Direct observation of the geometric isomer selectivity of a reaction controlled via adsorbed bromine

Topology selectivity of a conformationally flexible precursor through selenium doping

Conformational arrangements within nanostructures play a crucial role in shaping the overall configuration and determining the properties, for example in covalent/metal organic frameworks. In on-surface synthesis, conformational diversity often leads to uncontrollable or disordered structures. Therefore, the exploration of controlling and directing the conformational arrangements is significant in achieving desired nanoarchitectures. Herein, a conformationally flexible precursor 2,4,6-tris(3-bromophenyl)-1,3,5-triazine is employed, and a random phase consisting of C3h and Cs conformers is firstly obtained after deposition of the precursor on Cu(111) at room temperature to 365 K. At low coverage (0.01 ML) selenium doping, we achieve the selectivity of the C3h conformer and improve the nanopore structural homogeneity. The ordered two-dimensional metal organic nanostructure can be fulfilled by selenium doping from room temperature to 365 K. The formation of the conformationally flexible precursor on Cu(111) is explored through the combination of high-resolution scanning tunneling microscopy and non-contact atomic force microscopy. The regulation of energy diagrams in the absence or presence of the Se atom is revealed by density functional theory calculations. These results can enrich the on-surface synthesis toolbox of conformationally flexible precursors, for the design of complex nanoarchitectures, and for future development of engineered nanomaterials.

Toward Two-Dimensional Tessellation through Halogen Bonding between Molecules and On-Surface-Synthesized Covalent Multimers

The ability to engineer sophisticated two-dimensional tessellation organic nanoarchitectures based on triangular molecules and on-surface-synthesized covalent multimers is investigated using scanning tunneling microscopy. 1,3,5-Tris(3,5-dibromophenyl)benzene molecules are deposited on high-temperature Au(111) surfaces to trigger Ullmann coupling. The self-assembly into a semi-regular rhombitrihexagonal tiling superstructure not only depends on the synthesis of the required covalent building blocks but also depends on their ratio. The organic tessellation nanoarchitecture is achieved when the molecules are deposited on a Au(111) surface at 145 °C. This halogen-bonded structure is composed of triangular domains of intact molecules separated by rectangular rows of covalent dimers. The nearly hexagonal vertices are composed of covalent multimers. The experimental observations reveal that the perfect semi-regular rhombitrihexagonal tiling cannot be engineered because it requires, in addition to the dimers and intact molecules, the synthesis of covalent hexagons. This building block is only observed above 165 °C and does not coexist with the other required organic buildings blocks.

On-Surface Synthesis of Self-Assembled Covalently Linked Wavy Chains with Site-Selective Conformational Switching

Conformational arrangements in polymers on surfaces determine the overall shape as well as the potential properties. It is generally believed that conformational diversity leads to uncontrollable or disordered structures in on-surface synthesis. However, in this study, we obtain two well-ordered self-assembled covalently linked wavy chains with site-selective conformational switching via the Ullmann reaction of 1,2-bis(3-bromophenyl)ethane with multiple conformations on Ag(111). Two kinds of wavy chains exhibit distinct conformational arrangements, where chain I contains one repeating unit conformation of -cis-trans1-cis-trans1-cis-cis-trans1-, while the adjacent parallel parts in wavy chain II have two different conformational arrangements of -cis-cis-trans1- and -cis-cis-trans2-. Wavy chains coassemble with dissociated bromine atoms, suggesting that the Br···H-C interactions between Br atoms and molecular chains are crucial for the construction of ordered wavy chains. High-resolution scanning tunneling microscopy is employed to reveal the surface reaction process at the molecular scale. In depth growth mechanism analysis combined with density functional theory calculations unveils that the substrate also plays an important role in the fabrication of well-ordered wavy chains. The present work extends the surface reaction of conformational flexible precursors.

Chemically imaging nanostructures formed by the covalent assembly of molecular building blocks on a surface with ultrahigh vacuum tip-enhanced Raman spectroscopy

Surface-bound reactions have become a viable method to develop nanoarchitectures through bottom-up assembly with near atomic precision. However, the bottom-up fabrication of nanostructures on surfaces requires careful consideration of the intrinsic properties of the precursors and substrate as well as the complex interplay of any interactions that arise in the heterogeneous two-dimensional (2D) system. Therefore, it becomes necessary to consider these systems with characterization methods sensitive to such properties with suitable spatial resolution. Here, low temperature ultrahigh vacuum scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS) were used to investigate the formation of 2D covalent networks via coupling reactions of tetra(4-bromophenyl)porphyrin (Br₄TPP) molecules on a Ag(100) substrate. Through the combination of STM topographic imaging and TERS vibrational fingerprints, the conformation of molecular precursors on the substrate was understood. Following the thermally activated coupling reaction, STM and TERS imaging confirm the covalent nature of the 2D networks and suggest that the apparent disorder arises from molecular flexibility.

Anchoring and Reacting On-Surface to Achieve Programmability

On-surface synthesis has developed into a modern method to fabricate low-dimensional molecular nanostructures with atomic precision. It impresses the chemistry community mostly via its simplicity, selectivity, and programmability during the synthesis. However, an insufficient mechanistic understanding of on-surface reactions and the discriminations in methodologies block it out from the conventional cognition of reaction and catalysis, which inhibits the extensive implication of on-surface synthesis. In this Perspective, we summarize the empirical paradigms of conceptually appealing programmability in on-surface synthesis. We endeavor to deliver the message that the impressive programmability is related to chemical heterogeneity which can also be coded at the molecular level and deciphered by the catalytic surfaces in varying chemical environments as specific chemical selectivity. With the assistance of structure-sensitive techniques, it is possible to recognize the chemical heterogeneity on surfaces to provide insight into the programmable on-surface construction of molecular nanoarchitectures and to reshape the correlation between the mechanistic understanding in on-surface synthesis and conventional chemistry.

Stepwise on-surface synthesis of thiophene-based polymeric ribbons by coupling reactions and the carbon-fluorine bond cleavage

The rational synthesis of thiophene-based cross-coupled polymers on surfaces has been attracting more attention recently. Here, we report the stepwise activation of 5,5'-(2,3-difluoro-1,4-phenylene)bis(2-bromothiophene) as a precursor to synthesize thiophene-based polymeric ribbons on the Au(111) surface. Scanning tunneling microscopy studies showed that the precursor adopted different conformations in the self-assembled structure, organometallic species, and covalent polymers. On annealing the sample at a relatively low temperature (150 °C), the conversion of the organometallic structure into a covalent product with straight lines was observed, in which the Br adatoms arranged between the neighboring chains. On further annealing the sample at 270 °C, the detached Br adatoms played a key role in promoting the C-H bond activation. The cross-linked polymer was achieved by a combination of Ullmann and dehydrogenative coupling. When the annealing temperature was up to 390 °C, the C-F bond activation was triggered, which led to the formation of polymeric ribbons resulting from the cyclodehydrogenation of the fluorinated polymer. This study further supplements the reaction mechanism of thiophene-based dehalogenative, dehydrogenative and defluorinative coupling, and provides us a rational way for synthesizing cross-linked functional materials.

On-Surface Synthesis and Molecular Engineering of Carbon-Based Nanoarchitectures

On-surface synthesis via covalent coupling of adsorbed precursor molecules on metal surfaces has emerged as a promising strategy for the design and fabrication of novel organic nanoarchitectures with unique properties and potential applications in nanoelectronics, optoelectronics, spintronics, catalysis, etc. Surface-chemistry-driven molecular engineering (i.e., bond cleavage, linkage, and rearrangement) by means of thermal activation, light irradiation, and tip manipulation plays critical roles in various on-surface synthetic processes, as exemplified by the work from the Ernst group in a prior issue of ACS Nano. In this Perspective, we highlight recent advances in and discuss the outlook for on-surface syntheses and molecular engineering of carbon-based nanoarchitectures.

Optical scanning tunneling microscopy based chemical imaging and spectroscopy

Through coupling optical processes with the scanning tunneling microscope (STM), single-molecule chemistry and physics have been investigated at the ultimate spatial and temporal limit. Electrons and photons can be used to drive interactions and reactions in chemical systems and simultaneously probe their characteristics and consequences. In this review we introduce and review methods to couple optical imaging and spectroscopy with scanning tunneling microscopy. The integration of the STM and optical spectroscopy provides new insights into individual molecular adsorbates, surface-supported molecular assemblies, and two-dimensional materials with subnanoscale resolution, enabling the fundamental study of chemistry at the spatial and temporal limit. The inelastic scattering of photons by molecules and materials, that results in unique and sensitive vibrational fingerprints, will be considered with tip-enhanced Raman spectroscopy. STM-induced luminescence examines the intrinsic luminescence of organic adsorbates and their energy transfer and charge transfer processes with their surroundings. We also provide a survey of recent efforts to probe the dynamics of optical excitation at the molecular level with scanning tunneling microscopy in the context of light-induced photophysical and photochemical transformations.