On the Impact of Geometrical Factors on Hot Electron-Induced Tautomerization

The ability to controllably switch molecules, for example, by the application of external stimuli, such as charge carriers or photons, has fascinated the scientific community since the advent of nanoscience. A prominent example is isomerization, that is, the transformation of a molecule with a given shape into another atomic arrangement without changing the chemical formula. This rearrangement often only involves a small subunit of the molecule, the active part, whereas the inactive part remains unchanged. Here, we present a systematic low-temperature scanning tunneling microscopy investigation of the influence the inactive part has on hot electron-induced tautomerization processes. In our study, we investigate crosslike molecules, namely, deprotonated phthalocyanine and naphthalocyanine molecules, which exhibit the same central active unit but possess molecule arms of different lengths. If deposited on a sixfold symmetric Ag(111) surface, the four molecule arms loose degeneracy, resulting in two pairs of opposing arms with different tautomerization efficiencies. Our data reveal that the electron yield changes significantly depending on the specific molecular arm the charge carriers reach after injection into the Ag(111) surface state. To partially separate the fraction of hot electrons reaching the different arms, we performed distance- and angle-dependent molecular nanoprobe measurements. The experimental results are interpreted within a very simple geometrical model assuming rectangular shaped molecular arms, which nicely explains the effects observed for phthalocyanine and naphthalocyanine molecules.