Sum-frequency generation (SFG) vibrational spectroscopy is employed to investigate the reversible photoinduced trans/cis-isomerization of an azobenzene functionalized self-assembled mono- layer (SAM) on a gold substrate. A CΞN-marker group at the outer phenyl ring is used as a direct measure of the switching state. The azobenzene unit is connected to the surface by a tripodal linker system with an adamantane core, which results in both a sufficient decoupling of the functional azobenzene unit from the metallic substrate and a free volume to prevent steric hin- derance thus allowing the isomerization process. Optical excitation at 405 nm induces the trans -> cis-isomerization, whereas light exposure at 470 nm leads to the back reaction. The effective cross sections for the reactions are σeff(cis) = 4 ± 1 × 1018cm2 at 405 nm (trans -> cis) and σeff(trans) = 2.5 ± 0.9 × 1019cm2 at 470 nm (cis -> trans), respectively. We propose that the photoisomerization is driven by a direct (intramolecular) electronic excitation of the azobenzene conjugate analogous to the free molecules in solution.
The original publication is available at link DOI: 10.1039/B823330F