Understanding Photocatalytic Overall Water Splitting of ß-Ketoamine COFs through the N-C Site Synergistic Mechanism.

Overcoming the sluggish reaction kinetics of the oxygen evolution reaction (OER) is a determining factor for the practical application of photocatalysts for overall water splitting. Two-dimensional covalent organic frameworks (2D-COFs) offer an ideal platform for catalyst design in the field of overall water splitting for their exceptional chemical tunability and high efficiency of light capture. In this work, four ß-ketoamine 2D-COFs, consisting of 1,3,5-triformylphloroglucinol (Tp) groups and different linkers with pyridine segments, were constructed and optimized. By means of first-principles calculations, the band structures, free energy changes of photocatalytic hydrogen evolution reaction (HER) and OER, and charge density distributions were calculated and investigated systemically to discuss the visible-light response, overall water splitting activities on active sites, and the characteristic of charge transfer and separation. The protonated pyridine N derived from the double-H2O closed-ring H-bond adsorption model could efficiently induce N-C sites' synergistic effect between pyridine N and its ortho-position C to minimize the OER energy barrier and to enhance the charge transfer and separation. A N-C site synergistic mechanism has been proposed to provide a comprehensive explanation for the experimental results and a new strategy to design novel 2D-COF photocatalysts for overall water splitting.

Engineering ß-ketoamine covalent organic frameworks for photocatalytic overall water splitting.

Covalent organic frameworks (COFs) are an emerging type of crystalline and porous photocatalysts for hydrogen evolution, however, the overall water splitting activity of COFs is rarely known. In this work, we firstly realized overall water splitting activity of ß-ketoamine COFs by systematically engineering N-sites, architecture, and morphology. By in situ incorporating sub-nanometer platinum (Pt) nanoparticles co-catalyst into the pores of COFs nanosheets, both Pt@TpBpy-NS and Pt@TpBpy-2-NS show visible-light-driven overall water splitting activity, with the optimal H2 and O2 evolution activities of 9.9 and 4.8 µmol in 5 h for Pt@TpBpy-NS, respectively, and a maximum solar-to-hydrogen efficiency of 0.23%. The crucial factors affecting the activity including N-sites position, nano morphology, and co-catalyst distribution were systematically explored. Further mechanism investigation reveals the tiny diversity of N sites in COFs that induces great differences in electron transfer as well as reaction potential barriers.

Novel quadrilateral-pore 2D-COFs as visible-light driven catalysts evaluated by the descriptor of integrated pz-orbital population.

In the past few decades, strategies for designing new two-dimensional covalent organic framework (2D-COF) structures have been limited to the shape of positive hexagonal pores, and the underlying relationship between their structure and electronic properties still remains unclear. Herein, novel 2D-COFs with C, N and H elements confined to the quadrilateral-pore skeleton based on first-principles calculations and the topological assembly of different benzene-based building blocks were designed and studied. These 2D-COFs enriched the topology types and can offer an ideal platform for band engineering aimed at spontaneously driving the hydrogen evolution reaction (HER) under visible light irradiation. The approach for regulating pore structures on nodes, linkers and linkages can effectively tune band gaps, and thus the 2D-COF, consisting of benzene building blocks and imine linkages, has the optimal activity for the photocatalytic HER under common visible light conditions. Furthermore, the integrated pz-orbital population was found to evaluate the photocatalytic activity efficiently. We demonstrate that the pz-orbital population is in linear relationship with the intensity of H+ adsorption, indicating that the total contribution of the pz-orbital electrons can be an efficient descriptor for screening suitable 2D-COF structures for use as photocatalysts for the HER. Therefore, this work presents a new strategy for designing novel quadrilateral-pore 2D-COFs as visible-light photocatalysts and provides an important insight into the relationship between catalytic activity and the population of activated electrons.

Construction of Six-Oxygen-Coordinated Single Ni Sites on g-C3 N4 with Boron-Oxo Species for Photocatalytic Water-Activation-Induced CO2 Reduction.

The configuration regulation of single-atom photocatalysts (SAPCs) can significantly influence the interfacial charge transfer and subsequent catalytic process. The construction of conventional SAPCs for aqueous CO2 reduction is mainly devoted toward favorable activation and photoreduction of CO2 , however, the role of water is frequently neglected. In this work, single Ni atoms are successfully anchored by boron-oxo species on g-C3 N4 nanosheets through a facile ion-exchange method. The dative interaction between the B atom and the sp2 N atom of g-C3 N4 guarantees the high dispersion of boron-oxo species, where O atoms coordinate with single Ni (II) sites to obtain a unique six-oxygen-coordinated configuration. The optimized single-atom Ni photocatalyst, rivaling Pt-modified g-C3 N4 nanosheets, provides excellent CO2 reduction rate with CO and CH4 as products. Quasi-in-situ X-ray photoelectron spectra, transient absorption spectra, isotopic labeling, and in situ Fourier transform infrared spectra reveal that as-fabricated six-oxygen-coordinated single Ni (II) sites can effectively capture the photoelectrons of CN along the BO bridges and preferentially activate adsorbed water to produce H atoms to eventually induce a hydrogen-assisted CO2 reduction. This work diversifies the synthetic strategies for single-atom catalysts and provides insight on correlation between the single-atom configuration and reaction pathway.

Ultrathin Phosphate-Modulated Co Phthalocyanine/g-C3N4 Heterojunction Photocatalysts with Single Co-N4 (II) Sites for Efficient O2 Activation.

Realization of solar-driven aerobic organic transformation under atmospheric pressure raises the great challenge for efficiently activating O2 by tailored photocatalysts. Guided by theoretical calculation, phosphate groups are used to induce the construction of ultrathin Co phthalocyanine/g-C3N4 heterojunctions (CoPc/P-CN, ≈4 nm) via strengthened H-bonding interfacial connection, achieving an unprecedented 14-time photoactivity improvement for UV-vis aerobic 2,4-dichlorophenol degradation compared to bulk CN by promoted activation of O2. It is validated that more •O2 - radicals are produced through the improved photoreduction of O2 by accelerated photoelectron transfer from CN to the ligand of CoPc and then to the abundant single Co-N4 (II) catalytic sites, as endowed by the matched dimension, intimate interface even at the molecular level, and high CoPc dispersion of resulted heterojunctions. Interestingly, CoPc/P-CN also exhibits outstanding photoactivities in the aerobic oxidation of aromatic alcohols. This work showcases a feasible route to realize efficient photocatalytic O2 activation by exploiting the potential of ultrathin metal phthalocyanine (MPc) assemblies with abundant single-atom sites. More importantly, a universal facile strategy of H-bonding-dominating construction of MPc-involved heterojunctions is successfully established.

Rational Design of MOF/COF Hybrid Materials for Photocatalytic H2 Evolution in the Presence of Sacrificial Electron Donors.

Crystalline and porous covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) materials have attracted enormous attention in the field of photocatalytic H2 evolution due to their long-range order structures, large surface areas, outstanding visible light absorbance, and tunable band gaps. In this work, we successfully integrated two-dimensional (2D) COF with stable MOF. By covalently anchoring NH2 -UiO-66 onto the surface of TpPa-1-COF, a new type of MOF/COF hybrid materials with high surface area, porous framework, and high crystallinity was synthesized. The resulting hierarchical porous hybrid materials show efficient photocatalytic H2 evolution under visible light irradiation. Especially, NH2 -UiO-66/TpPa-1-COF (4:6) exhibits the maximum photocatalytic H2 evolution rate of 23.41 mmol g-1 h-1 (with the TOF of 402.36 h-1 ), which is approximately 20 times higher than that of the parent TpPa-1-COF and the best performance photocatalyst for H2 evolution among various MOF- and COF-based photocatalysts.

Transport and Photoelectric Properties of 2D Silicene/MX2 (M = Mo, W; X = S, Se) Heterostructures.

The transport and photoelectric properties of four two-dimensional (2D) silicene/MX2 (M = Mo, W; X = S, Se) heterostructures have been investigated by employing density functional theory, nonequilibrium Green's function, and Keldysh nonequilibrium Green's function methods. The stabilities of silicene (SiE) are obviously improved after being placed on the MX2 (M = Mo, W; X = S, Se) substrates. In particular, the conductivities of SiE/MX2 are enhanced compared with free-standing SiE and MX2. Moreover, the conductivities are increased with the group number of X, i.e., in the order of SiE < SiE/MS2 < SiE/MSe2. An evident current oscillation phenomenon is observed in the SiE/WX2 heterostructures. When a linear light illumination is applied, SiE/MSe2 shows a stronger photoresponse than SiE/MS2. The maximum photoresponse with a value of 9.0a 0 2/photon was obtained for SiE/WSe2. More importantly, SiE/MS2 (M = Mo, W) heterostructures are good candidates for application in designing solar cells owing to the well spatial separation of the charge carriers. This work provides some clues for further exploring 2D SiE/MX2 heterostructures involving tailored photoelectric properties.

Tunable transport and optoelectronic properties of monolayer black phosphorus by grafting PdCl2 quantum dots.

The electronic, transport, and optoelectronic properties of monolayer black phosphorus (MLBP) are much influenced by grafting PdCl2 groups, demonstrated here by using density functional theory (DFT) and non-equilibrium Green's function (NEGF) as well as the Keldysh Nonequilibrium Green's Functions (KNEGF) methods. We find that the PdCl2 groups prefer to locate over the furrow site of MLBP and form a planar quadridentate structure of . The PdCl2 groups serve as quantum dots by introducing discrete flat levels between the MLBP valence band and the Fermi level (E f). The conductivity is much lowered after attaching PdCl2 quantum dots, due to the fact that the scattering effect of PdCl2 plays a major role in the process of electron transporting. A threshold voltage is found for the functionalized system with a large density of PdCl2 quantum dots, a valuable clue for exploring current switches. However, no evident threshold voltage is found for the pure MLBP. Electrons permeate easier through the armchair direction compared with the zigzag either in the pure MLBP or in the functionalized composites. More importantly, grafting PdCl2 quantum dots is very beneficial for enhancing photoresponse. The values of photoresponse for the modified species are about 20 times higher than the free MLBP. A significant photoresponse anisotropy is observed for both MLBP and nPdCl2-BP (n = 1, 2, and 4), contrary to the conductivity, the zigzag direction shows much stronger photoresponse than the armchair. All of the aforementioned unique properties make these new two-dimensional (2D) MLBP based materials especially attractive for both electronic and optoelectronic devices.

Electronic and transport properties of the (VBz)n@MoS2NT nanocable.

The electronic structure of a novel inorganic (8, 8) MoS2 nanotube nanocable, (VBz)n@MoS2NT, (where Bz refers as C6H6), is investigated using density functional theory. Transport property calculations are further performed employing non-equilibrium Green's function methods by modeling a two-probe device with a finite-sized nanocable sandwiched between two electrodes of its own. It is found that the transport properties of the nanocable agree well with its electronic structure. The core (VBz)n nanowire in the (VBz)n@MoS2NT plays a significant role in electron transportation, meanwhile, the sheath MoS2NT also participates in electron transportation. This phenomenon is different from those of (VBz)n@CNT and (VBz)n@BNNT nanocables. For the (VBz)n@CNT, the transport properties are majorly dominated by the metallic CNT sheath, while for the (VBz)n@BNNT, it is merely decided by the core (VBz)n. The conductivity of the (VBz)n@MoS2NT is slightly better in comparison with pure (VBz)n. Similar to pure (VBz)n, the (VBz)n@MoS2NT shows spin-polarized transport properties: the spin-down state gives a higher conductivity than the spin-up state. The values of the spin filter efficiency of the (VBz)n@MoS2NT can be up to >80%, indicating it to be a good candidate for spin filters. In addition, it is also found that encapsulating (VBz)n into the MoS2NT could introduce magnetism. More importantly, the ferromagnetic (VBz)n@MoS2NT is thermally rather stable. Therefore, encapsulating (VBz)n into the MoS2NT can effectively tune the electronic and transport properties for exploring novel functional nanodevices.

Electronic and transport properties of PSi@MoS2 nanocables.

Electronic structures and transport properties of prototype MoS2 nanotube (15, 0) nanocables, including undoped PSi@MoS2 and B- and P-doped PSi@MoS2 (where PSi refers to polysilane), are investigated using the density functional theory (DFT) and the non-equilibrium Green's function (NEGF) methods. It is found that transport properties of two-probe systems by sandwiching finite long nanocables between two Au electrodes are basically in agreement with the electronic structures of their corresponding infinitely long systems. Encapsulating undoped and doped PSi nanowires inside the MoS2 nanotubes could not significantly affect the electronic and transport properties. B-doping and P-doping upon PSi play different roles in the electronic and transport properties. B-doping may exert constructive and destructive effects on electron transport depending on its position and applied bias direction, while P-doping displays a negligible effect. In addition, we found that bi-doping by two adjacent B atoms could slightly enhance the conductivity. These results could offer some clues for conducting experiments to achieve nanoelectronic devices with intrinsic transport properties of MoS2 nanotubes.

Enantioselective desymmetrization via carbonyl-directed catalytic asymmetric hydroboration and Suzuki-Miyaura cross-coupling.

The rhodium-catalyzed enantioselective desymmetrization of symmetric γ,δ-unsaturated amides via carbonyl-directed catalytic asymmetric hydroboration (directed CAHB) affords chiral secondary organoboronates with up to 98% ee. The chiral γ-borylated products undergo palladium-catalyzed Suzuki-Miyaura cross-coupling via the trifluoroborate salt with stereoretention.

Mechanistic Insights into Carbonyl-Directed Rhodium-Catalyzed Hydroboration: ab Initio Study of a Cyclic γ,δ-Unsaturated Amide.

A two-point binding mechanism for the cationic rhodium(I)-catalyzed carbonyl-directed catalytic asymmetric hydroboration of a cyclic γ,δ-unsaturated amide is investigated using density functional theory. Geometry optimizations and harmonic frequency calculations for the model reaction are carried out using the basis set 6-31+G** for C, O, P, B, N, and H and LANL2DZ for Rh atoms. The Gibbs free energy of each species in THF solvent is obtained based on the single-point energy computed using the PCM model at the ECP28MWB/6-311+G(d,p) level plus the thermal correction to Gibbs free energy by deducting translational entropy contribution. The Rh-catalyzed reaction cycle involves the following sequence of events: (1) chelation of the cyclic γ,δ-unsaturated amide via alkene and carbonyl complexation in a model active catalytic species, [Rh(L2)2S2]+, (2) oxidative addition of pinacol borane (pinBH), (3) migratory insertion of the alkene double bond into Rh-H (preferred pathway) or Rh-B bond, (4) isomerization of the resulting intermediate, and finally, (5) reductive elimination to form the B-C or H-C bond with regeneration of the catalyst. Free energy profiles for potential pathways leading to the major γ-borylated product are computed and discussed in detail. The potential pathways considered include (1) pathways proceeding via migratory insertion into the Rh-H bond (pathways I, I-1, and I-2), (2) a potential pathway proceeding via migratory insertion into the Rh-B bond (pathway II), and two potential competing routes to a ß-borylated byproduct (pathway III). The results find that the Rh-H migratory insertion pathway I-2, followed in sequence by an unanticipated isomerization via amide rotation and reductive elimination, is the most favorable reaction pathway. A secondary consequence of amide rotation is access to a competing ß-hydride elimination pathway. The pathways computed in this study are supported by and help explain related experimental results.

Theoretical investigation of one- and two-photon absorption properties of platinum acetylide chromophores.

In this paper, we have theoretically investigated bis((4-phenylethynyl)phenyl) ethynyl)bis(trimethylphosphine)platinum(II) (PE2) and its analogs three platinum acetylide complexes (1-3) that feature highly pi-conjugated ligands (alkynyl-dimethylfluorene substituted with electron-donating or -withdrawing moieties). The geometrical and electronic structures are calculated at the ECP60MWB//6-31G*(H, C, P, N, S) basis set level by the density functional theory (DFT) method; one-photon absorption properties have been calculated by using time-dependent DFT (TDDFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods, and two-photon absorption (TPA) properties are obtained with the ZINDO/sum-over-states method. The values of beta(sp) and beta(d) for Pt are adjusted to -1 eV and -28.5 eV, respectively, to make one-photon absorption spectra calculated by ZINDO closest to the experimental data and TDDFT results. The calculated results indicate that all molecules in this work (involving cis isomers of molecules 1-3) take on two TPA peaks in the 600-800 nm region. The peak at 700-750 nm should not be simply attributed to the appearance of noncentrosymmetric cis isomers in solution, although trans and cis isomers adhere to a different selection rule. Every TPA peak results from its transition character. Molecules 1-3 show greater two-photon absorption strength compared with PE2 and retain good transparency.

Theoretical investigation of one-photon and two-photon absorption properties for multiply N-confused porphyrins.

We have theoretically investigated a series of multiply N-confused porphyrins and their Zn or Cu complexes for the first time by using DFT(B3LYP/6-31G*) and ZINDO/SOS methods. The electronic structure, one-photon absorption (OPA), and two-photon absorption (TPA) properties have been studied in detail. The calculated results indicate that the OPA spectra of multiply N-confused porphyrins are red-shifted and the OPA intensities decrease compared to normal porphyrin. The maximum two photon absorption wavelengths lambda(max) are blue-shifted and the TPA cross sections delta(max) are increased 22.7-112.1 GM when the N atoms one by one are inverted from core to beta position to form multiply N-confused porphyrins. Especially delta(max) of N3CP get to 164.7 GM. The electron donors -C6F5s at meso-position can make the TPA cross section delta(max) increase. After forming metal complexes with Cu or Zn, the TPA properties of multiply N-confused porphyrins are further increased except for N3CP, N4CP. Our theoretical findings demonstrate that the multiply N-confused prophyrins as well as their metal complexes and derivatives are promising molecules that can be assembled series of materials with large TPA cross section, and are sure to be the subject of further investigation.