On-Surface Pseudo-High Dilution Synthesis of Macrocycles: Principle and Mechanism

TL;DR

This paper demonstrates a novel on-surface pseudo-high dilution method in ultrahigh vacuum to synthesize macrocycles with high yield, revealing a distinct mechanism from solution synthesis due to surface confinement effects.

Contribution

It introduces the first application of pseudo-high dilution on a surface for macrocycle synthesis and elucidates the unique formation mechanism via STM and XPS analysis.

Findings

Achieved up to 84% yield of macrocycles on Ag(111) surface.

Identified stepwise desilverization as key to macrocycle formation.

Revealed surface confinement alters cyclisation mechanism compared to solution.

Abstract

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 first successful application of the (pseudo-)high dilution method to the conditions of on-surface synthesis in ultrahigh vacuum (UHV). 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 (STM) and X-ray photoemission spectroscopy (XPS). We propose that hyperbenzene (MTP)6 forms majorly by stepwise desilverization of an organometallic (MTP-Ag)6 macrocycle, which preforms via cyclisation of (MTP-Ag)6 chains under pseudo-high dilution condition. The high probability of cyclisation 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 cyclisation typically occurs on the stage of covalently bonded open-chain precursor. This difference in the cyclisation mechanism on a surface compared to that in solution stems mainly from the 2D confinement exerted by the surface template, which to a large extent prevents the flipping of chain segments necessary for cyclisation.