|The IBM Zurich people have been treating us to a pic of naphthalocyanine at the nanoscale exposing its electronic picture (DOI). |
But what exactly do we see? First of all, naphthalocyanine is a curious molecule, first investigated as a molecular logic gate. The molecule looks like the letter X with one axis (SW to NE) definitely aromatic and full of electrons and the other axis non-aromatic (compare to isoindole but look at all resonance structures) and devoid of electrons. The axis can interchange by a process of tautomerism where the two inner hydrogen atoms change position.
This molecule has already been scanned with regular atomic force microscopy and the new images are obtained by an AFM variation called Kelvin Probe Force Microscopy where a voltage is applied between the AFM tip and the surface (Where does Kelvin come in?). The naphthalocyanine molecules are isolated from the surface by double monolayer of NaCl.
As the electrons inside the molecule will interfere with the voltage, the local electron density can be measured. In addition to that, when the applied voltage matches the LUMO energy, a tautomerisation is induced in the molecule and the electron density of both states can be compared. The images clearly confirms high/low electron density where you would expect it. The amazing thing is that the contrast is maximized not by closing up on the molecule but by hovering over it from a small distance. Contrast therefore is mainly determined by the electron density at a certain height above the molecule. In the article round-up the IBM people expect in the near future to be able to monitor electron distribution changes in actual chemical reactions between molecules taking place on surfaces.