Enhancing Light-Emitting Efficiency of Blue Through-Space Charge Transfer Emitters via Fixing Configuration Induced by Intramolecular Hydrogen Bonding.

Closely aligned configuration of the donor (D) and acceptor (A) is crucial for the light-emitting efficiency of thermally activated delayed fluorescence (TADF) materials with through-space charge transfer (TSCT) characteristics. However, precisely controlling the D-A distance of blue TSCT-TADF emitters is still challenging. Herein, an extra donor (D*) located on the side of the primary donor (D) is introduced to construct the hydrogen bonding with A and thus modulate the distance of D and A units to prepare high-efficiency blue TSCT emitters. The obtained "V"-shaped TSCT emitter presents a minimal D-A distance of 2.890 Å with a highly parallel D-A configuration. As a result, a high rate of radiative decay (>107 s-1) and photoluminescence quantum yield (nearly 90%) are achieved. The corresponding blue organic light-emitting diodes show maximum external quantum efficiencies (EQEmax) of 27.9% with a Commission Internationale de L'Eclairage (CIE) coordinate of (0.16, 0.21), which is the highest device efficiency of fluorene-based blue TSCT-TADF emitters. In addition, the TSCT-TADF emitter-sensitized OLEDs also achieve a high EQEmax of 29.3% with a CIE coordinate of (0.12, 0.16) and a narrow emission.

Thermally Activated Delayed Fluorescence Emitters Based on a Special Tetrahedral Silane Core.

Based on the tetraphenylsilane skeleton, a new class of thermally activated delayed fluorescence (TADF) molecules have been designed and synthesized. Benefiting from the unique tetrahedron architecture of tetraphenylsilane, the intermolecular distance between TADF units can be enlarged and thus weakened the aggregation-induced quenching of triplet excitons. By adjusting the numbers of TADF subunits, the spin-orbit coupling processes can be controlled, leading to efficient up-conversion processes. The related OLEDs are fabricated through the solution processing technology, and pure-blue and green electroluminescence were observed with maximum external quantum efficiencies (EQEmax) of 6.6 and 13.8% as well as Commission Internationale de l'Eclairage coordinates of (0.14, 0.15) and (0.25, 0.45), respectively. This study provides a new idea for designing color-tunable TADF emitters through spatial structure regulation.

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.

Mechanical properties of rooted soil under freeze-thaw cycles and extended binary medium constitutive model.

In seasonally frozen soil, soil sometimes is affected by freeze-thaw cycles and root systems. In order to study its mechanical characteristics, a series of consolidation drained triaxial tests under different confining pressures (25, 50, 100, 200 kPa), different freeze-thaw cycles (N = 0, 1, 5, 15) and different root-containing conditions (r = 0, 1, 3) were carried out. The test results show that the specimens exhibit strain softening behavior and volumetric dilatancy phenomena and shear failure under lower confining pressure, and strain hardening and volumetric contraction, bulging failure under higher confining pressure. With the increase of freeze-thaw cycles, the bearing capacity of the sample decreases and the volume strain increases. With the increase of volume ration of roots in the sample, the bearing capacity increases and the volume strain decreases. Based on the binary medium model, the soil is abstracted into bonded elements and frictional elements. At the same time, the bonded elements are transformed into frictional element when the bonded elements are broken during the loading process. Also, the root is abstracted into another non-destructive bonded elements material, which bears the load together. The linear elastic constitutive model is used for root and bonded elements, and the double-hardening model is used for friction elements. Considering the influence of freeze-thaw cycles, the extended binary model is derived here. Finally, the experimental results show that the predicted results of this model are in good agreement with the experimental results, and the new model can relatively well simulate the strain softening and volumetric dilatancy phenomena.

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.

Influence of inorganic and organic salts on the hydration mechanism of montmorillonite based on molecular simulation.

The molecular dynamics method is used to further reveal, from the molecular point of view, the mechanisms of salt inhibiting the hydration of Na-MMT. The interaction between water molecules, salt molecules, and montmorillonite are calculated by establishing the adsorption models. According to the simulation results, the adsorption conformation, interlayer concentration distribution, self-diffusion coefficient, ion hydration parameters, and other data are compared and analyzed. The simulation results show that the volume and basal spacing increase in a stepwise manner with the increase of water content, and water molecules have different hydration mechanisms. The addition of salt will enhance the hydration properties of compensating cations of montmorillonite and affect the mobility of particles. The addition of inorganic salts mainly reduces the adsorption tightness between water molecules and crystal surfaces, thereby reducing the thickness of water molecules layer, while the organic salts can better inhibit migration by controlling interlayer water molecules. The results of molecular dynamics simulations reveal the microscopic distribution of particles and the influence mechanism when the swelling properties of montmorillonite are modified by chemical reagents.

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.

Robust motion trajectory optimization of overhead cranes based on polynomial chaos expansion.

In this paper, we present a polynomial chaos-based framework for the trajectory optimization of an overhead crane system under uncertainty. The main research described in this paper is as follows. First, the deterministic trajectory optimization problem formulation of a two-dimensional overhead crane model is constructed. Based on this basic mathematical formulation, the uncertainty trajectory optimization problem is formed considering the uncertainty of initial state and system parameter. Then, to solve the uncertainty trajectory optimization problem efficiently, a robust trajectory optimization problem formulation is proposed. However, it is difficult to solve the robust trajectory optimization problem directly because it contains stochastic function terms, such as stochastic dynamic equations, constraint functions and objective functions. We consider both the system state and control input as functions of uncertainty and use polynomial chaos expansion to quantify these stochastic functions. An augmented deterministic trajectory optimization problem which can be solved directly is finally obtained. Based on the proposed robust trajectory optimization formation, the motion trajectory optimization of an overhead crane system under two different uncertainty types of is solved. All simulation results are compared with traditional sampling-based Monte Carlo simulations to demonstrate the feasibility and effectiveness of the proposed method.