Erik McNellis

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Berlín, Berlin, Germany

Are you Erik McNellis?

Claim your profile

Publications (9)44.45 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The isomerization behavior of photochromic molecular switches is strongly influenced by adsorption on metal surfaces. For (E)-3,5-di-tert-butyl-N-(3,5-di-tert-butylbenzylidene)aniline (abbreviated as TBI for tetra-tert-butyl imine) it is found that a layer adsorbed on Au(111) can undergo an isomerization from the trans- to the cis- and back to the trans-configuration when continuously increasing the sample temperature and accordingly decreasing the sample coverage. The conformation and adsorption geometry of TBI are determined from near-edge X-ray absorption fine structure measurements in agreement with density-functional-theory calculations taking into account the van-der-Waals interaction between adsorbate and metal surface. The coverage- and temperature-controlled conformational transitions are reversible and are driven by the higher packing density of the less stable cis-isomer in combination with the low thermal activation barrier of the trans to cis isomerization typical for imine derivatives. This unexpected scenario is corroborated by thermal desorption and vibrational spectroscopy, as well as scanning tunneling microscopy.
    Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface bonding. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the -N[double bond, length as m-dash]N- moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate.
    Physical Chemistry Chemical Physics 04/2010; 12(24):6404-12. · 4.20 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Optical properties and the geometric structure of self-assembled monolayers of azobenzene-functionalized alkanethiols have been investigated by UV/visible and near edge X-ray absorption fine structure spectroscopy in combination with density-functional theory. By attaching a trifluoro-methyl end group to the chromophore both the molecular tilt and twist angle of the azobenzene moiety are accessible. Based on this detailed structural analysis the energetic shifts observed in optical reflection spectroscopy can be qualitatively described within an extended dipole model. This substantiates sizable excitonic coupling among the azobenzene chromophores as an important mechanism that hinders trans to cis isomerization in densely packed self-assembled monolayers.
    Journal of the American Chemical Society 02/2010; 132(6):1831-8. · 10.68 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We employ normal-incidence x-ray standing wave and temperature programed desorption spectroscopy to derive the adsorption geometry and energetics of the prototypical molecular switch azobenzene at Ag(111). This allows us to assess the accuracy of semiempirical correction schemes as a computationally efficient means to overcome the deficiency of semilocal density-functional theory with respect to long-range van der Waals (vdW) interactions. The obtained agreement underscores the significant improvement provided by the account of vdW interactions, with remaining differences mainly attributed to the neglect of electronic screening at the metallic surface.
    Physical Review Letters 01/2010; · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We examine the influence of the adsorption geometry and electronic coupling between azobenzene-based molecular switches and metal surfaces on their photoisomerization ability. Using the normal-incidence X-ray standing wave technique and large-scale density functional theory (DFT) calculations we determine adsorption geometries for azobenzene and 3,3´,5,5´-tetra-tert-butyl-azobenzene (TBA) adsorbed on Ag(111). Comparing the experimental determined and calculated vertical bonding distances between the photochemically active diazo (-N=N-) moiety of both molecules, reveals that the photoisomerization ability is rather insensitive on the adsorption height, as the N-Ag adsorption distance in TBA/Ag(111) is only 0.14 Å (0.13 Å for the calculated value) larger than the corresponding value for azobenzene. Our DFT calculations predict also similar adsorption heights of the diazo-bridge for azobenzene and TBA adsorbed on Au(111) even though TBA undergoes a photoinduced isomerization while these process is suppressed in azobenzene/Au(111). The photoisomerization ability of TBA/Au(111) and its suppression for azobenzene on Au(111), Ag(111) as well as for TBA on Ag(111) thus demonstrate that a purely geometrical argumentation explaining the isomerization properties fails. Thus the electronic structure of the complete adsorbate/substrate complex has to be taken into account in order to control molecular functionality at surfaces.
    Chemical Physics Letters 01/2010; · 2.15 Impact Factor
  • Source
    Erik McNellis, Jorg Meyer, Karsten Reuter
    [Show abstract] [Hide abstract]
    ABSTRACT: We use different semi-empirical dispersion correction schemes to assess the role of long-range van der Waals interactions in the adsorption of the prototypical molecular switch azobenzene (C6H5-N2-C6H5) at the coinage metal surfaces Cu(111), Ag(111) and Au(111). Compared to preceding density-functional theory results employing a semi-local exchange and correlation functional we obtain partly sizable changes of the computed adsorption geometry and energetics. The discomforting scatter in the results provided by the different schemes is largely attributed to the unknown form of the damping function in the semi-empirical correction expression. Using the congeneric problem of the adsorption of benzene as a vehicle to connection with experiment, we cautiously conclude that the account of dispersive interactions at the metal surfaces provided by the various schemes is in the right ballpark, with the more recent, general schemes likely to overbind.
    Physical Review B 09/2009; 80:205414. · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a density-functional theory trend study addressing the binding of the trans-cis conformational switch azobenzene (C6H5-N=N-C6H5) at three coinage metal surfaces. From the reported detailed energetic, geometric, and electronic structure data we conclude that the governing factor for the molecule-surface interaction is a competition between covalent bonding of the central azo (-N=N-) bridge on the one hand and the surface interaction of the two closed-shell phenyl (-C6H5) rings on the other. With respect to this factor the cis conformer exhibits a more favorable gas-phase geometric structure and is thus more stabilized at the studied surfaces. With the overall binding still rather weak the relative stability of the two isomers is thereby reduced at Ag(111) and Au(111). This is significantly different at Cu(111), where the cis bonding is strong enough to even reverse the gas-phase energetic order at the level of the employed semi-local electronic exchange and correlation (xc) functional. While this actual reversal may well be affected by the deficiencies due to the approximate xc treatment, we critically discuss that the rationalization of the general effect of the surface on the meta-stable molecular states is quite robust. This should equally hold for the presented analysis of recent tip-manipulation and photo-excitation isomerization experiments from the view point of the derived bonding mechanism.
    Physical Review B 03/2009; 80:035414. · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The adsorption of benzene (C6H6) at the Cu(111) surface is a much studied model system for the interaction of larger pi-conjugated molecules with solid surfaces. At first glance a simple system, the suspected predominantly van der Waals type bonding at the extended metal surface poses a severe challenge for accurate first-principles calculations. Density-Functional Theory (DFT) with local and semi-local exchange-correlation (xc) functionals is uncertain to properly account for this type of bonding, while the system sizes required to correctly grasp the metallic band structure are computationally untractable with correlated wave function techniques. We overcome these limitations with a recently introduced ``local xc correction'' scheme [1], correcting the adsorption energetics from present-day DFT xc functionals with hybrid functional and Møller-Plesset perturbation theory calculations for small clusters. From the obtained convergence of the xc correction with cluster size we can disentangle short-range and dispersion type contributions to the bonding of the molecule at different heights above the surface. This enables us to qualify the role played by the two contributions in determining the binding energetics and geometry. [1] Q.-M. Hu, K. Reuter, and M. Scheffler, PRL 98, 176103 (2007) and 99, 169903 (2007); C. Tuma and J. Sauer, CPL 387, 388 (2004).
    03/2008;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Self-assembled monolayers (SAMs) of 4-trifluoromethyl-azobenzene-4′-methyleneoxy-alkanethiols (CF3– C6H4–N=N–C6H4–O–(CH2) n –SH on (111)-oriented poly-crystalline gold films on mica were examined by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The spectra are analyzed with the help of density-functional-theory calculations of the isolated molecule. Only one doublet is detected in the sulphur 2p spectra of the investigated SAMs, consistent with a thiolate bond of the molecule to the gold surface. The C1s XP spectra and the corresponding XAS π * resonance exhibit a rich structure which is assigned to the carbon atoms in the different chemical surroundings. Comparing XPS binding energies of the azobenzene moiety and calculated initial-state shifts reveals comparable screening of all C1s core holes. While the carbon1s XPS binding energy lies below the π *-resonance excitation-energy, the reversed order is found comparing core ionization and neutral core excitation of the nitrogen1s core-hole of the azo group. This surprising difference in core-hole binding energies is interpreted as site-dependent polarization screening and charge transfer among the densely packed aromatic moieties. We propose that a quenching of the optical excitation within the molecular layer is thus one major reason for the low trans to cis photo-isomerization rate of azobenzene in aromatic-aliphatic SAMs.
    Applied Physics A 01/2008; 93(2):267-275. · 1.69 Impact Factor

Publication Stats

126 Citations
44.45 Total Impact Points

Institutions

  • 2010
    • Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
      Berlín, Berlin, Germany
    • Forschungszentrum Jülich
      Jülich, North Rhine-Westphalia, Germany
  • 2008–2010
    • Fritz Haber Institute of the Max Planck Society
      Berlín, Berlin, Germany
    • Freie Universität Berlin
      • Department of Physics
      Berlin, Land Berlin, Germany