Fractals, being "exactly the same at every scale or nearly the same at different scales" as defined by Benoit B. Mandelbrot, are complicated yet fascinating patterns that are important in aesthetics, mathematics, science and engineering. Extended molecular fractals formed by the self-assembly of small-molecule components have long been pursued but, to the best of our knowledge, not achieved. To tackle this challenge we designed and made two aromatic bromo compounds (4,4″-dibromo-1,1':3',1″-terphenyl and 4,4‴-dibromo-1,1':3',1″:4″,1‴-quaterphenyl) to serve as building blocks. The formation of synergistic halogen and hydrogen bonds between these molecules is the driving force to assemble successfully a whole series of defect-free molecular fractals, specifically Sierpiński triangles, on a Ag(111) surface below 80 K. Several critical points that govern the preparation of the molecular Sierpiński triangles were scrutinized experimentally and revealed explicitly. This new strategy may be applied to prepare and explore various planar molecular fractals at surfaces.

A hexagonal macrocycle consisting of 18 phenylene units (hyperbenzene) was synthesized on a Cu(111) surface in ultrahigh vacuum by Ullmann coupling of six 4,4''-dibromo-m-terphenyl molecules. The large diameter of 21.3 Å and the ability to assemble in arrays makes hyperbenzene an interesting candidate for a nanotrough that could enclose metallic, semiconducting, or molecular quantum dots.

The formation, structure, and dynamics of planar organometallic macrocycles (meta-terphenyl-Cu)n and zigzag-shaped one-dimensional organometallic polymers on a Cu(111) surface were studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Vapor deposition of 4,4″-dibromo-meta-terphenyl (DMTP) onto Cu(111) at 300 K leads to C-Br bond scission and formation of C-Cu-C bonds, which connect neighboring meta-terphenyl fragments such that room-temperature stable macrocycles and zigzag chains are formed. The chains self-assemble to form islands, which are elongated in the direction of the chains. If DMTP is deposited onto Cu(111) held at 440 K, the island size is drastically increased (>200 × 200 nm(2)). STM sequences show the formation of ordered structures through reversible scission and reformation of the C-Cu-C bonds. The cyclic organometallic species such as the hexamer (meta-terphenyl-Cu)6 may represent intermediates in the surface-confined Ullmann synthesis of hydrocarbon macrocycles such as the recently discovered hyperbenzene.

Macrocycles have attracted much attention due to their specific "endless" topology, which results in extraordinary properties compared to related linear (open-chain) molecules. However, challenges still remain in their controlled synthesis with well-defined constitution and geometry. Here, we report the successful application of the (pseudo-)high-dilution method to the conditions of on-surface synthesis in ultrahigh vacuum. This approach leads to high yields (up to 84%) of cyclic hyperbenzene ([18]-honeycombene) via an Ullmann-type reaction from 4,4″-dibromo-meta-terphenyl (DMTP) as precursor on a Ag(111) surface. The mechanism of macrocycle formation was explored in detail using scanning tunneling microscopy and X-ray photoemission spectroscopy. We propose that the dominant pathway for hyperbenzene (MTP) formation is the stepwise desilverization of an organometallic (MTP-Ag) macrocycle, which forms via cyclization of (MTP-Ag) chains under pseudo-high-dilution conditions. The high probability of cyclization on the stage of the organometallic phase results from the reversibility of the C-Ag bond. The case is different from that in solution, in which cyclization typically occurs on the stage of a covalently bonded open-chain precursor. This difference in the cyclization mechanism on a surface compared to that in solution stems mainly from the 2D confinement exerted by the surface template, which hinders the flipping of chain segments necessary for cyclization.

The selective temperature-controlled surface-assisted synthesis of covalent, organometallic, and halogen-bonded nanomeshes based on a 3,5,3″,5″-tetrabromo-para-terphenyl (TBrTP) precursor was studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (STM) in ultrahigh vacuum. Vapor deposition of TBrTP onto Cu(111) at 90 K leads to a highly ordered organic monolayer stabilized by Br···Br and Br···H intermolecular bonds between the intact T-type assembled TBrTP molecules, as confirmed by density functional theory (DFT) calculations. Annealing the monolayer to 300 K results in C–Br bond scission and the formation of C–Cu–C bonds, which link adjacent para-terphenyl fragments such that stable organometallic frameworks are formed. Pore sizes correlate with the number of enclosed adatoms (most likely Br atoms), which presumably play a size-determining role during the process of the pore formation. Larger islands of the organometallic framework are obtained by deposition of TBrTP onto the copper surface held at 460 K. A further increase in sample temperature to 570 K during deposition gives rise to the formation of covalent organic frameworks with pores of tetragonal and trigonal symmetry. The covalent nanostructures are not completely planar, but contain phenylene units which are tilted relative to the surface plane, most likely due to steric hindrance between the C–H bonds inside the pores. Comparison of the three different bonding regimes reveals that the degree of long-range order correlates inversely with the strength of the bonds between the building blocks.