Studying an antiaromatic polycyclic hydrocarbon adsorbed on different surfaces

Zsolt Majzik,Enrique Guitián,Niko Pavliček,Dolores Pérez,Diego Peña,Nikolaj Moll,Leo Gross,Manuel Vilas-Varela,Gerhard Meyer

doi:10.1038/s41467-018-03368-9

Abstract

Antiaromatic and open-shell molecules are attractive because of their distinct electronic and magnetic behaviour. However, their increased reactivity creates a challenge for probing their properties. Here, we describe the on-surface and in-solution generation and characterisation of a highly reactive antiaromatic molecule: indeno[1,2-b]fluorene (IF). In solution, we generated IF by KI-induced dehalogenation of a dibromo-substituted precursor molecule and found that IF survives for minutes at ambient conditions. Using atom manipulation at low temperatures we generated IF on Cu(111) and on bilayer NaCl. On these surfaces, we characterised IF by bond-order analysis using non-contact atomic force microscopy with CO-functionalised tips and by orbital imaging using scanning tunnelling microscopy. We found that the closed-shell configuration and antiaromatic character predicted for gas-phase IF are preserved on the NaCl film. On Cu(111), we observed significant bond-order reorganisation within the s-indacene moiety because of chemisorption, highlighting the importance of molecule surface interactions on the π-electron distribution.

Highlights

Antiaromatic and open-shell molecules are attractive because of their distinct electronic and magnetic behaviour
The concept was later extended to polycyclic conjugated hydrocarbons (PCHs), based on the number of Clar sextets[5], or the presence of conjugated circuits with [4n + 2] π-electrons within a particular structure[6]
One important aspect is the bond order, which can be resolved directly with atomic force microscopy (AFM) by comparing the apparent length and contrast of individual bonds within the molecule[31], which has already been applied for the on-surface characterisation of biphenylenes[35] and highly reactive molecules such as arynes[29] or diradicals[30]

Summary

Introduction

Antiaromatic and open-shell molecules are attractive because of their distinct electronic and magnetic behaviour. One important aspect is the bond order, which can be resolved directly with AFM by comparing the apparent length and contrast of individual bonds within the molecule[31], which has already been applied for the on-surface characterisation of biphenylenes[35] and highly reactive molecules such as arynes[29] or diradicals[30]. In-solution IF was generated by iodide-induced decomposition of dibromo-substituted precursor 2, whereas on-surface IF was obtained by tip-induced dehydrogenation of the polycyclic hydrocarbon 3.

Results

Conclusion