Isomerization of One Molecule Observed through Tip-Enhanced Raman Spectroscopy

While exploring photoisomerization of azobenzyl
thiols (ABT) adsorbed on Au(111), through joint scanning
tunneling microscopy (STM) and tip-enhanced Raman
scattering (TERS) studies, the reversible photoisomerization
of one molecule is captured in TERS trajectories. The unique
signature of single molecule isomerization is observed in the
form of anticorrelated flip-flops between two distinct spectra
with two discrete, on- and off-levels. The apparently
heterogeneously photocatalyzed reaction is assigned to cis−
trans isomerization of an outlier, which is chemisorbed on the
silver tip of the STM. Otherwise, the ensemble of ABT molecules that lie flat on Au(111) remain strongly coupled to the surface, excluding the possibility of photoisomerization or detection through TERS.