Jean M.J. Frechet- Dendrimers and Other Dendritic Macromolecules: From Building Blocks to Functional Assemblies in Nanoscience and Nanotechnology

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HIGHLIGHT Dendrimers and Other Dendritic Macromolecules: From Building Blocks to Functional Assemblies in Nanoscience and Nanotechnology 1 ´ JEAN M. J. FRECHET1,2 Department of Chemistry, University of California, Berkeley, California 94720-1460 2 Division of Materials Science, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received 8 August 2003; Accepted 8 August 2003 ABSTRACT: Given their size, in the single-digit nanometer range, and the versatility of their functionali
  HIGHLIGHT Dendrimers and Other Dendritic Macromolecules: FromBuilding Blocks to Functional Assemblies in Nanoscienceand Nanotechnology JEAN M. J. FRE´CHET 1,2 1 Department of Chemistry, University of California, Berkeley, California 94720-1460 2 Division of Materials Science, Lawrence Berkeley National Laboratory,Berkeley, California 94720 Received 8 August 2003; Accepted 8 August 2003 ABSTRACT: Given their size, inthe single-digit nanometer range,and the versatility of their function-ality, dendrimers and other dendriticmacromolecules are poised to makea significant contribution to the rap-idly expanding fields of nanoscienceand nanotechnology. This highlightfocuses on nascent applications of dendrimers that take advantage of their structural features and polyva-lent character. In particular, the con-cept of dendritic encapsulation of function,borrowedfromNature,canbe applied to the design of a variedarray of energy-harvesting, light-emitting, or catalytic macromole-cules. Similarly, the compact sizeand hierarchical ordering of compo-nents within dendrimers make themideal for exploring the limits of nanolithography. Finally, the pres-ence of differentiated functionalitiesand the polyvalent character of den-drons and dendrimers constitutestrong assets for their use in polymertherapeutics. © 2003 Wiley Periodicals,Inc. J Polym Sci Part A: Polym Chem 41:3713–3725, 2003 Keywords: dendrimers; encapsu-lation; catalysis; nanolithography;light harvesting; antenna; light-emitting diodes (LED); drug car-rierBorn in France, Jean M. J. Fre´chet moved to the United States in 1967 topursue graduate work in carbohydrate chemistry at the State University of New York and Syracuse University under the outstanding mentorship of Conrad Schuerch. From 1971 to 1986, he taught chemistry, carried outpolymer research, and assumed administrative functions at the University of Ottawa. In 1987, he joined Cornell University, where he remained for 10stimulating years, first as the IBM Professor of Polymer Chemistry and thenas the first holder of the P. J. Debye Chair of Chemistry. Having joined theBerkeley faculty in 1997, he currently holds the Henry Rapoport Chair of Organic Chemistry and a research appointment at Lawrence Berkeley Na-tional Laboratory. His research interests largely focus on functional poly-mers: their design, synthesis, properties, and applications. JEAN M. J. FRE´CHET 3713 Correspondence to: J. M. J. Fre´chet (E-mail: Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 41, 3713–3725 (2003) © 2003 Wiley Periodicals, Inc.  INTRODUCTION Benefiting from their unique architectural, structural, andfunctional features, dendritic macromolecules are poisedto make significant contributions in several areas of thephysical and biological sciences and engineering. Theirnanometer size, globular shape, and multivalent charac-ter and the modularity of their assembly suggest andenable their use in a host of biomimetic and nanotech-nological applications. This highlight does not attemptencyclopedic coverage; instead, it mostly focuses on ourown work exploring early applications that exploit theunique nature of dendrimers and derived dendritic mac-romolecules. These applications include encapsulatingmedia for nanoscale devices, unimolecular nanoreactors,antennae, and functional arrays for optoelectronics, en-ergy harvesting and transduction, imaging materials andresists for molecular patterning, and nanosized carriersfor diagnostics or therapeutic applications. As a result of the lack of general availability of most dendrimers, manyof these applications are still distant, and it is likely thatcommercial acceptance of dendritic molecules will con-tinue to involve only high-added-value applications forsome time to come. MOLECULAR FEATURES OF DENDRIMERS Dendrimers are monodisperse and highly branched glob-ular macromolecules that are typically 1–10 nm in sizeand carry a multiplicity of functional groups at theirperiphery. Their globular shape results from an internalstructure 1–3 in which all bonds emerge radially from acentral core or focal point with repeat units that eachconstitute a branch point and are arranged in a regular,layered, branching pattern. Most dendrimers are assem-bled through covalent bonds with a divergent 1 or con-vergent 4,5 synthetic strategy, although some may also beobtained through the self-assembly of mutually comple-mentary molecular building blocks. 6–16 Figure 1 showstwo stylized three-dimensional views of a dendrimerpointing to the connectivity of its various building blocksand the globular shape that it may achieve, along with atwo-dimensional representation of an actual fourth-gen-eration poly(benzyl ether) convergent dendrimer.Numerous macromolecules possess some of the fea-tures of dendrimers, including the high degree of branch-ing that leads to a multiplicity of reactive sites or chainends. These include some naturally occurring polysac-charides, synthetic hyperbranched or comb-burst poly-mers, and hybrid dendritic–linear and dendronized poly-mers. 2,3 Yet none of these dendritic macromolecules iscapable of matching the ultimate properties of dendrim-ers. Even with true dendrimers, the properties of thedendritic state, 3 such as core encapsulation 17,18 and un-usual solution viscosity behavior, 19 are only accessedwhen globularity is achieved at a certain size threshold.The rigidity of dendrimers generally increases with thenumber of layers of repeat units or the generation of thedendrimer. 20,21 However, it is greatly affected by thechoice, intrinsic flexibility, and branching multiplicity of the monomer repeat unit, the number and type of bondsbetween branches, and the degrees of freedom availableto interbranch bonds. In general, dendrimers onlyachieve their globular, near-spherical shape in solution orat very high generations. Lower generation dendrimersare less rigid and, depending on their building blocks andthe interactions (both intramolecular and intermolecular)that prevail, may adopt elongated and flattened ovoidalshapes when spread on a surface. 21–23 As an artifact of the method by which they are drawn,two-dimensional representations of dendrimers (Fig. 1, Figure 1. Three dendrimer representations highlighting its major components. 3714 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 41 (2003)  right) suggest the existence of multiple cavities withinthe volume that they occupy. The possible existence of such cavities has remained a topic of some controversy.Unlike a micelle, which is a dynamic supramolecularassembly quite capable of incorporating a variable pay-load, covalent dendrimers are generally static structureswith an internal volume that may be used to accommo-date guest molecules, 24–29 particularly when they areenlarged by solvation with a good solvent. However,with the possible exception of very specialized structuressuch as shell-crosslinked dendrimers, 30,31 they do notpossess a permanent and rigid cavity. Small guests,which can penetrate the volume of a dendrimers as aresult of favorable enthalpic interactions, may remainencapsulated after the collapse of the solvated structure.Encapsulation may become permanent, as in Meijer etal.’s dendritic box, 29 if the peripheral density of thedendritic structure is increased to rigidify the wholemacromolecule while guest molecules are located withinthe extended volume of a dendrimer. Such encapsulationmay conceivably be used to prepare sensors, diagnosticbeacons, or functional components of molecular ma-chines. CONCEPT OF DENDRITICENCAPSULATION Nature is a wonderful source of inspiration, and our veryfractional understanding of natural processes such asenergy production, harvesting, and conversion, informa-tion storage, chemical synthesis, reproduction, and amyriad of other highly sophisticated processes can guideus in the design of functional molecular assemblies.Mimicry of Nature, combined with our enhanced abili-ties to mesh chemical structure and function, is respon-sible for many of the scientific and technological ad-vances that have taken place over the past decades.One area of natural mimicry of particular relevance tothis highlight is that of function derived from site isola-tion. 18 Numerous biological systems make use of theconcept of site isolation, by which an active center orcatalytic site is encapsulated, frequently within a protein,to afford properties that would not be encountered in thebulk state. For example, the heme moieties of cyto-chrome C or hemoglobin would not be active and wouldnot be able to perform their natural functions (catalysisand oxygen transport) were they not encapsulated insite-isolating proteins.The dendritic shell is similarly capable of encapsulat-ing functional core moieties to create specific site-iso-lated nanoenvironments, thereby affecting molecularproperties. Taking advantage of the radially emanatingarchitecture of dendrimers, researchers have placed ac-tive sites that have photophysical, photochemical, elec-trochemical, or catalytic function at the core. For exam-ple, Figure 2 shows a dendrimer used to encapsulate aporphyrin moiety analogous to the hemes of many en-zymes. Encapsulation is critical to function because itprevents the deactivation of catalytic activity that wouldresult from intermolecular interactions if two porphyrinrings came close enough to each other to effect   -stack-ing. Applying the general concept of site isolation toproblems in materials research is likely to prove ex-tremely fruitful in the long term, with short-term appli-cations readily accessed in areas such as the constructionof improved optoelectronic devices. DENDRIMERS AND LIGHT: FROMHARVESTING TO EMISSION The self-assembly of dendritic carboxylate ligandsaround a single lanthanide ion serving as a core was usedby Kawa 32,33 to improve the luminescence properties of the lanthanide metals, which are widely used as emittersin optical communications. In Kawa’s work, three spe-cially designed dendrons, each with an interacting car-boxylate focal point, self-assemble around erbium(III),europium(III), or terbium(III) ions, leading in each caseto an enhancement of luminescence efficiency with in-creasing generation because of site isolation of the lu-mophores, which drastically reduces the normally trou-blesomely high rate of self-quenching. Although it is of fundamental significance, this demonstration is also im-portant for its implications in the context of fiber-opticapplications. In such an application, an excellent matchexists between the 1350-nm wavelength used to carry asignal through the fiber and the wavelength emitted byEr 3  after excitation by light at 980 nm being pumpedinto the amplifier module (Fig. 3). As the emission of theerbium(III) core dendrimers matches the wavelengthused for signal transmission and dendrimer encapsula- Figure 2. Two views of a porphyrin encapsulated in a den-drimer. The dendrons are marked [G-n], with n indicating thegeneration number in the structure on the right side. HIGHLIGHT 3715  tion eliminates self-quenching of the emission fromEr 3  , they are extremely attractive for use in fiber-opticamplifiers and other optoelectronic devices. 34 Additionalstructure-related antenna effects attributed specifically tothe Fre´chet-type dendrons (Fig. 3) used for encapsulationhave also been noted in the aforementioned systems andmay lead to useful device applications.In addition to steric protection, which is responsiblefor the site isolation used by Kawa et al., 32–33 a den-drimer molecule is uniquely suited to arrange multipleperipheral functional groups around a single core unit.Through the introduction of an energy-transfer interac-tion or similar electronic link between the periphery andthe core, the design of dendritic light-harvesting anten-nae becomes feasible. 35 In a dendritic antenna, an arrayof terminal donor chromophores collects many photonsand transfers their energy through space (Fo¨rster energytransfer) to an acceptor unit located at the core or focalpoint of the dendrimer (Fig. 4). The acceptor, which canbe excited independently of the periphery, also contrib-utes to overall light harvesting. Because an emission isobserved from the core only, the system serves as aspatial and spectral energy concentrator; in other words,it acts as a molecular lens.Such light harvesting is important as it mimics theprimary events in photosynthesis, in which the light-harvesting complex funnels its excitation energy to thespecial pair, leading to subsequent charge separation.Light-harvesting dendrimers can be used to effect severaltypes of energy transformation: light into light (i.e.,broad band into monochromatic, or upconversion of low-energy radiation into high-energy radiation with mul-tiphoton processes), light into electricity with applica-tions in a variety of novel photovoltaic systems, and lightinto chemical energy as done so masterfully in Nature.In energy harvesting, two main types of systems canbe envisioned: one involving the dendritic architecturesolely as a scaffold 36–43 and another in which the den-drimer backbone itself participates in the energy-transferevent. 32,33,44–55 Using the former system, Gilat and Ad-ronov 36–40 have shown that a useful form of light am-plification can be achieved as the core acceptor moietyemits more light energy—transferred from the peripheraldonor chromophores—than it ever could by its directexcitation. This amplification effect, which has its srcin Figure 3. Encapsulation of an erbium ion in a self-assembled supramolecular dendrimer. The siteisolation of the erbium ion negates self-quenching and enables its use in fiber optics as an opticalsignal amplifier. Figure 4. Light-harvesting antenna. Light harvested by allthe chromophores (blue and red) is concentrated at the focal-point (red) acceptor chromophore and re-emitted as monochro-matic radiation. 3716 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 41 (2003)
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