Isomerization in single molecules of azobenzene probed by Surface-enhanced Raman Scattering

Trans↔cis isomerization in single azobenzene molecules were captured by Surface-enhanced Raman Scattering (SERS) using nanoAg-on-Ge substrates. Isomerization was observed to occur more frequently in the presence of 365 nm LED excitation in addition to the 532 nm Raman probe laser. Under these excitation conditions, the trans→cis isomerization frequency (i.e., ~0.1 s^-1) is observed to be much lower than the theoretical estimate (i.e., ~100 s^-1 without the surface enhancement effect). This “slowing down” effect allows us to resolve azobenzene´s conformational changes at a time scale of 40 ms. We attribute this dramatic reduction in isomerization frequency to adsorption of azobenzene to silver that restrains its rotational freedom. As deduced from statistical analysis of the single molecule spectra, this restraint is higher for the trans form that we explain its stronger chemisorption due to its planar geometry.