纳米孔缺陷导致单层黑磷电荷局域极大抑制非辐射电子-空穴复合的时域模拟

通常认为缺陷加速黑磷的非辐射电子-空穴复合,阻碍器件性能的持续提高。实验打破了这一认识。采用含时密度泛函理论结合非绝热分子动力学,我们发现P-P伸缩振动驱动非辐射电子-空穴复合,使纳米孔修饰的单层黑磷的激发态寿命比完美体系延长了约5.5倍。这主要归因于三个因素。一,纳米孔结构不但没有在禁带中引入深能级缺陷,而且由于价带顶下移使带隙增加了0.22 eV。二,除了带隙增加,纳米孔减小了电子和空穴波函数重叠,并抑制了原子核热运动,从而使非绝热耦合降低至完美体系的约1/2。三,退相干时间比完美体系延长了1.5倍。前两个因素战胜了第三个因素,使纳米孔结构激发态寿命延长至2.74 ns,而其在完美体系中约为480 ps。我们的研究表明可以制造合理数量和形貌的缺陷,如纳米孔,降低黑磷非辐射电子-空穴复合,提高光电器件效率。这一研究对于理解和调控黑磷和其它二维材料的激发态性质有重要意义。     

基于分子指纹对黄酮类化合物及其抗DPPH自由基活性的QSAR分析

应用定量构效关系（Quantitative structure activity relationship, QSAR）研究阐明黄酮类化合物（Flavonoid compounds, FCs）的子结构指纹（Substructure fingerprint）与1,1-二苯基-2-三硝基苯肼（1,1-Diphenyl-2-picrylhydrazyl, DPPH）自由基清除能力之间的关系,从而指导高效抗氧化物质的设计和发现。在PubMed数据库中收集77个具有明确抗氧化活性的黄酮类化合物,而在ChEMBL数据库中收集86个无抗DPPH活性的黄酮类化合物。这163个黄酮类化合物的子结构指纹由PubChem系统生成,然后通过卡方检验筛选出与黄酮类化合物的抗氧化活性显著相关的分子指纹,最后通过判别分析建立预测QSAR模型,并采用回代法和交叉验证法对已建立的模型进行准确性和稳健性的验证。结果表明,黄酮类化合物抗DPPH自由基活性与ESSSR环的计数、简单相邻原子的类型和简单的SMARTS模式等因素有关。此外,所建立的QSAR模型能较好地预测黄酮类化合物的DPPH自由基清除活性,可用于评价候选抗氧化剂的潜力。     

Monitoring the Thiol/Thiophenol Molecule-Modulated Plasmon-Mediated Silver Oxidation with Dark-Field Optical Microscopy

Surface plasmon can trigger or accelerate many photochemical reactions, especially useful in energy and environmental industries. Recently, molecular adsorption has proven effective in modulating plasmon-mediated photochemistry, however the realized chemical reactions are limited and the underlying mechanism is still unclear. Herein, by using in situ dark-field optical microscopy, the plasmon-mediated oxidative etching of silver nanoparticles (Ag NPs), a typical hot-hole-driven reaction, is monitored continuously and quantitatively. The presence of thiol or thiophenol molecules is found essential in the silver oxidation. In addition, the rate of silver oxidation is modulated by the choice of different thiol or thiophenol molecules. Compared with the molecules having electron donating groups, the ones having electron accepting groups accelerate the silver oxidation dramatically. The thiol/thiophenol modulation is attributed to the modulation of the charge separation between the Ag NPs and the adsorbed thiol or thiophenol molecules. This work demonstrates the great potential of molecular adsorption in modulating the plasmon-mediated photochemistry, which will pave a new way for developing highly efficient plasmonic photocatalysts.     

Functional Groups Assisted Tunable Dielectric Permittivity of Guest-Free Zn-Based Coordination Polymers for Gate Dielectrics

The dielectric properties of coordination polymers has been a topic of recent interest, but the role of different functional groups on the dielectric properties of these polymers has not yet been fully addressed. Herein, the effects of electron-donating (R=NH₂) and electron-withdrawing (R=NO₂) groups on the dielectric behavior of such materials were investigated for two thermally stable and guest-free Zn-based coordination polymers, [Zn(L₁)(L₂)]_n ( 1 ) and [Zn(L₁)(L₃)]_n ( 2 ) [L₁=2-(2-pyridyl) benzimidazole (Pbim), L₂=5-aminoisophthalate (Aip), and L₃=5-nitroisophthalate (Nip)]. The results of dielectric studies of 1 revealed that it possesses a high dielectric constant (κ=65.5 at 1 kHz), while compound 2 displayed an even higher dielectric constant (κ=110.3 at 1 kHz). The electron donating and withdrawing effects of the NH₂ and NO₂ substituents induce changes in the polarity of the polymers, which is due to the inductive effect from the aryl ring for both NO₂ and NH₂. Theoretical results from density functional theory (DFT) calculations, which also support the experimental findings, show that both compounds have a distinct electronic behavior with diverse wide bandgaps. The significance of the current work is to provide information about the structure-dielectric property relationships. So, this study promises to pave the way for further research on the effects of different functional groups on coordination polymers on their dielectric properties.     

Elucidating the Nature of the External Acid Sites of ZSM-5 Zeolites Using NMR Probe Molecules

The identification of acid and nonacid species at the external surface of zeolites remains a major challenge, in contrast to the extensively-studied internal acid sites. Here, it is shown that the synthesis of zeolite ZSM-5 samples with distinct particle sizes, combined with solid-state NMR and computational studies of trimethylphosphine oxide (TMPO) adsorption, provides insight into the chemical species on the external surface of the zeolite crystals. ¹H–³¹P HETCOR NMR spectra of TMPO-loaded zeolites exhibit a broad correlation peak at δ_P ∼35–55 ppm and δ_H ∼5–12 ppm assigned to external SiOH species. Pore-mouth Brønsted acid sites exhibit ³¹P and ¹H NMR resonances and adsorption energies close to those reported for internal acid sites interacting with TMPO. The presence of an external tricoordinate Al-Lewis site interacting strongly with TMPO is suggested, resulting in ³¹P resonances that overlap with the peaks usually ascribed to the interaction of TMPO with Brønsted sites.     

Enhanced Electrocatalytic Water Oxidation by Interfacial Phase Transition and Photothermal Effect in Multiply Heterostructured Co9S8/Co3S4/Cu2S Nanohybrids

The rational design of advanced nanohybrids (NHs) with optimized interface electronic environment and rapid reaction kinetics is pivotal to electrocatalytic schedule. Herein, we developed a multiple heterogeneous Co₉S₈/Co₃S₄/Cu₂S nanoparticle in which Co₃S₄ germinates between Co₉S₈ and Cu₂S. Using high-angle annular-dark-field imaging and theoretical calculation, it was found that the integration of Co₉S₈ and Cu₂S tends to trigger the interface phase transition of Co₉S₈, leading to Co₃S₄ interlayer due to the low formation energy of Co₃S₄/Cu₂S (−7.61 eV) than Co₉S₈/Cu₂S (−5.86 eV). Such phase transition not only lowers the energy barrier of oxygen evolution reaction (OER, from 0.335 eV to 0.297 eV), but also increases charge carrier density (from 7.76×10¹⁴ to 2.09×10¹⁵ cm⁻³), and creates more active sites. Compared to Co₉S₈ and Cu₂S, the Co₉S₈/Co₃S₄/Cu₂S NHs also demonstrate notable photothermal effect that can heat the catalyst locally, offset the endothermic enthalpy change of OER, and promote carrier migrate, reaction intermediates adsorption/deprotonation to improve reaction kinetics. Profiting from these favorable factors, the Co₉S₈/Co₃S₄/Cu₂S catalyst only requires an OER overpotential of 181 mV and overall water splitting cell voltage of 1.43 V to driven 10 mA cm⁻² under the irradiation of near-infrared light, outperforming those without light irradiation and many reported Co-based catalysts.     

Palladium-Catalyzed Fluorinative Bifunctionalization of Aziridines and Azetidines with gem-Difluorocyclopropanes

An unprecedented Pd-catalyzed fluorinative bifunctionalization of aziridines and azetidines was successfully developed via regioselective C−C and C−F bond cleavage of gem-difluorocyclopropanes, leading to various β,β′-bisfluorinated amines and β,γ-bisfluorinated amines. This reaction was achieved by incorporating a 2-fluorinated allyl group and a fluorine atom scissored from gem-difluorocyclopropane in 100 % atom economy for the first time. The mechanistic investigations indicated that the reaction underwent amine attacking 2-fluorinated allyl palladium complex to generate η²-coordinated N-allyl aziridine followed by fluoride ligand transfer affording the final β- and γ-fluorinated amines.     

Transition Metal-Free Aromatic C−H,C−N,C−S and C−O Borylation

Aromatic organoboron compounds are highly valuable building blocks in organic chemistry. They were mainly synthesized through aromatic C−H and C−Het borylation, in which transition metal-catalysis dominate. In the past decade, with increasing attention to sustainable chemistry, numerous transition metal-free C−H and C−Het borylation transformations have been developed and emerged as efficient methods towards the synthesis of aromatic organoboron compounds. This account mainly focuses on recent advances in transition metal-free aromatic C−H, C−N, C−S, and C−O borylation transformations and provides insights to where further developments are required.     

定标粒子理论计算非水溶液的盐效应常数

本文应用定标粒子理论计算了非电解质溶质在盐(NaI、或KI)和环丁砜组成的非水电解质溶液中溶解度的盐效应常数。硬球作用项采用Masterton-Lee的方法。软球作用项采用胡英等的径向分布函数处理方法, 并考虑进了偶极-偶极、偶极-诱导偶极、电荷-偶极和电荷-诱导偶极等相互作用。分子的硬球直径σ和能量参数∈/k由经验方程计算。由理论值和实验结果比较得出: 当σ_2取0.563 nm、离子半径取电子密度标度时, 理论值与实验值符合得较好。