No abstract

In this letter, we describe the preparation of a versatile polymer ligand, which can be attached to CdSe/ZnS semiconductor nanocrystals via a phase transfer reaction. The ligand is based on a chain of reactive esters, which can, in principle, be substituted by any compound containing amino-functionalities. The polymer/nanocrystal complexes are characterized in terms of structure and photostability.

Colloidal semiconductor nanocrystals (NCs) are color tunable inorganic fluorophores, which consist of an inorganic core and an outer shell of molecular ligands.[1] While the optical properties can be adjusted by the composition [2] and the size of the core, [3] the ease with which the NCs can be chemically processed is mainly governed by the molecular structure of the ligands. In this context we define a ligand as consisting of three parts: a functional anchor group that attaches to the inorganic NC surface, an outer functional group, and a spacer in between. The architecture of the ligand is of major importance for assembling the NCs into larger supramolecular structures [4] or conjugating them to biological samples where they serve as fluorescence labels.[5] For these purposes it is essential that both the anchor and outer functionality can form strong chemical bonds. We will address this problem by introducing a novel type of polymer ligand, where the bond strength is increased by the presence of multiple anchor groups. Since our model ligands are decorated with dye molecules, we can employ fluorescence resonance energy transfer (FRET) to study the formation and stability of the NC-dye couples, where the NCs are the donors and the dyes the acceptors [6,7] . Based on the ensemble absorption and fluorescence spectra, the amount of dye attached to a given NC [6] and the average distance between the NC and the dye molecules can be estimated.[8] However, since the relative orientation and structure of the FRET couples can differ significantly from one complex to the next, we extend our measurements to single NC-dye complexes. Our experiments clearly show that static and dynamic heterogeneities occur in the ensemble of NC-dye couples, a result which is of importance for a FRET-based analysis of intermolecular distances.The CdSe/ZnS core-shell NCs are synthesized by an established method [3,9] in the presence of phosphonic acids [10] and trioctylphosphine oxide (TOPO) as ligands. The surface molecules can be subsequently cross linked to form multianchor ligands to stabilize the ligand shell.

The synthesis of new chiral polyisocyanates is described. For this purpose a new chiral azobenzene containing monomer with the chiral center in R-position to the isocyanate group was synthesized. The anionic copolymerization was carried out in THF as solvent with potassium cyanide that was complexed by 18-crown-6 as initiator. This allowed a better control of the reaction and thereby the synthesis of polyisocyanates with a fairly low polydispersity. The copolymers show an extremely high transfer of chirality from the chiral side groups to the helical backbone in dilute solution. Copolymers with only 1.6 mol % of chiral side groups show nearly the full optical rotation and exist predominantly in the P-helical conformation. It was found that the isomerization of the azobenzene groups hardly has any influence on the conformation of the helix. Lyotropic liquid crystalline phases were formed from the polymers and 1,2-dichlorobenzene. It could be shown that even polymers with a very low content of chiral side groups (1.6 mol %) form cholesteric phases with a selective reflection in the visible range. The pitch of the cholesteric phase and thereby the wavelength of the selective reflection bands could be reversibly altered by the isomerization of the azobenzene groups.

Summary: We report on the synthesis of polyacrylamides that were prepared by the reaction of a reactive ester polymer with a mesogen‐containing secondary amine and N‐methylpiperazine. As the polymers do not form hydrogen bonds near the amide groups, their mobility is significantly higher than that of poly(N‐monoalkylacrylamides). The initially nonionic polymer shows no liquid‐crystalline behavior in bulk and in mixture with ethylene glycol. This is due to the low polarity difference between the different side chains. The polarity difference can be increased by protonation or quarternization of the tertiary amino groups, and liquid‐crystalline behavior is observed. The self‐assembly multilayer build‐up of the polymer with quarternized amino groups, and an anionic polyelectrolyte proceeds regularly. magnified image