The effects of alkyl substitution on the aggregation of π-conjugated dyes: spectroscopic study and modelling

A family of dithienosilole-based dyes with alternating donor and acceptor conjugated groups, decorated with linear or branched alkyl chains at different positions on the backbone, have been obtained and investigated in different aggregation states. These dyes are characterized by almost panchromatic absorption and by near-IR emission, with good quantum yields in a variety of solvents with different polarity. We demonstrate that the nature and position of the alkyl substituents strongly govern the self-assembly of the dyes, whose packing is also sensitive to external stimuli, such as grinding and water addition. Thanks to computational results and theoretical modelling, we are able to interpret the results based on two possible preferential packings, characterized by distinct spectroscopic behaviour, whose abundance can be tuned according to the nature and position of the alkyl chains, as well as via external stimuli.

Boosting Hole Mobility: Indenofluorene-Arylamine Copolymers and Their Impact on Solution-Processed OLED Performance

We introduce three newly designed thermally cross-linkable hole transport copolymers (PIF-TPD, PIF-F2PCz, and PIF-TPAPCz) for improving the performance of solution-processed organic light-emitting diodes (s-OLEDs). These copolymers, designed through a strategic molecular approach with benzocyclobutene (BCB) and styrene-based cross-linking monomers, show high solvent resistance at a low cross-linking temperature (150 °C). Furthermore, these conjugated copolymers based on planar indenofluorene with three different hole transport (HT) units, exhibit outstanding charge carrier mobility (1.61 × 10-2 cm2 V-1s-1), demonstrated by comparing hole reorganization energy and electronic coupling strength of HT units. Despite these copolymers showing the overall vertical orientation in the horizontal dipole moment measurement results, we demonstrated that the HT units can exhibit the preferred orientation, contributing to high hole transport properties. As a result, they perform exceptionally well as hole transport layers in green phosphorescent s-OLEDs, achieving a maximum external quantum efficiency of 15.3% and a maximum current efficiency of 53.9 cd A-1 with a small efficiency roll-off despite their relatively low triplet energy levels. These results are comparable to vacuum-deposited OLEDs, highlighting the potential of these copolymers in advancing OLED technology for display panels and lighting applications.

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.

Azomethine-Containing Pyrrolo[3,2-b]pyrrole Copolymers for Simple and Degradable Conjugated Polymers

Conjugated polymers have received significant attention as potentially lightweight and highly tailorable alternatives to inorganic semiconductors, but their synthesis is often complex, produces toxic byproducts, and they are not typically designed to be degradable or recyclable. These drawbacks necessitate dedicated efforts to discover materials with design motifs that enable targeted and efficient degradation of conjugated polymers. In this vein, the synthetic simplicity of 1,4-dihydropyrrolo[3,2-b]pyrroles (DHPPs) is exploited to access azomethine-containing copolymers via a benign acid-catalyzed polycondensation protocol. Polymerizations involve reacting a dialdehyde-functionalized dihydropyrrolopyrrole with p-phenylenediamine as the comonomer using p-toluenesulfonic acid as a catalyst. The inherent dynamic equilibrium of the azomethine bonds subsequently enabled the degradation of the polymers in solution in the presence of acid. Degradation of the polymers is monitored via NMR, UV-vis absorbance, and fluorescence spectroscopies, and the polymers are shown to be fully degradable. Notably, while absorbance measurements reveal a continued shift to higher energies with extended exposure to acid, fluorescence measurements show a substantial increase in the fluorescence response upon degradation. Results from this study encourage the continued development of environmentally-conscious polymerizations to attain polymeric materials with useful properties while simultaneously creating polymers with structural handles for end-of-life management or/and recyclability.

Advances in Solution-Processed OLEDs and their Prospects for Use in Displays

This review outlines problems and progress in development of solution-processed organic light-emitting diodes (SOLEDs) in industry and academia. Solution processing has several advantages such as low consumption of materials, low-cost processing, and large-area manufacturing. However, use of a solution process entails complications, such as the need for solvent resistivity and solution-processable materials, and yields SOLEDs that have limited luminous efficiency, severe roll-off characteristics, and short lifetime compared to OLEDs fabricated using thermal evaporation. These demerits impede production of practical SOLED displays. This review outlines the industrial demands for commercial SOLEDs and the current status of SOLED development in industries and academia, and presents research guidelines for the development of SOLEDs that have high efficiency, long lifetime, and good processability to achieve commercialization.

Octalithium, Tetrasodium, and Decalithium Compounds Based on Pyrrolyl Ligands: Synthesis, Structures, and Activation of the C-H Bonds of Pyrrolyl Rings and C═N Bonds of a Series of Ligands by Organolithium Reagents

The organometallic compounds of lithium ions have garnered continuous interest as indispensable precursors for the syntheses of organometallic complexes of main-group metals, transition metals, lanthanide metals, and actinide metals. In this work, we present a strategy for the preparation of a series of polynuclear lithium complexes. This methodology features the utilization of organolithium reagents both as metal sources to coordinate with the ligands and as nucleophilic reagents to undergo nucleophilic addition to the C═N bonds of the ligands. Reaction of a ligand HL1 [HL1 = 2-(((1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)methylene)amino)phenol] with n-BuLi produced complex [Li8(L1a)4]·1.5Tol (1·1.5Tol) [H2L1a = 2-((1-(1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)pentyl)amino)phenol]. One prominent feature regarding the formation of 1·1.5Tol is the occurrence of nucleophilic addition of n-BuLi to the C═N bond of HL1, leading to the generation of a new [L1a]2- ligand that contains both aminophenol and 1-(2-pyrrolyl)alkylamine scaffolds. The developed protocol can be adapted to a series of organolithium reagents. Compounds [Li8(L1b)4] (2) and [Li8(L1c)4] (3) were afforded by treatment of HL1 with sec-BuLi and LiCH2SiMe3, respectively. Reaction of an analogous ligand HL2 [HL2 = 2-(((1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)methylene)amino)-4-methylphenol] with n-BuLi generated compound [Li8(L2a)4] (4). C═N bond activation was not observed in the reaction of HL1 with NaOtBu, and the complex [Na4(L1)4]·Tol (5·Tol) was obtained. A decanuclear complex [Li10(L3a)2(L3b)2] (6) was also prepared via the reaction of HL3 [HL3 = 2-(2-((((1H-pyrrol-2-yl)methylene)amino)methyl)-1H-pyrrol-1-yl)-N,N-dimethylethan-1-amine] with t-BuLi. A remarkable feature in terms of the synthesis of 6 is the simultaneous occurrence of hydrogen atom abstraction from the C-H bond of the pyrrolyl ring and nucleophilic addition to the C═N bond of the HL3 ligand by t-BuLi. A series of amines containing biologically and physiologically important moieties were achieved by hydrolysis of the crude products from the reactions of the HL1-HL3 ligands and organolithium reagents. This work provides an efficient approach to high-nuclearity lithium compounds as well as a series of amines.

OLED Structure Optimization for Pure and Efficient NIR Electroluminescence of Nd3+ Complexes Bearing Fluorinated 1,3-Diketones

NIR emitting OLEDs (organic light-emitting diodes) with high photoluminescence quantum yields were developed on the basis of fluorinated 1,3-diketonate coordination compounds of the Nd³⁺ ion. Both thermal evaporation and spin-coating techniques were successfully employed for active layer deposition resulting in electroluminescence quantum yields up to 1.38·10^-2%. Blueish-green emission from exciplex and electroplax formations was almost suppressed with the topology optimization of the cell.

Probing Through-Bond and Through-Space Interactions in Singlet Fission-Based Pentacene Dimers

Interchromophoric interactions such as Coulombic coupling and exchange interactions are crucial to the functional properties of numerous π-conjugated systems. Here, we use magnetic circular dichroism (MCD) spectroscopy to investigate interchromophoric interactions in singlet fission relevant pentacene dimers. Using a simple analytical model, we outline a general relationship between the geometry of pentacene dimers and their calculated MCD response. We analyze experimental MCD spectra of different covalently bridged pentacene dimers to reveal how the molecular structure of the "bridge" affects the magnitude of through-space Coulombic and through-bond exchange interactions in the system. Our results show that through-bond interactions are significant in dimers with conjugated molecules as bridging units and these interactions promote the overall electronic coupling in the system. Our generalized approach paves the way for the application of MCD in investigating interchromophoric interactions across a range of π-conjugated systems.

Integrated 3D printing of flexible electroluminescent devices and soft robots

Flexible and stretchable light emitting devices are driving innovation in myriad applications, such as wearable and functional electronics, displays and soft robotics. However, the development of flexible electroluminescent devices via conventional techniques remains laborious and cost-prohibitive. Here, we report a facile and easily-accessible route for fabricating a class of flexible electroluminescent devices and soft robotics via direct ink writing-based 3D printing. 3D printable ion conducting, electroluminescent and insulating dielectric inks were developed, enabling facile and on-demand creation of flexible and stretchable electroluminescent devices with good fidelity. Robust interfacial adhesion with the multilayer electroluminescent devices endowed the 3D printed devices with attractive electroluminescent performance. Integrated our 3D printed electroluminescent devices with a soft quadrupedal robot and sensing units, an artificial camouflage that can instantly self-adapt to the environment by displaying matching color was fabricated, laying an efficient framework for the next generation soft camouflages.

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.

Imidazole Acceptor for Both Vacuum-Processable and Solution-Processable Efficient Blue Thermally Activated Delayed Fluorescence

The members of the imidazole family have been widely used for electron transporting, host, conventional fluorescent, and phosphorescent materials. Although the imidazole core also has great potential as an acceptor segment of deep-blue thermally activated delayed fluorescence (TADF) owing to its high triplet energy, the emission color of imidazole-based TADF organic light-emitting diodes (OLEDs) has so far been limited to blue to green. In this work, four acridan-imidazole systems are theoretically designed aiming for deep- or pure-blue emitters. All four emitters exhibit deep-blue to blue emission owing to the high energy levels of the lowest excited singlet states, exhibiting y coordinates of Commission Internationale de l'Eclairage coordinates between 0.06 and 0.26. The molecule composed of a trifluoromethyl-substituted benzimidazole acceptor in combination with a tetramethyl-9,10-dihydroacridine donor (named MAc-FBI) achieves a high maximum external quantum efficiency (EQE_MAX) of 13.7% in its application to vacuum-processed OLEDs. The emitter has high solubility even in ecofriendly nonhalogenated solvents, which motivates us to fabricate solution-processed MAc-FBI-based OLEDs, resulting in an even higher EQE_MAX of 16.1%.

Indolo[3,2,1-jk]carbazole-Derived Narrowband Violet-Blue Fluorophores: Tuning the Optical and Electroluminescence Properties by Chromophore Juggling

The development of rigid polyaromatic building blocks for narrowband violet fluorophores has received tremendous attention. Herein, we designed and synthesized two new triangle-shaped rigid building blocks, namely, 2,5-di-tert-butylindolo[3,2,1-jk]carbazole (tBuICz) and 2,11-di-tert-butylindolo[3,2,1-jk]carbazole-4-carbonitrile (tBuICzCN), and tethered them with different chromophores to yield a series of violet-blue fluorophores, viz., ICzTPA-ICzPICN, and studied their structure-function relationship. The appended chromophores and cyano unit played a vital role in controlling the optical and electrical properties of the compounds. Except triphenylamine-substituted derivatives, the compounds showed pure violet emission (λ_em ≤ 403 nm). Intriguingly, the compounds exhibited narrow-band emission with a full-width at half-maximum ≤ 40 nm, attributed to the rigidity of the ICz core. The emission of the compounds displayed positive solvatochromism, which is ascribed to the photoinduced intramolecular charge transfer in the excited state. The compounds revealed excellent thermal robustness with T_5d ≥ 363 °C. The triphenylamine-featuring derivatives displayed a high-lying HOMO compared to their congeners due to their electron-rich nature. When we applied these materials in organic light-emitting diodes, ICzPI outperformed in the series with an EQE_max of 3.07% and a current efficiency of 1.04 cd/A. Notably, its CIEy ∼ 0.046 precisely matched with the Rec.2020 standard of deep-blue color (CIEy ∼ 0.046).

Metal- and Solvent-Free Synthesis of Tetrahydrobenzo[c]carbazolones through NaI-Catalyzed Formal [4 + 2] Annulation

A novel strategy for the preparation of functional carbazoles through NaI-catalyzed formal [4 + 2] annulation of 2-(indol-3-yl)cyclohexanones and alkynes/alkenes has been developed. The present approach started from easily available raw materials and provided a variety of tetrahydrobenzo[c]carbazolones in satisfactory yields under metal- and solvent-free conditions. Furthermore, the products could be further transformed into structurally valuable carbazole-based conjugated derivatives.

The Root Causes of the Limited Electroluminescence Stability of Solution-Coated Versus Vacuum-Deposited Small-Molecule OLEDs: A Mini-Review

Using solution-coating methods for the fabrication of organic light-emitting devices (OLEDs) offers a tremendous opportunity for enabling low-cost products and new applications. The electroluminescence (EL) stability of solution-coated (SOL) OLEDs, however, is significantly lower than that of vacuum-deposited (VAC) OLEDs, causing their operational lifetimes to be much shorter—an issue that continues to hamper their commercialization. The root causes of the lower EL stability of these devices remain unclear. This article briefly reviews and summarizes some of the work that has been done to-date for elucidating the root cause of the lower EL stability of SOL OLEDs, giving special attention to studies where side-by-side comparisons of SOL and VAC devices of the same materials have been conducted. Such comparisons allow for more-reliable conclusions about the specific effects of the solution-coating process on device stability to be made. The mini-review is intended to introduce the work done to-date on the causes of lower stability in SOL OLEDs and to stimulate further work for the purpose of closing the existing knowledge gap in this area and surmounting this long-standing challenge in the SOL OLED technology.

Record efficiency of 1000 nm electroluminescence from a solution-processable host-free OLED

New ytterbium complexes K(Solv)_x[Yb(Lⁿ)₂] (Solv = ethanol and/or water) with 2-tosylaminobenzylidene-aryloylhydrazones (H₂L¹, aryloyl = benzoyl; H₂L², aryloyl = 2-naphthoyl) demonstrated high solubility and hole mobility (ca. 2.6 × 10^-6 cm² V^-1 s^-1), while their electron mobility and PLQY were different. The substitution of a benzoyl substituent with naphthoyl resulted in a significant increase of the electron mobility (6.9 × 10^-7vs. 1.7 × 10^-6 cm² V^-1 s^-1) and a decrease of the quantum yield (1.2% vs. 0.6%). As a result, the optimized OLEDs based on the K[Yb(Lⁿ)₂] layer demonstrated efficiencies up to 385 μW W^-1 and 441 μW W^-1, indicating the superior importance of charge mobility over the quantum yield. These are the highest efficiencies of the Yb electroluminescence.

Role of Guest Materials in the Lower Stability of Solution-Coated versus Vacuum-Deposited Phosphorescent OLEDs

Utilizing different phosphorescent materials as emitter guests, this work investigates the root causes of the lower electroluminescence (EL) stability of solution-coated (SOL) organic light-emitting devices (OLEDs) relative to their vacuum-deposited (VAC) counterparts. The results show that emitter guest molecules aggregate under electrical stress, leading to the emergence of new longer-wavelength bands in the EL spectra of the devices over time. However, the intensity of these aggregation emission bands is much stronger in the case of SOL host:guest systems than that of their VAC counterparts, indicating that guest aggregation occurs much faster in the former. The results reveal that the phenomenon arises from differences in the initial morphologies and are likely associated with the use of solvents in the solution-coating process. Moreover, although excitons can drive this aggregation in the case of SOL emissive layer (EML) devices, the coexistence of excitons and polarons accelerates this phenomenon significantly. The results uncover one of the main causes of the lower stability of OLEDs made by solution coating and reveal the importance of adopting new molecular designs that make them less susceptible to aggregation for the development of SOL OLEDs with high performance.

Linked Rotaxane Structure Restricts Local Molecular Motions in Solution to Enhance Fluorescence Properties of Tetraphenylethylene

The restriction of local molecular motions is critical for improving the fluorescence quantum yields (FQYs) and the photostability of fluorescent dyes. Herein, we report a supramolecular approach to enhance the performance of fluorescent dyes by incorporating a linked rotaxane structure with permethylated α-cyclodextrins. Tetraphenylethylene (TPE) derivatives generally exhibit low FQYs in solution due to the molecular motions in the excited state. We show that TPE with linked rotaxane structures on two sides displays up to 15-fold higher FQYs. Detailed investigations with variable temperature ¹ H NMR, UV-Vis, and photoluminescence spectroscopy revealed that the linked rotaxane structure rigidifies the TPE moiety and thus suppresses the local molecular motions and non-radiative decay. Moreover, the linked rotaxane structure enhances the FQY of the dye in various solvents, including aqueous solutions, and improves the photostability through the inhibition of local molecular motions in the excited TPE.

Synthesis and luminescence properties of functionalized V-shaped bis-coumarin derivatives

Three new functionalized V-shaped bis-coumarin derivatives were synthesized for OLED applications.

Rigid Polymer Network-Based Autonomous Photoswitches Working in the Solid State Encoded by Room-Temperature Phosphorescence

Autonomous molecular switches with self-recoverability are of great theoretical and experimental interest since they can avoid additional chemical or energy imposition during the working process. Due to the high energy barrier, however, the solid state is generally unfavorable for materials to exhibit the autonomous switch behavior. To promote the practical usage of the autonomous molecular switch, herein, we propose a prototype of an autonomous photoswitch that can work in the solid state based on a rigid polymer network. A hexacarboxylic sodium-modified hexathiobenzene compound was employed as a photoexcitation-driven unit, which can undergo molecular aggregation upon irradiation because of the distinct conformational difference between the ground state and the photoexcited state. Then, we selected a relatively rigid polymer named poly(dimethyldiallylammonium)chloride (PDDA) to complex with the hexacarboxylic sodium-modified hexathiobenzene through electrostatic coupling. Through optimization, the photoexcitation-controlled molecular aggregation and its self-recovery can work well in the solid matrix of PDDA under rhythmical photoirradiation. This process can be easily encoded by a self-recoverable room-temperature phosphorescence, featuring an excellent performance of the autonomous switch.

Asymmetrical-Dendronized TADF Emitters for Efficient Non-doped Solution-Processed OLEDs by Eliminating Degenerate Excited States and Creating Solely Thermal Equilibrium Routes

The mechanism of thermally activated delayed fluorescence (TADF) in dendrimers is not clear. We report that fully‐conjugated or fully‐nonconjugated structures cause unwanted degenerate excited states due to multiple identical dendrons, which limit their TADF efficiency. We have synthesized asymmetrical “half‐dendronized” and “half‐dendronized‐half‐encapsulated” emitters. By eliminating degenerate excited states, the triplet locally excited state is ≥0.3 eV above the lowest triplet charge‐transfer state, assuring a solely thermal equilibrium route for an effective spin‐flip process. The isolated encapsulating tricarbazole unit can protect the TADF unit, reducing nonradiative decay and enhancing TADF performance. Non‐doped solution‐processed devices reach a high external quantum efficiency (EQE_max) of 24.0 % (65.9 cd A⁻¹, 59.2 lm W⁻¹) with CIE coordinates of (0.24, 0.45) with a low efficiency roll‐off and EQEs of 23.6 % and 21.3 % at 100 and 500 cd m⁻².

Beads on a Chain Fluorescent Oligomeric Materials: Interactions of Conjugated Organic Cross-Linkers with Silsesquioxane Cages

Organic electronic materials have advantages over inorganics in terms of versatility, cost, and processability. Recent advancements in organic materials for light-emitting diodes (OLED), field effect transistors (OFET), and photovoltaics have engendered extensive innovation potential on this field. In this research, we focus on synthesizing SQ (silsesquioxane) based oligomers cross-linked by dibromo-aromatic linkers and explore how the cross-linker influences their photophysical properties. Bis-trialkoxy silyl (linker) model compounds were synthesized to compare noncage photophysical properties with the oligomers. Several techniques such as UV/vis, fluorescence, FTIR, and thermal gravimetric analysis (TGA) have been used to characterize the systems. Time-resolved fluorescence and femtosecond transient absorption spectroscopy were used to understand the excited state dynamics of these materials. Studies were carried out to understand the differences between monomers and oligomers and potential energy transfer and charge transfer between the cages and cross-linking chromophores. Transient absorption showed lower energy absorption from the excited states, suggesting short-range communication between moieties. Single photon counting studies have shown distinct lifetime differences between most linkers and cages display possible excitation energy transfer through these materials. Transient absorption anisotropy measurements have shown signatures for excitation energy transfer between linker chromophores for oligomeric compounds. The silsesquioxane (SQ) backbone of the oligomers gives substantial thermal stability as well as solution processability, giving better flexibility for achieving energy transfer between linking chromophores.

Leveraging Fleeting Strained Intermediates to Access Complex Scaffolds

Arynes, strained cyclic alkynes, and strained cyclic allenes were validated as plausible intermediates in the 1950s and 1960s. Despite initially being considered mere scientific curiosities, these transient and highly reactive species have now become valuable synthetic building blocks. This Perspective highlights recent advances in the field that have allowed access to structural and stereochemical complexity, including recent breakthroughs in asymmetric catalysis.

Molecular Engineering for the Development of a Discotic Nematic Mesophase and Solid-State Emitter in Deep-Blue OLEDs

A unique strategy for the attainment of a discotic nematic (N_D) mesophase is reported consisting of a central benzene core to which are attached two 4-alkylphenyl and two 4-pentylbiphenyl moieties diagonally via alkynyl linkers. The rotational nature and incompatibility of unequal phenylethynyl units led to the disruption of π-π interactions within cores that aids to the realization of N_D phase and favors high solid-state emission. When used in OLEDs, compounds act as an efficient solid-state pure deep-blue emitter with Commission Internationale de L'Eclairage (CIE_x,y) coordinates of (0.16, 0.07).

Effects of MEH-PPV Molecular Ordering in the Emitting Layer on the Luminescence Efficiency of Organic Light-Emitting Diodes

We investigated the effects of molecular ordering on the electro-optical characteristics of organic light-emitting diodes (OLEDs) with an emission layer (EML) of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The EML was fabricated by a solution process which can make molecules ordered. The performance of the OLED devices with the molecular ordering method was compared to that obtained through fabrication by a conventional spin coating method. The turn-on voltage and the luminance of the conventional OLEDs were 5 V and 34.75 cd/m², whereas those of the proposed OLEDs were 4.5 V and 120.3 cd/m², respectively. The underlying mechanism of the higher efficiency with ordered molecules was observed by analyzing the properties of the EML layer using AFM, SE, XRD, and an LCR meter. We confirmed that the electrical properties of the organic thin film can be improved by controlling the molecular ordering of the EML, which plays an important role in the electrical characteristics of the OLED.

New Electroactive Polymers with Electronically Isolated 4,7-Diarylfluorene Chromophores as Positive Charge Transporting Layer Materials for OLEDs

A group of polyethers containing electroactive pendent 4,7-diarylfluorene chromophores have been prepared by the multi-step synthetic route. Full characterization of their structures has been presented. The polymeric materials represent derivatives of high thermal stability with initial thermal degradation temperatures in a range of 392-397 °C. Glass transition temperatures of the amorphous polymers range from 28 °C to 63 °C and depend on structures of the 4,7-diarylfluorene chromophores. Electron photoemission spectra of thin layers of the electroactive derivatives showed ionization potentials in the range of 5.8-6.0 eV. Hole injecting/transporting properties of the prepared polymeric materials were confirmed during formation of organic light-emitting diodes with tris(quinolin-8-olato)aluminium (Alq₃) as a green emitter, which also serves as an electron transporting layer. The device using hole-transporting polymer with electronically isolated 2,7-di(4-biphenyl)fluorene chromophores demonstrated the best overall performance with low turn on voltage of 3 V, high current efficiency exceeding 1.7 cd/A, and with maximum brightness over 200 cd/m². The organic light-emitting diode (OLED) characteristics were measured in non-optimized test devices. The efficiencies could be further improved by an optimization of device structure, formation conditions, and encapsulation of the devices.

An indenocarbazole-based host material for solution processable green phosphorescent organic light emitting diodes

We designed and synthesized a new host material with a highly soluble and thermally stable indenocarbazole derivative that can make green phosphorescent organic light-emitting diodes (PHOLEDs) in a solution process.

Effect of different imidazole derived moieties on the photo- and electro-luminescence properties of 2,7,12-trisubstituted triazatruxene derivatives

Three 2,7,12-trisubstituted triazatruxene derivatives containing different imidazole derived moieties were synthesized for OLED applications.

Synthetic Bilayers on Mica from Self-Assembly of Hydrogen-Bonded Triazines

This study describes organic thin films prepared under a range of conditions from a model series of bis-N-alkyl chloro-triazines functionalized with short alkyl chains from ethyl to hexyl. The pure films were characterized using atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). When cast on mica, these compounds assemble as crystalline sheets made up of a synthetic bilayer along the crystallographic ab-plane with an internal hydrogen-bonded domain between external alkyl chains. These micron-scale surfaces stack along the c-axis, and increasing the alkyl chain length results in changes to the crystal morphology from needles to nanoscale plates. Thicker films produce nanoscale, pyramidal stacks of bilayers. Compared to atomically flat mica, a rougher, unetched silicon substrate produced irregular domains in the secondary bilayer. Films of mixtures comprising the ethyl derivative with butyl, pentyl, or hexyl derivative were imaged using time-of-flight secondary-ion mass spectrometry (ToF-SIMS) that indicated a trend toward a constant stoichiometry with increasing alkyl chain length. AFM of mixed films on mica showed single bilayers of height <2 nm, with an acceptable correlation to the XRD measurements, supporting a constant stoichiometry. These materials permit easy modification of mica to a micron-scale, atomically flat hydrophobic surface, and the use of mixtures with different alkyl chain lengths suggests a method to improve the quality of functional organic thin films.

Sustainable Access to π-Conjugated Molecular Materials via Direct (Hetero)Arylation Reactions in Water and under Air

Direct (hetero)arylation (DHA) is playing a key role in improving the efficiency and atom economy of C-C cross coupling reactions, so has impacts in pharmaceutical and materials chemistry. Current research focuses on further improving the generality, efficiency and selectivity of the method through careful tuning of the reaction conditions and the catalytic system. Comparatively fewer studies are dedicated to the replacement of the high-boiling-point organic solvents dominating the field and affecting the overall sustainability of the method. We show herein that the use of a 9:1 v/v emulsion of an aqueous Kolliphor 2 wt% solution while having toluene as the reaction medium enables the preparation of relevant examples of thiophene-containing π-conjugated building blocks in high yield and purity.

Optimizing small conjugated molecules for solar-cell applications using an inverse-design method

Small organic conjugated molecules are key elements for low-cost photovoltaic devices. One example is cyanopyridone molecules. By modifying these molecules, for instance through optimally chosen functional groups attached to the backbone, their properties can be improved. However, the very large number of possible modifications makes it difficult to identify the best performing molecules. In the present work, we have used a computational inverse-design approach (PooMa) to identify the positions and types of functional groups attached to a modified cyanopyridone that lead to the best performance in solar-energy harvesting. A QSPR model based on five electronic descriptors has been used to determine the properties of solar cells. Our approach uses a genetic algorithm to search the chemical space containing 18⁴ (104,976) substituted cyanopyridone systems and predicts out of those the best 20 molecules with optimal performance efficiencies (PCE). PooMa uses the Density-Functional Tight-Binding (DFTB) method for calculating the electronic properties. DFTB is a fast method with acceptable accuracy and, therefore, can be used on a normal desktop without expensive hard- or software. In order to get further information about our suggested systems, a DFT method and its derivative TD-DFT are applied.

Molecular Stacking Effect on Small-Molecular Organic Light-Emitting Diodes Prepared with Solution Process

The light-emitting layer (EML) is generally prepared by mixing the host and dopant to realize an organic light-emitting diode (OLED). However, phase separation is often observed during the fabrication process to prepare OLEDs, depending on the structure of the host materials. In particular, phase separation because of π-π stacking is frequently observed during thermal annealing for the solution process. The annealing process is required for solvent removal and complete relaxation of the molecule. Hence, the materials with a high glass transition temperature (T_g) are ideal because phase separation occurs because of π-π stacking during the annealing process, if T_g is too low. To understand this phenomenon, we compared two host materials with similar molecular weights but different three-dimensional connectivity, which causes different rotational freedom. Then, we investigated the effect on the device properties, depending on the annealing conditions. In both materials, when the annealing temperature rises above 120 °C, the dopant completely escaped from the EML. However, the material that does not disturb the molecular stacking order by annealing because of its limited free rotation through the internal bond shows much better device characteristics even after annealing at a higher temperature than T_g. The results show that interdiffusion at the interface and unstable internal density distribution with annealing temperature are responsible for the device degradation behavior.

Various Coating Methodologies of WO3 According to the Purpose for Electrochromic Devices

Solution-processable electrochromic (EC) materials have been investigated widely for various applications, such as smart windows, reflective displays, and sensors. Among them, tungsten trioxide (WO₃) is an attractive material because it can form a film via a solution process and relative low temperature treatment, which is suitable for a range of substrates. This paper introduces the slot-die and electrostatic force-assisted dispensing (EFAD) printing for solution-processable methods of WO₃ film fabrication. The resulting films were compared with WO₃ films prepared by spin coating. Both films exhibited a similar morphology and crystalline structure. Furthermore, three different processed WO₃ film-based electrochromic devices (ECDs) were prepared and exhibited similar device behaviors. In addition, large area (100 cm²) and patterned ECDs were fabricated using slot-die and EFAD printing. Consequently, slot-die and EFAD printing can be used to commercialize WO₃ based-ECDs applications, such as smart windows and reflective displays.

Emerging Self-Emissive Technologies for Flexible Displays

Featuring a combination of ultrathin and lightweight properties, excellent mechanical flexibility, low power-consumption, and widely tunable saturated emission, flexible displays have opened up a new possibility for optoelectronics. The demands for flexible displays are growing on a continual basis due not only to their successful commercialization but, more importantly, their endless possibilities for wearable integrated systems. Up to now, self-emissive technologies for displays, flexible active-matrix organic light-emitting diodes (flex-AMOLED), flexible quantum dot light-emitting diodes (flex-QLEDs), and flexible perovskite light-emitting diodes (flex-PeLEDs) have been widely reported, but despite the significant progress made in these technologies, enormous obstacles and challenges remain for the vision of truly wearable applications, in particular with flex-QLEDs and flex-PeLEDs. Here, a review of the recent progress of all three self-emissive technologies for flexible displays is conducted, including the emissive active materials, device structures and approaches to manufacturing, the flexible substrates, and conductive electrodes, as well as the encapsulation techniques. The fast-paced improvement made to the efficiency of flexible devices in recent years is also summarized. The review concludes by making suggestions on the future development in this area, and is expected to help researchers in gaining a comprehensive understanding about the newly emerging technologies for flexible displays.

Improving the Electroluminescent Performance of Blue Light-Emitting Polymers by Side-Chain Modification

Blue light-emitting polymers are in urgent demand for new-generation display and solid-state lighting devices fabricated through low-cost wet processing. However, their current performances are far from satisfactory. Here, we developed a series of poly(fluorene-co-dibenzothiophene-S,S-dioxides) (PFSOs) bearing different alkyl chains, alkoxyphenyl chains, or both alkylaryl and alkoxyphenyl side chains. The introduction of alkoxyphenyl groups moderately enhanced the electron-donating ability of the polymers, leading to more balanced charge carrier fluxes. Meanwhile, asymmetric bulky side chains enabled more pronounced variation of molecular conformation while restraining the intermolecular aggregation of polymers, resulting in a lower refractive index, thus facilitating light extraction compared with polymers based on the same two alkyl or alkoxyphenyl side chains. Polymer light-emitting devices based on PFSO-BMD with asymmetric side chains exhibited a maximum luminous efficiency of 8.58 cd A^-1, associated with pure blue Commission Internationale de l'Eclairage coordinates of (0.14, 0.14). These findings demonstrated that side-chain modification can be an effective strategy for developing efficient solution-processable blue light-emitting polymers.

Rational Design of Carbazole- and Carboline-Based Polymeric Host Materials for Realizing High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diode

Recently, various host materials have been developed for solution-processed thermally activated delayed fluorescent organic light-emitting diodes (TADF-OLEDs). Compared with small-molecule hosts, polymeric hosts are advantageous for inducing a uniform distribution and segregation of dopant molecules in the emissive layer without undesired large-scale phase separation. In this study, new polymer hosts were demonstrated, in which a bipolar conjugative moiety consisting of a carbazole (Cz) donor and an α-carboline (α-Cb) acceptor was bound to the polystyrene backbone through a non-conjugated linker. They exhibited high triplet energies of >2.8 eV, and their emission spectra overlapped with the absorption spectrum of a green TADF emitter, which allowed facile energy transfer from the polymeric host to the small-molecule dopants. High device performance was observed, with external quantum efficiencies (EQEs) of 13.65, 17.09, and 17.48% for solution-processed green TADF-OLEDs using PSCzCz, PSCzCb, and PSCbCz, respectively, as hosts for the EML. The EQEs of bipolar host (PSCzCb and PSCbCz)-based devices were higher than those of unipolar host (poly(N-vinylcarbazole) and PSCzCz)-based devices owing to the well-balanced charge-carrier transport. According to these results, the polymeric host bearing a bipolar Cz and α-Cb coupled moiety is a promising material for solution-processable TADF-OLEDs.

Color-Tunable Thermally Activated Delayed Fluorescence in Oxadiazole-Based Acrylic Copolymers: Photophysical Properties and Applications in Ratiometric Oxygen Sensing

Polymer-based emitters are a promising route to the production of low-cost, scalable solution-processable luminescent materials. Here we describe a series of acrylic oxadiazole-based donor-acceptor monomers with tunable emission from blue to orange, with quantum yields as high as 96%. By introducing structural constraints that limit donor-acceptor orbital overlap, thermally activated delayed fluorescence (TADF) was observed in these materials. Polymerization by Cu(0) reversible deactivation radical polymerization (RDRP) gave high-molecular-weight copolymers (M_n > 20 kDa) with dispersities ranging from 1.10 to 1.45, using a room-temperature procedure with Cu wire as a catalyst. One of these materials, which had phenothiazine as donor moiety, exhibited conformationally dependent dual emission, giving a mixture of prompt fluorescence and delayed fluorescence peaks, whose relative ratios varied based on the amount of O₂ present during measurement. We demonstrate that this material can combine prompt and delayed fluorescence to act as a single-component, all-organic, ratiometric oxygen sensor without external calibrant. Application to ratiometric oxygen sensing is demonstrated both using a polymer thin film and via incorporation of this material into water-soluble polymer dots (Pdots), with a ratiometric response to O₂ throughout the range of partial pressures relevant to biological environments.

All-Solution-Processed Red and Orange-Red Organic Light-Emitting Diodes with High-Efficiencies: The Effect of Mixed-Host Emissive Layers and Thermal Annealing Treatment

The instability of the organic light-emitting diodes (OLEDs) during operation can be attributed to the existence of point defects on the organic layers. In this work, the effect of mixed-host emissive layer and the thermal annealing treatment were investigated to eliminate defects and to boost the device performance. The mixed-host system includes 4,4',4''-tri (9-carbazoyl) triphenylamine (TCTA) and 2,7-bis(diphenylphosphoryl)-9, 9'-spirobi[fluorene] (SPPO13). The mixed-host emissive layer with thermal annealing treatment showed low roughness and few pinholes, and the devices fabricated from this emissive layer exhibited high efficiencies, high stabilities, and long lifetimes. The red and orange-red OLEDs exhibited efficiencies of 13.9 cd/A and 24.35 cd/A, respectively. The longest half-lifetime (L₀ =500 cd/m² ) of the red and orange-red OLEDs were 158 h and 180 h, respectively. Efforts were made to solve problems in large-area coating and to reduce the number of defects on in organic layer. Large-active-area (active area=3 cm×4 cm) red phosphorescent OLEDs (PhOLEDs) devices were realized with very high current efficiency up to 9 cd/A.