On-surface generation and imaging of arynes by atomic force microscopy

Abstract

Reactive intermediates are involved in many chemical transformations. However, their characterization is a great challenge because of their short lifetimes and high reactivities. Arynes, formally derived from arenes by the removal of two hydrogen atoms from adjacent carbon atoms, are prominent reactive intermediates that have been hypothesized for more than a century. Their rich chemistry enables a widespread use in synthetic chemistry, as they are advantageous building blocks for the construction of polycyclic compounds that contain aromatic rings. Here, we demonstrate the generation and characterization of individual polycyclic aryne molecules on an ultrathin insulating film by means of low-temperature scanning tunnelling microscopy and atomic force microscopy. Bond-order analysis suggests that a cumulene resonance structure is the dominant one, and the aryne reactivity is preserved at cryogenic temperatures. Our results provide important insights into the chemistry of these elusive intermediates and their potential application in the field of on-surface synthesis.

Synthesis and characterization of triangulene

“…This observation is in strong contrast to previously investigated diradicals that form strong covalent bonds when adsorbed on copper under the same conditions 14,15 , and can be rationalized by the fact that these comprise σ radicals, whereas triangulene features π radicals. Finally, Fig.…”

“…Instead, the brighter appearance (less attractive) of the peripheral carbons suggests a slightly bent adsorption with the outer carbons further away from the surface, as observed previously for pentacene on Cu(111) 18 . This observation is in strong contrast to previously investigated diradicals that form strong covalent bonds when adsorbed on copper under the same conditions 14,15 , and can be rationalized by the fact that these comprise σ radicals, whereas triangulene features π radicals. Finally, Fig.…”

“…The chemical structure of reactants and products can be resolved by means of AFM with functionalized tips 13 . Even molecules too elusive to be studied by other means 14,15 can be stabilized by using an ultrathin insulating film as a decoupling 2 layer. A decoupling layer also facilitates studying the frontier molecular orbitals of the free molecule by means of STM and scanning tunnelling spectroscopy (STS) 16 .…”

“…The central carbon of the three adjacent CH 2 group in 3 adopts the expected tetrahedral bond angle for sp 3 carbon leading to sharp ridges in both STM and AFM mode (Figs. 2c, g) because of strong tilting of the CO molecule at the tip apex.We dehydrogenated promising candidates (2a and 2b molecules) to obtain triangulene by means of atomic manipulation 14,15,19 . To this end, we first positioned the tip above a molecule.…”

“…We dehydrogenated promising candidates (2a and 2b molecules) to obtain triangulene by means of atomic manipulation 14,15,19 . To this end, we first positioned the tip above a molecule.…”