The design of molecular switches and trigger elements offers a formidable challenge on the road towards miniaturization in future technology and the development of materials for information storage and retrieval at the molecular level. Stilbene (1,2-Diphenylethylene) can be considered as a prototype molecule for such applications, with an extended ?-orbital system that provides conduction channels and also favors a stacking of the molecules with their aromatic planes parallel to each other. The cis-trans-isomerisation of the free molecule follows an in-plane "hula-twist" mechanism around the C=C-double bond upon UV-irradiation, which may work as well in constrained systems such as on a surface, thus realizing a "molecular switch" functionality.
We use Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy as well as core and valence level photoemission for an identification and differentiation of the two isomers. Gas phase NEXAFS-spectra of the molecules show spectral features in the ?*-resonance that allow for a clear distinction of the isomers. Upon adsorption on surfaces, this spectral feature remains largely unchanged, while the ?*-resonances and the higher ?*-resonances undergo changes. These changes can be used, in combination with theoretical calculations, to identify the interaction of the molecules with the substrates and to reveal the adsorption geometry. Spectra of the molecular orbitals recorded at low binding energies show that both cis- and trans-stilbene adsorb undissociated on the Si(100) and Cu(100) surface.
Analysis of the NEXAFS spectra for gas- and adsorbed phase in combination with theoretical calculations suggest that the interaction of the stilbene with the Si(100) surface inhibits the excitation of electrons into the unoccupied ?-orbital, a process required for isomerisation. Spectra of stilbene molecules on Cu(110), on the other hand, indicate that this excitation may not be hindered. We report on the cis-trans-isomerisation of stilbene on Si(100) under exposure to monochromatized UV light of different wavelengths (between 250 and 450 nm) in mono- and multilayers, which reveal significant differences for the two surfaces.
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