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.

We present a comparative study of copper-hexadecafluorophthalocyanine (F16CuPc) films grown on bare SiO2 and on SiO2 functionalized by octadecyltrimethoxisilane (OTMS), a methyl terminated molecular layer with a lower surface energy than SiO2. We show that the functionalization of SiO2 by OTMS leads to the formation of highly structurally ordered F16CuPc crystallites. The extended lateral order and subsequent reduction of grain boundaries result in an improved field-effect mobility for electrons by almost an order of magnitude in thin-film transistors made of these films.

An innovative approach has been developed to measure small molecule diffusion in polyelectrolyte multilayers (PEMs) assembled on colloidal particles by means of flow cytometry (FACS). FACS allows changes in fluorescence emission as a function of time to be recorded per particle in a colloidal dispersion. Dithionite, SO, diffusion in PEMs composed of polyallylamine hydrochloride (PAH) and poly styrene sulfonate (PSS) assembled on silica particles has been studied by recording the quenching of (7-nitrobenz-2-oxa-1,3-diazol-4yl)amino (NBD) labelled PAH layers by FACS. NBD is reduced when it encounters dithionite, and is therefore no longer fluorescent. The decay in fluorescence will be used to follow the kinetics of dithionite diffusion. The fluorescence decay curves show slow diffusion that does not follow classical Fickean law. However, by assuming that the diffusion coefficient is time dependent and follows an inverse power law in an atypical diffusion case, it was possible to obtain an excellent fit for the decay curves.

enhancement of polymer-side-chain mobility at room temperature, and the saturation dose is largely dependent on film thickness. We also show that polymer rewriteability is directly correlated to the degree of photobleaching that occurs during writing. The very large birefringence and high optical transparency in the 650±1600 nm wavelength range is quite promising for integrated photonic device applications of photoaddressable polymer thin films. ExperimentalSample Preparation: Amorphous isotropic polymer films with thicknesses ranging from 160±780 nm were prepared by spinning PAP solutions in tetrahydrofuran on glass substrates. The sample thickness was determined with an Alpha-Step surface profiler, while the refractive index was measured with a spectroscopic ellipsometer.Optical Characterization: Film birefringence was quantified in situ using a standard experimental setup [7] where the reading source (633 nm HeNe or 670 nm diode) was polarized 45 with respect to the write source (488 nm line of the Ar + beam). Both beams are directed onto the sample surface in normal incidence (write diameter 1.4 mm, read diameter~0.5 mm), and subsequently pass through polarized beam splitters. The write beam and the perpendicular-polarized component of the read beam are then directed to germanium photodetectors to obtain intensity data for the calculation of photobleaching and birefringence, respectively.

No abstract