Science 365, 142–145 (2019)

Charge can induce changes of molecular behaviour in photosynthesis, organic photovoltaic devices and molecular electronics. Existing characterization techniques such as X-ray diffraction and vibrational and optical microscopy can only provide us with the average information of a large number of molecules. Now, Fatayer et al. use Atomic Force Microscopy (AFM) to monitor changes on the single-molecule level upon charging and discharging.

Credit: AAAS

They adsorb four different molecules on insulating NaCl multilayers, which allows them to study the molecules in different charge states. Controlled transfer of electrons from/to the CO functionalized AFM tip charges/discharges the molecule. High-resolution AFM images unveil changes in molecular conformation, bond order and aromaticity in multiple charge states. They find that the addition of electrons distorts azobenzene from planar to non-planar. The change of bond lengths in the dianion pentacene suggests the formation of radical anions in its second and fourth rings. The AFM and DFT results reveal changes in the geometry of dicyano moieties in tetracyanoquinodimethane upon charge-state changes. Furthermore, they observe changes of the aromaticity and conjugation pathway of porphine with altering bond order at different oxidation states.