Bond-Order Discrimination by Atomic Force Microscopy

Visualizing Bond Order Bond lengths in conjugated molecules closely reflect individual bond order and are usually determined by diffraction methods. It is valuable to know bond order for rationalizing aromaticity, and reactivity and for chemical structure determination. Gross et al. (p. 1326; see the Perspective by Perez and the cover) differentiated the bond orders in individual molecules in the fullerene C60 and in polyaromatic hydrocarbons by imaging with noncontact atomic force microscopy (AFM). The molecules were adsorbed onto a copper surface, and the AFM tip was decorated with a CO molecule, which was used to measure tip frequency shifts above the bonds and their apparent lengths. Multiple bonds appeared brighter in the images because of stronger Pauli repulsion, and their shorter length was amplified by bending of the CO at the tip apex. Images detected with an atomic force microscope tip decorated with a carbon monoxide molecule could distinguish Pauling bond order. We show that the different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes can be distinguished by noncontact atomic force microscopy (AFM) with a carbon monoxide (CO)–functionalized tip. We found two different contrast mechanisms, which were corroborated by density functional theory calculations: The greater electron density in bonds of higher bond order led to a stronger Pauli repulsion, which enhanced the brightness of these bonds in high-resolution AFM images. The apparent bond length in the AFM images decreased with increasing bond order because of tilting of the CO molecule at the tip apex.