Single Dendrimer Research Articles

Inorganic Nanocrystals with Dendrimer Templates: Mesoscopic Model System and Route to New Nanocomposites

ABSTRACTWe investigate nanostructures that are formed when dendrimers act as hosts. Poly(amidoamine) (PAMAM) dendrimers can be used as templates for inorganic nanocrystals, both in aqueous solution1-3 and in a polymeric matrix4. SANS, SAXS and TEM are used to characterize the resulting hybrid structures. Different inorganic colloids like noble metal and cadmium sulfide colloids are studied1,2. With increasing dendrimer generation, we observe a transition from low molecular colloid stabilizing to an effective polymer templating in terms of a “host-guest nanoscale synthesis”. For these higher generation dendrimers, inorganic colloids are formed inside single dendrimer molecules and the size of the nanocrystal can precisely be controlled by the dendrimer generation (“fixed loading law”). Hydrophobically modified dendrimers in organic solvent represent a further, different kind of host system that we have investigated using scattering methods5.

Probing Photophysical Processes in Individual Multichromophoric Dendrimers by Single-Molecule Spectroscopy

Individual multichromophoric dendrimer molecules, bearing eight perylenecarboximide chromophores at the rim, immobilized in a thin polyvinylbutyral (PVB) film were studied by far-field fluorescence microscopy. Fluorescence intensity trajectories as a function of time (transients), spectra, and decay traces were recorded separately or simultaneously. For comparison, similar measurements have been performed on a model compound containing one perylenecarboximide chromophore. Collective on/off jumps of the fluorescence intensity were observed for single dendrimer molecules, resembling previously reported collective jumps for the emission of single light-harvesting antenna systems. Spectra and decays of both non-interacting and dimer-like interacting chromophoric sites could be distinguished within an individual dendrimer. Transitions between the different spectral forms and decay times, observed for a single molecule, underline the dynamic character of the interactions among the chromophores. Evidence for a stepwise bleaching process of the multichromophoric system was found. Furthermore, the single-molecule data incontestably prove the assumptions stated in the ensemble model.

Properties of Single Dendrimer Molecules Studied by Atomic Force Microscopy

Well-separated, individual polyphenylene dendrimer molecules have been prepared by spin coating on a mica surface, and subsequently imaged by noncontact atomic force microscopy (NCAFM). The observed height is in good agreement with the size of a single dendrimer molecule, as calculated by molecular dynamics simulation. By using pulsed force mode (PFM) AFM, stiffness and adhesion properties of individual polyphenylene dendrimers have been studied. They could be related to the molecular structure and the chemical nature of the outer surface of the dendrimers and the thin film of water adsorbed on mica when imaged under ambient conditions. Finally, by changing the concentration of the spin-coating solution, two different kinds of aggregates have been characterized.

STM imaging of a heptanuclear ruthenium(II) dendrimer, mono-add layer on graphite

Scanning tunneling microscopy (STM) and molecular mechanics calculations were used to investigate the long-range packing and the structure of an heptanuclear ruthenium (II) dendritic species, as a PF6- salt. STM imaging was carried out on a mono-add layer of the ruthenium dendrimer formed by physisorption from a 1,2,4-trichlorobenzene solution at the liquid-graphite interface. The packing of the molecules on the surface was visualised by the formation of ordered patterns and a distance of 27 +/- 2 A was measured between two adjacent lamellae. The comparison of this dimension with the molecular-modelling data indicates that the lamellae were formed by rows of dendrimer molecules in which the counterions (PF6-) were strongly associated with the Ru atoms. The images acquired with higher spatial resolution revealed the presence of repeating units within the lamellae. The comparison of the STM images with the modelling results allowed the attribution of the repeating units observed in the imaged pattern to the STM signature of single dendrimer molecules.

Fluorescence Detection from Single Dendrimers with Multiple Chromophores.

The differences in the fluorescence behavior of a polyphenylene dendrimer with eight peryleneimides chromophores (1) and a single hexaphenylperyleneimide chromophore have been investigated at a single-molecule level through the combination of ultrasensitive fluorescence detection and microscopy.

Detection of a Single Dendrimer Macromolecule with a Fluorescent Dihydropyrrolopyrroledione (DPP) Core Embedded in a Thin Polystyrene Polymer Film

Three generations of dendrimers with a fluorescent core were synthesized by attaching dendrons of the Frechet type to a dihydropyrrolopyrroledione (DPP) molecule. Samples were prepared by spin-coating a toluene solution containing 2 × 10-9 M of the dendrimers and 3 mg/mL polystyrene (MW 45 000) on a glass cover slip. By exciting near the absorption maximum of the chromophore in the dendrimer, single dendrimer molecules could be imaged via a confocal microscope. Using an electrooptical modulator (EOM) in the excitation beam path, linear polarized light with a slowly rotating polarization direction (1 Hz modulation frequency) was produced. Two APD's (avalanche photodiodes) and a polarizing beam splitter were placed in the detection path. In this way we were able to distinguish single molecules from small clusters. Furthermore, from variations of the two detected intensities and from the phase shift of the signal with respect to the modulation, it was shown that the orientation of the absorption transition d...

Mono- and Multilayers of Mesogen-Substituted Carbosilane Dendrimers on Mica

Self-assembled ultrathin (5−15 nm) films of carbosilane dendrimers with mesogenic units on the periphery, obtained after deposition on mica surfaces, have been studied with atomic force microscopy. Dendritic polymers with 12, 36, and 108 mesogenic cholesteryl end groups, first, second, and third generation, respectively, were employed. Solvent casting was used to generate the films. At high concentrations of the dendrimers in solution, flat, homogeneous films of 2−4 dendrimer layers were found. For low dendrimer concentrations, a single dendrimer monolayer exhibiting an irregular cellular pattern of holes was observed. The thickness of the monolayer correlated well with the diameter of the dendrimers and with wide-angle X-ray scattering results obtained on crystalline powders. Annealing of the films of G1 and G2 in the liquid crystalline phase caused reorientation of the molecules at the surface. The third-generation dendrimer with 108 end groups (G3), the periphery of which is densely packed with mesogens, behaved differently and did not show dewetting or reorientation upon annealing, which is attributed to lower molecular mobility.