Author: chemistadmin

A research team has proposed a new strategy for designing pure-red organic light emitting diodes (OLED) materials. These materials have achieved a milestone with electroluminescence efficiencies exceeding 43%, marking a significant step toward high-performance ultrahigh-definition OLED displays. The study was published online in the Journal of the American Chemical Society.

OLEDs have emerged as a leading technology due to their unique features such as flexibility and bright self-emission. However, the performance of red OLEDs, especially in the saturated red region, has lagged behind that of their blue and green counterparts. The development of efficient red emitters with high color purity has been a major challenge in the field.

Focusing on overcoming the challenge of red light emitters, the research team proposed a new strategy for the design of pure-red OLED materials with high luminous efficiency, excellent color purity, and long-term stability. The key innovation lies in the molecule BNTPA, which was designed to incorporate secondary electron-donating units and extend the π-skeleton within multiresonance cores. This structural modification significantly enhances intramolecular charge transfer, enabling the molecule to more efficiently handle the excitation energy.

As a result, light emission is effectively shifted into the red spectrum, while still maintaining narrowband characteristics for ensuring high color fidelity, which is a key requirement for high-definition displays. To further improve the molecular design, the team optimized the reverse inter-system crossing (RISC) process. BNTPA’s refined structure not only accelerates the RISC rate but also ensures a balanced combination of short-range and long-range charge transfer characteristics.

This balance is particularly important for improving the overall photophysical performance of the emitter, as it minimizes energy loss and improves both the luminous efficiency and stability of the OLEDs. Additionally, the integration of secondary electron-donating units stabilizes the excited states of BNTPA, reducing non-radiative decay and preventing energy loss that often occurs in red-emitting materials.

This enhancement of the molecular architecture ensures that BNTPA-based OLEDs achieve greater operational stability and longer lifetimes, making them suitable for practical, long-term use in real-world applications.

OLEDs based on BNTPA achieved a record-breaking external quantum efficiency exceeding 43%. Its CIE value is (0.657, 0.343), and aligns closely with NTSC standards (0.67, 0.33), achieving excellent color purity. These advancements are attributed to the molecule’s optimized design, which enhances energy efficiency and operational stability. This establishes BNTPA as a benchmark for next-generation high-performance red MR-TADF emitters.

This research sets a precedent for future research and practical deployment in high-definition displays and next-generation electronic devices. It also contributes to the development of energy-efficient and durable lighting systems, enabling OLED displays to meet stringent color standards.

The team was led by Prof. Cui Songlin at University of Science and Technology of China (USTC), in collaboration with Prof. Zhou Meng’s team from Beijing Information Science and Technology University (BISTU).

A research team led by Prof. Kang Yanbiao from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences has made a significant discovery in the field of environmental chemistry. They developed a novel photocatalyst named KQGZ, which can photocatalytically defluorinate polyfluoroalkyl and perfluoroalkyl substances (PFAS) at a low temperature range of 40°C–60°C. This finding has been published in Nature.

PFAS, referred to as “forever chemicals,” possess high thermal and chemical stabilities as well as hydrophobic and oleophobic properties because of the inert carbon-fluorine (C–F) bonds. As a result, they are widely used in various fields such as chemicals, electronics and medical devices.

However, the inertness of the C–F bonds also makes it difficult for PFAS to decompose through defluorination under natural environment or mild conditions. For example, pyrolysis of Teflon usually proceeds at over 500°C and toxic gases are released. The disposal of PFAS into the natural environment has led to a series of environmental and health issues.

To address the challenges, the team designed and created an organic super-photoreductant called KQGZ based on the characteristics of photoreductants’ strong reducibility under specific light conditions. As a type of photoreductant, KQGZ can be excited by absorbing light and transfer an electron from its excited state to other organic molecules.

By adding KQGZ to the reaction system and experimenting with various reaction conditions, the team achieved complete defluorination and mineralization of Teflon and small molecule PFAS at low temperatures for the first time, efficiently recycling them into inorganic fluoride salts and carbon resources.

More specifically, the study’s core experiments involved the application of KQGZ as a photocatalyst under visible light to defluorinate a range of PFASs, including polytetrafluoroethylene (PTFE), perfluorocarbons (PFCs), perfluorooctane sulfonic acid (PFOS), polyfluorooctanoic acid (PFOA), and their derivatives.

The process resulted in the formation of amorphous carbon and fluoride salts from PTFE, while oligomeric PFASs yielded a variety of carbonate, formate, oxalate, and trifluoroacetate products. This not only addresses the degradation of PFASs but also enables the recycling of fluorine in the form of inorganic fluoride salts.

A detailed mechanistic investigation was also conducted to understand the reaction behavior and product composition differences between PTFE and oligomeric PFAS. The researchers discovered that the photocatalytic reduction ability is not directly correlated with the excited oxidation potential of the photocatalyst, challenging the traditional paradigm in the field.

This insight suggests that the electron transfer ability of the photocatalyst may be related to the torsion of the carbazole ring, a finding that could guide the design of more effective photocatalysts in the future.

Finally, the study also meticulously investigated the effects of various reducing reagents, finding that most demonstrated good reactivity, with γ-terpinene and cesium formate yielding the highest results. Control experiments confirmed the indispensable role of light, photocatalyst, and reducing reagent in the defluorination process.

This study not only reports for the first time the promoting effect of highly twisted carbazole-cores on the electron transfer of super-photoreductants, but also shows that the excited oxidative potential of photoreductants is not directly related to their reduction ability, and therefore should not be the only standard for the photoreduction ability. In addition, the ability to completely defluorinate Teflon and other PFAS can serve as a standard for the reduction ability of organic reductants.

Have you ever imagined that high-density metals could be converted into an ultralight aerogel? This counterintuitive idea was presented in 2009 by Eychmüller’s group, where all-metal-made aerogels, i.e., metal aerogels (MAs), were produced by assembling metal nanoparticles in a controlled manner. Since then, these special and promising materials have been explored by global scientists, gradually forming a new field in materials science.

MAs composed of more than one metal, namely multimetallic aerogels (MMAs), have received particular attention, because MMAs feature widely tunable properties stimulated by the synergy of multiple metals. For a material structured from multiple components, the first thought might be whether this material will have attributes stemming from each constituent, or if it will feature enhanced performance because of the synergy of different constituents.

Indeed, many research articles demonstrate that MMAs are often better than single-component MAs in, for example, electrocatalysis. Better performance was primarily achieved by tuning the difference in electrical conductivity, lattice parameters and electronic structure of dissimilar metals.

I am interested in controlled synthesis because I believe the synthesis ability dictates how far a material goes. Therefore, instead of the application aspect, I am concentrating on the synthesis aspect incurred by multimetallic effects. This is the motivation of our paper published in Matter titled “Manipulating multimetallic effects: Programming size-tailored metal aerogels as self-standing electrocatalysts.”

In our study, we found that multimetallic effects concurrently impacted the sol-gel process of metals and the ligament size of the resulting MMAs.

We discovered an unconventional, gravity-driven gelation behavior of metal systems in a Science Advances paper five years ago. We found that the gelation process of metal systems is similar to a precipitation process. Driven by the high density of metals (e.g., the density of gold is ~19.3 g cm^-3), the as-formed metal aggregates eventually settle down with the lapse of time and form a monolithic gel at the vessel bottom.

If the metal aggregate is not solely made up of gold, for example, what will happen for a gold-silver bimetallic system? The incorporation of relatively low-density silver (~10.5 g cm^-3) will reduce the average density of metals and thus slow down the sedimentation process, leading to a prolonged gelation time.

This was proven by our experiments and characterizations using a variety of metal combinations (single, binary and triple metals). It not only offers a way to tune the sol-gel process but also confirms the generality of our proposed gravity-driven gelation mechanism.

The most exciting and important part is the ligament size control via multimetallic effects. The ligament size is a critical parameter for MAs, for it dictates the nano effects and thus many physicochemical properties of materials.

Historically, the ligament size is tuned by modulating the initiators or introducing ligands, which may contaminate the resulting MAs. Taking a glance at all reported MAs since 2009, one will recognize that some MAs (e.g., Au, Ag) often feature large ligament sizes while others (e.g., Pd, Pt, Ru, Rh) often feature small ligament sizes. However, almost all alloy aerogels feature small ligament sizes. Then the question arises: What happens when two metals come together?

We thoroughly studied the ligament size change by controllably introducing different types and amounts of auxiliary metals into the main metal systems (e.g., introducing nickel sources to gold sources before conducting the gelation process). We found that 1% auxiliary metals drastically reduced the ligament size by ~ 30% to 78%, which worked for Au, Ag and Cu-based aerogels.

This impressive phenomenon was rationalized by the atomic radius mismatch between the main metal and the auxiliary metal. The mismatch will retard the layer-type deposition of metal atoms. Instead, the ligament growth will follow an island-type deposition style, thus producing more branches and thinning the ligament size (see image above). Depending on the mismatch degree and the proportion of the auxiliary metal atoms, the ligament size can be well adjusted.

Finally, using the gravity-driven gelation behavior, we developed a sedimentation-based, non-destructive strategy to boost the electrocatalytic performance of MMAs. This technique avoids the sonication-led structure destruction that was suffered by previously reported MA-based electrocatalysts.

Briefly, several pieces of carbon paper were placed at the bottom of the reaction vessel, accepting the settled metal aggregates. The in-situ-generated metal aggregates will gradually sediment and enrich on the carbon paper, thus forming a CP-supported intact gel film (the Au-Pt system was used as an example).

This CP-supported intact Au-Pt gel film was directly used as the working electrode to catalyze the alcohol oxidation reaction. Because of its well-retained network, this intact metal gel manifested record-high performance for both methanol- and ethanol- oxidation reactions.

In summary, our study not only provides a fresh viewpoint on using multimetallic effects for tuning the preparation and structure of MMAs but also solves the long-lasting challenge of preparing intact metal gel-based electrocatalysts for high-performance catalysis.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.

Ran Du received his B.E. in 2011 from Beijing Institute of Technology and PhD degree in 2016 from Peking University. After successive research stays at Nanyang Technological University (2016–2017), TU Dresden (2017–2019, sponsored by Humboldt fellowship), and Hong Kong University (2020–2021), he joined the Beijing Institute of Technology as a professor in 2021. His research interest lies in the creative synthesis of advanced aerogels (e.g., metal aerogels, nanocarbon aerogels, etc.) and exploring their smart applications in catalysis, environment remediation, and smart materials.

Currently employed computational methods to simulate materials and their mechanical behavior are based on molecular dynamics (MD) with atomistic force-fields. These methods provide an excellent description of the thermodynamically stable phases of materials with arbitrary chemical and microstructural complexity.

However, simulating the mechanical deformation behavior of materials at the atomistic level, or, in general, the response of a material to an external time-dependent stimulus, has been an open challenge for a long time. The main bottleneck is represented by the inevitably short time scale of integration of the equations of motion (just a few femtoseconds) that atomistic MD methods rely on. This is a necessary step in order to discretize the equations of motion that govern atomic motions and collisions, in order to solve them on a computer.

This limitation makes it impossible to simulate the dynamical deformation of materials on long time scales encountered in experiments, i.e., for deformation rates lower than ~10 to 100 gigahertz. This fundamental time-scale bridging problem is currently unsolved, and prevents the computational prediction of material mechanics in the regimes that are experimentally accessible in standard mechanical tests and rheology.

With my post-doc, Dr. Vinay Vaibhav, and with my long-time collaborator at the US Army Research Lab, Dr. Tim Sirk, I have now developed a computational framework that provides a working solution to this problem, arguably one of the biggest problems in molecular simulations of materials under deformations and external stimuli.

The key idea of our approach is that the mechanical response at the low frequencies (e.g., around the Hertz) is dominated by atomic displacements known as nonaffine displacements. A nonaffine displacement is a swerve in the trajectory of an atom, which thus deviates from the trajectory prescribed by the externally imposed deformation (akin to Epicurus’s “clinamen,” if you are familiar with Greek philosophy).

The origin of this swerve is the necessity to enforce mechanical equilibrium at every step in the deformation. In other words, at each step, the atom receives forces from its neighbor atoms, which need to be relaxed via an extra motion, the nonaffine swerve.

As my collaborators and I have come to realize over the years, implementing this description of atomic trajectories implies computing the vibrational normal modes of the system, which can be done with modern computational techniques.

This has now allowed us, in a paper published in the journal Macromolecules, to achieve a parameter-free agreement with the viscoelastic moduli of a real complex material, a crosslinked epoxy polymer glass in its amorphous solid state, at frequencies that are about 10 orders of magnitude lower than those that can be achieved by simulating the deformation process in standard molecular dynamic simulations.

The agreement with experimental data from mechanical tests is striking, considering that no adjustable parameters are involved in the comparison.

Our approach can still be refined in future work, e.g., by taking larger snapshots of the material configuration, with an increasing number of atoms, which will improve our predictions and reduce the noise from numerical fluctuations.

An exciting prospect offered by this method is that of being able to single out the atomic and molecular vibrations, and motions, that are mostly responsible for the stiffness and hardness of a given material (or, conversely, for its softness), with plenty of opportunities for the development of new materials with high-performance properties for many technological and engineering applications.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.