Bond Order Via Microscopy

A scanning-probe microscopy method can reveal subtle bond order differences between individual bonds in single organic molecules, according to a study led by researchers at IBM and published in Science (DOI: 10.1126/science.1225621). The advance may enable scientists to map single-bond electron distributions at key circuit junctions in future molecular electronic devices. The work may also deepen understanding of electron transport processes and chemical reactivity in thin films.

Bond order is a classic chemistry concept that’s useful for describing bonding differences between atoms in molecules. The order of the C–C bond in ethylene, for example, is 2, and in ethane it’s 1. That difference reflects the increased electron density and shorter length of C–C double bonds relative to single ones. For the simplest conjugated molecule, benzene, bond order is 1.5, between single and double bonds.

Knowing the bond order in more complex molecules, such as fullerenes and polycyclic aromatic hydrocarbons (PAHs), can help predict geometry, aromaticity, reactivity, and other molecular properties. Bond order in fullerenes, which are made up of fused hexagons and pentagons, can fall anywhere between single- and double-bond values. Bond length, which is related to bond order, can be measured in some cases by using diffraction methods, but that approach yields values averaged over large numbers of molecules.

The new study, led by Leo Gross, Fabian Mohn, and coworkers at IBM’s Zurich research center, shows that scanning microscopy can now be used to directly measure the small differences in electronic charge distribution and bond length associated with bond order variations. The team, which also includes researchers based in Spain and France, made the measurements using an atomic force microscopy (AFM) instrument with a CO-functionalized metal tip operating in a noncontact mode.

In one case, they probed bonds in C₆₀ that fuse two hexagons and a hexagon and a pentagon. The bond joining the hexagons is known to be slightly electron-rich relative to the other bond. The imaging results underscored that subtle difference: The bond fusing the C₆ rings appeared shorter and brighter than the bond joining C₆ to C₅.

The group also probed two PAHs that feature a variety of bonding configurations with corresponding bond order variations. In one case, they examined bonding in hexabenzocoronene, a molecule with a central C₆ ring surrounded symmetrically by other C₆ rings. Theoretical analysis shows that bonds within the central ring are of slightly higher bond order than bonds connecting the central ring to the outside rings. The team reports that those subtle differences were captured directly by the AFM method. They obtained similar results for a complex PAH made up of nine six-membered rings.

Noting that recent AFM developments have made atomic-scale imaging possible for many types of materials, Rubén Pérez, a professor of condensed matter physics at the Autonomous University of Madrid, comments that the new study “pushes the imaging capability of AFM still further.” The technique was applied to PAHs and fullerenes. But in principle, he says, by customizing the molecule used to functionalize the microscope tip, the technique could be applied to a wide variety of systems.