On-Surface Synthesis of Porous Carbon Nanoribbons from Polymer Chains.

Maximilian Ammon,Sabine Maier,Tim Sander

doi:10.1021/jacs.7b04783

Abstract

We demonstrate the on-surface synthesis of porous carbon nanoribbons on Ag(111) via a preprogrammed isomerization of conformationally flexible polymer chains followed by dehydrogenation reactions using thermal annealing. The carbon chains are fabricated by polymerization of prochiral 1,3,5-tris(3-bromophenyl)benzene (mTBPB) directly on the surface using an Ullmann-type reaction. At room temperature, mTBPB partially self-assembles in halogen-bonded 2D networks, which transform into organometallic chains and rings after debromination. The chain and ring formation is facilitated by conformational switching from a C3h to Cs symmetry of mTBPB via rotation of m-phenylene units. The high conformational selectivity toward Cs-conformers is templated by the twofold coordination to Ag adatoms. After thermally induced covalent-linking through aryl-aryl coupling, well-ordered nanoporous chains are created. Finally, the rotation of single phenylene units in combination with dehydrogenation cross-linking reactions within the polymer chains leads to the unexpected formation of porous carbon nanoribbons. We unveil the reaction mechanism in a low-temperature scanning tunneling microscopy study and demonstrate that the rotation of m-phenylene units is a powerful design tool to promote structural control in the synthesis of cyclic covalent organic nanostructures on metal surfaces.

Highlights

Self-assembly by controlled folding of extended polymer structures is the basis for the biological molecular functionality in protein structures, and is an integral design tool for supramolecular architectures on surfaces for example in DNA origamis.[1]
We demonstrate the on-surface synthesis of porous carbon nanoribbons on Ag(111) via a preprogrammed isomerization of conformationally flexible polymer chains followed by dehydrogenation reactions using thermal annealing
We report the formation of porous polymer chains from 1,3,5-tris(3-bromophenyl)benzene precursors and their transformation into porous carbon Scheme 1. 1,3,5-Tris(3-bromophenyl)benzene Conformers with C3h and Cs Symmetrya aThe conformation of mTBPB can be switched through a rotation of the bromophenyl groups around the interconnecting carbon−carbon σ-bonds to the central phenyl ring.[8] nanoribbons via a isomerization reaction followed by C−C bond formation on Ag(111) in ultrahigh vacuum (UHV) environment

Summary

■ INTRODUCTION

Self-assembly by controlled folding of extended polymer structures is the basis for the biological molecular functionality in protein structures, and is an integral design tool for supramolecular architectures on surfaces for example in DNA origamis.[1]. We conclude that some of the phenylene units of the debrominated mTBPB rotated around the respective interconnecting carbon−carbon σ-bonds while annealing to 350 K. organometallic structures with a twofold Ag-coordination (Figure S3b); the unit cell vectors of the potential porous network would have to increase by nearly 50% compared to the halogen-bonded self-assembly, which might be the reason why it was not observed in the STM experiments. (a,b) STM images and (c) tentative model of the self-assembled covalently linked chains composed of cyclohexa-m-phenylene (CHP) rings bridged by two m-phenylene units after annealing at 550 K. We note that the rotation of the m-phenylene units in the polymer chains after the covalent bond formation allows for an isomerization between trans and cis structures (see Figure S9). We note that phenyl units bind stronger to Ag(111) than to Au(111); the rotation of phenylene units should proceed even easier on Au(111).[41,66] This corroborates that the organometallic intermediates facilitate the conformational selectivity toward Cs-mTBPB on Ag(111)

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES