On the electronic and geometrical structure of the trans- and cis-isomer of tetra-tert-butyl-azobenzene on Au(111) Roland Schmidt, a Sebastian Hagen, b Daniel Brete, a Robert Carley, a Cornelius Gahl, a Jadranka Dokic´, c Peter Saalfrank, c Stefan Hecht, d Petra Tegeder* b and Martin Weinelt* ab Received 19th November 2009, Accepted 17th February 2010 First published as an Advance Article on the web 23rd March 2010 DOI: 10.1039/b924409c Near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy have been employed to follow the reversible trans to cis isomerization of tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). For one monolayer the molecules adopt an adsorption geometry characteristic of the trans-TBA isomer. The azo-bridge (N = N) is aligned nearly parallel to the surface and the phenyl rings exhibit a planar orientation with a small tilt angle r41 with respect to the surface normal. Illumination of the molecular layer at 455 nm triggers the trans to cis isomerization which is associated with a pronounced change of the geometrical and electronic structure. The N1s to p* transition of the central azo-bridge shifts by 0.45  0.05 eV to higher photon energy and the transition dipole moment (TDM) is tilted by 59  51 with respect to the surface normal. The p-system of one phenyl ring is tilted by about 301 with respect to the surface normal, while the second ring plane is oriented nearly perpendicular to the surface. This reorientation is supported by a shift and broadening of the C–H resonances associated with the tert-butyl legs of the molecule. These findings support a configuration of the photo-switched TBA molecule on Au(111) which is comparable to the cis-isomer of the free molecule. In the photo-stationary state 53  5% of the TBA molecules are switched to the cis configuration. Thermal activation induces the back reaction to trans-TBA. I. Introduction In recent years molecular switches such as azobenzenes adsorbed on (metal) surfaces have attracted considerable attention due to their potential use in nanotechnology with implementations ranging from information storage and processing to adaptive surfaces. 1–12 Azobenzene and its derivatives permit the control of molecular geometry and functional properties via optical excitation in the ultraviolet and visible regime. This light-induced switching is based on a reversible trans to cis isomerization between the planar thermodynamically more stable trans-isomer and the staggered cis form. 13–15 In addition, the back reaction from cis- to trans-isomer can be thermally activated, overcoming an energy barrier of around 1 eV. While in the liquid phase the isomerization behavior of azobenzenes is well understood, it remains a key question to determine the switching properties of the surface-bound molecules. With future applications in molecule-based devices in mind, a detailed knowledge of the adsorption geometry, i.e. the molecular orientation, seems essential. In general the structure of the adsorption complex is determined by the molecule–substrate interaction. Moreover, the electronic coupling plays an important role for optically driven isomerization processes since the picosecond lifetimes of excited states in solution decrease strongly when the molecules are bound to a (metal) substrate. Photoisomerization of a molecular switch in direct contact with a metal surface has so far been demonstrated for a specifically designed azobenzene derivative, namely the 3,3 0 ,5,5 0 -tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). 16–18 For TBA it is believed that the bulky tert-butyl- groups lead to a decoupling of the functional azobenzene backbone from the metallic substrate. This allows for the photoinduced trans to cis conformational change and the thermally activated back reaction. 16,19–22 For the free TBA molecule conformations of the trans- and cis-isomers as well as a representative transition state TS reached in the thermal back reaction have been calculated in ref. 23. The corresponding chemical structures are illustrated in Fig. 1a. In the B3LYP/6-31G* calculations, the free TBA molecule has been optimized for the cis, trans, and TS geometries. To vary the nature of the respective stationary states, normal mode analyses have been carried out which gave only real frequencies for the cis and trans geometries, and a single imaginary frequency for the TS. Further computational details are given in ref. 23. Note that the photoinduced isomerization of bare azobenzene, i.e. the molecule bound to Au(111) without tert-butyl spacer, a Max-Born-Institut fu ¨r Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Str. 2A, 12489 Berlin, Germany. E-mail: weinelt@mbi-berlin.de b Freie Universita ¨t Berlin, Fachbereich Physik, Arnimallee 14, D-14195 Berlin, Germany. E-mail: petra.tegeder@physik.fu-berlin.de c Institut fu ¨r Chemie, Universita ¨t Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany d Humboldt-Universita ¨t zu Berlin, Department of Chemistry, Brook-Taylor-Str. 2, 12489 Berlin, Germany 4488 | Phys. Chem. Chem. Phys., 2010, 12, 4488–4497 This journal is  c the Owner Societies 2010 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics