Dendrimer

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Figure 1. Dendrimer and dendron
Crystal structure of a first-generation polyphenylene dendrimer reported by Müllen et al.[1]

Dendrimers are repeatedly branched molecules. The huge number of papers on dendritic architectures such as dendrimers, dendronized, hyperbranched and brush-polymers has generated a vast variety of inconsistent terms and definitions making a clear and concise unfolding of this topic highly difficult. The purpose of this section is to provide the vocabulary required for the description of chemical and physical phenomena as well as application aspects associated with the research in the area of dendritic molecules.

Dendritic molecules are repeatedly branched species that are characterised by structural perfection. This is based on the evaluation of both symmetry and polydispersity. The field of dendritic molecules can roughly be divided into low-molecular weight and high-molecular weight species. The first category includes dendrimers and dendrons, and the second includes dendronised polymers, hyperbranched polymers and brush-polymers (called also bottle-brushes).

The name comes from the Greek "δενδρον"/dendron, meaning "tree". Synonymous terms are arborols and cascade molecules. Dendrimer is an internationally accepted term. Dendrimers and dendrons are repeatedly branched, monodisperse and usually highly symmetric compounds. There is no apparent distinction between the definitions dendrimer and dendron. A dendron usually contains a single chemically addressable group called the focal point. Because of the absence of molar mass distribution, high-molar-mass dendrimers and dendrons are macromolecules but not polymers.

The first dendrimers were synthesised divergently by Vögtle in 1978[2], by Denkewalter and coworkers at Allied Corporation as polylysine dendrimers in 1981[3], by Tomalia at Dow Chemical in 1983[4] and in 1985[5], and by Newkome in 1985[6]. In 1990 a convergent synthesis was introduced by Fréchet[7]. Dendrimers then experienced an explosion of scientific interest because of their unique molecular architecture (Fig 1). This resulted in more than 5,000 scientific papers and patents published by the end of 2005.

Contents

[edit] Properties and applications

The properties of dendrimers are dominated by the functional groups on the molecular surface, however, there are examples of dendrimers with internal functionality[8] [9] [10].Dendritic encapsulation of functional molecules allows for the isolation of the active site, a structure that mimics the structure of active sites in biomaterials because dendritic scaffolds separate internal and external functions.[11][12][13]. For example, a dendrimer can be water-soluble when its end-group is a hydrophilic group, like a carboxyl group. It is theoretically possible to design a water-soluble dendrimer with internal hydrophobicity, which would allow it to carry a hydrophobic drug in its interior. Recently it has been shown that redox-active nanoparticles can be synthesized, placing the redox molecules between the nanoparticle core and the dendritic wedges; despite their isolation, some of the redox molecules (COOH in this case) remained uncoupled, and thus still reactive.[citation needed]

Another property is that the volume of a dendrimer increases when it has a positive charge. If this property can be applied, dendrimers can be used for drug delivery systems (DDS) that can give medication to the affected part inside a patient's body directly[14].

[edit] Photonic excited molecules

The inside of a dendrimer has a unique chemical environment because of its high density. From this property, it has been discovered that azobenzene is photoisomerized by very weak infrared rays when covered by a dendrimer [15]. Through the discovery of a function that catches light and conveys this energy using excitation of the molecule, attempts have recently been made to synthesize dendrimers that insert porphyrin, absorb light, and photosynthesize artificially. In addition, the development of organic electroluminescent devices and their applications has been undertaken by researchers all over the world.

[edit] Synthesis

In the synthesis of dendrimers, monomers lead to a monodisperse polymer, tree-like, or generational structure. There are two defined methods of dendrimer synthesis, divergent synthesis and convergent synthesis. Divergent syntheses assemble the molecule from the core, extending radially to the periphery and in contrast convergent methods start at the surface and proceed inwards, before the attachment of pre-synthesised dendrons to the core.

However, because a repeated reaction which consists of many steps is needed to protect the active site, it is difficult to synthesize dendrimers even if both methods are used. This is why there are obstacles to the synthesis of large quantities of dendrimers. Up to date, there are only a few companies that sell dendrimers; Polymer Factory[16] commercializes biocompatible bis-MPA dendrimers and Dendritech[17] is the only kilogram-scale producers of PAMAM dendrimers.

The original Newkome dendrimer or arborol (1985) started by nucleophilic substitution of 1-bromopentane by triethyl sodiomethanetricarboxylate in dimethylformamide and benzene. The ester groups were then reduced by lithium aluminium hydride to a triol in a deprotection step. Activation of the chain ends was achieved by converting the alcohol groups to tosylate groups with tosyl chloride and pyridine. The tosyl group then served as leaving groups in another reaction with the tricarboxylate, forming generation two.

The Newkome 1985 dendrimer system

This sequence can be repeated many times.

[edit] Click chemistry

Dendrimers have been prepared via click chemistry employing Diels-Alder reactions [18] , thiol-ene reactions [19] and azide-alkyne reactions [20] [21] [22]. An example is the synthesis of certain polyphenylene dendrimers [23]:

Dendrimer DA reaction Mullen 1996

[edit] See also

Applications:

[edit] References

  1. ^ Roland E. Bauer, Volker Enkelmann, Uwe M. Wiesler, Alexander J. Berresheim, Klaus Müllen (2002). "Single-Crystal Structures of Polyphenylene Dendrimers". Chemistry: A European Journal 8: 3858. doi:10.1002/1521-3765(20020902)8:17<3858::AID-CHEM3858>3.0.CO;2-5. 
  2. ^ Egon Buhleier, Winfried Wehner, Fritz Vögtle (1978). ""Cascade"- and "Nonskid-Chain-like" Syntheses of Molecular Cavity Topologies". Synthesis 1978: 155–158. doi:10.1055/s-1978-24702. 
  3. ^ Patent 4,289,872 (published 1981, filed 1979) and 4,410,688 (published 1983, filed 1981)
  4. ^ Dow patent is 4,507,466 (published 1985, filed 1983)
  5. ^ D. A. Tomalia, H. Baker, J. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J. Ryder and P. Smith (1985). "A New Class of Polymers: Starburst-Dendritic Macromolecules". Polymer Journal 17: 117. doi:10.1295/polymj.17.117. 
  6. ^ George R. Newkome, Zhongqi Yao, Gregory R. Baker, Vinod K. Gupta (1985). "Micelles. Part 1. Cascade molecules: a new approach to micelles. A [27]-arborol". J. Org. Chem. 50: 2003. doi:10.1021/jo00211a052. 
  7. ^ Hawker, C. J.; Fréchet, J. M. J. (1990). "Preparation of polymers with controlled molecular architecture. A new convergent approach to dendritic macromolecules". J. Am. Chem. Soc. 112: 7638. doi:10.1021/ja00177a027. 
  8. ^ Bifunctional Dendrimers: From Robust Synthesis and Accelerated One-Pot Postfunctionalization Strategy to Potential Applications P. Antoni, Y. Hed, A. Nordberg, D. Nyström, H. von Holst, A. Hult and M. Malkoch Angew. Int. Ed., 2009, 48 (12), pp 2126-2130 doi:10.1002/anie.200804987
  9. ^ J. R. McElhanon and D. V. McGrath JOC, 2000, 65 (11), pp 3525-3529
  10. ^ C. O. Liang and J. M. J. Fréchet Macromolecules, 2005, 38 (15), pp 6276-6284
  11. ^ S. Hecht, J. M. J. Fréchet (2001). "Dendritic Encapsulation of Function: Applying Nature's Site Isolation Principle from Biomimetics to Materials Science". Angew. Chem. Int. Ed. 40: 74. doi:10.1002/1521-3773(20010105)40:1<74::AID-ANIE74>3.0.CO;2-C. 
  12. ^ J. M. J. Fréchet, D. A. Tomalia,Dendrimers and Other Dendritic Polymers, John Wiley & Sons, Ltd. NY, NY.
  13. ^ M. Fischer, F. Vogtle (1999). "Dendrimers: From Design to Application—A Progress Report". Angew. Chem. Int. Ed. 38: 884. doi:10.1002/(SICI)1521-3773(19990401)38:7<884::AID-ANIE884>3.0.CO;2-K. 
  14. ^ DendrimerWeb
  15. ^ Dong-Lin Jiang, Takuzou Aida (1997). "Photoisomerization in dendrimers by harvesting of low-energy photons". Nature 388: 454–456. doi:10.1038/41290. 
  16. ^ Polymer Factory AB, Stockholm, Sweden.Polymer Factory
  17. ^ Dendritech Inc., from Midland, Michigan, USA.Dendritech.
  18. ^ Diels–Alder “Click” Chemistry in Designing Dendritic Macromolecules, Gregory Franc and Ashok K. Kakkar Chem. Eur. J. 2009 doi:10.1002/chem.200900252
  19. ^ Robust, Efficient, and Orthogonal Synthesis of Dendrimers via Thiol-ene “Click” Chemistry Kato L. Killops, Luis M. Campos and Craig J. Hawker J. Am. Chem. Soc., 2008, 130 (15), pp 5062–5064 doi:10.1021/ja8006325
  20. ^ A chemoselective approach for the accelerated synthesis of well-defined dendritic architectures P. Antoni, D. Nyström, C. J. Hawker, A. Hult and M. Malkoch Chem. Comm., 2007, 22, pp 2249-2251 doi:10.1039/b703547k
  21. ^ New methodologies in the construction of dendritic materials A. Carlmark, C. J. Hawker, A. Hult and M. Malkoch Chem. Soc. Rev., 2009, 38, pp 352 - 362 doi:10.1039/b711745k
  22. ^ Dendrimer Design Using Cu(I)-Catalyzed Alkyne-Azide Click Chemistry, G. Franc, A. Kakkar Chem. Comm., 2008, pp 5267 - 5276 doi:10.1039/b809870k
  23. ^ Polyphenylene Dendrimers: From Three-Dimensional to Two-Dimensional Structures Angewandte Chemie International Edition in English Volume 36, Issue 6, Date: April 4, 1997, Pages: 631-634 Frank Morgenroth, Erik Reuther, Klaus Müllen doi:10.1002/anie.199706311
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