Investigations of photoswitchable platform molecules on Au(111) surfaces by scanning tunneling microscopy
The functionalization of surfaces by self-assembled monolayers is of great interest for the bottom-up preparation of functional nanosystems. In this thesis structural investigations of photoswitchable self-assembled monolayers on Au(111) and investigations of their switching behavior via scanning tunneling microscopy under ambient conditions are presented. Initially, adsorbate layers of five different triazatriangulenium-based (TATA) azobenzene derivatives with varying coupling units between the photoswitch and the surface were investigated. They all exhibit highly-ordered hexagonal monolayers with a (√19 x √19) R23.4 superstructure and IRRAS measurements confirm that the reversible isomerization mechanism is retained upon metal-surface adsorption. When exploring the gold mediated thermal relaxation a significant dependency of the relaxation rates on the coupling of the azobenzene to the gold surface was shown. An electronically isolating linker group therefore enables a high stability of the cis configuration against thermal backswitching. Based on these results, the trans-to-cis isomerization of monolayers consisting of the isolated azobenzene derivative was investigated by molecular scale STM. The analysis of the high-resolution STM images revealed that the trans-to-cis isomerization does not occur randomly but is dependent on the isomeric state of the neighboring molecules. Therefore, the irradiation with UV-light results in a formation of trans and cis clusters. Furthermore, the coadsorption of the highly stable isolated azobenene TATA with TATA-based molecules with different vertical functions is shown. The molecules can be mixed without affecting the in-plane order, allowing a facile preparation of stochastically mixed bifunctional adsorbate layers. Finally, a molecule composed of an azobenzene unit spanned between two molecular pillars mounted on TATA platforms was adsorbed on Au(111). A preparation under defined deposition conditions results in a (√31 x √31) R8.9 superstructure. UV/Vis and NEXAFS measurements show the reversible switching of the azobenzene and indicate differences in the rotatability of the isomers, making it a horizontally oriented switch as well as an altitudinal rotor.
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