Polycyclic aromatic chains on metals and insulating layers by repetitive [3+2]\xa0cycloadditions

Alexander Riss,Knud Seufert,Klaus Müllen,Reinhard Berger,Rajesh Raju,Manuela Garnica,Michael Wuttke,Jacob Ducke,Yuanqin He,Johannes V Barth,Willi Auwärter,Carlos-Andres Palma,Akimitsu Narita,Marcus Richter,Xiao-Ye Wang,Eduardo Corral,Xinliang Feng,Angel Rubio,Alejandro PéRez Paz

doi:10.1038/s41467-020-15210-2

Abstract

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces. On-surface fabrication of such materials on non-metal surfaces remains to be demonstrated with high yield and selectivity. Here we present the synthesis of polyaromatic chains on metallic substrates, insulating layers, and in the solid state. Scanning probe microscopy shows the formation of azaullazine repeating units on Au(111), Ag(111), and h-BN/Cu(111), stemming from intermolecular homo-coupling via cycloaddition reactions of CN-substituted polycyclic aromatic azomethine ylide (PAMY) intermediates followed by subsequent dehydrogenation. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry demonstrates that the reaction also takes place in the solid state in the absence of any catalyst. Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces.

Highlights

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces
Scanning tunneling microscopy (STM) measurements revealed units that resemble the shape of the polyaromatic ring system of the cation and the cycloaddition products of head-to-tail coupled molecules (Fig. 2a–c)
To explore the influence of the different substrates on the reaction yield and oligomer length, we investigated via DFT the reaction energies for the coupling of two species of 1a towards the dimer 2a (Fig. 1, gray) on Au(111), Ag (111) and hexagonal boron nitride (h-BN) (Fig. 6a–d)

Summary

Introduction

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces. Matrix-assisted laser desorption/ ionization (MALDI) mass spectrometry demonstrates that the reaction takes place in the solid state in the absence of any catalyst Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces. We present a covalent intermolecular cycloaddition reaction of CN-functionalized PAMY with itself, into onedimensional polyaromatic chains with azaullazine[26] repeating units on metals, insulating layers, and in the solid state. Scanning tunneling microscopy (STM) and bond-resolved atomic force microscopy (AFM) with CO-functionalized tips demonstrates that on all investigated substrates, i.e., Au(111), Ag(111), and hexagonal boron nitride (h-BN) on Cu(111), phenanthridinium precursors predominantly form covalently coupled chains with repeating units of 1-azadibenzo[d,k]ullazine via the cycloaddition reaction[27,28] between the cyano group and the azomethine ylide moiety, as well as subsequent dehydrogenation (see Fig. 1)[24,29,30,31,32,33,34]. This result suggests that the catalytic effect of the substrate is not crucial, and instead, the intrinsic reactivity of the molecules plays the dominant role, facilitating the cycloaddition reaction in the absence of catalysts, i.e., on insulating and inert substrates

Methods

Results

Conclusion