Surface-confined alternating copolymerization with molecular precision by stoichiometric control.

Keen desires for artificial mimicry of biological polymers and property improvement of synthesized ones have triggered intensive explorations for sequence-controlled copolymerization. However, conventional synthesis faces great challenges to achieve this goal due to the strict requirements on reaction kinetics of comonomer pairs and tedious synthetic processes. Here, sequence-controlled alternating copolymerization with molecular precision is realized on surface. The stoichiometric control serves as a thermodynamic strategy to steer the polymerization selectivity, which enables the selective alternating organometallic copolymerization via intermolecular metalation of 4,4"-dibromo-p-terphenyl (P-Br) and 2,5-diethynyl-1,4-bis(phenylethynyl)benzene (A-H) with Ag adatoms on Ag(111) at P-Br: A-H = 2, as verified by scanning tunneling microscopy and density functional theory studies. In contrast, homopolymerization yield increases as the stoichiometric ratio deviates from 2. The microscopic characterizations rationalize the mechanism, providing a delicate explanation of the stoichiometry-dependent polymerization. These findings pave a way to actualizing an efficient sequence control of copolymerization by surface chemistry.

Assembling Surface Molecular Sierpinski Triangle Fractals via K+-Invoked Electrostatic Interaction.

Molecular Sierpinski triangles (STs), a family of elegant and well-known fractals, can be prepared on surfaces with atomic precision. Up to date, several kinds of intermolecular interactions such as hydrogen bond, halogen bond, coordination, and even covalent bond have been employed to construct molecular STs on metal surfaces. Herein a series of defect-free molecular STs have been fabricated via electrostatic attraction between potassium cations and electronically polarized chlorine atoms in 4,4″-dichloro-1,1':3',1″-terphenyl (DCTP) molecules on Cu(111) and Ag(111). The electrostatic interaction is confirmed both experimentally by scanning tunneling microscopy and theoretically by density functional theory calculations. These findings illustrate that electrostatic interaction can serve as an efficient driving force to construct molecular fractals, which enriches our toolbox for the bottom-up fabrication of complex functional supramolecular nanostructures.

Molecular insight into on-surface chemistry of an organometallic polymer.

A molecular investigation of Cu-elimination and subsequent C-C coupling of DCTP (4,4''-dichloro-1,1':3',1''-terphenyl)-Cu organometallic (OM) polymers on Cu(111) is conducted by scanning tunneling microscopy and spectroscopy, revealing that the Cu adatoms embedded in the DCTP-Cu chains are located at the hollow and bridge sites on the Cu(111) surface. The difference in the catalytic activities of these surface sites leads to stepwise elimination of Cu adatoms in the OM chains. Moreover, the interchain interaction plays an important role in the Cu-elimination process of the DCTP-Cu chains as well. The interchain steric hindrance, on the one hand, induces the formation of Cu-eliminated intermediates that are scarcely observed in other Ullmann coupling systems, and on the other hand, promotes the cooperative Cu-elimination and C-C coupling of the OM segments in neighboring chains. These findings demonstrate the key role of the molecule-substrate and intermolecular interactions in mediating the reaction processes of the extended molecular systems on the surface.

Steering Effect of Bromine on Intermolecular Dehydrogenation Coupling of Poly(p-phenylene) on Cu(111).

Among the multitudinous methodologies to steer on-surface reactions, less attention has been paid to the effect of externally introduced halogen atoms. Herein, highly selective trans-dehydrogenation coupling at the specific meta-C-H site of two poly(p-phenylene) molecules, p-quaterphenyl (Ph4) and p-quinquephenyl (Ph5), is achieved on Cu(111) by externally introduced bromine atoms. Scanning tunneling microscopy/spectroscopy experiments reveal that the formed molecular assembly structure at a stoichiometric ratio of 4:1 for Br to Ph4 or 5:1 for Br to Ph5 can efficiently promote the reactive collision probability to trigger the trans-coupling reaction at the meta-C-H site between two neighboring Ph4 or Ph5 molecules, leading to an increase in the coupling selectivity. Such Br atoms can also affect the electronic structure and adsorption stability of the reacting molecules. It is conceptually demonstrated that externally introduced halogen atoms, which can provide an adjustable halogen-to-precursor stoichiometry, can be employed to efficiently steer on-surface reactions.

Long-Range Ordered Structures of Corannulene Governed by Electrostatic Repulsion and Surface-State Mediation.

The adsorption and assembly of sub-monolayered bowl-shaped corannulene (COR) on Cu(111) and Ag(111) are investigated by scanning tunneling microscopy (STM). Three COR configurations, namely, up, down, and tilted ones, are formed on Cu(111), as unraveled by high-resolution STM images. It is also experimentally revealed that monodispersed, hexagonal, and evenly spaced stripe patterns develop on both Cu(111) and Ag(111). A quantitative evaluation of the long-range intermolecular interaction on Cu(111) mediated by electrostatic repulsion and surface-state mediation is presented. At 0.05 monolayer (ML), the long-range monodispersed pattern is mainly induced by electrostatic interaction. At 0.24 and 0.47 ML, however, surface-state mediation plays a dominant role, and the electrostatic interaction is leveled due to the identical static environment for each molecule.

Chiral features of metal phthalocyanines sitting atop the pre-assembled TiOPc monolayer on Ag(111).

The chiral features of the top-layer TiOPc molecules on monolayered TiOPc assembly on Ag(111) were carefully investigated by scanning tunnelling microscopy and local work function measurements. Combined with the density functional theory calculations, systematic experimental explorations of the TiOPc/TiOPc, CuPc/TiOPc and TiOPc/CuPc systems on Ag(111) revealed that the chirality originated from asymmetric electronic interactions rather than conformational change, which might be related to the high performance of the photoelectronic devices based on the MPc complexes.

Dechlorinated Ullmann Coupling Reaction of Aryl Chlorides on Ag(111): A Combined STM and XPS Study.

Surface-assisted Ullmann coupling was widely applied to construct various molecular nanostructures on surfaces due to its reliability and controllability. By using 4,4''-dichloro-1,1':3',1''-terphenyl (DCTP) as the precursor, covalently bonded zig-zag oligophenylene chains and hexagonal hyperbenzene rings, e. g., [18]-honeycombenes, were successfully fabricated on Ag(111) via dechlorinated Ullmann coupling reaction. Stepwise annealing was applied to investigate the reaction process in detail. Scanning tunneling microscopy and synchrotron X-ray photoemission spectroscopy were utilized to explore the thermal evolution of the DCTP molecules on Ag(111) under ultrahigh vacuum conditions, evidencing the existence of intact DCTP molecules, chemisorbed Cl atoms, covalently bonded DCTP dimers as well as organometallic C-Ag-C-containing intermediates. These results may help understand dechlorinated Ullmann coupling reaction of aryl chlorides on metal surfaces.

Steering the Achiral into Chiral with a Self-Assembly Strategy.

Chirality transfer from self-assembly of achiral titanyl phthalocyanine (TiOPc) to its top-sitting TiOPc molecule has been successfully achieved. The TiOPc molecules first assemble into a porous network on Au(111) that contains periodic chiral voids, each being fenced by four axially rotating TiOPc molecules in upward adsorption geometry where their ending O atoms exclusively point away from the substrate. The additional top-sitting TiOPc molecule turns out to be chiral upon adsorption on a chiral void with its ending O atom toward the substrate. The chirality of the top-sitting TiOPc is associated with a charge transfer between its indole rings and the ending O atoms of the underlying TiOPc molecules that form the chiral void, resulting in asymmetric electronic density of the indole rings in the top-sitting molecule and accordingly the chirality of the molecular orbitals. Such a scenario also validates other planar achiral metallophthalocyanines such as copper phthalocyanine that become chiral upon adsorption on the chiral voids in the underlying TiOPc assembly, indicating that the chirality transfer mechanism from assembly to the top-sitting molecule is not uncommon.

On Controllability and Applicability of Surface Molecular Self-Assemblies.

Molecular self-assembly (MSA) refers to spontaneous arrangement of molecular building blocks into ordered structures governed by weak interactions. Due to its high versatility and reversibility, MSA has been widely employed as a robust bottom-up approach to fabricating low-dimensional functional nanostructures, which are used in various applications in nanoscience and technology. To date, tremendous effort has been devoted to constructing various MSAs at surfaces, ranging from self-assembled monolayers and two-dimensional (2D) nanoporous networks to complex 2D quasicrystals and Sierpinski triangle fractals. However, precise control of the assembled structures and efficient achievement of their full applicability remain two major challenges in the MSA field. As another widely employed bottom-up approach to fabricating nanostructures, on-surface reaction (OSR) refers to a reaction that occurs on the surface and is two-dimensionally confined. OSR offers the possibility to synthesize compounds that may not be feasibly achieved in solution chemistry. Compared with MSA based on weak intermolecular interactions, OSR-based structures possess high thermal and chemical stabilities due to internal strong covalent bonds. In this Account, we briefly overview recent achievements of MSAs on single crystal metal surfaces with a focus on their controllability and applicability in tweaking the properties of the molecular building blocks involved. Emphasis will be particularly placed upon mediation of OSRs with the MSA strategy. To explore surface MSAs, on the one hand, scanning tunneling microscopy and spectroscopy have been routinely employed as the experimental tools to probe the intermolecular interactions as well as geometric and electronic structures of the assemblies at the atomic and molecular levels. On the other hand, density functional theory and molecular dynamics have been theoretically applied to model and calculate the assembling systems, furthering our understanding of the experimental results. In principle, MSA is primarily balanced by molecule-molecule and molecule-substrate interactions under vacuum conditions. In terms of the assembling methodologies, people have been attempting to achieve rational design, accurate prediction, and controllable construction of assembled molecular nanostructures, namely, tentative design of specific backbones and functional groups of the molecular building blocks, and careful control of the assembling parameters including substrate lattice, temperature, coverage, and external environment as well. An obvious goal for the development of these methodologies lies in the ultimate applications of these MSAs. MSA can retrospectively affect the properties of the assembling molecules. For instance, self-assembled structures not only can serve as secondary templates to host guest molecules but also can stabilize surface metal adatoms. In fact, the electronics, magnetism, and optics of MSAs have been successfully explored. In surface chemistry, the MSA strategy can be further applied to mediate OSRs in at least three aspects: tweaking reaction selectivity, changing reaction pathway, and restricting reaction site. The governing principle lies in that the self-assembled molecules are confined in the assemblies so that the pre-exponential factors and the energy barriers in the Arrhenius equation of the involved reactions could be substantially varied because the subtle reaction mechanisms may change upon assembling. In this sense, the MSA strategy can be efficiently exploited to tune the properties of the assembling molecules and mediate OSRs in surface chemistry.

On the shuttling mechanism of a chlorine atom in a chloroaluminum phthalocyanine based molecular switch.

An intermediate shuttling structure of a chloroaluminum phthalocyanine(ClAlPc)-based molecular switch is transiently created and analyzed by combined scanning tunneling microcopy/spectroscopy and density-functional theory calculations, which suggests that the Cl atom is squeezed into the space between the central Al atom and the inner N-containing ring in ClAlPc.

Preparation of novel porphyrin nanomaterials based on the pH-responsive shape evolution of porphyrin microspheres.

The shapes and properties of self-assembled materials can be adjusted easily using environmental stimuli. Yet, the stimulus-triggered shape evolution of organic microspheres in aqueous solution has rarely been reported so far. Here, a novel type of poly(allylamine hydrochloride)-g-porphyrin microspheres (PAH-g-Por MPs) was prepared by a Schiff base reaction between 2-formyl-5,10,15,20-tetraphenylporphyrin (Por-CHO) and PAH doped in 3.5-µm CaCO3 microparticles, followed by template removal. The PAH-g-Por MPs had an average diameter of 2.5 µm and could be transformed into one-dimensional nanorods (NRs) and wormlike nanostructures (WSs) after being incubated for different times in pH 1-4 HCl solutions. The rate and degree of hydrolysis had a significant effect on the formation and morphologies of the nanorods. The NRs@pH1, NRs@pH2, and NRs@pH3 were all composed of the released Por-CHO and the unhydrolyzed PAH-g-Por because of the incomplete hydrolysis of the Schiff base. However, the WSs@pH4 were formed by a pure physical shape transformation, because they had the same composition as the PAH-g-Por MPs and the Schiff base bonds were not hydrolyzed. The self-assembled NRs and WSs exhibited good colloidal stability and could emit stable red fluorescence over a relatively long period of time.