Alex Adronov

Light Harvesting and Energy Transfer in Novel Convergently Constructed Dendrimers

By the attachment of interacting laser dyes to the chain ends and focal point of a dendritic macromolecule it is possible to funnel energy harvested by the large peripheral antenna of the dendrimer (see picture on the left) efficiently and directly to the central fluorescent core (picture in the middle) by a process that does not involve the dendrimer inner backbone. The energy is then emitted as a narrow band of fluorescent radiation from the core (picture on the right).

Light-harvesting dendrimers

Natural photosynthetic systems collect sunlight using a vast array of light-harvesting chromophores that channel the absorbed energy to a single reaction center. Recently, it has been realized that dendritic macromolecules can exhibit similar properties, though on a more modest scale. The preparation of dendritic structures and assemblies composed of numerous light-collecting chromophores that transfer their energy to a single energy ‘sink’ at the core has been achieved in a number of diverse and creative ways. These novel structures are being used as model systems in light-emitting diodes, signal amplifiers, fluorescent sensors, frequency converters, and other photonic devices.

Polymer Grafting of Carbon Nanotubes Using Living Free‐Radical Polymerization

The chemistry of carbon nanotubes has become an area of intense research as chemical derivatization is the only means for modifying the properties of these highly interesting and technologically promising materials. Specifically, numerous researchers have focused on improving the solubility of carbon nanotubes through chemical grafting. To this end, significant recent effort has been devoted to the attachment of polymers to the nanotube surface, as macromolecules can be more effective in modifying nanotube solubility properties than small molecules. In addition, the use of functional polymers has enabled the preparation of polymer‐nanotube composite materials that demonstrate a variety of interesting properties, such as responsiveness to environmental stimuli (solvent, temperature, pH), the ability to complex metal ions, and photoinduced electron transport. A variety of different techniques have now been developed for the functionalization of carbon nanotubes with polymers, including “grafting to”, “grafting from”, and supramolecular interactions. This review will focus on recent developments in the use of living radical polymerization methods for the functionalization of carbon nanotubes with well‐defined polymers.

Synthesis, radiolabeling, and bio-imaging of high-generation polyester dendrimers.

A series of aliphatic polyester dendrons, generations 1 through 8, were prepared with a core p-toluenesulfonyl ethyl (TSe) ester as an easily removable protecting group that can be efficiently replaced with a variety of nucleophiles. Using amidation chemistry, a tridentate bis(pyridyl)amine ligand which is known to form stable complexes with both Tc(I) and Re(I) was introduced at the dendrimer core. Metalation of the core ligand with (99m)Tc was accomplished for generations 5 through 7, and resulted in regioselective radiolabeling of the dendrimers. The distribution of the radiolabeled dendrimers was evaluated in healthy adult Copenhagen rats using dynamic small-animal single photon emission computed tomography (SPECT). The labeled dendrimers were cleanly and rapidly eliminated from the bloodstream via the kidneys with negligible nonspecific binding to organs or tissues being observed. These data were corroborated by a quantitative biodistribution study on the generation 7 dendrimer following necropsy. The quantitative biodistribution results were in excellent agreement with the data obtained from the dynamic SPECT images.

Selective and reversible noncovalent functionalization of single-walled carbon nanotubes by a pH-responsive vinylogous tetrathiafulvalene-fluorene copolymer.

A vinylogous tetrathiafulvalene (TTFV) monomer was prepared and copolymerized with fluorene to give a conformationally switchable conjugated copolymer. This copolymer was shown to undergo a conformational change upon protonation with trifluoroacetic acid (TFA). When mixed with single-walled carbon nanotubes (SWNTs), this TTFV-fluorene copolymer exhibited strong interactions with the SWNT surface, leading to stable, concentrated nanotube dispersions in toluene. Photoluminescence excitation mapping indicated that the copolymer selectively disperses low-diameter SWNTs, as would be expected from its ability to form a tightly coiled conformation on the nanotube surface. Addition of TFA to the copolymer-SWNT dispersion resulted in a rapid conformational change and desorption of the polymer from the SWNT surface, resulting in precipitation of pure SWNTs that were completely free of polymer. Importantly, the nanotubes isolated after dispersion and release by the TTFV-fluorene copolymer were more pure than the original SWNTs that were initially dispersed.

Reinforcement of collagen with covalently-functionalized single-walled carbon nanotube crosslinkers

Poly(ethyleneimine) (PEI) functionalized single-walled carbon nanotubes (SWNTs) and free PEI were investigated as collagen crosslinking agents to determine their ability to improve the Youngs modulus of a collagen hydrogel. The crosslinked collagen matrices were prepared by blending Type I bovine collagen with either PEI-SWNT or free PEI and crosslinked utilizing carbodiimide chemistry. The resulting SWNT enriched material was a crosslinked collagen hydrogel with sufficient mechanical strength to be manipulated and transferred without damaging the matrix. Raman spectroscopy confirmed the presence of SWNTs within the PEI-SWNT/collagen hydrogels. In addition, it confirmed that crosslinking did not alter the SWNT delocalized network. Dynamic scanning calorimetry confirmed a change in the denaturation temperature for hydrogels prepared by PEI-SWNT or PEI crosslinkers. Water uptake analysis suggested very loosely crosslinked matrices were produced regardless of the crosslinking agent used. However, evaluating the Youngs modulus, it was found that collagen hydrogels produced with PEI-SWNTs as the crosslinking agent had a modulus 4.5 to 9 times that of collagen hydrogels produced with PEI of the same amine concentration or in the absence of any crosslinking agent. In addition, PEI-SWNT crosslinked collagen hydrogels exhibited superior Youngs modulus to control samples in which only the SWNTs or unlinked mixtures of SWNTs and PEI were used as crosslinkers. This comparative study confirms that PEI-SWNT crosslinked collagen gels exhibited Youngs moduli significantly higher than collagen gels cross-linked with just nanotubes, PEI, or blends of nanotubes and PEI.

Metal-Free Reduction of Secondary and Tertiary N-Phenyl Amides by Tris(pentafluorophenyl)boron-Catalyzed Hydrosilylation

Tris(pentafluorophenyl)boron B(C6F5)3 is an effective catalyst for the hydrosilylative reduction of tertiary and N-phenyl secondary amides. It allows for the mild reduction of a variety of these amides in near quantitative yield, with minimal purification, at low temperatures, and with short reaction times. This reduction shows functional group tolerance for alkenes, nitro groups, and aryl halides, including aryl iodides.

Das Sammeln von Licht und die Energieübertragung in neuen konvergent aufgebauten Dendrimeren

Durch den Einbau von miteinander wechselwirkenden Laserfarbstoffen an den Kettenenden und im Brennpunkt eines dendritischen Makromolekuls kann Energie unterschiedlicher Wellenlangen, die durch die grose periphere Antenne des Dendrimers gesammelt wurde (siehe linkes Bild), direkt und mit hohem Wirkungsgrad auf den zentralen Kern ubertragen werden (mittleres Bild), ohne das das innere Skelett des Dendrimers am Ubertragungsprozes beteiligt ist. Die Energie wird dann als schmalbandige Fluoreszenzstrahlung vom Kern emittiert (rechtes Bild).

A study of the dynamics of the branch ends of a series of pyrene-labeled dendrimers based on pyrene excimer formation.

A series of pyrene-labeled dendrimers were prepared from generation n = 1 to n = 4 where the pyrenes were attached to the end groups of the dendrimers. Pyrene excimer formation was monitored by steady-state and time-resolved fluorescence spectroscopy as a function of generation number and in terms of the I(E)/I(M) ratio and the average rate constant of excimer formation . To account for the unconventional distribution of pyrene labels which were neither randomly distributed throughout the macromolecule nor limited to just two units which are the only two pyrene-labeling schemes that can be dealt with in a straightforward manner, a Model Free (MF) analysis was applied to the global analysis of the fluorescence decays. Within experimental error, the I(E)/I(M) ratios and obtained from, respectively, the steady-state fluorescence spectra and the time-resolved fluorescence decays were found to increase linearly with increasing generation number. This result is inconsistent with the fact that both the I(E)/I(M) ratio and are proportional to the local concentration of pyrene inside the dendrimer ([Py](loc)) which is not expected to increase with generation number if the excited pyrene is assumed to diffuse freely throughout the dendrimer interior. Since the core-dense model predicts that the dendrimer terminal ends can occupy any position throughout the dendrimer interior, these results suggest that excimer formation between the pyrene-labeled ends is enhanced due to the branched nature of the dendrimer.

Supramolecular complexes of single walled carbon nanotubes with conjugated polymers

We have synthesized a series of conjugated polymers, including poly[(2,7-(9,9-dioctylfluorene)-alt-2,7-(9,9-didodecylfluorene)] (PF), poly[(2,7-(9,9-dioctylfluorene)-alt-2,5-(3-dodecylthiophene)] (PFT), and poly(3-dodecylthiophene) (PT). Non-covalent functionalization of single walled carbon nanotubes (SWNTs) with these polymers can impart good solubility to nanotubes in a number of organic solvents, including THF, dichlorobenzene, chloroform, and toluene. Solution and solid-state characterization of the resulting polymer–SWNT composites are described, including UV-Vis absorption and Raman spectroscopy. It was found that the UV-Vis absorption maximum of the polymers was red-shifted in their corresponding composites due to the planarization of the polymer backbone following adsorption upon the SWNT surface. Polymer–SWNT complexes also exhibited good solution stability at elevated temperature in THF and dichlorobenzene, with no significant SWNT sedimentation observed at elevated temperatures. Both UV-Vis absorption and Raman spectroscopy results indicated that the interaction of PT with the nanotubes was different from those of PF and PFT, suggesting that the choice of aromatic ring in the polymer structures plays an important role in the supramolecular complex formation with carbon nanotubes.