Two-dimensional self-assembly and co-assembly of two tetracarboxylic acid derivatives investigated by STM.

In this work, the two-dimensional self-assembly and co-assembly behaviors of two tetracarboxylic acid derivatives (H4BDETP and H4BTB) were investigated by scanning tunneling microscopy (STM). H4BDETP molecules self-assembled into linear nanostructures, and H4BTB molecules formed lamellar and tetragonal nanostructures. The formation of a H4BDETP/H4BTB co-assembly nanostructure was closely related to the deposition sequence of H4BDETP and H4BTB on highly oriented pyrolytic graphite (HOPG). The introduction of H4BTB into the self-assembly system of H4BDETP resulted in the emergence of the H4BDETP/H4BTB nanostructure, while the addition of H4BDETP had no effect on the self-assembly system of H4BTB and a H4BDETP/H4BTB co-assembly nanostructure was not obtained.

Ultra-stable two-dimensional metal-organic frameworks for photocatalytic H2 production.

Two-dimensional (2D) metal-organic frameworks (MOFs) are some of the most promising photocatalysts owing to their high numbers of exposed active sites and excellent charge mobility. However, the synthesis of highly stable 2D MOF photocatalysts involves challenges, and examples have been rarely reported. Herein, a new kind of material, 2D indium-based porphyrin MOF cubic nanosheets (2D In-TCPP NS) with an average thickness of ∼3.97 nm, is synthesized via a surfactant-assisted approach, and it shows good chemical stability in the pH range of 2-11 in aqueous solution. In photocatalytic H2-generation experiments, 2D In-TCPP NS exhibits activity that is enhanced by over one order of magnitude compared with the 3D bulk In-TCPP MOF, arising from its highly enhanced electron-hole separation abilities. Moreover, after 40 h of continuous photocatalysis testing, 2D In-TCPP NS shows nearly no activity decrease, which suggests its great potential for practical commercial use.

Regulation of the Assembled Structure of a Flexible Porphyrin Derivative Containing Tetra Isophthalic Acids by Coronene or Different Pyridines.

Based on previous research, a new coassembly formed by a porphyrin derivative (IPETPP), which contains a flexible substituent of m-phthalic acid, is observed with coronene (COR) molecules at a higher concentration. Besides, a fresh IPETPP self-assembly formed at a lower concentration and another new coassembly with COR molecules are obtained. Moreover, the addition of a series of bipyridines alters the diamond arrangement of IPETPP, which enhances the stability of the two-component structures. It is unprecedented that bipyridine derivatives break intermolecular hydrogen bonds containing m-phthalic acid substituents. All the coassemblies are investigated by scanning tunneling microscopy on a highly oriented pyrolytic graphite. Combined with density functional theory, the formation mechanism of the assembled structures is revealed. These results would contribute to understanding the interfacial crystal behaviors and probably provide an efficient pathway to regulate the binary structures.

Regulation of a Porphyrin Derivative Containing Two Symmetric Benzoic Acids by Different Pyridines.

A porphyrin derivative called 5,15-di(4-carboxyphenyl)porphyrin (H2DCPp) with carboxyl groups successfully self-assembled on a highly oriented pyrolytic graphite (HOPG) surface and its co-assembly structures with three kinds of pyridine molecules were investigated by scanning tunneling microscopy (STM) with atomic resolution. H2DCPp arranged in a long-range ordered structure, and both 1,4-bis (pyridin-4-ylethynyl) benzene (BisPy), 4,4'-bipyridine (BP) and 1,3,5-tris(pyridin-4-ylethynyl) benzene (TPYB) molecules successfully regulated the host molecules as guest molecules. The well-organized model optimized by density functional theory (DFT) calculations reveals the detailed behavior of the assembly characteristics and regulation of porphyrin derivatives, which is helpful for the research and development of solar cells and nanodevices.

Efficient Schottky Junction Construction in Metal-Organic Frameworks for Boosting H2 Production Activity.

Manipulation of the co-catalyst plays a vital role in charge separation and reactant activation to enhance the activity of metal-organic framework-based photocatalysts. However, clarifying and controlling co-catalyst related charge transfer process and parameters are still challenging. Herein, three parameters are proposed, V transfer (the electron transfer rate from MOF to co-catalyst), D transfer (the electron transfer distance from MOF to co-catalyst), and V consume (the electron consume rate from co-catalyst to the reactant), related to Pt on UiO-66-NH2 in a photocatalytic process. These parameters can be controlled by rational manipulation of the co-catalyst via three steps: i) Compositional design by partial substitution of Pt with Pd to form PtPd alloy, ii) location control by encapsulating the PtPd alloy into UiO-66-NH2 crystals, and iii) facet selection by exposing the encapsulated PtPd alloy (100) facets. As revealed by ultrafast transient absorption spectroscopy and first-principles simulations, the new Schottky junction (PtPd (100)@UiO-66-NH2) with higher V transfer and V consume exhibits enhanced electron-hole separation and H2O activation than the traditional Pt/UiO-66-NH2 junction, thereby leading to a significant enhancement in the photoactivity.

π-Conjugated Macrocycle Host-Guest Coassembly with C60 on HOPG.

Two kinds of π-conjugated macrocycles with aggregation-induced emission (AIE) properties were investigated by scanning tunneling microscopy (STM) to elucidate their self-assembly behaviors and interaction with C60 on a highly oriented pyrolytic graphite (HOPG) surface. Both TPEMC and TPEMCS could self-assemble into orderly cavity structures. However, C60 guest molecules could only successfully enter the cavity of TPEMC to form a stable TPEMC + C60 host-guest coassembly structure. Density functional theory (DFT) calculations were also used to interpret the assembly mechanisms. This work disclosed the assembly characteristic of these new types of conjugated macrocyclic compounds, which was helpful to develop new structural porous luminescent materials.

Progress in self-assemblies of macrocycles at the liquid/solid interface.

Macrocyclic self-assemblies have gained great interest for diversified structures and potential applications, such as catalysis, magnetism, photovoltaic devices, organic light-emitting diodes. Macrocycles can present regular assembly systems at the liquid/solid interface due to theπ-conjugated structures. Furthermore, suitable guest molecules can be selected for constructing multi-component supramolecular co-assemblies. This review mainly summarizes macrocyclic self-assembly structures with different shapes in recent years. All of the studies are completed with the assistance of scanning tunneling microscope at the liquid/solid interface.

Optimization of plasmonic metal structures for improving the hydrogen production efficiency of metal-organic frameworks.

The introduction of plasmonic metals of gold (Au) nanoparticles onto metal-organic frameworks (MOFs) has been testified as an efficient approach to boost the H2 evolution ability because the excited Au particles can generate hot electrons that can then be injected into MOFs to inhibit the recombination of charges. Generally, Au particles possess two modes of polarization, which are transverse and longitudinal polarizations. However, which of the two modes is more efficient remains unclear and is yet to be disclosed. Herein, we report a strategy of finely controlling the transverse and longitudinal polarizations by gradually reducing the length, without changing the width of Au rods, and then combining with MOF Pt@MIL-125 to construct Pt@MIL-125/Au Schottky junctions, for investigating the polarization mode effects. The results indicate that with the reduction in the length of Au rods from 67 nm to 38 nm, the transverse polarization will be increased, which can lead to a significant enhancement in the electron-hole separation efficiency and H2 generation activity. When Au is in spherical shape, the transverse polarization effect reaches the highest level, thereby achieving the highest H2 production rate of 265.1 µmol g-1 h, which is 1.6 times more than that of 67 nm Au rods. The enhancement can be attributed to the significant accelerated electron transfer rate induced by transverse polarization and evidenced by ESR analysis. The results indicate that the transverse polarization is more efficient for hot electron injection and highlight that the Au sphere is a more appropriate candidate for producing the maximum SPR effect during photocatalysis.

Solvent-Dependent Core-Modified Rubyrin Self-Assembly at Liquid/Solid Interfaces.

Scanning tunneling microscopy (STM) was utilized to disclose four novel core-modified rubyrin self-assembly behaviors on the highly-oriented pyrolytic graphite (HOPG) surface, of which N2S4-OR(1)/N2Se4-OR(2) had no phenanthrene pyrrole ring and N2S4-OR(3)/N2Se4-OR(4) had phenanthrene-fused pyrrole rings and meso-aryl substituents. It was discovered that the core-modified rubyrin could self-assemble into either face-on or edge-on monolayer structures selectively at the liquid/HOPG interface in different solvents. There was an obvious solvent-dependent self-assembly for N2S4-OR(3)/N2Se4-OR(4), which adopted an edge-on and face-on structure in 1-phenyloctane and 1-heptanoic acid solvents, respectively, whereas N2S4-OR(1)/N2Se4-OR(2) showed no obvious difference in the assembly structure, which both adopted a face-on structure in the two solvents. Density functional theory (DFT) calculations were also utilized to reveal the relevant self-assembly mechanisms. This study shows a typical solvent effect regulating core-modified rubyrin self-assembly, which is essential for porphyrin-based functional devices' design and manufacture.

Self-assembled flower structures formed by C3-symmetric aromatic carboxylic acids with meta-carboxyl groups.

In this study, two kinds of C3-symmetric hexacarboxylic acids containing three pairs of meta-carboxyl groups self-assembled into similar flower structures with two different sizes of cavities. The bi- and tri-component host-guest systems were built to show no molecular selectivity for the guest molecule COR in the co-assembly process.

Synthesis of Well-Defined Internal-Space-Controllable UiO-66 Spherical Nanostructures Used as Advanced Nanoreactor.

Controllable well-defined metal-organic frameworks (MOFs) spherical structures are expected to be potential good platforms for the heterogeneous catalysis, guest storage, and drug delivery. However, the synthesis faces a challenge. In this paper, a series of well-defined MOFs spherical structures including core-shell, yolk-shell, and hollow spheres were successfully constructed with colloidal carbon as template. Meanwhile, the shapes of MOFs unit (bricklike or octahedral) and sizes of the formed internal space can be controlled simultaneously by rationally changing the modulator and removing template in a good manner. The as-prepared structures could not only keep intact crystal textures but also be used as advanced nanoreactors for enhancing raw materials conversion in the heterogeneous reaction of synthesizing benzoin ethyl ether. Particularly, by our internal-space-controllable nanoreactors, for the first time, the quantifiable relation between nanoreactors' internal space sizes and raw material conversion was revealed.

Temperature-Triggered Self-Assembled Structural Transformation: From Pure Hydrogen-Bonding Quadrilateral Nanonetwork to Trihexagonal Structures.

Adequate control over the structures of molecular building blocks plays an important role in the fabrication of desired supramolecular nanostructures at interfaces. In this study, the formation of a pure hydrogen-bonding co-assembly supramolecular nanonetwork on a highly oriented pyrolytic graphite surface was demonstrated by means of a scanning tunneling microscope. The thermal annealing process was conducted to monitor the temperature-triggered structural transformation of the self-assembled nanonetwork. On the basis of the single-molecule-level resolution scanning tunneling microscopy images, together with the density functional theory calculations, the formation mechanisms of the formed nanoarrays were proposed. The results have great significance with regard to controlled construction of complex nanostructures on the surface.

Unravelling the Self-Assembly of Diketopyrrolopyrrole-Based Photovoltaic Molecules.

The nanostructure of bulk heterojunction in an organic solar cell dominating the electron transport process plays an important role in improving the device efficiency. However, there is still a great need for further understanding the local nanostructures from the viewpoint of molecular design because of the complex alignment in the solid film. In this work, four kinds of photovoltaic materials containing a diketopyrrolopyrrole (DPP) unit combined with other different building blocks were selected and their self-assembled structures on a solid surface were studied by scanning tunneling microscopy technique in combination with theory calculations. The results reveal these DPP-based photovoltaic molecules self-assembled into different nanostructures, which strongly depend on the chemical structure, in particular the backbones and alkyl side chains. The planarities of backbones are affected both by molecule-substrate interaction and steric hindrance induced by the substituted thiophene or benzo[ b]thiophene units on DPP and porphyrin building blocks. The substituted branched alkyl side chains are out of the plane, which are influenced by the alignments of molecular backbones. In addition, the solution concentration also shows a large effect on the self-assembled nanostructures. This systematic research on the self-assembled structures of DPP-based semiconductors on a surface would provide guidance for designing materials and controlling the morphology of a donor/acceptor heterojunction system.

A Strategy to Boost H2 Generation Ability of Metal-Organic Frameworks: Inside-Outside Decoration for the Separation of Electrons and Holes.

Inhibiting the recombination of electron and holes plays an essential role in photocatalytic process, particularly for metal-organic frameworks (MOFs), which had long been anticipated as high-efficient photocatalysts. Herein, we introduce a new strategy to make efficient separation of electrons and holes for the MOF-based photocatalyst, UiO-66-NH2 . At first, encapsulation of Pt nanoparticles (NPs) into UiO-66-NH2 (Pt@U6N) to shorten the electrons transport distance inside of MOF crystals, then using graphene oxide to wrap the external surface of Pt@U6N to facilitate the electrons transfer on the surface. The designed structure was found to possess superior H2 -generation ability compared to only inside or outside decorated samples, highlighting that the enhanced property strongly correlates with the inhibited recombination of electrons and holes by the inside/outside modification strategy. These findings suggest a synergistic effect of Pt NPs and graphene oxide on UiO-66-NH2 and reveal a new modification strategy to enhance the catalytic activity of the photocatalysts.

Two-dimensional (2D) self-assembly of oligo(phenylene-ethynylene) molecules and their triangular platinum(ii) diimine complexes studied using STM.

In this investigation, the two-dimensional (2D) self-assembly nanostructures of a series of cyclic oligo(phenylene-ethynylene) (OPE) molecules (L1, L2-6 and L2-12) at the 1-phenyloctane/highly oriented pyrolytic graphite (HOPG) interface were thoroughly studied using scanning tunneling microscopy (STM). Comparative STM studies with their triangular Pt(ii) diimine complexes (C1, C2-6 and C2-12) were also carried out. Based on careful measurements on single molecule level STM images and density functional theory (DFT) calculations, the formation mechanisms of the nanoarrays formed were revealed.

STM analysis of surface-adsorbed conjugated oligo(p-phenylene-ethynylene) (OPE) nanostructures.

The nanostructures of a series of conjugated oligo(p-phenylene-ethynylene)s (OPE) adsorbed on a surface were thoroughly studied using scanning tunneling microscopy (STM). These oligomers have different backbone lengths and side chains. As a result, various nanostructures displaying periodic linear patterns at a single molecule level were obtained. Based on careful measurements on the STM images in combination with density functional theory (DFT) calculations, it could be found that the vertical and parallel distances between neighboring oligomers were responsible for the specific arrangement of the backbone and side chains. The results showed that these molecular designs strongly affect their self-assembled structure, which is important to clarify the structure-property relationship in the nanoscience field.

A size, shape and concentration controlled self-assembling structure with host-guest recognition at the liquid-solid interface studied by STM.

In the present investigation, we reported the fabrication of host networks formed by two newly prepared phenanthrene-butadiynylene macrocycles (PBMs) at the liquid-solid interface. Size, shape and concentration controlled experiments have been performed to investigate the PBMs/coronene (COR) host-guest system with the structural polymorphism phenomenon. Initially, PBM1 could form a regular linear network structure and PBM2 form a well-ordered nanoporous network structure. When the COR molecules were introduced, the self-assembled structure of PBM1 remained unchanged, while COR could be entrapped into the cavities of the PBM2 nanoporous network, and the co-assembly of the PBM2/COR host-guest systems underwent a structural transformation with the increase of concentration of COR. Scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations are utilized to reveal the formation mechanism of the molecular nanoarrays controlled by the solution concentration.

Synthesis and optimization of ZnPc-loaded biocompatible nanoparticles for efficient photodynamic therapy.

Zinc(ii) phthalocyanine (ZnPc) is a promising photosensitizer for PDT but suffers from aggregation in a physiological aqueous environment. In this paper, a class of biocompatible polymeric nanoparticles (NPs) was prepared to encapsulate ZnPc molecules. Mostly because of the planar structure, ZnPc molecules were difficult to be encapsulated into the polymeric NPs unless further coated with a thick poly-l-lysine (PLL) layer. The PLL shell endowed the NPs with good biocompatibility, efficient cellular uptake, and potential bioconjugation. The degree of aggregation (DOA) of ZnPc molecules in PLL-NPs was thoroughly investigated based on self-defined relative DOA, and a loading capacity of 4 wt% was deduced as the turning point for aggravating aggregation. Similarly, the optimal loading capacity of ZnPc was determined to be 4% according to the 1O2 generation rate, demonstrating the feasibility of the DOA approach. Polymers with large rigid units (PVK and PFO) were also utilized to relieve the aggregation of ZnPc in NPs. Taking advantage of the optimized ZnPc-loaded NPs, high PDT efficacy was demonstrated in HepG2 cells and in tumor-bearing mice as well. Both high in vitro and in vivo PDT efficacy and biocompatibility are demonstrated. Aside from affording a class of efficient biocompatible nanophotosensitizers, this work is also instructive to design other types of ZnPc-based nanocarriers, in which aggregation should be well considered.

Solution concentration controlled self-assembling structure with host-guest recognition at the liquid-solid interface.

In the present investigation, we reported the fabrication of a chicken-wire porous 2D network formed by triphenylene-2,6,10-tricarboxylic acid (H3TTCA) at the liquid-solid interface. When coronene (COR) molecules were added into the system, the H3TTCA honey-comb network was broken and the reconstructed structures of the H3TTCA/COR host-guest systems were subsequently formed. Scanning tunneling microscopic (STM) measurements and density function theory (DFT) calculations were utilized to reveal the structural variety in the co-assembly of H3TTCA/COR controlled by the solution concentration at 1-heptanoic acid/HOPG interface.

Intense deep-blue electroluminescence from ITO/Y2O3/Ag structure.

ITO/Y2O3/Ag devices were fabricated using Y2O3 films as insulator. Four intense and sharp lines with half-peak width of 4 nm were observed for the 293.78 nm InI, 316.10 nm InI, 444.82 nm InII and 403.07 nm InIII transitions. Luminescence mechanism was illustrated by cross-section of the devices based on the analysis of surface morphology. Under the action of strong electric field, the loss of K-shell electrons led to the occurrence of characteristic radiation of indium ions. In addition, the device with turn-on voltage of 10V demonstrates typical I-V diode characteristics. Moreover, Y2O3/In2O3 multiple films as the insulation layer instead of single Y2O3 films was found to improve the device performance with excellent CIE (x, y) coordinates (0.16, 0.03).