Correlated Triplet Pair Formation Activated by Geometry Relaxation in Directly Linked Tetracene Dimer (5,5'-Bitetracene)

Singlet fission (SF) materials have the potential to overcome the traditional external quantum efficiency limits of organic light-emitting diodes (OLEDs). In this study, we theoretically designed an intramolecular SF molecule, 5,5'-bitetracene (55BT), in which two tetracene units were directly connected through a C-C bond. Using quantum chemical calculation and the Fermi golden rule, we show that 55BT undergoes efficient SF induced by geometry relaxation in a locally excited singlet state, ¹(S₀S₁). Compared with another high-performing SF system, the tetracene dimer in the crystalline state, 55BT has advantages when used in doped systems owing to covalent bonding of the two tetracene units. This feature makes 55BT a promising candidate triplet sensitizer for near-infrared OLEDs.

Thiophene-Fused Naphthodiphospholes: Modulation of the Structural and Electronic Properties of Polycyclic Aromatics by Precise Fusion of Heteroles

For polycyclic aromatics with heterole-fused structures, the orientation of fused heterole rings as well as the geometry of their fused structures has a large impact on the physicochemical properties. In this study, a series of isomers of thiophene-fused naphthodiphospholes was designed and synthesized. Systematic investigation unveiled the explicit impact of heterole-fused structures on their structural and electronic properties. The isomers with 1,2/5,6-fused structure display phosphorescence due to enhanced spin-orbit coupling, whereas the isomers with 2,3/6,7-fused structure exhibit intense fluorescence. The trans isomers exhibited 1D slip π-stacked arrangement. In contrast, the cis isomers displayed 2D herringbone structure or columnar structure with a cavity. Therefore, the precisely controlled fusion of heterole rings is a universal approach to uncover their intrinsic properties for versatile applications as organic functional materials.

Conformation Control of Iminodibenzyl-Based Thermally Activated Delayed Fluorescence Material by Tilted Face-to-Face Alignment With Optimal Distance (tFFO) Design

In organic light-emitting diodes (OLEDs), all triplet excitons can be harvested as light via reverse intersystem crossing (RISC) based on thermally activated delayed fluorescence (TADF) emitters. To realize efficient TADF, RISC should be fast. Thus, to accomplish rapid RISC, in the present study, a novel TADF emitter, namely, TpIBT-tFFO, was reported. TpIBT-tFFO was compared with IB-TRZ, which contains the same electron donor and acceptor segments, specifically iminodibenzyl and triazine moieties. TpIBT-tFFO is based on a recently proposed molecular design strategy called tilted face-to-face alignment with optimal distance (tFFO), whereas IB-TRZ is a conventional through-bond type molecule. According to quantum chemical calculations, a very large RISC rate constant, k _RISC, was expected for TpIBT-tFFO because not only the lowest triplet state but also the second lowest triplet state were close to the lowest excited singlet state, as designed in the tFFO strategy. IB-TRZ has two different conformers, leading to dual emission. Conversely, owing to excellent packing, the conformation was fixed to one in the tFFO system, resulting in single-peaked emission for TpIBT-tFFO. TpIBT-tFFO displayed TADF type behavior and afforded higher photoluminescence quantum yield (PLQY) compared to IB-TRZ. The k _RISC of TpIBT-tFFO was determined at 6.9 × 10⁶ s^-1, which is one of the highest values among molecules composed of only H, C, and N atoms. The external quantum efficiency of the TpIBT-tFFO-based OLED was much higher than that of the IB-TRZ-based one. The present study confirms the effectiveness of the tFFO design to realize rapid RISC. The tFFO-based emitters were found to exhibit an additional feature, enabling the control of the molecular conformations of the donor and/or acceptor segments.

Lamellar Structure in Alanine-Glycine Copolypeptides Studied by Solid-State NMR Spectroscopy: A Model for the Crystalline Domain of Bombyx mori Silk Fibroin in Silk II Form

Bombyx mori silk fibroin (SF) fibers with excellent mechanical properties have attracted widespread attention as new biomaterials. However, the structural details are still not conclusive. Here, we propose a lamellar structure for the crystalline domain of the SF fiber based on structural analyses of the Ala Cβ peaks in the ¹³C cross-polarization/magic angle spinning NMR spectra of (Ala-Gly)_m (m = 9, 12, 15, and 25) and ¹³C selectively labeled (Ala-Gly)₁₅ model peptides. Namely, three Ala Cβ peaks with relative intensities of 1:2:1 obtained by deconvolution were assigned to two kinds of β-sheet and a β-turn, which are interpreted as a lamellar structure formed by repetitive folding using β-turns every eighth amino acid, for which the basic structure is (Ala-Gly)₄ in an antipolar arrangement. The dynamics and intermolecular arrangement were further studied using ¹³C solid-state spin-lattice relaxation time observations and the rotational echo double resonance experiments, respectively.

Noise Reduction in Solid-State NMR Spectra Using Principal Component Analysis

A noise reduction method was developed for solid-state nuclear magnetic resonance spectroscopy using multivariate analysis. Principal component analysis was first applied for cross-polarization/magic angle spinning and ¹³C spin-lattice relaxation measurements of solid-state nuclear magnetic resonance array spectra. The contact time of cross-polarization/magic angle spinning and the delay time in spin-lattice relaxation measurements were continuously changed to obtain a series of spectra, which were used for noise reduction using principal component analysis. The noise reduction method successfully produced spectra with improved signal-to-noise ratios. This noise reduction method shortens the measurement time and allows for detection of components with minute signals.

The effect of gas emission on the strength of composite products derived using alkali-activated municipal solid waste incineration fly ash/pyrophyllite-based systems

We explored the effects of gas emission by mixtures undergoing alkali-activation of municipal solid waste incineration fly ash (MSWIFA) and pyrophyllite (the mixtures included dehydrated pyrophyllite, MSWIFA, 14 mol/L aqueous sodium hydroxide, and sodium silicate; curing proceed at 105 °C for 24 h). We measured the compressive strengths of the derived solid composites. The causes of gas emission, and the physical and chemical properties of products created under controlled gas emission, were investigated. Hydrogen was emitted after mixing MSWIFA and alkali. The compressive strength of products prepared when gas emission was complete was 2-3.4-fold greater than that of products prepared when gas emission was incomplete. X-ray micro-tomography and mercury intrusion porosimetry showed that products formed during complete gas emission tended to have smaller pores. X-ray diffraction and nuclear magnetic resonance (²⁷Al and ²⁹Si) indicated that the aluminum substitution levels in tectosilicate differed under such conditions, although the minerals were identical. Thus, complete gas emission after mixing improved ultimate products.

Thermally Activated Delayed Fluorescent Materials Combining Intra- and Intermolecular Charge Transfers

A novel thermally activated delayed fluorescent (TADF) compound, 9-(3-((4,6-diphenyl-1,3,5-triazin-2-yl)oxy)phenyl)-3,6-diphenyl-9 H-carbazole (PhCz- o-Trz), with a donor-σ-acceptor (D-σ-A) motif is developed. A flexible small space σ-junction is adopted to partly suppress the intramolecular charge transfer (intra-CT) while inversely enhancing the intermolecular charge transfer (inter-CT) between D/A moieties, realizing the coexistence of both intra-CT and inter-CT in an amorphous aggregate. The coexistence of dual CTs increases the complexity of the singlet and triplet state mixing, enhancing the triplet-to-singlet spin-flip transition and thereby the TADF emission. Additionally, PhCz- o-Trz is evaluated not only as an emitter but also as a sensitizing host for fluorescent and phosphorescent dopants, all exhibiting high efficiencies with alleviated efficiency roll-offs. These results shed light on the development of new TADF materials with dual CTs and may further deepen our understanding about TADF mechanisms.

Unveiling the Role of Langevin and Trap-Assisted Recombination in Long Lifespan OLEDs Employing Thermally Activated Delayed Fluorophores

Recent research studies on noble-metal-free thermally activated delayed fluorescent (TADF) materials have boosted the efficiencies of organic light-emitting diodes (OLEDs) to unity. However, the short lifespan still hinders their further practical application. Carrier recombination pathways have been reported to have a significant influence on the efficiencies of TADF devices, though their effects on device lifetimes remain rarely studied. Here, we have designed and synthesized five pyrimidine or pyrazine/carbazole isomers as hosts for TADF OLEDs to explore the inherent role of Langevin recombination (LR) and trap-assisted recombination (TAR) in device lifetimes. It is revealed that for LR dominant devices, lifetimes would increase by reducing the host triplet energy levels, whereas for TAR dominant devices, lifetimes are insensitive to the host triplet excitons as recombination mainly takes place on dopants. Still, LR dominant devices are favored as they offer more room for optimization. We further apply this concept in designing a stable LR dominant blue TADF device, achieving a long LT50 (lifespan up to 50% of the initial luminance) of 269 h and high external quantum efficiency of 17.9% at 1000 cd m^-2 simultaneously.

Ambient-dried highly flexible copolymer aerogels and their nanocomposites with polypyrrole for thermal insulation, separation, and pressure sensing

Highly flexible copolymer and copolymer/polypyrrole nanocomposite aerogels have been synthesized via ambient pressure drying for superinsulation, separation and pressure sensing.

Superflexible Multifunctional Polyvinylpolydimethylsiloxane-Based Aerogels as Efficient Absorbents, Thermal Superinsulators, and Strain Sensors

Aerogels are porous materials but show poor mechanical properties and limited functionality, which significantly restrict their practical applications. Preparation of highly bendable and processable aerogels with multifunctionality remains a challenge. Herein we report unprecedented superflexible aerogels based on polyvinylpolydimethylsiloxane (PVPDMS) networks, PVPDMS/polyvinylpolymethylsiloxane (PVPMS) copolymer networks, and PVPDMS/PVPMS/graphene nanocomposites by a facile radical polymerization/hydrolytic polycondensation strategy and ambient pressure drying or freeze drying. The aerogels have a doubly cross-linked organic-inorganic network structure consisting of flexible polydimethylsiloxanes and hydrocarbon chains with tunable cross-linking density, tunable pore size and bulk density. They have a high hydrophobicity and superflexibility and combine selective absorption, efficient separation of oil and water, thermal superinsulation, and strain sensing.

Effects of Structural and Energetic Disorders on Charge Transports in Crystal and Amorphous Organic Layers

Understanding charge transports in organic films is important for both fundamental science and practical applications. Here, contributions of off-diagonal (structural) and diagonal (energetic) disorders to charge transports were clarified using molecular-based multiscale simulation. These disorders, important for understanding charge transport in organic systems, are investigated by comparing crystal and amorphous aggregates of N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPD). Although NPD has been used as a hole transport material, it also exhibits comparable electron mobility experimentally. The experimental mobility and its electric field dependence in amorphous layers were reasonably reproduced by the multiscale simulation, confirming the electron transport properties of NPD. We assumed that the structural disorder would lower mobilities; however, the mobilities were found to be independent of the degree of structural disorder. Energetic disorder markedly lowered charge mobility instead. Charge migration in crystals was dominated by maximum electronic coupling pairs, whereas small electronic coupling pairs significantly contributed to charge transport in amorphous aggregate.

Adamantyl Substitution Strategy for Realizing Solution-Processable Thermally Stable Deep-Blue Thermally Activated Delayed Fluorescence Materials

Highly efficient solution-processable emitters, especially deep-blue emitters, are greatly desired to develop low-cost and low-energy-consumption organic light-emitting diodes (OLEDs). A recently developed class of potentially metal-free emitters, thermally activated delayed fluorescence (TADF) materials, are promising candidates, but solution-processable TADF materials with efficient blue emissions are not well investigated. In this study, first the requirements for the design of efficient deep-blue TADF materials are clarified, on the basis of which, adamantyl-substituted TADF molecules are developed. The substitution not only endows high solubility and excellent thermal stability but also has a critical impact on the molecular orbitals, by pushing up the lowest unoccupied molecular orbital energy and triplet energy of the molecules. In the application to OLEDs, an external quantum efficiency (EQE) of 22.1% with blue emission having Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.19) is realized. A much deeper blue emission with CIE (0.15, 0.13) is also achieved, with an EQE of 11.2%. These efficiencies are the best yet among solution-processed TADF OLEDs of CIE y < 0.20 and y < 0.15, as far as known. This work demonstrates the validity of adamantyl substitution and paves a pathway for straightforward realization of solution-processable efficient deep-blue TADF emitters.

Versatile Indolocarbazole-Isomer Derivatives as Highly Emissive Emitters and Ideal Hosts for Thermally Activated Delayed Fluorescent OLEDs with Alleviated Efficiency Roll-Off

Maintaining high efficiency at high brightness levels is an exigent challenge for real-world applications of thermally activated delayed fluorescent organic light-emitting diodes (TADF-OLEDs). Here, versatile indolocarbazole-isomer derivatives are developed as highly emissive emitters and ideal hosts for TADF-OLEDs to alleviate efficiency roll-off. It is observed that photophysical and electronic properties of these compounds can be well modulated by varying the indolocarbazole isomers. A photoluminescence quantum yield (η_PL ) approaching unity and a maximum external quantum efficiency (EQE_max ) of 25.1% are obtained for the emitter with indolo[3,2-a]carbazolyl subunit. Remarkably, record-high EQE/power efficiency of 26.2%/69.7 lm W^-1 at the brightness level of 5000 cd m^-2 with a voltage of only 3.74 V are also obtained using the same isomer as the host in a green TADF-OLED. It is evident that TADF hosts with high η_PL values, fast reverse intersystem crossing processes, and balanced charge transport properties may open the path toward roll-off-free TADF-OLEDs.

Transparent, Superflexible Doubly Cross-Linked Polyvinylpolymethylsiloxane Aerogel Superinsulators via Ambient Pressure Drying

Aerogels have many attractive properties but are usually costly and mechanically brittle, which always limit their practical applications. While many efforts have been made to reinforce the aerogels, most of the reinforcement efforts sacrifice the transparency or superinsulating properties. Here we report superflexible polyvinylpolymethylsiloxane, (CH₂CH(Si(CH₃)O_2/2))_n, aerogels that are facilely prepared from a single precursor vinylmethyldimethoxysilane or vinylmethyldiethoxysilane without organic cross-linkers. The method is based on consecutive processes involving radical polymerization and hydrolytic polycondensation, followed by ultralow-cost, highly scalable, ambient-pressure drying directly from alcohol as a drying medium without any modification or additional solvent exchange. The resulting aerogels and xerogels show a homogeneous, tunable, highly porous, doubly cross-linked nanostructure with the elastic polymethylsiloxane network cross-linked with flexible hydrocarbon chains. An outstanding combination of ultralow cost, high scalability, uniform pore size, high surface area, high transparency, high hydrophobicity, excellent machinability, superflexibility in compression, superflexibility in bending, and superinsulating properties has been achieved in a single aerogel or xerogel. This study represents a significant progress of porous materials and makes the practical applications of transparent flexible aerogel-based superinsulators realistic.

Aerogels from Chloromethyltrimethoxysilane and Their Functionalizations

Reactions of chloromethyltrimethoxysilane (CMTMS) and its derived colloidal network polychloromethylsilsesquioxane (PCMSQ) have been investigated to extend the material design strategy toward functionalized and mechanically reinforced aerogels. In a carefully designed sol-gel system, CMTMS has afforded transparent aerogels in the presence of cationic surfactant. The surface chloromethyl groups with polarity and reactivity are shown to be useful for supporting nanostructures, with photoluminescent carbon dots (C-dots) prepared from polyethylenimine and citric acid as an example. Furthermore, since nucleophilic substitution (S_N2) reactions on the surface chloromethyl groups are found to control the equilibrium of formation/dissociation of siloxane bonds, a new gelation strategy triggered by S_N2 reactions in sol-gel has been developed. In the presence of nucleophilic organic species such as polyamines, a hybrid network consisting of PCMSQ cross-linked with a polyamine nucleophile can be prepared to enhance mechanical properties of aerogel.

Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy

Molecular orientation in amorphous organic semiconducting thin-film devices is an important issue affecting device performance. However, to date it has not been possible to analyze the "distribution" of the orientations. Although solid-state NMR (ssNMR) spectroscopy can provide information on the "distribution" of molecular orientations, the technique is limited because of the small amount of sample in the device and the low sensitivity of ssNMR. Here, we report the first application of dynamic nuclear polarization enhanced ssNMR (DNP-ssNMR) spectroscopy for the orientational analysis of amorphous phenyldi(pyren-1-yl)phosphine oxide (POPy₂ ). The ³¹ P DNP-ssNMR spectra exhibited a sufficient signal-to-noise ratio to quantify the distribution of molecular orientations in amorphous films: the P=O axis of the vacuum-deposited and drop-cast POPy₂ shows anisotropic and isotropic distribution, respectively. The different molecular orientations reflect the molecular origin of the different charge transport behaviors.