Berlin 2008 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 34: Metal Substrates: Adsorption of Organic/Bio Molecules II

O 34.6: Vortrag

Dienstag, 26. Februar 2008, 15:00–15:15, MA 043

Why a ’decoupled’ switch doesn’t switch. Self-assembled monolayers of Azobenzenethiol on gold — •Roland Schmidt¹, Daniel Brete^1,2, Wolfgang Freyer¹, Cornelius Gahl¹, and Martin Weinelt^1,2 — ¹Max-Born-Institut, Max-Born-Straße 2A, 12489 Berlin — ²FU-Berlin, Arnimallee 14, 14195 Berlin

We have investigated well oriented, self-assembled monolayers of alkanethiols on gold, functionalized by the molecular switch azobenzene (HS−(CH₂)_n−O−C₆H4−N₂−C₆H₄−CF₃, n=3,6,10). Using X-ray absorption spectroscopy the tilt angle between the backbone of the azobenzene group and the surface normal was shown to be ∼ 15^∘, with only slight dependence on the length of the alkane chain n. The evaluation of autoionization spectra of the N1s⁻¹π^{∗ +1} state in terms of the core-hole-clock approach gave a charge-transfer time τ_CT of the LUMO of several 10 fs. Also τ_CT showed no significant dependence on the length of the alkane chain n. Hence the switches are significantly decoupled from the substrate. However, a strong quenching of the SAM’s π−π^∗-excitation is observed in UV-Vis spectroscopy, pointing out that lateral interactions among the azobenzene chromophores in the crystal-like layer dominate both the charge transfer and the geometrical structur of the SAM. This is supported by the energy dependence of τ_CT that is atypical for tunnelling into the substrate. We suppose that the ultrafast delocalization of the S₂ excitation causes the observed inefficiency of photochemical switching of alkanethiol based SAMs.