32 research outputs found
Quantification of finite-temperature effects on adsorption geometries of -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 â 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 diÂźerent 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 â2.2 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 â2.2 eV and above hnâ4.4 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 â 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 âInvertedâ 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 âinvertedâ 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