Abstract
On-surface synthesis provides a route for the production of 1D and 2D covalently bonded polymeric structures. Such reactions are confined to the surface of a substrate and the catalytic properties of the substrate are often utilised to initiate the reaction. Recent studies have focused on the properties of various crystallographic planes of metallic substrates, as well as native surface features such as step-edges, in an effort to provide control over the pathway of the reaction and the resultant products. An alternative approach is to template the catalytic surface with a porous molecular overlayer; giving rise to well-defined surface regions within which an on-surface reaction may be confined. Here we present a methodology where macromolecular templates are used to confine an on-surface reaction. Cyclic porphyrin polymers, nanorings - consisting of 40 porphyrin units with internal diameter 13 nm, are used to form a template on a Au(111) surface, and an on-surface Ullmann-type coupling reaction is initiated within the nanoring template. The surface confined template and covalently coupled reaction products are investigated and characterised with scanning tunnelling microscopy.
Highlights
IntroductionThe on-surface synthesis of 1D and 2D covalently bonded structures, performed on metal substrates under ultra-high vacuum (UHV) conditions, has been achieved through a variety of different strategies[1,2,3,4,5]: facilitating the synthesis of molecular chains[6,7,8,9,10,11], graphene nanoribbons[12,13], and 2D molecular frameworks[14,15,16]
Porphyrin nanorings are employed to act as areas of confinement for the on-surface Ullmann-type coupling reaction of 1,3,5-tris(4-iodophenyl)benzene (TIPB) on Au(111) [NB: the terminology Ullmann-type reaction is used here to refer to on-surface reactions employing Au, Ag and Cu substrates and halogen functionalised reactant molecules, as the original Ullmann reaction was defined using a Cu catalyst]
When deposited onto a separate, clean, Au(111) surface the c-P40 rings were observed to form stacks on the surface, typically 2–4 units high; similar to that observed in previous experiments (~0.4 nm for double stacks, ~0.7 nm for triple stacks, and ~0.1 nm for single height rings)[31] where the number of rings in a stack can be affected by the composition of the solvent used for electrospray deposition34. c-P40 is known to be flexible and very few circular rings were observed, with most arranged in stacks of ovals
Summary
The on-surface synthesis of 1D and 2D covalently bonded structures, performed on metal substrates under ultra-high vacuum (UHV) conditions, has been achieved through a variety of different strategies[1,2,3,4,5]: facilitating the synthesis of molecular chains[6,7,8,9,10,11], graphene nanoribbons[12,13], and 2D molecular frameworks[14,15,16]. Many studies have focused on the transfer of classical solution-phase reactions (e.g. Ullmann coupling and Glaser coupling)[2] onto supporting substrates held under UHV conditions Such reactions are often initiated via the interaction between the vacuum deposited monomer species and the catalytic surface, resulting in the breaking of covalent bonds (e.g. carbon-halogen bonds in Ullmann coupling) to produce a reactive intermediate species that may participate in the on-surface formation of a covalently bonded product. In summary: A porous hydrogen-bonded supramolecular network was formed on a Ag(111) surface, with the substrate acting as a catalyst for the breaking of intramolecular carbon-halogen bonds of deposited reactant species confined within the pores This approach has the potential to allow catalytic surfaces to be templated by molecular systems, leading to surfaces with well-defined www.nature.com/scientificreports/. Similar approaches have been applied to investigate a range of molecules including proteins[37], single molecule magnets[36,38], and a variety of other species[39,40]
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