An atomic- and molecular-level understanding of heterogeneous catalysis is required to characterize the nature of active sites and improve the rational design of catalysts. Achieving this level of ...characterization requires techniques that can correlate catalytic performances to specific surface structures, so as to avoid averaging effects. Tip-enhanced Raman spectroscopy combines scanning probe microscopy with plasmon-enhanced Raman scattering and provides simultaneous topographical and chemical information at the nano/atomic scale from ambient to ultrahigh-vacuum and electrochemical environments. Therefore, it has been used to monitor catalytic reactions and is proposed to correlate the local structure and function of heterogeneous catalysts. Bimetallic catalysts, such as Pd-Au, show superior performance in various catalytic reactions, but it has remained challenging to correlate structure and reactivity because of their structural complexity. Here, we show that TERS can chemically and spatially probe the site-specific chemical (electronic and catalytic) and physical (plasmonic) properties of an atomically well-defined Pd(sub-monolayer)/Au(111) bimetallic model catalyst at 3nm resolution in real space using phenyl isocyanide as a probe molecule (Fig. 1a). We observe a weakened NC bond and enhanced reactivity of phenyl isocyanide adsorbed at the Pd step edge compared with that at the Pd terrace. Density functional theory corroborates these observations by revealing a higher d-band electronic profile for the low-coordinated Pd step edge atoms. The 3nm spatial resolution we demonstrate here is the result of an enhanced electric field and distinct electronic properties at the step edges.
The high performance and low cost of dye-sensitized solar cells (DSSCs) have drawn great interest from both academic and industrial circles. The research on exploring novel efficient sensitizers, ...especially on inexpensive metal-free pure organic dyes, has never been suspended. The donor-π bridge-acceptor (D-π-A) configuration is mainstream in the design of organic sensitizers due to its convenient modulation of the intramolecular charge-transfer nature. Recently, it has been found that incorporation of additional electron-withdrawing units (such as benzothiadiazole, benzotriazole, quinoxaline, phthalimide, diketopyrrolopyrrole, thienopyrazine, thiazole, triazine, cyanovinyl, cyano- and fluoro-substituted phenyl) into the π bridge as internal acceptors, termed the D-A-π-A configuration, displays several advantages such as tuning of the molecular energy levels, red-shift of the charge-transfer absorption band, and distinct improvement of photovoltaic performance and stability. We apply the D-A-π-A concept broadly to the organic sensitizers containing additional electron-withdrawing units between electron donors and acceptors. This
review
is projected to summarize the category of pure organic sensitizers on the basis of the D-A-π-A feature. By comparing the structure-property relationship of typical photovoltaic D-A-π-A dyes, the important guidelines in the design of such materials are highlighted.
This review article provides an insight into how molecular tailoring in D-A-π-A featured sensitizers influences their absorption, energy levels and photovoltaic performances in DSSCs.
Covalent organic frameworks (COFs) have recently emerged as a new generation of porous polymers combining molecular functionality with the robustness and structural definition of crystalline solids. ...Drawing on the recent development of tailor-made semiconducting COFs, we report here on a new COF capable of visible-light driven hydrogen generation in the presence of Pt as a proton reduction catalyst (PRC). The COF is based on hydrazone-linked functionalized triazine and phenyl building blocks and adopts a layered structure with a honeycomb-type lattice featuring mesopores of 3.8 nm and the highest surface area among all hydrazone-based COFs reported to date. When illuminated with visible light, the Pt-doped COF continuously produces hydrogen from water without signs of degradation. With their precise molecular organization and modular structure combined with high porosity, photoactive COFs represent well-defined model systems to study and adjust the molecular entities central to the photocatalytic process.
•The thermal stability and pyrolysis mechanism of cured BPR are investigated.•The high char yield of BPR results from the formed phenyl borates during curing.•Boron oxide is formed on the surface of ...carbonized product during pyrolysis.•The formed boron oxide revealed the cleavage of O–C bonds from phenyl borates.•The graphitization degree and graphite crystallites of PR are improved by introducing boron.
Boron-containing phenolic resin (BPR) is a kind of the ablative resins with high-performance. Due to the lack of the exact knowledge concerning the pyrolysis mechanism of BPR, its development and application are greatly impeded. In the present paper, the chemical structure of the cured BPR and its structural evolution at high temperatures are investigated to clarify the reason for the high char yield of BPR. The results indicate that the high char yield of BPR is mainly attributed to the phenyl borates formed during curing, which can block parts of phenolic hydroxyl groups, and effectively inhibit their thermal decomposition reaction. Boron oxide is formed on the surface of carbonization products by the cleavage of O–C bonds from phenyl borates via pyrolysis, which avoids the release of volatile carbon dioxide and reduces the development of micro-structural defects of carbonization products. Introducing boron into PR improves the graphitization degree and graphite crystallites of carbonization products, which promotes the formation of a more ordered glassy carbon during pyrolysis. This study provides a new vision for the understanding of the high char yield of BPR, which makes it possible to develop a new ablative resin through molecular design.
A series of asymmetric zinc porphyrin (ZnPy) derivatives bearing different external substituents were synthesized and used to sensitize Pt-loaded graphitic carbon nitride (Pt/g-C3N4) for ...photocatalytic H2 production. Among them, ZnPy-1 has one benzoic acid and three phenyls as peripheral substituents, while ZnPy-2, ZnPy-3, and ZnPy-4 contain one benzoic acid and three pseudo-pyridines with different N-atom positions. The experimental results indicate that the pseudo-pyridine substitution for the phenyls in ZnPy-1 lead to enhanced photosensitization with an order of ZnPy-1 < ZnPy-2 < ZnPy-3 < ZnPy-4 under visible light (λ > 420 nm) irradiation. ZnPy-4-sensitized Pt/g-C3N4 (ZnPy-4-Pt/g-C3N4) exhibits the best average H2 production activity of 524 μmol h-1 with an extremely high turnover number (TON) of 11 089 h-1, which is much higher than that (328 μmol h-1) of ZnPy-2-Pt/g-C3N4 with a TON of 6942 h-1. Also, ZnPy-4-Pt/g-C3N4 shows a much higher apparent quantum yield (AQY) of 32.3% than that (11.5%) of ZnPy-2-Pt/g-C3N4 under 420 nm monochromatic light irradiation. The different N-atom positions in the pseudo-pyridines result in different interactions of the ZnPy dyes with a sacrificial reagent, which then strongly influences the photoactivity for H2 production. The present results demonstrate the molecular engineering aspect of ZnPy dyes in which fine-tuning of molecular structures is crucial for improving the photocatalytic H2 production activity of dye-sensitized semiconductors.
The performance of anion exchange membrane fuel cells (AEMFCs) employing Pt or PtRu electrocatalyst and ionomers with different polyaromatic backbones is correlated with the density functional theory ...(DFT)-calculated adsorption energies of the ionomer fragments on the metal surfaces. The performance of the AEMFCs tested in this work significantly changes depending on the backbone structure of polyaromatic ionomer or the type of the catalyst used at the anode. For the same anode catalyst, the performance decreases in the order poly(fluorene) > poly(p-biphenyl alkylene) > poly(terphenyl alkylene)s, which is in excellent agreement with the decrease in the DFT-calculated interaction energies between the catalyst surface and the corresponding ionomer fragment. Namely, DFT-calculated adsorption energies decrease in the order: p-terphenyl ≥ m-terphenyl > biphenyl > diphenyl ether > benzene ≥ o-terphenyl > 9,9-dimethyl fluorene. The trend in the adsorption energies is explained on the basis of the structural and conformational features of the ionomer fragments. Namely, strong adsorption of the polyaromatic ionomer fragments correlates with the number of benzene rings with a low rotational barrier that can bind parallel to the metal surfaces, leading to strong interaction and hybridization of the aromatic π-orbitals with metal electronic states. The results of this work suggest, therefore, that the interaction between the ionomer and electrocatalyst should be taken into account when designing high-performing ionomers even before considering other factors such as hydroxide conductivity, gas permeability, and water uptake of the ionomeric binder.
N-Heterocyclic carbenes (NHCs) are widely used ligands and reagents in modern inorganic synthesis as well as in homogeneous catalysis and organocatalysis. However, NHCs are not always innocent ...bystanders. In the last few years, more and more examples were reported of reactions of NHCs with main-group elements which resulted in modification of the NHC. Many of these reactions lead to ring expansion and the formation of six-membered heterocyclic rings involving insertion of the heteroatom into the C-N bond and migration of hydrides, phenyl groups or boron-containing fragments. Furthermore, a few related NHC rearrangements were observed some decades ago. In this Perspective, we summarise the history of NHC ring expansion reactions from the 1960s till the present.
Expanding the belt! NHC ring expansion reactions and E-E activation from the 1960s till the present are summarized.
Research on aggregation-induced emission (AIE) has been a hot topic. Due to enthusiastic efforts by many researchers, hundreds of AIE luminogens (AIEgens) have been generated which were mainly based ...on archetypal silole, tetraphenylethene, distyrylanthracene, triphenylethene, and tetraphenyl-1,4-butadiene,
To enlarge the family of AIEgens and to enrich their functions, new AIEgens are in high demand. In this work, we report a new kind of AIEgen based on tetraphenylpyrazine (TPP), which could be readily prepared under mild reaction conditions. Furthermore, we show that the TPP derivatives possess a good thermal stability and their emission could be fine-tuned by varying the substituents on their phenyl rings. It is anticipated that TPP derivatives could serve as a new type of widely utilized AIEgen, based on their facile preparation, good thermo-, photo- and chemostabilities, and efficient emission.
Synthesis of a carbon nanobelt Povie, Guillaume; Segawa, Yasutomo; Nishihara, Taishi ...
Science (American Association for the Advancement of Science),
04/2017, Letnik:
356, Številka:
6334
Journal Article
Recenzirano
Odprti dostop
The synthesis of a carbon nanobelt, comprising a closed loop of fully fused edge-sharing benzene rings, has been an elusive goal in organic chemistry for more than 60 years. Here we report the ...synthesis of one such compound through iterative Wittig reactions followed by a nickel-mediated aryl-aryl coupling reaction. The cylindrical shape of its belt structure was confirmed by x-ray crystallography, and its fundamental optoelectronic properties were elucidated by ultraviolet-visible absorption, fluorescence, and Raman spectroscopic studies, as well as theoretical calculations. This molecule could potentially serve as a seed for the preparation of structurally well-defined carbon nanotubes.
Half-sandwich pseudo-octahedral pentamethylcyclopentadienyl Ir
complexes of the type (η
-Cp
)Ir(C^C)ClPF
, where Cp
is pentamethylcyclopentadienyl (Cp*), or its phenyl (Cp
= C
Me
C
H
) or biphenyl ...(Cp
= C
Me
C
H
C
H
) derivatives, and the C^C-chelating ligands are different N-heterocyclic carbene (NHC) ligands, have been synthesized and characterized. Three X-ray crystal structures have been determined. Except for Cp* complex 1A, the other eleven complexes 1B-4C all showed potent cytotoxicity, with IC
values ranging from 2.9 to 46.3 μM toward HeLa human cervical cancer cells. The potency toward HeLa cells increased with additional phenyl substitution on Cp*: Cp
> Cp
> Cp*, and increased with the size of chain substitution on the C^C-ligand in the order: ph > butyl > ethyl > methyl. Complex (η
-C
Me
C
H
C
H
)Ir(L4)ClPF
(4C) displayed the highest potency, and was about 3 times more active than the clinical platinum drug cisplatin. Complexes 1A-4C all undergo hydrolysis and their kinetics was studied. DNA binding appears not to be the major mechanism of action. The ability of these iridium complexes to catalyze hydride transfer from the coenzyme NADH to NAD
was studied. Complexes (η
-C
Me
C
H
C
H
)Ir(L2)ClPF
(2C) and (η
-C
Me
C
H
C
H
)Ir(L3)ClPF
(3C) cause cell apoptosis and arrest the cell cycle at the G
phase and G
/M phase when HeLa cancer cells are treated with different IC
concentrations of the complexes, and increase the amount of reactive oxygen species (ROS) dramatically, which appears to contribute to the anticancer activity. This class of organometallic Ir complexes has unusual features worthy of further exploration in the design of novel anticancer drugs.
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