Quantification of finite-temperature effects on adsorption geometries of π\pi-conjugated molecules

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of non-local collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial.Comment: 12 pages, 3 figure

X-ray standing wave simulations based on Fourier vector analysis as a method to retrieve complex molecular adsorption geometries

We present an analysis method of normal incidence x-ray standing wave (NIXSW) data that allows detailed adsorption geometries of large and complex molecules to be retrieved. This method (Fourier vector analysis) is based on the comparison of both the coherence and phase of NIXSW data to NIXSW simulations of different molecular geometries as the relevant internal degrees of freedom are tuned. We introduce this analysis method using the prototypical molecular switch azobenzene (AB) adsorbed on the Ag(111) surface as a model system. The application of the Fourier vector analysis to AB/Ag(111) provides, on the one hand, detailed adsorption geometries including dihedral angles, and on the other hand, insights into the dynamics of molecules and their bonding to the metal substrate. This analysis scheme is generally applicable to any adsorbate, it is necessary for molecules with potentially large distortions, and will be particularly valuable for molecules whose distortion on adsorption can be mapped on a limited number of internal degrees of freedom

Azobenzene/Ag(111)

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of nonlocal collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial

X-ray-Induced Reversible Switching of an Azobenzene Derivative Adsorbed on Bi(111)

We report on the adsorption of a submonolayer of di-m-cyanoazobenzene (DMC) on Bi(111) and on the reversible switching of these molecules induced by resonant X-ray illumination. DMC adsorbs in at least two configurations, the flat trans and the nonflat cis isomer. We find that in 0.8 monolayers at least 26% of the molecules change their configuration at 110 K by excitation of the N1s → LUMO transition at the azo group, and by a thermally induced back reaction at 120 K. Nonresonant excitation with X-ray light does not induce any reversible changes

Excitation mechanism in the photoisomerization of a surface-bound azobenzene derivative: Role of the metallic substrate

Two-photon photoemission (2PPE) spectroscopy is employed to elucidate the electronic structure and the excitation mechanism in the photoinduced isomerization of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). Our results demonstrate that the optical excitation and the mechanism of molecular switching at a metal surface is completely di®erent compared to the corresponding process for the free molecule. In contrast to direct (intramolecular) excitation operative in the isomerization in the liquid phase, the conformational change of the surface-bound TBA is driven by a substrate-mediated charge transfer process. We find, that photoexcitation above a threshold hn ≈2.2 eV leads to hole formation in the Au d-band followed by a hole transfer to the highest occupied molecular orbital (HOMO) of TBA. This transiently formed positive ion resonance subsequently results in a conformational change. The photon energy dependent photoisomerization cross section exhibit an unusual shape for a photochemical reaction of an adsorbate on a metal surface. It shows a threshold like behavior below hn ≈2.2 eV and above hn≈4.4 eV. These thresholds correspond to the minimum energy required to create single or multiple hot holes in the Au d-bands, respectively. This study provides important new insights into the use of light to control the structure and function of molecular switches in direct contact with metal electrodes

Adsorption and switching properties of a N-benzylideneaniline based molecular switch on a Au(111) surface

High resolution electron energy loss spectroscopy is employed to analyze the adsorption geometry and the photoisomerization ability of the molecular switch carboxy-benzylideneaniline (CBA) adsorbed on Au(111). CBA adopts on the Au(111) surface a planar (trans) configuration in the first monolayer (ML) as well as for higher coverages (up to 6 ML), contrary to the geometry in solution, which is strongly non-planar. Illumination with UV light of CBA in direct contact with the Au(111) surface (≤ 1 ML) caused no changes in the vibrational structure, whereas at higher coverages ( > 1 ML) pronounced modifications of vibrational features are observed, which we assign to a trans → cis isomerization. Thermal activation induced the back reaction to trans-CBA. We propose that the photoisomerization is driven by a direct (intramolecular) electronic excitation of the adsorbed CBA molecules in the second ML (and above) analogous to CBA in the liquid phase

Imine Derivatives on Au(111): Evidence for “Inverted” Thermal Isomerization

Molecules that undergo reversible isomerization between trans and cis states, typically upon illumination with light at appropriate wavelengths, represent an important class of molecular switches. In this combined scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS) study, we report on self-assembled arrays of imine derivatives on a Au(111) surface. Most of the molecules are found in the trans state after deposition at room temperature, but many of them change their conformation upon heating, which we assign to switching to the cis state. As for many molecular switches, the trans isomer is the energetically more stable compound in solution, resulting in thermal cis to trans relaxation upon sufficient heating. On the surface, however, the number of cis isomers increases with temperature, pointing toward an “inverted” thermal isomerization behavior. The reason for this surface-mediated effect could be a stronger coupling, as compared to the trans state, of the central imine part of the molecule to the gold surface, which is sterically only possible in the cis state

Reversible photoisomerization of an azobenzene-functionalized self-assembled monolayer probed by sum-frequency generation vibrational spectroscopy

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