Regulating the Nature of Triplet Excited States of Thermally Activated Delayed Fluorescence Emitters.

ConspectusCharacterized by the reverse intersystem crossing (RISC) process from the triplet state (T1) to the singlet state (S1), thermally activated delayed fluorescence (TADF) emitters, which produce light by harvesting both triplet and singlet excitons without noble metals, are considered to be third-generation organic electroluminescent materials. Rapid advances in molecular design criteria, understanding the photophysics underlying TADF, and applications of TADF materials as emitters in organic light-emitting diodes (OLEDs) have been achieved. Theoretically, enhanced spin-orbit coupling (SOC) between singlet and triplet states can result in a fast RISC process and thus a high light-emitting efficiency according to Fermi's golden rule. Therefore, regulating the nature of triplet excited states by elaborate molecular design to improve SOC is an effective approach to high-efficiency TADF-based OLEDs. Generally, on one hand, the increased local excited (LE) populations of the excited triplet state can significantly improve the nature flips between S1 and T1. On other hand, the reduced energy gap between S1 and the lowest triplet with a charge transfer (CT) characteristic can also enhance their vibronic coupling. Consequently, it is vital to determine how to regulate the nature of triplet excited states by molecular design to guide the material synthesis, especially for polymeric emitters.In this Account, we focus on modulating the strategy of triplet excited states for TADF emitters and an in-depth understanding of the photophysical processes, leading to optimized OLED device performance. We include several kinds of strategies to control the nature of triplet excited states to guide the synthesis of small-molecule and polymer TADF emitters: (1) Modulating the electronic distribution of conjugated polymeric backbones by copolymerizing the electron-donating host: accordingly, the nature of excited states can be changed, especially for triplets. Meanwhile, the utilization of excitons can be systematically improved by adjusting the electronic structure of triplet states with long-range distribution in the conjugated polymeric backbones. (2) Halogenating acceptors of TADF units: the introduced halogen atoms would reestablish the electronic distribution of the triplet and relocate the hole orbits, resulting in a CT and LE hybrid nature of a triplet transformed into a LE-predominant state, which favors the RISC process. (3) Stereostructure regulation: by constructing a diverse arrangement of three-dimensional spatial configurations or conjugated architectures, the nature of the triplet can also be finely tuned, such as hyperbranched structures with multiple triplet-singlet vibration couplings, half-dendronized-half-encapsulated asymmetric systems, trinaphtho[3,3,3] propeller-based three-dimensional spatial interspersed structures, intramolecular close-packed donor-acceptor systems, and so on. We hope that this Account will provide insights into new structures and mechanisms for achieving high-performance OLEDs based on regulating the nature of triplet excited states.

High Mobility Emissive Excimer Organic Semiconductor Towards Color-Tunable Light-Emitting Transistors.

Organic light-emitting transistors (OLETs) are highly integrated and minimized optoelectronic devices with significant potential superiority in smart displays and optical communications. To realize these various applications, it is urgently needed for color-tunable emission in OLETs, but remains a great challenge as a result of the difficulty for designing organic semiconductors simultaneously integrating high carrier mobility, strong solid-state emission, and the ability for potential tunable colors. Herein, a high mobility emissive excimer organic semiconductor, 2,7-di(2-anthryl)-9H-fluorene (2,7-DAF) was reasonably designed by introducing a rotatable carbon-carbon single bond connecting two anthracene groups at the 2,7-sites of fluorene, and the small torsion angles simultaneously guarantee effective conjugation and suppress fluorescence quenching. Indeed, the unique stable dimer arrangement and herringbone packing mode of 2,7-DAF single crystal enables its superior integrated optoelectronic properties with high carrier mobility of 2.16 cm2 ⋅ V-1 ⋅ s-1 , and strong excimer emission with absolute photoluminescence quantum yield (PLQY) of 47.4 %. Furthermore, the voltage-dependent electrically induced color-tunable emission from orange to blue was also demonstrated for an individual 2,7-DAF single crystal based OLETs for the first time. This work opens the door for a new class of high mobility emissive excimer organic semiconductors, and provides a good platform for the study of color-tunable OLETs.

Synthesis and Properties of Bipolar Ladder-Like Polysiloxane with Carbazole and Triphenylphosphine Oxygen Groups.

In this study, a series of ladder-like polysiloxanes are synthesized by introducing double-chain Si-O-Si polymer as the backbone and the carbazole and triphenylphosphine oxide with high triplet energy as side groups. The ladder-like structures of polysiloxanes are achieved through a controlled polymerization method that involves the monomer self-assembly and subsequent surface-restricted solid-phase in situ condensation through freeze-drying. The introduction of siloxane improves thermal stability of the polymers and inhibits the conjugation of the polymers between the side groups, leading to an increase in the triplet energy level. Therefore, all these polymers perform higher triplet energy levels than phosphorescent emitter (FIrpic). The cyclic voltammetry measurements demonstrate that the bipolar polymer exhibits a high highest occupied molecular orbital (HOMO) value of -5.32 eV, which is consistent with the work function of ITO/PEDOT:PSS, consequently facilitating hole injection. Furthermore, the incorporation of triphenylphosphine oxide promotes electron injection. Molecular simulations reveal that the frontier orbital distributions of the bipolar polymer are located on the carbazole and triphenylphosphine groups, respectively, which facilitate the transport of electrons and holes.

Design, Synthesis and Second-Order Nonlinear Optical Properties of Azobenzene-Based Polysiloxanes.

To study the effect of polymeric structures on second-order nonlinear optical properties, polysiloxanes materials based on azobenzene as chromophore have been designed and synthesized successfully. Herein, the siloxane monomer is directly bonded to azobenzene units by palladium catalysis, which avoids the influence of flexible chains on the photoelectric properties of azobenzene. According to the different positions of azobenzene units in the polymers, it is divided into side-chain, main-chain, and alternative-type polymers. The chemical structures of obtained polysiloxanes are confirmed by nuclear magnetic resonance spectra and mass spectra. Three polymers present high thermal decomposition temperatures and the medium glass transition temperatures. The effects of polymeric structures on the second-order nonlinear properties are compared. The main-chain polysiloxane possesses the highest thermal stability because of its rigid architecture. The side-chain polysiloxane shows the fastest isomerization transformation rate due to the large free volume. Besides, the alternative polysiloxane displays the best second-order nonlinear performance with second harmonic generation coefficient (d33 ) value of 47.6 pm V-1 , which is 3 times higher than the side-chain one.

Correction: Synthesis and properties of siloxane modified perylene bisimide discotic liquid crystals.

Correction for 'Synthesis and properties of siloxane modified perylene bisimide discotic liquid crystals' by Tingjie Zhang et al., Soft Matter, 2013, 9, 10739-10745, https://doi.org/10.1039/C3SM52054D.

Combining Polymer Zwitterions and Zinc Oxide for High-Performance Inverted Organic Solar Cells.

Zinc oxide (ZnO) is a widely used cathode interlayer material in inverted organic solar cells (OSCs). However, there are lots of surfaces or bulk film defects in ZnO layers, which degrade solar cell performance. Here, the typical phosphorylcholine- and sulfobetaine-based polymer zwitterions (PMPC and PDMAPS) are synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization to modify ZnO interlayers for inverted OSCs. The polymer zwitterions can efficiently passivate the defects in ZnO films and thus increase the conductivity of the ZnO interlayers. Both PMPC and PDMAPS modified ZnO interlayers show some general advantages in improving the performance of fullerene-based and non-fullerene-based OSCs. The highest efficiency of 16.69% is achieved by using PMPC modified ZnO interlayers in PM6:Y6 based solar cell devices, which is among the best performance in inverted OSCs. Such an improvement in device performance is attributed to the work function reduction of the polymer zwitterions modified ZnO films, which provides an efficient cathode platform to extract and transport electrons from the active layers, to the benefit of suppressing interfacial charge recombination. As a result, the organic-inorganic hybrid composites (ZnO: polymer zwitterions) show efficient interfacial modification to align energy levels at the device interface, which have promising application prospects in organic electronics.

Thermally Activated Delayed Fluorescence Polysiloxanes with Short Delay Fluorescence Lifetimes.

Blue-emitting thermally activated delayed fluorescence (TADF) polymers are still in demand for high-efficiency display materials. Through-space charge transfer (TSCT) strategy is promising for keeping color purity of blue-emitting polymers with nonconjugated main chains. It is, however, hard to synthesize copolymers with well-dispersed donors or acceptors utilizing traditional polyethylene backbones via radical polymerization. Herein, two series of blue-emitting polysiloxane with TADF properties, random and order-controlled copolysiloxanes, are successfully designed and synthesized and their photophysical properties are investigated and compared in detail. All of them display short prompt and delay fluorescence lifetimes and a very fast reverse intersystem crossing (RISC) rate of 107 s-1 . Compared with random copolysiloxanes, acceptors are well separated by donors for order-controlled copolysiloxanes, which exhibit the faster RISC processes and the higher photoluminescence quantum yield. Therefore, the order-controlled architecture provides a guide for improving light-emitting efficiency of TSCT-type TADF polymers.

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 (EQEmax ) 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 .

Tuning Intramolecular Stacking of Rigid Heteroaromatic Compounds for High-Efficiency Deep-Blue Through-Space Charge-Transfer Emission.

A series of ultrapure-blue thermally activated delayed fluorescence (TADF) emitters featuring through-space charge transfer (TSCT) have been constructed by close stacking between the donor and acceptor moieties in rigid heteroaromatic compounds. The obviously accelerated radiative transition of singlet excitons, the diminished vibrionic relaxation of ground and excited states, and the consequent reduced Stokes shift and the narrow emission are evident. The corresponding organic light-emitting diodes (OLEDs) based on AC-BO realize the best performance among all deep-blue TSCT-TADF emitters, with an external quantum efficiency (EQEmax ) of 19.3 %. Furthermore, the OLEDs based on QAC-BO display an EQEmax of 15.8 %, and achieve the first high-efficiency ultrapure-blue TSCT-TADF material with an excellent Commission Internationale de L'Eclairage coordinate (CIE) of (0.145, 0.076) which perfectly matches the ultrapure-blue CIE requirements (0.14, 0.08) defined by the National Television System Committee.

AlGaN-based deep ultraviolet micro-LED emitting at 275 nm.

The investigation of electrical and optical properties of micro-scale AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) emitting at ∼275nm was carried out, with an emphasis on fabricated devices having a diameter of 300, 200, 100, 50, and 20 µm, respectively. It was revealed that the LED chips with smaller mesa areas deliver considerably higher light output power density; meanwhile, they can sustain a higher current density, which is mainly attributed to the enhanced current spreading uniformity in micro-scale chips. Importantly, when the diameter of LED chips decreases from 300 µm to 20 µm, the peak external quantum efficiency (EQE) increases by 20%, and the EQE peak current density can be boosted from 8.85A/cm2 and 99.52A/cm2. Moreover, we observed a longer wavelength emission with enlarged full-width at half-maximum (FWHM) in the LEDs with smaller chip sizes because of the self-heating effect at high current injection. These experimental observations provide insights into the design and fabrication of high-efficiency micro-LEDs emitting in the DUV regime with different device geometries for various future applications.

Photoluminescent Behaviors of Thermally Activated Delayed Fluorescence Polymeric Emitters in Nanofibers.

Anisotropic 1D nanostructures with high surface-area-to-volume ratio display the enhanced optoelectronic properties of light-emitting compounds compared to bulk or 2D systems. To study the effect of nanometer-constrained space on photoluminescent behavior of thermally activated delayed fluorescence (TADF) polymeric emitters, electrospinning technique is used to produce nanofibers of TADF emitters. Herein, two TADF polymer (P1 and P3) nanofibers with 90% polyacrylonitrile (PAN) are fabricated and their photophysical properties are studied and compared with their spin-coated film counterparts. The distinguishing polarized photoluminescencent properties of P1/PAN or P3/PAN electrospun nanofibers are obtained due to high orientation degree and superior molecular arrangement. Moreover, the better TADF properties in nanofibers can be observed comparing with their spin-coated films, including longer-lived excited states, higher photoluminescence quantum efficiency, lower internal conversion decay rate, and higher reverse intersystem crossing rate constant.

Advantages of AlGaN-based deep-ultraviolet light-emitting diodes with an Al-composition graded quantum barrier.

AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) still suffer from poor quantum efficiency and low optical power. In this work, we proposed a DUV LED structure that includes five unique AlxGa1-xN quantum barriers (QBs); Each QB has a linear-increment of Al composition by 0.03 along the growth direction, unlike those commonly used flat QBs in conventional LEDs. As a result, the electron and hole concentration in the active region was considerably increased, attributing to the success of the electron blocking effect and enhanced hole injection efficiency. Importantly, the optical power was remarkably improved by 65.83% at the injection current of 60 mA. After in-depth device optimization, we found that a relatively thinner graded QB layer could further boost the LED performance because of the increased carrier concentrations and enhanced electron and hole wave function overlap in the QW, triggering a much higher radiative recombination efficiency. Hence, the proposed graded QBs, which have a continuous increment of Al composition along the growth direction, provide us with an effective solution to boost light output power in the pursuit of high-performance DUV emitters.

Synthesis and Cyclization-Induced Charge Transfer of Rectangular Bisterthiophenesiloxanes.

Cyclization-modified terthiophene displays the change of emission behavior from locally excited (LE) to the intramolecular charge transfer (ICT) state. The rectangular bisterthiophenesiloxanes (DSiTh) was successfully prepared by π-π-stacking-aided hydrogen-bonding interactions. Cyclization-induced ICT in DSiTh could be observed, which was confirmed by absorption spectra, fluorescence spectra, and quantum chemistry analysis. The cyclization produces a strong intramolecular electron redistribution of a highly packed π-conjugated terthiophene. Thus, a distinctive variation of the dipole moment and a through-space ICT happen.

Band Spacing in Poly(3-hydroxybutyrate) and Its Blends with Poly(propylene carbonate): Dependence on Thermal Processing.

The band spherulites grown in neat poly(3-hydroxybutyrate) (PHB) and its blends with poly(propylene carbonate) (PPC) were observed by polarized optical microscopy. For the spherulites in neat PHB, it is evident that the band spacing increases first and then decreases with melting time. As the melting time is within 7 min, the band spacing increases continuously, which should be attributed to increasing mobility of polymer chains or decreasing viscosity of the melt. When the melting time is prolonged, evident thermal degradation of PHB occurs and results in a great deal of noncrystalline fractions, which is similar with addition of miscible amorphous polymers in the melt, and the band spacing decreases accordingly. The thermal degradation of PHB cannot, however, be detected by a thermogravimetric analyzer because of less volatile productions. An evident decrease of molecular weight can be measured by gel permeation chromatography, indicating occurrence of serious degradation. The decrease of crystallization and melting temperature revealed by differential scanning calorimetry (DSC) also prove the thermal degradation. For spherulites in PHB/PPC blends, however, the variation of band spacing differs from that in neat PHB. The band spacing increases continuously when melting time is within 15 min. The crystallization and melting behaviors are not influenced greatly by prolonging melting time in PHB/PPC blends. The variations of Mw for PHB/PPC are slighter than those of the neat PHB and PPC upon heating at 190 °C. Combined with the corresponding DSC results, it is conjectured that blending may prohibit the degradation of PHB to some extent. An intermolecular interaction can be detected between PHB and PPC via Fouriertransform infrared spectra and should help to avoid degradation of PHB to a certain degree. The present results may help widen the applications of PHB and shed some light on understanding the formation mechanism of the band for aliphatic polyester polymers.

Highly Anisotropic P3HT Film Fabricated via Epitaxy on an Oriented Polyethylene Film and Solvent Vapor Treatment.

To improve the epitaxial crystallization ability of poly(3-hexylthiophene) (P3HT) on a highly oriented polyethylene (PE) substrate, controlled solvent vapor treatment (CSVT) is employed. The anisotropic structures and related optical properties depend not only on the solvent used to prepare the film but also on the subsequent solvent vapor treatment pressure and time. A highly oriented PE film facilitates the "side-on" chain orientation of P3HT with its c axis parallel to the drawing direction of the PE film. The dichroic ratio (DR) of the P3HT film reflected by UV-vis spectra can reach as high as 7.1, which is much larger than the value treated by thermal annealing. Moreover, the excitation bandwidth W, indicating the effective conjugation length and molecular order, shows significant anisotropic features. Solvent used for solution processing with a high boiling point is more favorable for inducing anisotropic multiscale structures. In particular, the oriented structures lead to obvious anisotropic carrier mobility. The carrier mobility of P3HT after CSVT along the PE molecular chain direction is 7.5 times higher than that measured perpendicular to the PE chain direction. This is of great importance in fabricating anisotropic thin films of conjugated polymeric semiconductors with enhanced performance.

Impact of Methoxy Substituents on Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence in All-Organic Donor-Acceptor Systems.

Thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) are known to occur in organic D-A-D and D-A systems where the donor group contains the phenothiazine unit and the acceptor is dibenzothiophene- S, S-dioxide. This study reports the synthesis and characterization of one new D-A and four new D-A-D systems with methoxy groups on the phenothiazine to examine their effect on emission properties in the zeonex matrix. X-ray analysis and highly specialized NMR techniques were used to characterize asymmetric methoxy-substituted derivative 3b, which is chiral at N because of an extremely high flipping barrier at the phenothiazine N atom. Based on hybrid-density functional theory computations, the methoxy substituents tune the relative stabilities of the axial conformers with respect to equatorial conformers of the phenothiazine units, depending on their substitution position. This conformational effect significantly influences both TADF and RTP contributions compared to the parent D-A-D system. It is also demonstrated that the equatorial forms of D-A-D and D-A systems in zeonex exhibit TADF. Additionally, the methoxy groups promote luminescence in D-A-D systems where only axial conformers exist. This work reveals further design opportunities for more efficient TADF and RTP molecules.

In Vivo Glycan Engineering via the Mannosidase I Inhibitor (Kifunensine) Improves Efficacy of Rituximab Manufactured in Nicotiana benthamiana Plants.

N-glycosylation has been shown to affect the pharmacokinetic properties of several classes of biologics, including monoclonal antibodies, blood factors, and lysosomal enzymes. In the last two decades, N-glycan engineering has been employed to achieve a N-glycosylation profile that is either more consistent or aligned with a specific improved activity (i.e., effector function or serum half-life). In particular, attention has focused on engineering processes in vivo or in vitro to alter the structure of the N-glycosylation of the Fc region of anti-cancer monoclonal antibodies in order to increase antibody-dependent cell-mediated cytotoxicity (ADCC). Here, we applied the mannosidase I inhibitor kifunensine to the Nicotiana benthamiana transient expression platform to produce an afucosylated anti-CD20 antibody (rituximab). We determined the optimal concentration of kifunensine used in the infiltration solution, 0.375 µM, which was sufficient to produce exclusively oligomannose glycoforms, at a concentration 14 times lower than previously published levels. The resulting afucosylated rituximab revealed a 14-fold increase in ADCC activity targeting the lymphoma cell line Wil2-S when compared with rituximab produced in the absence of kifunensine. When applied to the cost-effective and scalable N. benthamiana transient expression platform, the use of kifunensine allows simple in-process glycan engineering without the need for transgenic hosts.

Precisely Automatic Time Window Locating for an Interferometric Fiber-Optic Sensor Array Based on a TDM Scheme.

Interferometric fiber-optic sensors are often organized in the form of large-scale arrays by lending the technique of time division multiplexing (TDM) to reduce the system cost. Discriminating the time windows for different sensor units is the prerequisite to successfully demodulate the sensing message, but it traditionally calls for a very time-consuming manual calibration process. To combat this problem, a novel automatic time window locating method is proposed in this paper. It introduces the concept of shape function and carries out the cross-correlation operation between the shape function and the sensor signal. The shape function is defined as the function whose curve profile reflects the main data characteristics of the sensor signal. The time window information is then extracted from the correlation result. This whole process is carried out automatically by the interrogation controller of the sensor system without any manual intervene. Experiments are conducted to validate this method. The proposed method can greatly reduce the complexity of locating time windows in large-scale TDM sensor arrays, and make the practical use of the TDM scheme much more convenient.

Implementation of Glycan Remodeling to Plant-Made Therapeutic Antibodies.

N-glycosylation profoundly affects the biological stability and function of therapeutic proteins, which explains the recent interest in glycoengineering technologies as methods to develop biobetter therapeutics. In current manufacturing processes, N-glycosylation is host-specific and remains difficult to control in a production environment that changes with scale and production batches leading to glycosylation heterogeneity and inconsistency. On the other hand, in vitro chemoenzymatic glycan remodeling has been successful in producing homogeneous pre-defined protein glycoforms, but needs to be combined with a cost-effective and scalable production method. An efficient chemoenzymatic glycan remodeling technology using a plant expression system that combines in vivo deglycosylation with an in vitro chemoenzymatic glycosylation is described. Using the monoclonal antibody rituximab as a model therapeutic protein, a uniform Gal2GlcNAc2Man3GlcNAc2 (A2G2) glycoform without α-1,6-fucose, plant-specific α-1,3-fucose or ß-1,2-xylose residues was produced. When compared with the innovator product Rituxan®, the plant-made remodeled afucosylated antibody showed similar binding affinity to the CD20 antigen but significantly enhanced cell cytotoxicity in vitro. Using a scalable plant expression system and reducing the in vitro deglycosylation burden creates the potential to eliminate glycan heterogeneity and provide affordable customization of therapeutics' glycosylation for maximal and targeted biological activity. This feature can reduce cost and provide an affordable platform to manufacture biobetter antibodies.

Effects of Composition and Melting Time on the Phase Separation of Poly(3-hydroxybutyrate)/Poly(propylene carbonate) Blend Thin Films.

In this study, the effect of composition and melting time on the phase separation of poly(3-hydroxybutyrate)/poly(propylene carbonate) (PHB/PPC) blend thin films was investigated. Optical microscopy under phase contrast confirms the occurrence of phase separation of PHB/PPC blends at 190 °C. Polarized optical and scanning electron microscopies (POM and SEM) demonstrate that phase separation takes place along both horizontal and vertical film planes, which should be attributed to the two different interfaces and immiscible blends. A low PPC content (e.g. 30 wt %) results in the formation of compact PHB spherulites filling the whole space, whereas the noncrystallizable PPC spherical microdomains scatter in the PHB region, and their size shows a remarkable melting-time dependence. With the increasing PPC component and melting time, it is observed from POM that the separated PHB domains scattered in the continuous PPC phase, like the island structure. However, it can be revealed by SEM micrographs that the PHB thick domains are actually connected by its thin layer under the PPC layer. The real situation is, therefore, a large amount of PPC aggregates to the surface to form a network uplayer, whereas the PHB thick domains connected by its thin layer form a continuous PHB region, leading to a superimposed bilayer structure. There is also a small amount of PHB small domains scattered in the PHB phase. The PHB thick domains crystallize cooperatively with the PHB- or PHB-rich sublayer in a way just like the growth of pure PHB spherulites. This superimposed bilayer by interplay between phase separation and crystallization may provide availability to tailor the final structure and properties of crystalline/amorphous polymer blends.