Dendrimer 1335984 217448670 2008-06-06T01:54:29Z Shoy 459990 [[WP:UNDO|Undid]] revision 216445616 by [[Special:Contributions/70.16.229.161|70.16.229.161]] ([[User talk:70.16.229.161|talk]]) [[Image:graphs.jpg|thumbnail|230px|Figure 1. Dendrimer and dendron]] [[Image:Dendrimer_ChemEurJ_2002_3858.jpg|thumbnail|230px|Crystal structure of a first-generation polyphenylene dendrimer reported by Müllen and coworkers in Chem.-Eur. J., 2002, 3858-3864.]] '''Dendrimers''' are repeatedly branched [[molecule]]s. 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 characterized by their structure perfection. The latter is based on the evaluation of both symmetry and polydispersity. The area of dendritic molecules can roughly be divided into the low-molecular weight and the high-molecular weight species. The first category includes dendrimers and dendrons whereas the second encompasses dendronized polymers, hyperbranched polymers, and brush-polymers (also called bottle-brushes). The name comes from the [[Greek language|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 difference in defining dendrimer and dendron. A dendron usually contains a single chemically addressable group that is called the focal point. Because of the lack of the molar mass distribution high-molar-mass dendrimers and dendrons are macromolecules but not polymers. The first dendrimers were synthesized divergently by Vögtle in 1978<ref>"Cascade"- and "Nonskid-Chain-like" Syntheses of Molecular Cavity Topologies Egon Buhleier, Winfried Wehner, Fritz Vögtle Synthesis '''1978'''; 1978: 155-158 {{DOI|10.1055/s-1978-24702}}</ref>, by Denkewalter and coworkers at [[Allied Corporation]] as [[Poly-lysine|polylysine]] dendrimers in 1981<ref> Patent 4,289,872 (published 1981, filed 1979) and 4,410,688 (published 1983, filed 1981) </ref>, by Tomalia at [[Dow Chemical]] in 1983<ref>Dow patent is 4,507,466 (published 1985, filed 1983)</ref> and in 1985<ref>''A New Class of Polymers: Starburst-Dendritic Macromolecules'' D. A. Tomalia, H. Baker, J. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J. Ryder and P. Smith Polymer Journal, Vol. 17 ('''1985''') No. 1 pp.117-132. {{DOI|10.1295/polymj.17.117}}</ref>, and by Newkome in 1985<ref>Micelles. Part 1. Cascade molecules: a new approach to micelles. A [27]-arborol George R. Newkome, Zhongqi Yao, Gregory R. Baker, Vinod K. Gupta [[J. Org. Chem.]]; '''1985''', 50(11), 2003-2004. {{DOI|10.1021/jo00211a052}} </ref>. In 1990 a convergent synthesis was introduced by [[Jean_Fréchet|Fréchet]]<ref>''Preparation of polymers with controlled molecular architecture. A new convergent approach to dendritic macromolecules'' Hawker, C. J.; Fréchet, J. M. J. [[J. Am. Chem. Soc.]] '''1990''', 112, 7638. {{DOI|10.1021/ja00177a027}}</ref>. Dendrimers then experienced an explosion of scientific interest because of their unique molecular architecture (Fig 1). This resulted in over 5,000 scientific papers and patents published by the end of 2005. ==Properties and applications== The properties of dendrimers are dominated by the [[functional groups]] on the molecular [[surface]]. 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.<ref>S. Hecht, J. M. J. Fréchet, [[Angew. Chem. Int. Ed.]] '''2001''', 40, 74.</ref><ref>J. M. J. Fréchet, D. A. Tomalia,<I>Dendrimers and Other Dendritic Polymers</I>, John Wiley & Sons, Ltd. NY, NY.</ref><ref>M. Fischer, F. Vogtle, <I>Angew. Chem. Int. Ed.</I><B>1999</B>,<I>38</I>, 884</ref>. For example, a dendrimer can be water-soluble when its [[end-group]] is a [[Hydrophilicity|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.<ref>article in press</ref> 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<ref>[http://www.dendrimerweb.com DendrimerWeb<!-- Bot generated title -->]</ref>. === 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 [[photoisomerization|photoisomerized]] by very weak [[infrared]] rays when covered by a dendrimer <ref>Dong-Lin Jiang, Takuzou Aida, Nature 388, 454-456 (1997)</ref>. 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 [[photosynthesis|photosynthesize]] artificially. In addition, the development of [[organic light-emitting diode|organic electroluminescent device]]s and their applications has been undertaken by researchers all over the world. ==Synthesis== In the [[synthesis]] of dendrimers, [[monomer]]s 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. Presently, the only kilogram-scale producers of dendrimers is Dendritech <ref>Dendritech Inc., from Midland, Michigan, USA.[http://www.dendritech.com Dendritech].</ref> 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 [[organic reduction|reduced]] by [[lithium aluminium hydride]] to a [[alcohol|triol]] in a [[protective group|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 group]]s in another reaction with the tricarboxylate, forming generation two. [[Image:DendrimerNewkome1985.png|400px|center|The Newkome 1985 dendrimer system]] This sequence can be repeated many times. ==References== <div class="references-small"><references/><div> [[Category:Supramolecular chemistry]] [[de:Dendrimere]] [[fr:Dendrimère]] [[it:Dendrimero]] [[hu:Dendrimer]] [[nl:Dendrimeer]] [[ja:デンドリマー]] [[pl:Dendrymer]]