Dordaneh Zargarani

Photoswitching Behavior of Azobenzene-Containing Alkanethiol Self-Assembled Monolayers on Au Surfaces

The photoisomerization of self-assembled monolayers of azobenzene-containing alkanethiols, as well as of mixed monolayers of these substances with n-alkanethiol spacer molecules on Au surfaces, was studied by photoelectrochemical measurements and surface plasmon resonance spectroscopy. A strong dependence on the molecular structure of the adsorbates was found, specifically on the linker between the azobenzene moiety and the alkanethiol: while molecules with an amide group were photoinactive, those with an ether group exhibited pronounced, reversible photoisomerization in pure and mixed adlayers. Both trans-cis and cis-trans isomerization followed first-order kinetics with time constants that suggest high quantum efficiencies for these processes.

Gap-mode SERS studies of azobenzene-containing self-assembled monolayers on Au(111).

Self-assembled monolayers of azobenzene-containing thiols on smooth Au(111) surfaces were studied by gap-mode surface-enhanced Raman spectroscopy (gap-mode SERS). By adsorption of colloidal Au nanoparticles on top of the organic adlayer highly reproducible spectra with strongly enhanced intensities are obtained. The observed bands indicate a trans conformation of the azobenzene moieties and are in agreement with structural data for the molecular layer. A characteristic dependency on the terminal and the spacer groups of the molecules is found. Samples prepared during illumination with UV light show pronounced spectral differences that can be attributed to azobenzene in cis conformation.

Reversible light-controlled conductance switching of azobenzene-based metal/polymer nanocomposites

We present a new concept of light-controlled conductance switching based on metal/polymer nanocomposites with dissolved chromophores that do not have intrinsic current switching ability. Photoswitchable metal/PMMA nanocomposites were prepared by physical vapor deposition of Au and Pt clusters, respectively, onto spin-coated thin poly(methylmethacrylate) films doped with azo-dye molecules. High dye concentrations were achieved by functionalizing the azo groups with tails and branches, thus enhancing solubility. The composites show completely reversible optical switching of the absorption bands upon alternating irradiation with UV and blue light. We also demonstrate reversible light-controlled conductance switching. This is attributed to changes in the metal cluster separation upon isomerization based on model experiments where analogous conductance changes were induced by swelling of the composite films in organic vapors and by tensile stress.

Probing the switching state of a surface-mounted azobenzene derivative using femtosecond XUV photoemission

Photoemission spectroscopy using femtosecond XUV light pulses is applied to probe the isomerization state of the molecular switch 3-(4-(4-hexyl-phenylazo)-phenoxy)-propane-1-thiol deposited by liquid phase self-assembly on Au(111). Spectral shifts of valence-electronic signatures that we associate with the carbon C2s orbital enable us to distinguish the trans and the cis isomerization state of the adsorbed molecules. These preliminary results envision the potential to probe reversible switching processes of surface-mounted molecules in real time by tracking the temporal evolution of the electronic and nuclear degrees of freedom in a femtosecond XUV photoemission experiment.