On the electronic and geometrical structure of the trans- and cis-isomer of tetra-tert-butyl-azobenzene on Au(111)

Abstract

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 <or=4 degrees 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 pi* 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 +/- 5 degrees with respect to the surface normal. The pi-system of one phenyl ring is tilted by about 30 degrees 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.

UV/Vis Spectroscopy Studies of the Photoisomerization Kinetics in Self-Assembled Azobenzene-Containing Adlayers

“…Another explanation is the electronic quenching of the isomerization, caused by alternative pathways for the deexcitation of the optical excited electronic state, which compete with the photoisomerization reaction. The latter may occur due to electronic coupling with the metal substrate 24 , which is an important effect for molecules in which the azobenzene exhibits a planar adsorption geometry with small spacing to the surface 12,25 . However, for azobenzene, which is separated from the surface, these effects should not be able to suppress photoswitching according to a recent theoretical study by Benassi and Corni 26 .…”

Surface hopping dynamics of direct trans → cis photoswitching of an azobenzene derivative in constrained adsorbate geometries

“…A way to achieve this is by mounting the molecule on a surface. [4][5][6][7][8][9][10][11][12] Of course, the surface will affect the switching behaviour. This can be due to geometric constraints imposed by the substrate or by neighbouring molecules, 7 due to energy delocalization and phase relaxation within the molecule by coupling to substrate phonons, substrate electron-hole pairs, and other adsorbates, 8,9 or due to the active participation of the surface itself.…”

Sensitivity of photoelectron diffraction to conformational changes of adsorbed molecules: Tetra-tert-butyl-azobenzene/Au(111)

“…The trans isomer adsorbs in a nearly planar geometry with both phenyl rings parallel to the surface. In the cis isomer, one of the phenyl groups flaps up by ≈30° while the second one finds itself perpendicular with respect to the surface, 10 as shown in Fig. 1 (bottom part).…”

“…The pronounced structural changes in the TBA/Au(111) system associated with the trans-cis isomerization have been studied experimentally by means of near-edge x-ray absorption fine structure (NEXAFS) 10 and theoretically by density functional theory (DFT) 11 . The trans isomer adsorbs in a nearly planar geometry with both phenyl rings parallel to the surface.…”