gives outstanding performance with an EQE of nearly 11% with an extremely small efficiency roll-off, which was ascribed to the excellent charge balance in the emitting layer of the EL device. The TTA process of the SP-based dopants accounts significantly for the superior efficiencies of the EL devices.

Highly Efficient Nondoped Green Organic Light-Emitting Diodes with Combination of High Photoluminescence and High Exciton Utilization

Photoluminescence (PL) efficiency and exciton utilization efficiency are two key parameters to harvest high-efficiency electroluminescence (EL) in organic light-emitting diodes (OLEDs). But it is not easy to simultaneously combine these two characteristics (high PL efficiency and high exciton utilization) into a fluorescent material. In this work, an efficient combination was achieved through two concepts of hybridized local and charge-transfer (CT) state (HLCT) and "hot exciton", in which the former is responsible for high PL efficiency while the latter contributes to high exciton utilization. On the basis of a tiny chemical modification in TPA-BZP, a green-light donor-acceptor molecule, we designed and synthesized CzP-BZP with this efficeient combination of high PL efficiency of η(PL) = 75% in the solid state and maximal exciton utilization efficiency up to 48% (especially, the internal quantum efficiency of η(IQE) = 35% substantially exceed 25% of spin statistics limit) in OLED. The nondoped OLED of CzP-BZP exhibited an excellent performance: a green emission with a CIE coordinate of (0.34, 0.60), a maximum current efficiency of 23.99 cd A(-1), and a maximum external quantum efficiency (EQE, η(EQE)) of 6.95%. This combined HLCT state and "hot exciton" strategy should be a practical way to design next-generation, low-cost, high-efficiency fluorescent OLED materials.

Accurate description of hybridized local and charge-transfer excited-state in donor–acceptor molecules using density functional theory

The ωB97X was the most reliable functional for the accurate description of HLCT state at ground state and excited state.

Hybridization and de-hybridization between the locally-excited (LE) state and the charge-transfer (CT) state: a combined experimental and theoretical study

Gradual evolution of hybridization process between the LE and CT states is investigated in a series of TPA–PI (H_xV_y) derivatives by TD-DFT method.

Highly efficient near ultraviolet organic light-emitting diode based on a meta-linked donor-acceptor molecule

A meta-linked donor–acceptor (D–A) structure was utilized to achieve high-efficiency and colour-purity near ultraviolet (NUV) in organic light-emitting diodes (OLEDs).

A novel near ultraviolet (NUV) emitter with a meta-linked donor–acceptor (D–A) structure between triphenylamine (TPA) and phenanthroimidazole (PPI), mTPA–PPI, was designed and synthesized. This molecular design is expected to resolve the conflict between the non-red-shifted emission and the introduction of a charge-transfer (CT) state in the D–A system, aiming at NUV organic light-emitting diodes (OLEDs) with high-efficiency and colour-purity. Theoretical calculations and photophysical experiments were implemented to verify the unique excited state properties of mTPA–PPI. The mTPA–PPI device exhibited excellent NUV electroluminescence (EL) performance with an emission peak at 404 nm, a full width at half maximum (FWHM) of only 47 nm corresponding to a CIE coordinate of (0.161, 0.049), and a maximum external quantum efficiency (EQE) of 3.33%, which is among the best results for NUV OLEDs. This work not only demonstrates the promising potential of mTPA–PPI in NUV OLEDs, but also provides a valuable strategy for the rational design of NUV materials by using the meta-linked D–A architecture.

New insight into photo-induced electron transfer with a simple ubiquinone-based triphenylamine model

A ubiquinone-based triphenylamine system was designed as a simple model to study the photo-induced electron transfer (PET).

Vertically-expanded imidazo[1,2-a]pyridines and imidazo[1,5-a]pyridine via dehydrogenative coupling

The intramolecular dehydrogenative coupling mediated by potassium constitutes the general methodology leading to weakly emitting π-expanded heterocycles.