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Zusammenfassung:
Sum-frequency generation (SFG) vibrational spectroscopy is employed to investigate the reversible, photoinduced trans/cis isomerization of an azobenzene-functionalized self-assembled monolayer (SAM) on a gold substrate. A C≡N marker group at the outer phenyl ring is used as a direct measure of the switching state. The azobenzene unit is connected to the surface by a tripodal linker system with an adamantane core, which results in both a sufficient decoupling of the functional azobenzene unit from the metallic substrate and a free volume to prevent steric hinderance, thus allowing the isomerization process. Optical excitation at 405 nm induces the trans → cis isomerization, whereas light exposure at 470 nm leads to the back reaction. The effective cross sections for the reactions are σeff(cis) = 4 ± 1 × 1018 cm2 at 405 nm (trans → cis) and σeff(trans) = 2.5 ± 0.9 × 1019 cm2 at 470 nm (cis → trans). We propose that the photoisomerization is driven by a direct (intramolecular) electronic excitation of the azobenzene conjugate, analogous to the free molecules in solution.
