Nanoscale Chemical Imaging of Reversible Photoisomerization of an Azobenzene‐Thiol Self‐Assembled Monolayer by Tip‐Enhanced Raman Spectroscopy

Abstract

An understanding of the photoisomerization mechanism of molecules bound to a metal surface at the molecular scale is required for designing photoswitches at surfaces. It has remained a challenge to correlate the surface structure and isomerization of photoswitches at ambient conditions. Herein, the photoisomerization of a self-assembled monolayer of azobenzene-thiol molecules on a Au surface was investigated using scanning tunneling microscopy and tip-enhanced Raman spectroscopy. The unique signature of the cis isomer at 1525 cm observed in tip-enhanced Raman spectra was clearly distinct from the trans isomer. Furthermore, tip-enhanced Raman images of azobenzene thiols after ultraviolet and blue light irradiation are shown with nanoscale spatial resolution, demonstrating a reversible conformational change. Interestingly, the cis isomers of azobenzene-thiol molecules were preferentially observed at Au grain edges, which is confirmed by density functional theory.

Azo bond formation on metal surfaces

“…[12,13] Among these N-based functional groups, the azo group shows valuable properties in photo switching. [14][15][16] In solution phase chemistry, azo compounds are obtained through an azo coupling using diazonium salts, but the cross-coupling of nitro and amino compounds remains challenging. Herein, onsurface azo formation by nitroarene-arylamine redox crosscoupling (path 1 in Scheme 1a) is successfully realized on the Ag(111) surface under ultrahigh vacuum (UHV) conditions.…”

“…[12,13] Among these N-based functional groups, the azo group shows valuable properties in photo switching. [14][15][16] In solution phase chemistry,a zo compounds are obtained through an azo coupling using diazonium salts,b ut the cross-coupling of nitro and amino compounds remains challenging.Herein, on-surface azo formation by nitroarene-arylamine redox cross-coupling (path 1 in Scheme 1a)i s successfully realized on the Ag(111) surface under ultrahigh vacuum (UHV) conditions.O n-surface azo formation via reductive nitroarene or oxidative arylamine homocoupling (paths 2 and 3 in Scheme 1a)s hows poor efficiency.L owtemperature scanning tunneling microscopy (LT-STM) and X-ray photoelectron spectroscopy (XPS) were applied for product analysis.T he chemical structures of organic precursors used are depicted in Scheme 1b (for their synthesis,s ee the Supporting Information, SI). By using well-designed precursors,a zo polymers and alternating azo oligomers are accessible on-surface.I na ddition, the effect of the metal surface on the azo formation is investigated.…”

“…Only a few carbon−nitrogen and nitrogen−nitrogen bond forming processes have been reported, such as imine, [5–7] imide, [8] azo group formation [9–11] and azide–alkyne cycloaddition [12, 13] . Among these N‐based functional groups, the azo group shows valuable properties in photo switching [14–16] . In solution phase chemistry, azo compounds are obtained through an azo coupling using diazonium salts, but the cross‐coupling of nitro and amino compounds remains challenging.…”

Gewinner der Emanuel‐Merck‐Vorlesung und des Heinrich‐Emanuel‐Merck‐Preises

“…Zenobi befasst sich mit laserbasierter Massenspektrometrie und Rastersondenmikroskopie. Kürzlich berichtete er in der Angewandten Chemie über die Visualisierung einer reversiblen Photoisomerisierung in selbstorganisierten Monoschichten mithilfe von spitzenverstärkter Raman‐Spektroskopie …”

Azobindungsbildung auf Metalloberflächen

“…Unter diesen N-basierten funktionellen Gruppen zeigt die Azogruppe nützliche Eigenschaften als Photoschalter. [14][15][16] In der Lçsungsmittelchemie werden Azoverbindungen durch eine Azokupplung unter Verwendung von Diazoniumsalzen erhalten, die Kreuzkupplung von Nitro-und Aminoverbindungen bleibt jedoch herausfordernd. Hier wird die oberflächenchemische Azobildung durch Nitroaren-Arylamin-Redoxkreuzkupplung (Pfad 1 in Schema 1a)e rfolgreich auf der Ag(111)-Oberfläche unter Ultrahochvakuum(UHV)-Bedingungen realisiert.…”