K. Jeuris

Light stability of 3,4-ethylenedioxythiophene-based derivatives

Abstract The light stability of two 3,4-ethylenedioxythiophene-based derivatives, i.e. the oligomeric compound 5,5″-diphenyl-2,2′-5′,2″-ter(3,4-ethylenedioxythiophene) (1, 3-EDT) and the corresponding polymer of 2-tetradecyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine (2, PEDT-C14), was investigated. Their stability upon irradiation with a xenon lamp was studied by means of UV-Vis spectroscopy. These measurements lead to the proposal of a photooxidation mechanism that seems to be operative in chlorinated solvents, followed by H-abstraction. Furthermore, the experiments point out that it is mainly UV light that causes the instability of the oxidatively doped compounds.

Design of Functionalized Lipids and Evidence for Their Binding to Photosystem II Core Complex by Oxygen Evolution Measurements, Atomic Force Microscopy, and Scanning Near-Field Optical Microscopy

Photosystem II core complex (PSII CC) absorbs light energy and triggers a series of electron transfer reactions by oxidizing water while producing molecular oxygen. Synthetic lipids with different alkyl chains and spacer lengths bearing functionalized headgroups were specifically designed to bind the Q(B) site and to anchor this large photosynthetic complex (240 kDa) in order to attempt two-dimensional crystallization. Among the series of different compounds that have been tested, oxygen evolution measurements have shown that dichlorophenyl urea (DCPU) binds very efficiently to the Q(B) site of PSII CC, and therefore, that moiety has been linked covalently to the headgroup of synthetic lipids. The analysis of the monolayer behavior of these DCPU-lipids has allowed us to select ones bearing long spacers for the anchoring of PSII CC. Oxygen evolution measurements demonstrated that these long-spacer DCPU-lipids specifically bind to PSII CC and inhibit electron transfer. With the use of atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM), it was possible to visualize domains of PSII CC bound to DCPU-lipid monolayers. SNOM imaging has enabled us to confirm that domains observed by AFM were composed of PSII CC. Indeed, the SNOM topography images presented similar domains as those observed by AFM, but in addition, it allowed us to determine that these domains are fluorescent. Electron microscopy of these domains, however, has shown that the bound PSII CC was not crystalline.

Near-field scanning optical microscopy and polymers

Abstract Polymer composite films consisting of fluorescent nanometric particles of dye-labeled latex dispersed in poly(vinyl alcohol) matrices were imaged with an aperture Near-field Scanning Optical Microscope (NSOM). Different films of this type with a thickness of ∼ 25 nm containing latex particles with diameters of 103 nm ± 9 nm or of 14 nm ± 3.7 nm with low particle density were studied. During image acquisition with the NSOM the particles were excited by a tunable argon ion laser. In case of the 103 nm small particles the excitation wavelength, λ, was chosen to be at the maximum or at the red edge of the excitation band at λ = 458 nm or at λ = 488 nm, respectively. In case of the 14 nm small particles the respective films were excited at λ = 488 nm. In both cases strong fluorescence spots with FWHM diameters of λ 2 could be found. Additionally, photobleaching of a single 103 nm small fluorescent latex particle with a NSOM was performed representing a controlled photochemical reaction on a submicrometer length scale. Beside the presentation of the own work, references to the application of near-field optical microscopy to the investigation of thin polymer films are given.

Space and Time Resolved Spectroscopy of Two-Dimensional Molecular Assemblies

The fluorescence decays of several amphiphilic dyes incorporated in Langmuir-Blodgett films were determined and analyzed globally over different dye concentrations and/or emission wavelengths The highly non-exponential character of the fluorescence decay could be analyzed in a model based on two-dimensional energy transfer from the excited monomers to dimers of the dye Depending upon the miscibility of the dye and the matrix, homogeneous and two-phase multilayers were distinguished This distinction could be confirmed by the spatial distribution of the fluorescence as revealed by confocal fluorescence microscopy and near-field scanning optical microscopy For chromophores with important internal rotation (amphiphilic crystal violet) the fluorescence decay at low concentrations is determined by a distribution of free volumes rather than by energy transfer to dimers The homogeneous distribution assumed for an amphiphilic crystal violet in cadmium arachidate could be confirmed by the global analysis of the fluorescence decay of an amphiphilic pyronine in the presence of the amphiphilic crystal violet.

Excited State Probing of Supramolecular Systems on a Submicron Scale

Imaging techniques, such as Confocal Fluorescence Microscopy (CFM) or Near-field Scanning Optical Microscopy (NSOM) [1, 2] are essential techniques to study complex heterogeneous organic thin films by mapping their optical properties. They are complementary techniques having different advantages and disadvantages [3]. CFM is relatively easy to apply and combines a lateral resolution approaching λ/2 with the possibility of layered imaging in the z-direction. In contrast, NSOM has significantly better spatial resolution and offers simultaneous optical and topographic images. Although confocal microscopy has been mostly used for biological applications, the technique has proven to be useful, for example, in the study of colloids [4,5], polymer blends [6] and liquid crystals [7,8].