Synthesis of Photosensitive Dendrimer Molecules
Studies on nano-organized supramolecular and macromolecular systems have been accelerated by recent developments in both synthetic techniques and spectroscopic and scanning microscopic techniques. Dendrimers are hyperbranched, highly symmetric, monodisper
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5.1
Introduction
Studies on nano-organized supramolecular and macromolecular systems have been accelerated by recent developments in both synthetic techniques and spectroscopic and scanning microscopic techniques. Dendrimers are hyperbranched, highly symmetric, monodisperse oligomolecules or macromolecules with well-controlled globular nanostructure [1-5]. The present chapter deals with the synthesis of photosensitive dendrimer molecules. In the case of dendrimers, a dendrimer molecule itself can be regarded as a nanoparticle.
5.2
Dendrimers as Nano-organized Particles
The concept of dendrimers was first proposed by Tomalia in 1985 [6], wherein the name originates from the Greek word dendron which means tree. Dendrimers generally consist of a focal core, many building blocks (repeating units) and numbers of exterior end groups, and are characterized by the number of generations they are made of in addition to the chemical structure of the core, repeating units, and end groups. They can be prepared by two different approaches, i.e., divergent [6] and convergent [7] methods. These two methods can provide almost total control over critical molecular design parameters such as size (molecular weight), shape and disposition of certain functionalities introduced at the interior core, as building blocks, and/or on the exterior surface. There have been numerous examples reported of surface-functionalized dendrimers with self-assembling, chiral, liquid-crystalline, catalytic, redox-active, electroconductive or light-harvesting functionalities, while examples of corefunctional dendrimers are rather limited so far [8]. Unlike linear synthetic polymers, dendrimers have the following unique characteristics [5]: a. Uniform molecular weight without molecular weight distribution. b. Predictable three-dimensional structure (shape). c. Adjustable molecular size with a diameter of several nanometers to tens of nanometers. d. Core-to-surface gradient in branch density. e. Site-selective functionalization. f. Controllable solubility by surface functionalities. H. Masuhara et al. (eds.), Single Organic Nanoparticles © Springer-Verlag Berlin Heidelberg 2003
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Synthesis of Photosensitive Dendrimer Molecules
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Porphyrin-centered dendrimers were first synthesized in 1993 by Aida et al. [9], who utilized aryl ether dendrimers for covalent three-dimensional encapsulation of porphyrin functionality. Various types of aryl ether dendrimers have been prepared intensitively by Aida's group, e.g., with core zinc porphyrin for photoinduced electron transfer [10] and interaction [11] studies, with core azobenzene for harvesting of infrared photons for chemical reactions [1214]' with core ferrous porphyrin as a hemoprotein mimic [15], with a peptide core to form a micrometer-scale fibrous assembly [16], with a hybrid-forming core with fullerene [17], and with a poly(phenyleneethynylene) core for a blueluminescent dendritic rod [18,19]. Horie et al. have prepared aryl-ether type dendrimers with a free-base tetraphenylporphine core for photoch
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