Quantum Dot on a Rope

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Abstract
The conjugation of nanoparticles (NPs) typically yields supramolecular materials which are fairly rigid, and the electronic coupling between the NP and other structural units of these compounds is fixed by covalent bonds. Here, we report on a novel bichromophor system constructed from a quantum dot tethered to a semiconducting polymer, which demonstrates the possibility of the dynamic interunit coupling in the NP supramolecules. The NP bichromophoric system was made on the basis of the layer-by-layer assembled (LBL) films of an anionic polyelectrolyte with poly(p-phenylene ethynylene) backbone, aPPE, and poly(allylamine hydrochloride) PAH polycation. To conjugate CdTe NPs to the (aPPE/PAH)m LBL film, we took advantage of the reactive groups of NP stabilizer, that is, −COOH, and the aminogroups on PAH. Tethering of CdTe was accomplished by using poly(ethyleneglycole), PEG, chains with two reactive terminals such as t-BOC-NH-PEG-COO-NHS. The evidence for successful conjugation of NPs to the LBL films can be seen both in AFM images and in optical data. The latter also indicate that the light quanta emitted by the NPs originate from the light absorption of the polymer film, which proves the presence of the aPPE → NP energy-transfer process. The average separation distance between the NPs tethered to the LBL films can be changed by altering the dielectric properties of the solvent affecting PEG tether coiling (water/alcohol mixture). The reduced emission intensity of aPPE was found to follow the extension of the PEG tether. The quenching of aPPE is reversible when the original composition of the solvent mixture is restored. Thus, CdTe-PEG-aPPE is an example of an organized NP system with tunable optical coupling. Variable electronic coupling offers a convenient structural platform for new nanotechnological devices for which spatial control translates into a higher level of sophistication. PEG molecules afford a wide variety of polymer chain configurations with different reactive terminals, which makes possible the preparation of diverse NP superstructures.