Fluorination of porphyrin ß-periphery boosts nickel(II)-catalyzed hydrogen evolution reaction.

Tunichlorin, the naturally occurring chlorophyll cofactor containing Ni(II) ion, sets up a golden standard for designing the electrocatalysts for hydrogen evolution reaction (HER) via ß-peripheral modification. Besides the fine-tuning of the porphyrin ß-periphery such as adjusting the aromatics (the saturated level of tetrapyrrole) or installing hydroxyl group (hydrogen bond network) to enhance the catalytic HER efficiency, here we report that ß-fluorination of porphyrin is also an important approach to increase the reactivity of Ni(II) center. Benefiting the previously reported derivatization of ß-fluorinated porpholactones, we constructed a ß-fluorinated tunichlorin mimic (6). Compared with the non-fluorinated analogs (1, 3, and 5), we found that 2, 4, and 6 exhibit significant electrocatalytic HER reactivity acceleration (in terms of turnover frequencies, TOF, s-1) of ca. 37, 170, 133-fold, respectively. Mechanism studies suggested that ß-fluorination negatively shifts the metal complexes' reduction potentials and accelerates the electron transfer process, both contributing to the boosting of HER reaction. Notably, 6 showed an 890-fold increase of TOFs than 1, demonstrating the combining advantages of the of fluorination, hydrogenation, and hydroxylation at porphyrin ß-periphery.

Preparation, Characterization, and Antioxidant Activities of Extracts from Amygdalus persica L. Flowers.

A novel water-soluble Amygdalus persica L. flowers polysaccharide (APL) was successfully isolated and purified from Amygdalus persica L. flowers by hot water extraction. Its chemical components and structure were analyzed by IR, GC-MS, and HPLC. APL consisted of rhamnose, arabinose, mannose and glucose in a molar ratio of 0.17:0.034:1.0:0.17 with an average molecular weight of approximately 208.53 kDa and 15.19 kDa. The antioxidant activity of APL was evaluated through radical scavenging assays using 1,1-diphenyl-2-picrylhydrazyl (DPPH), 3-ethylbenzthiazoline-6-sulfonic acid (ABTS), Hydroxyl radical scavenging, Superoxide radical scavenging, and the reducing power activity was also determined in vitro. Besides, in vivo antioxidant experiment, zebrafish (Danio rerio) embryos were treated with different concentrations of APL and then exposed to LPS to induce oxidative stress. Treatment with APL at 50 or 100 µg/mL significantly reduced LPS-induced oxidative stress in the zebrafish, demonstrating the strong antioxidant activity of APL. Moreover, the effect of APL on zebrafish depigmentation was tested by analyzing the tyrosinase activity and melanin content of zebrafish embryos. APL showed a potential reduction in the total melanin content and tyrosinase activity after treatment. This work provided important information for developing a potential natural antioxidant in the field of cosmetics and food.

Terminal Group-Oriented Self-Assembly to Controllably Synthesize a Layer-by-Layer SnSe2 and MXene Heterostructure for Ultrastable Lithium Storage.

Heterostructured materials integrate the advantages of adjustable electronic structure, fast electron/ions transfer kinetics, and robust architectures, which have attracted considerable interest in the fields of rechargeable batteries, photo/electrocatalysis, and supercapacitors. However, the construction of heterostructures still faces some severe problems, such as inferior random packing of components and serious agglomeration. Herein, a terminal group-oriented self-assembly strategy to controllably synthesize a homogeneous layer-by-layer SnSe2 and MXene heterostructure (LBL-SnSe2 @MXene) is designed. Benefitting from the abundant polar terminal groups on the MXene surface, Sn2+ is induced into the interlayer of MXene with large interlayer spacing, which is selenized in situ to obtain LBL-SnSe2 @MXene. In the heterostructure, SnSe2 layers and MXene layers are uniformly intercalated in each other, superior to other heterostructures formed by random stacking. As an anode for lithium-ion batteries, the LBL-SnSe2 @MXene is revealed to possess strong lithium adsorption ability, the small activation energy for lithium diffusion, and excellent structure stability, thus achieving outstanding electrochemical performance, especially with high specific capacities (1311 and 839 mAh g-1 for initial discharge and charge respectively) and ultralong cycling stability (410 mAh g-1 at 5C even after 16 000 cycles). This work conveys an inspiration for the controllable design and construction of homogeneous layered heterostructures.

Stereoselective On-Surface Cyclodehydrofluorization of a Tetraphenylporphyrin and Homochiral Self-Assembly.

The thermally induced cyclodehydrofluorization of iron tetrakis(pentafluorophenyl)porphyrin proceeded highly stereoselectively to give a prochiral product on a gold surface in an ultrahigh vacuum, whereas dehydrocyclization of the respective iron tetrakisphenylporphyrin did not show such selectivity. Stereoselectivity was predominantly observed for closely packed layers, which is an indication of intermolecular cooperativity and steric constraints induced by adjacent species. Density functional theory identified intermolecular packing constraints as the origin of such selectivity during the reaction. Scanning tunneling microscopy revealed the formation of an enantiomerically pure two-dimensional self-assembly as a conglomerate of mirror domains. On-surface two-dimensional topochemistry, as reported herein, may open new routes for stereoselective synthesis.

Enhancing the reactivity of nickel(ii) in hydrogen evolution reactions (HERs) by ß-hydrogenation of porphyrinoid ligands.

Fine-tuning of the porphyrin ß-periphery is important for naturally occurring metal tetrapyrroles to exert diverse biological roles. Here we describe how this approach is also applied to design molecular catalysts, as exemplified by Ni(ii) porphyrinoids catalyzing the hydrogen evolution reaction (HER). We found that ß-hydrogenation of porphyrin remarkably enhances the electrocatalytic HER reactivity (turnover frequencies of 6287 vs. 265 s-1 for Ni(ii) chlorin (Ni-2) and porphyrin (Ni-1), and of 1737 vs. 342 s-1 for Ni(ii) hydroporpholactone (Ni-4) and porpholactone (Ni-3), respectively) using trifluoroacetic acid (TFA) as the proton source. DFT calculations suggested that after two-electron reduction, ß-hydrogenation renders more electron density located on the Ni center and thus prefers to generate a highly reactive nickel hydride intermediate. To demonstrate this, decamethylcobaltocene Co(Cp*)2 was used as a chemical reductant. [Ni-2]2- reacts ca. 30 times faster than [Ni-1]2- with TFA, which is in line with the electrocatalysis and computational results. Thus, such subtle structural changes inducing the distinctive reactivity of Ni(ii) not only test the fundamental understanding of natural Ni tetrapyrroles but also provide a valuable clue to design metal porphyrinoid catalysts.

Highly near-IR emissive ytterbium(iii) complexes with unprecedented quantum yields.

The design of highly near-infrared (NIR) emissive lanthanide (Ln) complexes is challenging, owing to the lack of molecular systems with a high sensitization efficiency and the difficulty of achieving a large intrinsic quantum yield. Previous studies have reported success in optimizing individual factors and achieving high overall quantum yields, with the best yield being 12% for Yb(iii). Herein we report a series of highly NIR emissive Yb complexes, in which the Yb is sandwiched between an octafluorinated porphyrinate antenna ligand and a deuterated Kläui ligand, which allowed optimization of two factors in the same system, and one of the complexes had an unprecedented quantum yield of 63% (estimated uncertainty 15%) in CD2Cl2 with a long lifetime (τobs) of 714 µs. Systematic analysis of the structure-photophysical properties relationship suggested that porphyrinates are effective antenna ligands with a sensitization efficiency up to ca. 100% and that replacement of the high-energy C-H oscillators in porphyrinate and Kläui ligands significantly improves the intrinsic quantum yield up to 75% (τobs/τrad), both of which contribute to enhancing the NIR emission intensity of Yb(iii) up to 25-fold. Besides the high luminescence efficiency, these Yb complexes have other attractive features such as excitation in the visible range and large extinction coefficients which make these Yb(iii) complexes outstanding optical materials in the NIR region.

Evidence for Ultralow-Energy Vibrations in Large Organic Molecules.

The quantum efficiency or the rate of conversion of incident photon to free electron in photosynthesis is known to be extremely high. It has long been thought that the origin of this efficiency are molecular vibrations leading to a very fast separation of electrons and holes within the involved molecules. However, molecular vibrations are commonly in the range above 100 meV, which is too high for excitations in an ambient environment. Here, we analyze experimental spectra of single organic molecules on metal surfaces at ∼4 K, which often exhibit a pronounced dip. We show that measurements on iron(II) [tetra-(pentafluorophenyl)]porphyrin resolve this single dip at 4 K into a series of step-shaped inelastic excitations at 0.4 K. Via extensive spectral maps under applied magnetic fields and corresponding theoretical analysis we find that the dip is due to ultralow-energy vibrations of the molecular frame, typically in the range below 20 meV. The result indicates that ultralow energy vibrations in organic molecules are much more common than currently thought and may be all-pervasive for molecules above a certain size.