Modulation of supramolecular structure by stepwise removal of tert-butyl groups from tetraazaperopyrene derivatives on Ag(111).

Tert-butyl functional groups can modulate the self-assembly behavior of organic molecules on surfaces. However, the precise construction of supramolecular architectures through their controlled thermal removal remains a challenge. Herein, we precisely controlled the removal amount of tert-butyl groups in tetraazaperopyrene derivatives by stepwise annealing on Ag(111). The evolution of 4tBu-TAPP supramolecular self-assembly from the grid-like structure composed of 3tBu-TAPP through the honeycomb network formed by 2tBu-TAPP to the one-dimensional chain co-assembled by tBu-TAPP and TAPP was successfully realized. This series of supramolecular nanostructures were directly visualized by high resolution scanning tunneling microscopy. Tip manipulation and density functional theory calculations show that the formation of honeycomb network structure can be attributed to the van der Waals interactions, N-Ag-N coordination bonds, and weak C-H⋯N hydrogen bonds. Further addition of two tert-butyl groups (6tBu-TAPP) leads to a completely different assembly evolution, due to the fact that the additional tert-butyl groups affect the molecular adsorption behavior and ultimately induce desorption. This work can possibly be exploited in constructing stable and long-range ordered nanostructures in surface-assisted systems, which can also promote the development of nanostructures in functional molecular devices.

Synthesis of Tridecacene by Multistep Single-Molecule Manipulation.

Acenes represent a unique class of polycyclic aromatic hydrocarbons that have fascinated chemists and physicists due to their exceptional potential for use in organic electronics. While recent advances in on-surface synthesis have resulted in higher acenes up to dodecacene, a comprehensive understanding of their fundamental properties necessitates their expansion toward even longer homologues. Here, we demonstrate the on-surface synthesis of tridecacene via atom-manipulation-induced conformational preparation and dissociation of a trietheno-bridged precursor on a Au(111) surface. The generated tridecacene has been investigated by scanning tunneling microscopy and spectroscopy (STM/STS), combined with first-principles calculations. We observe that the STS transport gap (1.09 eV) shrinks again following the gap reopening of dodecacene (1.4 eV). Spin-polarized density functional theory calculations confirm an antiferromagnetic open-shell ground-state electronic configuration for tridecacene in the gas phase. Interestingly, tridecacene's open-shell character is significantly reduced upon interaction with the Au(111) surface despite being only physisorbed. The interaction with the surface leads to a lowering of the magnetization of tridecacene, a reduced gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), compared to the gas phase, and a reduced relative energy to the nonmagnetic state, making it nearly isoenergetic. These observations show qualitatively that the influence of the Au(111) substrate on the properties of long acenes is significant, which is important for interpreting the measured STS transport gaps. Our work contributes to a fundamental understanding of the electronic properties of long acenes, confirming a nonmonotonous length-dependent HOMO-LUMO gap, and to the development of multistep tip-assisted synthesis of elusive compounds.

Highly Regioselective Cyclodehydrogenation of Diphenylporphyrin on Metal Surfaces.

Exploring the effect of porphin tautomerism on the regioselectivity of its derivatives is a big challenge, which is significant for the development and application of porphyrin drugs. In this work, we demonstrate the regioselectivity of 2H-diphenylporphyrin (H2-DPP) in the planarization reaction on Au(111) and Ag(111) substrates. H2-DPP monomer forms two configurations (anti- and syn-) via a dehydrogenation coupling, between which the yield of the anti-configuration exceeds 90%. Using high-resolution scanning tunneling microscopy, we visualize the reaction processes from the H2-DPP monomer to the final two planar products. Combined with DFT calculations of the potential reaction pathway and comparative experiments on Au(111) and Ag(111) substrates. Using M-DPP (M = Cu and Fe), we confirm that the regioselectivity of H2-DPP is derived from the reaction energy barrier during the cyclodehydrogenation reaction of different tautomers. This work reveals the regioselectivity mechanism of H2-DPP on the atomic scale, which holds great significance for understanding the chemical conversion process of organic macrocyclic molecules.

Adsorption configurations and electronic properties of self-assembled C60and C70molecules on a semiconductor CuSe monolayer with periodic nanopores.

Two-dimensional (2D) supramolecular self-assembly architectures are considered one of the most significant and challenging topics in nanotechnology and modern organic chemistry. The study of these processes on surfaces is vital to achieving a higher degree of control in the design of supramolecular architecture. Herein, we report on the 2D self-assembly monolayer architectures based on C60and C70molecules on a semiconductor CuSe monolayer with periodic nanopores, which are essential for providing ideas for surface template chemistry. With the aid of low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS) and density functional theory (DFT) calculation methods, we systematically investigate the adsorption configurations and electronic properties of C60and C70on CuSe monolayer with periodic nanopores. Our results show that both the C60and C70molecules above the nanopores will fall into the nanopores, while those on the CuSe surface will show well-defined self-assembly with various adsorption configurations. Besides, through STS measurement, the lowest unoccupied molecular orbitals (LUMOs) and characteristic peaks of fullerene molecules will be slightly different due to different adsorption configurations. This work helps us to study the adsorption behavior of the fullerene family on various kinds of semiconductor substrates, and also provides vigorous support for the development of fullerene electrical devices in the future.

Real-space imaging of a phenyl group migration reaction on metal surfaces.

The explorations to extend present chemical synthetic methods are of great importance to simplify synthetic routes of chemical species. Additionally, understanding the chemical reaction mechanisms is critical to achieve controllable synthesis for applications. Here, we report the on-surface visualization and identification of a phenyl group migration reaction of 1,4-dimethyl-2,3,5,6-tetraphenyl benzene (DMTPB) precursor on Au(111), Cu(111) and Ag(110) substrates. With the combination of bond-resolved scanning tunneling microscopy (BR-STM), noncontact atomic force microscopy (nc-AFM) and density functional theory (DFT) calculations, the phenyl group migration reaction of DMTPB precursor is observed, forming various polycyclic aromatic hydrocarbons on the substrates. DFT calculations reveal that the multiple-step migrations are facilitated by the hydrogen radical attack, inducing cleavage of phenyl groups and subsequent rearomatization of the intermediates. This study provides insights into complex surface reaction mechanisms at the single molecule level, which may guide the design of chemical species.

On-surface synthesis and characterization of nitrogen-doped covalent-organic frameworks on Ag(111) substrate.

Atomically precise fabrication of covalent-organic frameworks with well-defined heteroatom-dopant sites and further understanding of their electronic properties at the atomic level remain a challenge. Herein, we demonstrate the bottom-up synthesis of well-organized covalent-organic frameworks doped by nitrogen atoms on an Ag(111) substrate. Using high-resolution scanning tunneling microscopy and non-contact atomic force microscopy, the atomic structures of the intermediate metal-organic frameworks and the final covalent-organic frameworks are clearly identified. Scanning tunneling spectroscopy characterization reveals that the electronic bandgap of the as-formed N-doped covalent-organic framework is 2.45 eV, in qualitative agreement with the theoretical calculations. The calculated band structure together with the projected density of states analysis clearly unveils that the incorporation of nitrogen atoms into the covalent-organic framework backbone will remarkably tune the bandgap owing to the fact that the foreign nitrogen atom has one more electron than the carbon atom. Such covalent-organic frameworks may offer an atomic-scale understanding of the local electronic structure of heteroatom-doped covalent-organic frameworks and hold great promise for all relevant wide bandgap semiconductor technologies, for example, electronics, photonics, high-power and high-frequency devices, and solar energy conversion.

On-Surface Synthesis of a Nitrogen-Doped Graphene Nanoribbon with Multiple Substitutional Sites.

Doped graphene nanoribbons (GNRs) with heteroatoms are a principal strategy to fine-tune the electronic structures of GNRs for future device applications. Here, we successfully synthesized the N=9 nitrogen-doped armchair GNR on the Au(111) surface. Due to the flexibility of precursor molecules, three different covalent bonds (C-C, C-N, N-N) are formed in the GNR backbone. Scanning tunneling spectroscopy analysis together with band structure calculations reveals that the band gap of the N-9-AGNRs (C-C) will be enlarged compared to pristine 9-AGNRs, and the C-N bond and N-N bond at the isolated site of N-9-AGNR (C-C) will introduce new defect states near the Fermi level. DFT calculations reveal that the electronic structure of N-9-AGNR (C-C) shows semiconductor character, while N-9-AGNR (C-N) and N-9-AGNR (N-N) display metallic character. Our results provide a promising route for creating more complex molecular heterostructures with tunable band gaps, which may be useful for future molecular electronics and memory device applications.

Identification and electronic characterization of four cyclodehydrogenation products of H2TPP molecules on Au(111).

C-H bond activation and dehydrogenative coupling reactions have always been significant approaches to construct microscopic nanostructures on surfaces. By using scanning tunneling microscopy/spectroscopy (STM/STS) and non-contact atomic force microscopy (nc-AFM) combined with density functional theory (DFT), we systematically characterized the atomically precise topographies and electronic properties of H2TPP cyclodehydrogenation products on Au(111). Through surface-assisted thermal excitation, four types of cyclodehydrogenation products were obtained and clearly resolved in the nc-AFM images. The electronic characterization depicts the predominant resonances and their spatial distributions of the four products.

On-surface synthesis of one-type pore single-crystal porous covalent organic frameworks.

Employing a 1,3,5-tris(4-bromophenyl)benzene precursor as a building block, we successfully fabricate large-scale, non-multihole and single-layer pCOFs on the Ag(111) surface in a controllable manner via the on-surface reaction. We reveal that two main factors, the heating rate and growth temperature, have a strong impact on the size and quality of the pCOFs by STM. Furthermore, the band gap of the pCOFs has been further measured to be approximately 3.01 eV.