Abstracts Chemistry

Azobenzenes and Imines as Molecular Switches on Gold(111):Structure, Excitonic Coupling, and Photoisomerization

by Daniel Brete

Photo-responsive molecular switches are a promising class of materials. In this work two types of systems, in which the molecular switches were adsorbed on a surface, are investigated: self-assembled monolayers (SAMs) of azobenzenes on gold and (sub)monolayers of TBI (E)-3,5-di- tert -butyl-N-(3,5-di- tert -butyl-benzylidene)aniline vapour-deposited on gold. Azobenzene SAMs: SAMs of azobenzene alkanethiolates on Au(111) were investigated by X-ray photoemission spectroscopy (XPS) and near-edge X-ray-absorption fine-structure (NEXAFS) spectroscopy. We found that the chromophores in these SAMs are oriented predominately upright. We developed a structural model of the SAMs by combining the molecular orientation obtained from NEXAFS measurements with AFM/STM data fromliterature on the lateral structure of the SAMs. The optical properties of the SAMs were investigated by UV/vis differential reflectance (DR) spectroscopy. Strong excitonic coupling of the chromophores was observed in these densely packed layers. The trans - cis photoisomerization was found to be inhibited in these SAMs—presumably due to steric hindrance or excitonic coupling. Bicomponent SAMs were prepared by coadsorption from solution. The composition of the SAMs as a function of the ratio of the two components in the solution was analysed by quantitative XPS. We demonstrate that we can arbitrarily tune the composition of the SAM and we present a thermodynamic model describing the relation between the composition of the SAM and the solution. Mixed SAMs comprising two different azobenzene chromophores allowed to study the excitonic coupling in a two-component system. In order to enable photoisomerization in the SAM, mixed SAMs of azobenzene alkane-thiolates diluted with unsubstituted alkanethiolates were produced. Here a dependence of the excitonic coupling on the dilution of the chromophores and efficient photoisomerization was demonstrated. TBI: Aromatic imines are closely related to azobenzenes. In imines the N=N group of azobenzenes is replaced by the isoelectrical CH=N group, which only contains a single N atom. The lower trans - cis isomerization barrier in imines compared to azobenzenes allows for thermal isomerization near room temperature. The imine TBI adsorbed on Au(111) is partially decoupled form the surface by tert -butyl substituents. We show that this compound forms two different stable monolayers: an all- cis and an all- trans layer. The fraction of the two isomers in the layers of TBI on Au (111) depends on the temperature and the coverage. With increasing temperature and decreasing coverage the compound undergoes isomerization from trans to cis and back to trans . The coverage of the various layers was determined by thermal desorption (TD) spectroscopy. The conformation and adsorption geometry of TBI in the layers was determined from NEXAFS measurements. The conformational transitions are driven by the higher packing density of the less stable cis isomer. Furthermore, it is found that in the cis -form the phenyl ring attached to the…