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.

The Shear-Accelerated II-I Phase Transition of Isotactic Poly(1-Butene).

The II-I phase transition of isotactic poly(1-butene) (iPBu) leads to improved mechanical performance. However, this will take several weeks and increase storage and processing costs. In this work, shear forces are introduced into the supercooled iPBu melt, and the effects of isothermal crystallization temperature (Tc) and shear temperature (Tshear) on crystallization and phase transition are explored. Shear-induced transcrystalline morphology of Form II with a significantly shortened crystallization induction period can be observed at relatively high Tc (105 °C). Besides, the shear-induced Form II can transit to Form I faster than the unsheared one. In addition, the phase transition rate increases as the Tshear decreases, with the fastest rate occurring at Tshear of 120 °C. The half transition time (t1/2) is measured as 6.3 h when Tc = 105 °C, Tshear = 120 °C, which is much shorter than the 20.7 h required for unsheared samples. The accelerated phase transition of iPBu can be attributed to the stretching of molecular chains, resulting from shear treatment. This study provides a quantitative analysis of the influence of the shear treatment and the Tshear on the II-I phase transition rate. It also presents a cost-effective and straightforward approach for expediting the phase transition process.

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.

Strain-induced 3D-oriented crystallites in natural rubber/chitin nanofiber composites.

Natural rubber (NR) composites containing bio-based chitin nanofibers (ChNFs) exhibit a wide range of mechanical properties - from rubber to plastic behavior - with increasing chitin contents. A constrained 3-dimensional network can be formed by mixing natural rubber latex and a modified zwitterionic rigid chitin counterpart. By inclusion of highly anisotropic chitin nanofibers (30 wt%), strain-induced NR crystallization occurs at a much lower strain of 50%. More intriguingly, 2D-WAXD results reveal that the strain-induced crystallization of NR/ChNFs composites show 3-dimensionally oriented crystallite formation behaving similar to "3D-single crystals orientation" when the content of ChNFs is over 5 wt%. It is suggested that not only c-axis (NR chains) orients along the stretching direction, but also the a- and b-axes deliberately arrange along the normal direction and transverse direction, respectively. Structure and morphology in 3-dimensional spaces after strain-induced crystallization of the NR/ChNFs30 composite are investigated in detail. Therefore, this study might pave a new way to enhance mechanical properties by incorporation of ChNFs, obtaining 3-dimensionally oriented crystallites of novel multifunctional NR/ChNFs composite with shape memory ability.

Unravelling the Superiority of Nonbenzenoid Acepleiadylene as a Building Block for Organic Semiconducting Materials.

Acepleiadylene (APD), a nonbenzenoid isomer of pyrene, exhibits a unique charge-separated character with a large molecular dipole and a small optical gap. However, APD has never been explored in optoelectronic materials to take advantage of these appealing properties. Here, we employ APD as a building block in organic semiconducting materials for the first time, and unravel the superiority of nonbenzenoid APD in electronic applications. We have synthesized an APD derivative (APD-IID) with APD as the terminal donor moieties and isoindigo (IID) as the acceptor core. Theoretical and experimental investigations reveal that APD-IID has an obvious charge-separated structure and enhanced intermolecular interactions as compared with its pyrene-based isomers. As a result, APD-IID displays significantly higher hole mobilities than those of the pyrene-based counterparts. These results imply the advantages of employing APD in semiconducting materials and great potential of nonbenzenoid polycyclic arenes for optoelectronic applications.

Spatially Confined Fabrication of Polar Poly(Vinylidene Fluoride) Nanotubes.

Polar poly(vinylidene fluoride) (PVDF) nanotubes have attracted significant attention due to their excellent piezoelectric and ferroelectric properties, yet a tunable fabrication of homogeneous polar PVDF nanotubes remains a challenge. Here, a simple method is reported to fabricate polar PVDF nanotubes using anodize aluminum oxide (AAO) membranes as templates that are removed by etching in a potassium hydroxide (KOH) solution and then ageing at room temperature. PVDF nanotubes originally crystallized in the AAO membrane are pure α-crystals with very low crystallinity, yet after being released from the templates, the crystallinity of the nanotubes markedly increases with ageing at room temperature, leading to the formation of ß-PVDF crystals in a very short time, with the formation of γ crystals after longer ageing times. A large amount of γ crystals formed when the released PVDF nanotubes are heated to ≈130 °C. The formation of polar PVDF nanotubes released from the AAO templates treated with higher concentrations of alkaline solution results from the reaction of the surface of the PVDF nanotubes with the alkaline solution and structure reorganization under confined conditions. This large-scale preparation of ß- and γ-PVDF opens a new pathway to produce polar PVDF nanomaterials.

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.

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.

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.

Monocyclic and Dicyclic Dehydro[20]annulenes Integrated with Perylene Diimide.

A novel kind of monocyclic and dicyclic dehydro[20]annulenes exhibiting specific sizes and topologies from regioselective unilateral ortho-diethynyl PDI, is developed by Cu-catalyzed Glaser-Hay homo-coupling and cross-coupling. Through the integration of electron-deficient PDI chromophores into the dehydroannulene scaffolding, these macrocycles exhibit intense and characteristic absorption properties and the degenerated LUMO levels. The single-crystal X-ray diffraction analysis unambiguously revealed unique porous supramolecular structures, which display micropore characteristics with surface area of 120.74 m2 g-1 . A moderate electron mobility of 0.05 cm2 V-1 s-1 for chlorine-free dehydro[20]annulene based on micrometer-sized single-crystalline transistors was witnessed. The porous and yet semiconducting features signify the prospects of PDI-integrated dehydroannulenes in organic optoelectronics.

Crystallization Mechanism of 9,9-Diphenyl-dibenzosilole from Solids.

Organic semiconductor (OSC) crystals have great potential to be applied in many fields, as they can be flexibly designed according to the demands and show an outstanding device performance. However, OSCs with the capacity of solid-state crystallization (SSC) are developing too slowly to meet demands in productions and applications, due to their difficulties in molecular design and synthesis, unclear mechanism and high dependence on experimental conditions. In this work, in order to solve the problems, we synthesized an organic semiconductor capable of SSC at room temperature by adjusting the relationship between conjugated groups and functional groups. The thermodynamic and kinetic properties have been studied to discover the model of film SSC. Moreover, it can be purposefully controlled to prepare the high-quality crystals, and their corresponding organic electronic devices were further fabricated and discussed.

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.

The Effect of Epoxidation on Strain-Induced Crystallization of Epoxidized Natural Rubber.

The effect of epoxidation on strain-induced crystallization (SIC) of epoxidized natural rubber (ENR) and mechanism are studied with synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) and polarized infrared spectroscopy (P-IR). WAXD results reveal that appropriate epoxidation, for example, ENR-25 epoxidized with ≈25% isoprene units, can unexpectedly enhance the SIC of natural rubber (NR), resulting in the improvement of tear resistance. On the other hand, exorbitant epoxidation, for example, ENR-40 epoxidized with ≈40% isoprene units, depresses the SIC and weakens the mechanical properties of NR remarkably. P-IR studies reveal that epoxidation can promote the orientation of chain segments along the stretching direction, which plays a determining role on SIC of NR. Accordingly, hierarchical multiscale schematic models are proposed. This insight into epoxidation on SIC of ENR strongly suggests that ENR with appropriate epoxidation degree is a promising candidate material for the fabrication of high-performance engineering rubber products.

Oriented Overgrowths of Poly(l-Lactide) on Oriented Isotactic Polypropylene: A Sequence of Soft and Hard Epitaxies.

The crystallization behavior of an amorphous poly(l-lactide) (PLLA) layer deposited on uniaxially oriented isotactic polypropylene (iPP) substrate is been studied by atomic force microscopy (AFM) and electron microscopy combined with electron diffraction. A patterned PLLA structure with two fixed lamella and chain orientations is observed. Electron diffraction demonstrates that the major lamellar set is oriented with molecular chains perpendicular to the chain direction of the iPP. The minor lamellar set is inclined at ≈64° to both the iPP chain axis direction and the lamellae of the major set as judged from both the bright field electron micrograph and the AFM image. The orientation of the main set is explained in terms of "soft" epitaxy or graphoepitaxy, in which PLLA chains oriented parallel to the ditches of the iPP substrate caused by alternatively arranged crystalline and amorphous regions. The minor set is due to a homoepitaxy of PLLA with parallelism of the helical paths. The orientation of this minor set of lamellae therefore depends on and can help determine the chirality-l or d-of the PLA investigated.

The dependence of the ß-to-α phase transition behavior of poly(1,4-butylene adipate) on phase separated morphology in its blends with poly(vinylidene fluoride).

Synchrotron wide-angle X-ray diffraction was used to monitor the melting and ß-to-α transition behavior of ß-poly(1,4-butylene adipate) (ß-PBA) and its blends with poly(vinylidene fluoride) (PVDF). Two kinds of typical phase separated morphologies are prepared: interfabrillar (70/30 PBA/PVDF) and interlamellar (50/50 PBA/PVDF) morphologies. After melt recrystallization at 10 °C, ß-PBA crystals are obtained. The thermal expansion, phase transition and melting behavior of ß crystals highly depend on the phase separated morphology. The ß crystals show the highest melting temperature and ß-to-α phase transition temperature in 70% PBA, which is caused by the thick lamellae of PBA and/or the lower surface energy of chain folding, while the ß crystals show the lowest melting temperature and ß-to-α phase transition temperature in 50% PBA, which is caused by the thinner lamellae of PBA. An unexpectedly high melting temperature of α crystals transited from ß crystals observed in 50% PBA due to the lamellar thickening occuring in the heating process. The different phase separated morphologies lead to different lamellae thicknesses and surface energies of chain folding which consequently alter the stability of the crystals. The phase transition and melting behavior of ß-PBA in PBA/PVDF is also compared with its blends with poly(vinyl phenol) (PVPh). Completely different factors influence the phase transition behavior of ß crystals in these two systems.

Macroporous Graphene Thin Films as Electrochemical Electrodes: Enhancing the Sensitivity for Detection of Metal Ions.

A macroporous graphene thin films coated on ITO substrates (MGTFs@ITO) have been developed as electrodes for the electrochemical detection of heavy metal ions. The MGTF@ITO electrodes were characterized by scanning electron microscopy, Raman spectroscopy and contact angle measurements. The results demonstrated that the MGTF@ITO has a high specific area with robust macroporous framework and a hydrophilic surface. The cyclic voltammetry and electrochemical impedance spectroscopy using MGTFs@ITO as electrochemical electrodes indicated enhanced currents at the redox peaks, enlarged electrochemical surface area and a decreased charge transfer resistance. Based on these outstanding properties, the MGTF@ITO electrodes exhibited excellent stripping performace for the analysis of Ag(I) with a detection limit of 0.005 µg L-1. The high sensitivity of the MGTF@ITO electrodes can be ascribed to the well defined macroporous framework, high electrical conductivity, high specific area and good wettability. The MGTF@ITO electrodes were further demonstrated applicable to the simultaneous determination of Zn(II), Cd(II), Pb(II), Cu(II) and Ag(I) ions with outstanding sensing performance.