Spectroscopic evidence of a reversible, photoinduced trans ↔ cis photoisomerization is provided for an azobenzene-functionalized triazatriangulene (TATA) platform on Au(111). As shown by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), these molecules form a well-ordered self-assembled monolayer (SAM) on Au(111). The surface-adsorbed azo-TATA platforms are also investigated by infrared reflection absorption spectroscopy (IRRAS); a methoxy marker group at the upper phenyl ring of the azo moiety is employed to monitor the switching state. The IRRAS data are analyzed by comparison with theoretical and transmission IR spectra as well as bulk and surface-enhanced Raman spectroscopic (SERS) data. IRRAS shows that the methoxy group is oriented perpendicular to the surface in trans- and tilted with respect to the surface normal in cis-configuration. This indicates that the photoswitching capability of the azobenzene moieties is retained on the gold surface. The lifetime of the cis-configuration is, however, reduced by a factor of ∼10(3) with respect to the homogeneous solution.

Triazatriangulenium (TATA) platform molecules allow the preparation of functionalized surfaces with well-defined lateral spacings of freestanding functional groups. Using scanning tunneling microscopy, synchrotron-based X-ray photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy and complementary density functional theory calculations the chemical composition and orientational order of adlayers of functionalized azobenzene containing TATA platform molecules were characterized. According to these studies the molecules are chemically intact on the surface after self-assembly from solution and exhibit a well-defined adsorption geometry where the azobenzene units are oriented almost perpendicular to the surface.

Direct comparative studies of the photoisomerization of azobenzene derivatives in self-assembled adlayers on Au and as free molecules in dichloromethane solution were performed using UV/vis spectroscopy. For all studied systems a highly reversible trans-cis isomerization in the adlayer is observed. Quantitative studies of the absorbance changes and photoisomerization kinetics reveal that in azobenzenes mounted as freestanding vertical groups on the surface via triazatriangulene-based molecular platforms photoswitching is nearly uninhibited by the local environment in the adlayer. The blue-shift of the π-π* transition in adlayers of these molecules is in good agreement with theoretical studies of the effect of excitonic coupling between the molecules. In contrast, in azobenzene-containing thiol self-assembled monolayers the fraction of photoswitching molecules and the photoisomerization kinetics are significantly reduced compared to free molecules in solution.