Stepwise versus Concerted: Theoretical Insights into the Stereoselectivity in Aryl Imine Formation Assisted by Acid and Water.

The wide applications of the aryl Schiff base require extensive understanding of the mechanism of its formation, which remains unclear. In this work, the detailed formation mechanisms between benzaldehyde and aniline or 4-(9-anthryl) ethynyl aniline were investigated at the CCSD(T)//B3LYP level, and the influence of water molecules and acid catalysis and the stereoselectivity were addressed. The results show that the participation of explicit water molecules greatly accelerates the reactions by alleviating the ring tension of the transition states, and acid catalysis strongly favors the imine formation and provides driving force for the forward reaction. In acidic conditions, both N-protonated carbinolamine formations and imine formations are achieved under mild conditions with the assistance of water molecules, and the proton transfer is more advanced than the C-N and C═N bond formation, which is in good agreement with the experimental observations. In contrast, under neutral conditions, even with the assistance of two water molecules, the reaction is hard to take place at room temperature owing to the high Gibbs free energy barriers with the proton transfer and the C-N or C═N bond formation concerted. The analysis of stereoselectivity shows that the formation of trans imine is both kinetically and thermodynamically more favorable than the cis one under the acidic condition with the assistance of water molecules, and the presence of conjugated substituent 4-(9-anthryl) ethynyl of aniline marginally raises the energy barriers. This work provides a systematic view of the mechanism for the formation of aryl imine and is expected to offer insights for the control of the dynamic covalent chemistry and the synthesis of covalent organic frameworks.

Regulating the Synergistic Effect in Bimetallic Two-Dimensional Polymer Oxygen Evolution Reaction Catalysts by Adjusting the Coupling Strength Between Metal Centers.

Bimetallic electrocatalysts with its superior performance has a broad application prospect in oxygen evolution reaction (OER), but the fundamental understanding of the mechanism of synergistic effect is still limited since there lacks a practical way to decouple the influence factors on the intrinsic activity of active sites from others. Herein, a series of bimetallic Co-Ni two-dimensional polymer (2DP) model OER catalysts with well-defined architecture, monolayer characteristic, were designed and synthesized to explore the influence of the coupling strength between metal centers on OER performance. The coupling strength was regulated by adjusting the spacing between metal centers or the conjugation degree of bridge skeleton. Among the examined 2DPs, CoTAPP-Ni-MF-2DP, which has the strongest coupling strength between metal centers exhibited the best OER performance. These model systems can help to explore the precise structure-performance relationships, which is important for the rational catalyst design at the atomic/molecular levels.

Single Molecule Level and Label-Free Determination of Multibiomarkers with an Organic Field-Effect Transistor Platform in Early Cancer Diagnosis.

The single molecule level determination with a transistor (SiMoT) platform has attracted considerable attention in the recognition of various ultralow abundance biomolecules, while complicated labeling and testing processes limit its further applications. Recently, organic field-effect transistor (OFET)-based biosensors are good candidates for constructing an advanced label-free SiMoT platform due to their facile fabrication process, rapid response time, and low sample volume with a wide range of detection. However, the sensitivity of most OFET-based biosensors is in the order of nM and pM, which cannot meet the detection requirements of ultralow abundance protein. Herein, a label-free SiMoT platform is demonstrated by integrating pillar[n]arene as a signal amplifier, and the detection limit can reach 4.75 aM. Besides, by simultaneous determination of α-fetoprotein, carcinoembryonic antigen, and prostate antigen, the proposed multiplexed OFET-based SiMoT platform provides a key step in reliable early cancer diagnosis.

Spatial Well-defined Bimetallic Two-Dimensional Polymers with Single-Layer Thickness for Electrocatalytic Oxygen Evolution Reaction.

Innovative bimetallic materials provide more possibilities for further improving the performance of oxygen evolution reaction (OER) electrocatalysts. However, it is still a great challenge to rationally design bimetallic catalysts because there is not a practical way to decouple the factors influencing the intrinsic activity of active sites from others, thus hindering in-depth understanding of the mechanism. Herein, we provide a rational design of bimetallic Ni, Co two-dimensional polymer model OER catalyst. The well-defined architecture, identical density of active sites and monolayer characteristic allow us to decouple the intrinsic activity of active sites from other factors. The results confirmed that the relative position and local coordination environment has significant effect on the synergistic effect of the bimetallic centres. The highest electrocatalytic activity with the turnover frequency value up to 26.19 s-1 was achieved at the overpotential of 500 mV.

Red Phosphorescent Carbon Quantum Dot Organic Framework-Based Electroluminescent Light-Emitting Diodes Exceeding 5% External Quantum Efficiency.

Carbon quantum dots (CQDs) have developed into prospective nanomaterials for next-generation lighting and displays due to their intrinsic advantages of high stability, low cost, and environmental friendliness. However, confined by the spin-forbidden nature of triplet state transitions, the highest theoretical value of external quantum efficiency (EQE) of fluorescent CQDs is merely 5%, which fundamentally limits their further application in electroluminescent light-emitting diodes (LEDs). Soluble phosphorescent CQDs offer a means of breaking the shackle to achieve efficient monochromatic electroluminescence, especially red emission, which is a pivotal constituent in full-color displays. Here, the synthesis of red (625 nm) phosphorescent carbon quantum dot organic frameworks (CDOFs) with a quantum yield of up to 42.3% and realization of high-efficiency red phosphorescent electroluminescent LEDs are reported. The LEDs based on the CDOFs exhibited a red emission with a maximum luminance of 1818 cd m-2 and an EQE of 5.6%. This work explores the possibility of a new perspective for developing high-performance CQD-based electroluminescent LEDs.

Ternary Conductance Switching Realized by a Pillar[5]arene-Functionalized Two-Dimensional Imine Polymer Film.

Nowadays, most manufacturing memory devices are based on materials with electrical bistability (i. e., "0" and "1") in response to an applied electric field. Memory devices with multilevel states are highly desired so as to produce high-density and efficient memory devices. Herein, we report the first multichannel strategy to realize a ternary-state memristor. We make use of the intrinsic sub-nanometer channel of pillar[5]arene and nanometer channel of a two-dimensional imine polymer to construct an active layer with multilevel channels for ternary memory devices. Low threshold voltage, long retention time, clearly distinguishable resistance states, high ON/OFF ratio (OFF/ON1/ON2=1 : 10 : 103 ), and high ternary yield (75 %) were obtained. In addition, the flexible memory device based on 2DPTPAZ+TAPB can maintain its stable ternary memory performance after being bent 500 times. The device also exhibits excellent thermal stability and can tolerate a temperature as high as 300 °C. It is envisioned that the results of this work will open up possibilities for multistate, flexible resistive memories with good thermal stability and low energy consumption, and broaden the application of pillar[n]arene.

Surface Confined Synthesis of Hydroxy Functionalized Two-Dimensional Polymer: The Effect of the Position of Hydroxy Groups.

We report on the on-surface synthesis of a series of two-dimensional polymers (2DPs) and macrocycles containing hydroxyl groups on a highly oriented pyrolytic graphite surface. The formed 2DPs and macrocycles were visualized through scanning tunneling microscopy. By varying the solvent and reaction temperature, structural evolution from oligomers to well-ordered 2DPs or discrete macrocycles was directly followed. In addition, we discovered that the reaction outcome can be steered from extended 2DPs to discrete macrocycles or catenular structures by exchanging the position of the hydroxyl and aldehyde group. These results indicate that the relative positions of hydroxyl and aldehyde groups on the biphenyl ring play a determining role in the control and selection of the final products of the surface-confined Schiff base coupling reaction.

Tunable oligo-histidine self-assembled monolayer junction and charge transport by a pH modulated assembly.

Histidine works as an important mediator in the charge transport process through proteins via its conjugate side group. It can also stabilize a peptide's secondary structure through hydrogen bonding of the imidazole group. In this study, the conformation of the self-assembled monolayer (SAM) and the charge transport of the tailor-made oligopeptide hepta-histidine derivative (7-His) were modulated through the pH control of the assembly environment. Histidine is found to be an efficient tunneling mediator in monolayer junctions with an attenuation factor of ß = ∼0.5 Å-1. Successful theoretical model fitting indicates a linear increase in the number of tunneling sites as the 7-His SAM thickness increases, following the deprotonation of histidine. Combined with the ultraviolet photoelectron spectroscopy (UPS) measurements, a modulable charge transport pathway through 7-His with imidazole groups of histidine as tunneling foot stones is revealed. Histidine therefore possesses a large potential for modulable functional (bio)electronic devices.

Space-Confined Strategy toward Large-Area Two-Dimensional Single Crystals of Molecular Materials.

Two-dimensional molecular crystals (2DMCs) are a promising candidate for flexible and large-area electronics. Their large-area production requires both low nuclei density and 2D crystal growth mode. As an emerging type of material, their large-area production remains a case-by-case practice. Here we present a general, efficient strategy for large-area 2DMCs. The method grows crystals on water surface to minimize the density of nuclei. By controlling the interfacial tension of the water/solution system with a phase transfer surfactant, the spreading area of the solvent increases tens of times, leading to the space-confined 2D growth of molecular crystals. As-grown sub-centimeter-sized 2DMCs floating on the water surface can be easily transferred to arbitrary substrates for device applications.

Surface-Confined Dynamic Covalent System Driven by Olefin Metathesis.

Understanding how the constitutional dynamics of a dynamic combinatorial library (DCL) adapts to surfaces (compared to bulk solution) is of fundamental importance to the design of adaptive materials. Submolecular resolved scanning tunneling microscopy (STM) can provide detailed insights into olefin metathesis at the interface. Analysis of the distribution of products has revealed the important role of environmental pressure, reaction temperature, and substituent effects in surface-confined olefin metathesis. We also report an unprecedented preferred deposition and assembly of linear polymers, and some specific oligomers, on the surface that are hard to obtain otherwise.