Exploring the Reversibility of the Ring-Closing Metathesis Mediated Cross-linking of Dendrimers

Elmer, Stephanie L.; Lemcoff, N. Gabriel; Zimmerman, Steven C.

doi:10.1021/ma071233e

Cited by 16 publications

(12 citation statements)

References 29 publications

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“…50−150 per dot), helps prevent undesirable side reactions, such as previously reported dendronization and cross-linking. 73,74 UV−vis, PL, XRD, and TEM showed that metathesis proceeds without affecting the particle size or optical properties. After removal of free soluble unreacted styrene and the selfmetathesis byproduct by crashing and centrifugation, both 100% vinyl-capped and 50%/50% vinyl/methyl-capped CdS dots show ca.…”

Section: Chemistry Of Materialsmentioning

confidence: 99%

Surface Doping Quantum Dots with Chemically Active Native Ligands: Controlling Valence without Ligand Exchange

et al. 2012

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One remaining challenge in the field of colloidal semiconductor nanocrystal quantum dots is learning to control the degree of functionalization or "valence" per nanocrystal. Current quantum dot surface modification strategies rely heavily on ligand exchange, which consists of replacing the nanocrystal's native ligands with carboxylate-or amine-terminated thiols, usually added in excess. Removing the nanocrystal's native ligands can cause etching and introduce surface defects, thus affecting the nanocrystal's optical properties. More importantly, ligand exchange methods fail to control the extent of surface modification or number of functional groups introduced per nanocrystal. Here, we report a fundamentally new surface ligand modification or "doping" approach aimed at controlling the degree of functionalization or valence per nanocrystal while retaining the nanocrystal's original colloidal and photostability. We show that surface-doped quantum dots capped with chemically active native ligands can be prepared directly from a mixture of ligands with similar chain lengths. Specifically, vinyl and azide-terminated carboxylic acid ligands survive the high temperatures needed for nanocrystal synthesis. The ratio between chemically active and inactive-terminated ligands is maintained on the nanocrystal surface, allowing to control the extent of surface modification by straightforward organic reactions. Using a combination of optical and structural characterization tools, including IR and 2D NMR, we show that carboxylates bind in a bidentate chelate fashion, forming a single monolayer of ligands that are perpendicular to the nanocrystal surface. Moreover, we show that mixtures of ligands with similar chain lengths homogeneously distribute themselves on the nanocrystal surface. We expect this new surface doping approach will be widely applicable to other nanocrystal compositions and morphologies, as well as to many specific applications in biology and materials science. ABSTRACT: One remaining challenge in the field of colloidal semiconductor nanocrystal quantum dots is learning to control the degree of functionalization or "valence" per nanocrystal. Current quantum dot surface modification strategies rely heavily on ligand exchange, which consists of replacing the nanocrystal's native ligands with carboxylate-or amine-terminated thiols, usually added in excess. Removing the nanocrystal's native ligands can cause etching and introduce surface defects, thus affecting the nanocrystal's optical properties. More importantly, ligand exchange methods fail to control the extent of surface modification or number of functional groups introduced per nanocrystal. Here, we report a fundamentally new surface ligand modification or "doping" approach aimed at controlling the degree of functionalization or valence per nanocrystal while retaining the nanocrystal's original colloidal and photostability. We show that surface-doped quantum dots capped with chemically active native ligands can be prepared directly from a mixture of...

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Section: Chemistry Of Materialsmentioning

confidence: 99%

Surface Doping Quantum Dots with Chemically Active Native Ligands: Controlling Valence without Ligand Exchange

et al. 2012

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“…Putting all of the data together led to the conclusion that the relative preference of cross-links observed is type d ≥ type c > type b ≫ type a , indicating that larger loops are the favored cross-links. These preferred connections may reflect kinetic or thermodynamic factors or a mixture of the two because the RCM cross-linking may undergo dynamic exchange of cross-links. , …”

Section: Intra-macromolecular Reactionsmentioning

confidence: 99%

“…These preferred connections may reflect kinetic or thermodynamic factors or a mixture of the two because the RCM cross-linking may undergo dynamic exchange of crosslinks. 33,34 In collaboration with Rainer Haag, we extended this chemistry to polyglycerol dendrimers and hyperbranched polymers. 35−37 The former are an ideal choice for RCM cross-linking because in their iterative synthesis each generation proceeds through an intermediate with allyl ether end groups.…”

Section: Introductionmentioning

confidence: 99%

Intramolecularly Cross-Linked Polymers: From Structure to Function with Applications as Artificial Antibodies and Artificial Enzymes

2020

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Metrics & MoreArticle Recommendations CONSPECTUS: Cross-linking of polymers significantly alters their physical properties, greatly expanding their everyday utility. Indeed, the polymeric networks resulting from linkages between polymer chains are found in everyday materials from soft contact lenses and automobile tires to enamel coatings and highperformance adhesives. In contrast, intramolecularly cross-linked polymers have received far less attention until recent years, in large part because they are synthetically more challenging to prepare. In this Account, we trace our own efforts to develop the chemistry of intramolecularly cross-linked macromolecules, starting with dendrimers. Dendrimers provided an excellent starting point for investigating intramolecular cross-linking because they are single molecular entities. We showed that the end groups of dendrimers can be extensively cross-linked using the ring-closing metathesis reaction and that the discrete structure of the dendrimer provides unique opportunities for characterizing the number and location of the cross-links as well as some physical properties of the macromolecule such as its size and rigidity. Increasing the number of ring-closing metathesis reactions correlated with a reduction in size and an increase in rigidity. The general strategy applied to dendrimers was extended to star polymers and hyperbranched polyglycerols. Each of these macromolecules has a core or an initiating group from which the branches emanate. Linking the end groups or branches of these polymers presents a unique opportunity to chemically remove the core of the cross-linked macromolecule in a process that is reminiscent of that used to produce covalent molecular imprinted polymers. Recognizing this analogy, we sought a compelling application for cross-linked dendrimers, the first example of unimolecular imprinting, where a single polymer contains a single molecular imprint. The quality of the imprinting was mixed but pointed to an alternative general strategy for molecular imprinting in polymers. The effort also focused attention on synthetic antibodies and the general biomimicry provided by this class of macromolecules. Indeed, cross-linking of polymers either covalently or non-covalently bears a loose resemblance to folding of proteins into defined three-dimensional shapes. The synthesis and study of cross-linked linear polymers, often called single-chain nanoparticles (SCNPs), has emerged as a very active area of research in the past few years. Our experience with the cross-linking of branched polymers combined with an interest in performing organic synthesis within living cells led us to develop copper-containing SCNPs as artificial clickases. These polymeric clickases exhibit all of the hallmarks of enzymatic catalysis. One clickase containing a polyacrylamide backbone performs low-concentration copper-assisted alkyne−azide click reactions at unprecedented rates. Another performs click reactions within living cells. Other organic transformations can be performed intracel...

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“…In a related study, Zimmerman investigated the reversibility of the cross-linking reaction in dendrimers. 6 Although metathesis should in principle be reversible, the steric hindrance in the cross-linked structure could hinder reversibility. By attaching an alkene-terminated dendron to the cross-linked dendrimer through an ester group, the authors were able to detect the opening of the cross-linking structure during metathesis.…”

Section: Proximity Effect a Reactions Between Adjacent Groups In The ...mentioning

confidence: 99%

Reactivity in organised assemblies

Chechik

2008

Annu. Rep. Prog. Chem., Sect. B: Org. Chem.

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Exploring the Reversibility of the Ring-Closing Metathesis Mediated Cross-linking of Dendrimers

Cited by 16 publications

References 29 publications

Surface Doping Quantum Dots with Chemically Active Native Ligands: Controlling Valence without Ligand Exchange

Surface Doping Quantum Dots with Chemically Active Native Ligands: Controlling Valence without Ligand Exchange

Intramolecularly Cross-Linked Polymers: From Structure to Function with Applications as Artificial Antibodies and Artificial Enzymes

Reactivity in organised assemblies

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