Self-assembled molecular network with waterwheel-like architecture: experimental and theoretical evaluation toward electron transport capabilities for optoelectronic devices.

In recent times, self-assembled electron transport materials for optoelectronic devices, both solar cells and organic light-emitting diodes (OLEDs), have been gaining much interest as they help in fabricating high-efficiency devices. However, designing organic small molecular materials with star-shaped self-assembled networks is a challenge. To achieve this sort of target, we chose triazine and benzene-1,3,5-tricarbonyl cores for developing such architecture, and we developed four molecular systems, vizTCpCN, TCmCN, TmCN, and TpCN. Successful isolation of single crystals followed by structural analysis of TmCN revealed interesting molecular arrangements in the solid state resulting in the formation of a waterwheel type architecture with an extended network bearing characteristic voids. Theoretical calculations was carried out to check their electron transportability. The natural transition orbital calculation helped in understanding the locally excited and charge transfer excited states. The low electron reorganization energies of these molecules indicated that these materials may have potential to be used in electron transport layers of optoelectronic devices, particularly in OLEDs. Moreover, the assembled networks have a relatively wide surface area and linked structures, which are advantageous for the conduction of carriers with poor electron recombination inside the ETL, and these may offer a straightforward channel for electron conduction to the emissive layer. Finally, the fabricated electron-only device indicated that the synthesized materials may be used as ETMs in the electron transport layer of optoelectronic devices.

Bicarbazole-Benzophenone Based Twisted Donor-Acceptor Derivatives as Potential Blue TADF Emitters for OLEDs.

Over the past few decades, organic light-emitting diodes (OLEDs) find applications in smartphones, televisions, and the automotive sector. However, this technology is still not perfect, and its application for lighting purposes has been slow. For further development of the OLEDs, we designed twisted donor-acceptor-type electroactive bipolar derivatives using benzophenone and bicarbazole as building blocks. Derivatives were synthesized through the reaction of 4-fluorobenzophenone with various mono-alkylated 3,3'-bicarbazoles. We have provided a comprehensive structural characterization of these compounds. The new materials are amorphous and exhibit suitable glass transition temperatures ranging from 57 to 102 °C. They also demonstrate high thermal stability, with decomposition temperatures reaching 400 °C. The developed compounds exhibit elevated photoluminescence quantum yields (PLQY) of up to 75.5% and favourable HOMO-LUMO levels, along with suitable triplet-singlet state energy values. Due to their good solubility and suitable film-forming properties, all the compounds were evaluated as blue TADF emitters dispersed in commercial 4,4'-bis(N-carbazolyl)-1,10-biphenyl (CBP) host material and used for the formation of emissive layer of organic light-emitting diodes (OLEDs) in concentration-dependent experiments. Out of these experiments, the OLED with 15 wt% of the emitting derivative 4-(9'-{2-ethylhexyl}-[3,3']-bicarbazol-9-yl)benzophenone exhibited superior performance. It attained a maximum brightness of 3581 cd/m2, a current efficacy of 5.7 cd/A, a power efficacy of 4.1 lm/W, and an external quantum efficacy of 2.7%.

Tuning the Liquid Crystallinity and Electroluminescence via Sulfonation of S-Annulated Perylene Tetraester.

This study presents a comparative analysis of S-annulated perylene tetraester (PTE-S) and its sulfone (PTE-SO2) analogue. This sulfone modification reduced melting point and stabilized a room temperature columnar rectangular (Colr) phase in contrast to its parent PTE-S which showed a crystalline behaviour at room temperature. This molecular design also leads to red-shifted absorbance and emission in comparison to PTE-S, along with a tuning of photoluminescence from sky blue to green, achieving an impressive quantum yield of 85 %. OLED devices fabricated using PTE-SO2 as emitter material at concentrations of 0.2, 0.5, and 1 wt.% in CBP as host material. A maximum external quantum efficiency (EQE) of 2.9 % was observed with the 0.5 wt.% PTE-SO2 in CBP with CIE coordinates of (0.45, 0.35), accompanied by an orange luminance of 848 cd/m2. Notably, a device with a 0.5 wt% doping concentration of PTE-S demonstrates an EQE of 3.5 %, and cyan luminance of 2,598 cd/m2.

Sterically Crowded Donor-Rich Imidazole Systems as Hole Transport Materials for Solution-Processed OLEDs.

Imidazole, being an interesting dinitrogenic five-membered heterocyclic core, has been widely explored during the last several decades for developing various fascinating materials. Among the different domains where imidazole-based materials find wide applications, the area of optoelectronics has seen an overwhelming growth of functional imidazole derivatives developed through remarkable design and synthesis strategies. The present work reports a design approach for integrating bulky donor units at the four terminals of an imidazole core, leading to the development of sterically populated imidazole-based molecular platforms with interesting structural features. Rationally chosen starting substrates led to the incorporation of a bulky donor at the four terminals of the imidazole core. In addition, homo- and cofunctional molecular systems were synthesized through a suitable combination of initial ingredients. Our approach was extended to develop a series of four molecular systems, i.e., Cz3PhI, Cz4I, Cz3PzI, and TPA3CzI, containing carbazole, phenothiazine, and triphenylamine as known efficient donors at the periphery. Given their interesting structural features, three sterically crowded molecules (Cz4I, Cz3PzI, and TPA3CzI) were screened by using DFT and TD-DFT calculations to investigate their potential as hole transport materials (HTMs) for optoelectronic devices. The theoretical studies on several aspects including hole reorganization and exciton binding energies, ionization potential, etc., revealed their potential as possible candidates for the hole transport layer of OLEDs. Single-crystal analysis of Cz3PhI and Cz3PzI established interesting structural features including twisted geometries, which may help attain high triplet energy. Finally, the importance of theoretical predictions was established by fabricating two solution-process green phosphorescent OLED devices using TPA3CzI and Cz3PzI as HTMs. The fabricated devices exhibited good EQE/PE and CE of ∼15%/56 lm/W/58 cd/A and ∼13%/47 lm/W/50 cd/A, respectively, at 100 cd/m2.

Bicarbazole-Benzophenone-Based Twisted Donor-Acceptor-Donor Derivatives as Blue Emitters for Highly Efficient Fluorescent Organic Light-Emitting Diodes.

This paper delves into the development of a group of twisted donor-acceptor-donor (D-A-D) derivatives incorporating bicarbazole as electron donor and benzophenone as electron acceptor for potential use as blue emitters in OLEDs. The derivatives were synthesized in a reaction of 4,4'-difluorobenzophenone with various 9-alkyl-9'H-3,3'-bicarbazoles. The materials, namely, DB14, DB23, and DB29, were designed with different alkyl side chains to enhance their solubility and film-forming properties of layers formed using the spin-coating from solution method. The new materials demonstrate high thermal stabilities with decomposition temperatures >383 °C, glass transition temperatures in the range of 95-145 °C, high blue photoluminescence quantum yields (>52%), and short decay times, which range in nanoseconds. Due to their characteristics, the derivatives were used as blue emitters in OLED devices. Some of the OLEDs incorporating the DB23 emitter demonstrated a high external quantum efficiency (EQEmax) of 5.3%, which is very similar to the theoretical limit of the first-generation devices.

Status and Challenges of Blue OLEDs: A Review.

Organic light-emitting diodes (OLEDs) have outperformed conventional display technologies in smartphones, smartwatches, tablets, and televisions while gradually growing to cover a sizable fraction of the solid-state lighting industry. Blue emission is a crucial chromatic component for realizing high-quality red, green, blue, and yellow (RGBY) and RGB white display technologies and solid-state lighting sources. For consumer products with desirable lifetimes and efficiency, deep blue emissions with much higher power efficiency and operation time are necessary prerequisites. This article reviews over 700 papers covering various factors, namely, the crucial role of blue emission for full-color displays and solid-state lighting, the performance status of blue OLEDs, and the systematic development of fluorescent, phosphorescent, and thermally activated delayed fluorescence blue emitters. In addition, various challenges concerning deep blue efficiency, lifetime, and approaches to realizing deeper blue emission and higher efficacy for blue OLED devices are also described.

Naphtalimide-Based Bipolar Derivatives Enabling High-Efficiency OLEDs.

Organic light-emitting diodes (OLEDs) have revolutionized the world of technology, making significant contributions to enhancing our everyday lives. With their exceptional display and lighting capabilities, OLEDs have become indispensable in various industries such as smartphones, tablets, televisions, and automotives. They have emerged as a dominant technology, inspiring continuous advancements, and improvements. Taking inspiration from the remarkable advancements in OLED advancements, we have successfully developed naphtalimide-based compounds, namely RB-08, RB-09, RB-10, and RB-11. These compounds exhibit desirable characteristics such as a wide bandgap, high decomposition temperatures (306-366 °C), and very high glass transition temperatures (133-179 °C). Leveraging these exceptional properties, we have harnessed these compounds as green emitters in the aforementioned devices. Among the various fabricated OLEDs, the one incorporating the RB-11 emitter has exhibited superior performance. This specific configuration achieved maximum power efficacy of 7.7 lm/W, current efficacy of 7.9 cd/A, and external quantum efficiency of 3.3%. These results highlight the outstanding capabilities of our synthesized emitter and its potential for further advancements in the field.

Rationally heteroarylated pyridines as hole transport materials for OLEDs.

The advancement in developing highly efficient hole transport materials for OLED devices has been a challenge over the past several years. For an efficient OLED device, there should be an efficient promotion of charge carriers from each electrode and effective confinement of triplet excitons in the emissive layer of the phosphorescent OLED (PhOLED). Thus, the development of stable and high triplet energy hole transport materials is in urgent demand for high-performing PhOLED devices. The present work demonstrates the development of two hetero-arylated pyridines as high triplet energy (2.74-2.92 eV) multifunctional hole transport materials to reduce the exciton quenching and to enhance the extent of charge carrier recombination in the emissive layer. In this regard, we report the design, synthesis, and theoretical modeling with electro-optical properties of two molecules, namely PrPzPy and MePzCzPy, with suitable HOMO/LUMO energy levels and high triplet energy, by incorporating phenothiazine as well as other donating units into a pyridine scaffold, and finally developing a hybrid phenothiazine-carbazole-pyridine based molecular architecture. The natural transition orbital (NTO) calculations were done to analyze the excited state sensation in these molecules. The long-range charge transfer characteristics between the higher singlet and triplet states were also analyzed. The reorganization energy of each molecule was calculated to examine their hole transportability. The theoretical calculations for PrPzPy and MePzCzPy revealed that these two molecular systems could be promising materials for the hole transport layer of OLED devices. As a proof of concept, a solution-processed hole-only device (HOD) of PrPzPy was fabricated. The increase in current density with an increase in operating voltage in the range of ∼3-10 V supported that the suitable HOMO energy of PrPzPy can facilitate the hole transportation from the hole injection layer (HIL) to the emissive layer (EML). These results indicated the promising hole transportability of the present molecular materials.

Permissible viewing times of educational projector and TV.

Projectors have become one major medium in modern teaching, with large area-size displays emerging as an alternative. What concerns the general public is whether such eLearning would impose threat on eyes, by noting blue enriched white light to be hazardous to retina and else. Especially, little was known about their permissible viewing time under a certain viewing clarity. We had hence carried out a quantitative study with the use of a blue-hazard quantification spectrometer to determine the permissible viewing time when using a projector and a large size TV screen for displaying. Surprisingly, the large TV screen could permit a much longer viewing time, meaning which is more eye-friendly. It is plausibly because its resolution is much higher than that of the projector. Two dilemmas were observed in such eLearning; those sitting in the front would suffer a much higher illuminance, leading to a much shorter viewing time, while those sitting in the back would need a far much larger font size to see clearly. To ensure both viewing clarity and a sufficiently long permissible viewing time, orange text on black background is suggested to replace the defaulted black text on white background. The permissible viewing time could hence drastically increase from 1.3 to 83 h at 2 m by viewing a 30 pt font for the TV and from 0.4 to 54 h for the projection. At 6 m, the permissible viewing time was increased from 12 to 236 h for the TV and from 3 to 160 h for the projection, based on a viewable 94 pt font. These results may help educators and other e-display users to wisely apply the display tools with safety.

Bifunctional Bicarbazole-Benzophenone-Based Twisted Donor-Acceptor-Donor Derivatives for Deep-Blue and Green OLEDs.

Organic light-emitting diodes (OLEDs) have played a vital role in showing tremendous technological advancements for a better lifestyle, due to their display and lighting technologies in smartphones, tablets, television, and automotive industries. Undoubtedly, OLED is a mainstream technology and, inspired by its advancements, we have designed and synthesized the bicarbazole-benzophenone-based twisted donor-acceptor-donor (D-A-D) derivatives, namely DB13, DB24, DB34, and DB43, as bi-functional materials. These materials possess high decomposition temperatures (>360 °C) and glass transition temperatures (~125 °C), a high photoluminescence quantum yield (>60%), wide bandgap (>3.2 eV), and short decay time. Owing to their properties, the materials were utilized as blue emitters as well as host materials for deep-blue and green OLEDs, respectively. In terms of the blue OLEDs, the emitter DB13-based device outperformed others by showing a maximum EQE of 4.0%, which is close to the theoretical limit of fluorescent materials for a deep-blue emission (CIEy = 0.09). The same material also displayed a maximum power efficacy of 45 lm/W as a host material doped with a phosphorescent emitter Ir(ppy)3. Furthermore, the materials were also utilized as hosts with a TADF green emitter (4CzIPN) and the device based on DB34 displayed a maximum EQE of 11%, which may be attributed to the high quantum yield (69%) of the host DB34. Therefore, the bi-functional materials that are easily synthesized, economical, and possess excellent characteristics are expected to be useful in various cost-effective and high-performance OLED applications, especially in displays.

Bright Morning Lighting Enhancing Parasympathetic Activity at Night: A Pilot Study on Elderly Female Patients with Dementia without a Pacemaker.

Exposure to bright morning light (BML) entrains the master circadian clock, modulates physiological circadian rhythms, and reduces sleep-wake disturbances. However, its impact on the autonomic nervous system at night remains unclear. Here, we investigated the effects of BML exposure on parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS) activity at night in elderly women. This nonrandomized controlled pilot study included female participants aged ≥ 60 years who were diagnosed with a type of dementia or cognitive disorder, excluding individuals with pacemakers. The treatment group was exposed to 2500 lx of BML, whereas the control group was exposed to 200 lx of general lighting. We measured heart rate variability to quantify ANS activity. The treatment group displayed significant increases in high-frequency (HF) power (Roy's largest root = 1.62; p < 0.001) and nonsignificant decreases in normalized low-frequency (LF%) power. The corresponding nonsignificant decreases in the low-frequency/high-frequency (LF/HF) ratio and cognitive function were correlated with PSNS activity (Roy's largest root = 1.41; p < 0.001), which improved severe dementia. BML exposure reduced SNS activity and enhanced PSNS activity at night in female participants, which improved cognitive function. Thus, BML therapy may be a useful clinical tool for alleviating cognitive decline.

In Search of Hosts for Blue OLEDs: Computational Design and Experimental Validation.

Considering the difficulties associated with the conventional 'trial and error' method for a complete analysis of a giant molecular space, we took the aid of computational pathway (DFT) in screening a large space search of 780 (12×13×5) molecules to search for a host for the blue emitter. The selection process was completed in three Tiers with the conditions of highest theoretical triplet energy (>2.81 eV), aligned HOMO/LUMO levels w.r.t blue dopant (FIrpic), and position of substituents to meet the optimal requirements as host materials. Tier 1 screened twelve different imidazole heterocycle derivatives as base space groups which resulted in the selection of 4,5-diphenyl-1H-imidazole. Tier 2 process converged the search to mCN-CZ having the highest triplet energy and appropriate HOMO/LUMO level relative to FIrpic and ETL. Further, the carbazole of mCN-CZ was replaced with different aromatic hydrocarbons to find the other best compound in terms of triplet energy and HOMO/LUMO. Tier 3 resulted in another promising candidate (mCN-FL) as possible host materials. The band alignment with guest predicted mCN-FL and mCN-CZ to have optimal device performances compared to CZ-CZ and the experimentally observed device performance was in accordance with virtual screening results when TAPC was utilized as the hole transporter. The device results of mCN-CZ and mCN-FL were better than the reference host TCTA. The obtained results thus proved that a virtual screening process will be a useful tool for synthetic chemists in designing task-specific materials.

Minor Copper-Doped Aluminum Alloy Enabling Long-Lifetime Organic Light-Emitting Diodes.

Aluminum has been extensively used as a conductor material in numerous electronic devices, including solar cells, light-emitting diodes (LEDs), organic LEDs (OLEDs), and thin-film transistors. However, its spiking surface and easy electromigration have limited its performance. To overcome this, a trace amount of nonprecious copper dopant has been proven effective in enhancing device reliability. Nevertheless, a comprehensive investigation regarding the effect of copper doping on the morphology at the aluminum conductor-organic interface is yet to be done. We had hence fabricated a series of green OLED devices to probe how copper doping affected the aluminum conductor, morphologically and electrically, and the corresponding device's efficiency and lifetime performance. We found 4 wt % copper doping to be highly effective in enabling a spike-less and smoother aluminum interface, which in turn enabled the fabrication of devices with much higher efficiency and lifetime. Specifically, the corresponding power efficacy at 1000 cd/m2 was increased from 32 to 42 lm/W and the lifetime increased from 75 to 263 h, an increment of 250%. Atomic force microscopy confirmed that the copper doping did help smooth out the conductor interface as deposited and reduce electromigration upon operation.

Solution-processable organic light-emitting diodes utilizing electroluminescent perylene tetraester-based columnar liquid crystals.

Herein, we reveal a homologous series of liquid crystals involving perylene tetraesters as the core connected to the four trialkoxyphenyl units at the periphery using the triazole moiety as the linker. A thorough analysis using differential scanning calorimetry, polarized optical microscopy, and small- and wide-angle X-ray scattering studies confirm that all the mesogens 1a-c hold a stable enantiotropic columnar mesophase. Suitable molecular orbital levels and excellent material photophysical and thermal properties encouraged the study of their electroluminescent properties. Due to this, a well designed solution-processable organic light emitting diode device structure is configured as ITO (125 nm)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (35 nm)/host: x wt% emitter (x = 0.5, 1.0, 3.0, 5.0) (20 nm)/2,2'2''-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) (40 nm)/lithium fluoride (LiF) (1 nm)/aluminium (Al) (200 nm) using compounds 1a-c as emitters. 4,4',4''-Tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) and 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) were chosen as two different host materials. The current density-voltage-luminance and current efficacy-luminance-power efficacy plots suggest that m-MTDATA is a better host than CBP. Amongst, device based on 1 wt% emitter 1c doped in the m-MTDATA host matrix displayed the best performance, with a maximum power efficacy of 17.2 lm W-1, current efficacy of 18.5 cd A-1, and external quantum efficiency of 6.3%.

An easy-to-apply method for determining permissible exposure limit of retina to light.

Long exposure to intensive artificial light imposes threats to the retina. It is crucial to know the permissible exposure time to prevent photoretinitis. One complicated quantification method is available, which, however, also requires instrumentally measured spectrum and illuminance. We present herein an easy-to-apply formula and a ready-to-use table derived therefrom to determine the permissible exposure limit for any given desk lamp at any viewing distance. One only needs to acquire its color temperature and luminous flux labeled on the product. The method can be used to assist the general public in quickly assessing the potential hazardous status of their desk lamps, if any, and guide the field experts in designing human-eye-friendly lighting.

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

Correction for 'Luminescent columnar discotics as highly efficient emitters in pure deep-blue OLEDs with an external quantum efficiency of 4.7%' by Joydip De et al., Soft Matter, 2022, DOI: 10.1039/d1sm01558c.

Ambient bright lighting in the morning improves sleep disturbances of older adults with dementia.

BACKGROUND: Ambient light therapies are potentially effective in improving sleep disturbances and circadian rhythms. This study created a new lighting intervention model for elderly patients with dementia. It is hypothesized that exposure to bright ambient light in the morning is more effective than general lighting in improving sleep disturbances and circadian rhythms. METHODS: A single-blind longitudinal-group experimental design was employed. The dementia participants came from the community and nursing homes. Those in the experimental group were exposed to ambient light at 2500 lux, and those in the comparison group were exposed to 114-307 lux. The corresponding sleep disturbances and circadian rhythms were determined using an accelerometer (XA-5). A longitudinal experimental design was adopted to observe the time to an effective response. RESULTS: The covariates of benzodiazepine use and total activity during the day were analyzed using generalized estimating equations. The experimental group showed significant sleep efficiency improvement, with mean increases of 41.9% (P < 0.001) and 31.7% (P = 0.002), sleep time increases of 141 min (P = 0.001) and 135 min (P = 0.008), awakening time decreases of 116 min (P = 0.001) and 108 min (P = 0.002), and sleep onset advancements of 60-84 min/sleep offset delays of 57-79 min upon the fifth and ninth week, respectively. A 4-week bright ambient light intervention was the most effective. CONCLUSIONS: This study found that bright ambient light in the morning is beneficial for improving sleep disturbances and was driven by stabilizing circadian rhythms.

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.

Highly Efficient Candlelight Organic Light-Emitting Diode with a Very Low Color Temperature.

Low color temperature candlelight organic light-emitting diodes (LEDs) are human and environmentally friendly because of the absence of blue emission that might suppress at night the secretion of melatonin and damage retina upon long exposure. Herein, we demonstrated a lighting device incorporating a phenoxazine-based host material, 3,3-bis(phenoxazin-10-ylmethyl)oxetane (BPMO), with the use of orange-red and yellow phosphorescent dyes to mimic candlelight. The resultant BPMO-based simple structured candlelight organic LED device permitted a maximum exposure limit of 57,700 s, much longer than did a candle (2750 s) or an incandescent bulb (1100 s) at 100 lx. The resulting device showed a color temperature of 1690 K, which is significantly much lower than that of oil lamps (1800 K), candles (1900 K), or incandescent bulbs (2500 K). The device showed a melatonin suppression sensitivity of 1.33%, upon exposure for 1.5 h at night, which is 66% and 88% less than the candle and incandescent bulb, respectively. Its maximum power efficacy is 23.1 lm/W, current efficacy 22.4 cd/A, and external quantum efficiency 10.2%, all much higher than the CBP-based devices. These results encourage a scalable synthesis of novel host materials to design and manufacture high-efficiency candlelight organic LEDs.

Aggregation-Induced Enhanced Emission-Active Zinc(II) ß-Diketiminate Complexes Enabling High-Performance Solution-Processable OLEDs.

Earth-abundant and cheaper zinc-based organometallic molecules as luminophores are drawing significant research attention for solid-state lighting devices. In this paper, we report two air-stable zinc complexes, where the zinc is coordinated to two sterically encumbered ß-diketiminate ligands in a tetrahedral geometry. In such a geometry, eight phenyl/aryl rings from the ligand backbones are oriented in a propeller shape, augmenting the restricted rotation of the putative rings. Such an architecture harnesses aggregation-induced emission behavior with an excellent solid-state emission property. The rigidity of these molecules reduces the possibility of non-radiative transitions and makes them excellent fluorescence emitters. Both molecules exhibit electroluminescence (EL) in the yellowish-green region of the visible spectrum. We have utilized these molecules as emitters to fabricate multilayered organic light-emitting diode (OLED) devices. The emitter Zn-I in host m-MTDATA exhibits EL with a maximum external quantum efficiency of 4.4%. Among the handful of zinc-based OLEDs, the performance of this emitter is very commendable with power and current efficacies of 15.2 lm W-1 and 12.1 cd A-1, respectively, along with a brightness of 2426 cd m-2.