Shape persistence

Besides the classical search for linear, one-dimensional electronically active materials, synthetic approaches are now also focussed on the generation and characterization of two- and three-dimensional structures, especially shape-persistent molecules with a well-defined size and geometry on a nanometer-scale. It is therefore timely and adequate to extend concepts of materials synthesis and processing to meet the needs defined by nanochcmislry since the latter is now emerging as a subdiscipline of material sciences. 

This modular concept allows for the synthesis of monodisperse dendritic phen-ylene dendrimers of the first (43a, 22 benzene rings) and second (43b, 62 benzene rings) generation [65]. Due to the dense packing of the phenylene rings, shape-persistent nanostructures result. Several of these large phcnylene-type dendrimers (e.g. 43a) can be further cyclized to giant polycyclic PAHs. 

This article has been focussing on poly(phenylene)s with 1,4-(pnra-)phenylene units since these polymers play a key role in the synthesis-driven search for electronic materials. From this article it has become clear that poly(phenylene) chemistry has not restricted its attention to linear (1D-) structures, but has more recently developed into 2D- and 3D-structures as well, the latter serving as functional shape-persistent nanoparticlcs. 

The shape-persistent, structurally well-defined nature of PAMs and PDMs make them attractive models for binding guest molecules within their cavities. In 1995, Hoger and Enkelmann reported the construction of the first meta/para-PAM/PDM hybrid designed to possess hydrophobic and hydrophilic substituents for subsequent use in host guest chemistry [71]. Macrocyclic amphiphile 116 was assembled via the straightforward manner depicted in Scheme 26. 

Dendrimers with a polyphenyl core around a central biphenyl unit decorated at the rim with peryleneimide chromophores have been investigated both in bulk and at the single-molecule level in order to understand their time and space-resolved behavior [28]. The results obtained have shown that the conformational distribution plays an important role in the dynamics of the photophysical processes. Energy transfer in a series of shape-persistent polyphenylene dendrimers substituted with peryleneimide and terryleneimide chro-mophoric units (4-7) has been investigated in toluene solution [29]. 

Another, architecture as yet exotic, has been proposed by Sakamoto and Mullen, who designed original glycodendrimers based on shape-persistent polyphenylenes building... 

Yang J-S, Swager TM (1998) Porous shape persistent fluorescent polymer films an approach to TNT sensory materials. J Am Chem Soc 120 5321-5322... 

To characterize dendrimers, analytical methods used in synthetic organic chemistry as well as in macromolecular chemistry can be applied. Mass spectrometry and NMR spectroscopy are especially useful tools to estimate purity and structural perfection. To get an idea of the size of dendrimers, direct visualization methods such as atomic force microscopy (AFM) and transmission electron microscopy (TEM), or indirect methods such as size exclusion chromatography (SEC) or viscosimetry, are valuable. Computer aided simulation also became a very useful tool not only for the simulation of the geometry of a distinct molecule, but also for the estimation of the dynamics in a dendritic system, especially concerning mobility, shape-persistence, and end-group disposition. 

In contrast to dendrimers built up from aliphatic chains, polyphenylene dendrimers exist as shape persistent nanoparticles as we have demonstrated above. The preparation of functionalized dendrimers is the key step towards various applications. 

A current example of the value of shape persistent dendrimers in fundamental research is their use in the study of electron transfer processes between different redox centers. If one redox center is situated in the core and another redox center is located in the shell at a distinct distance from the other center, the distances which are necessary for an electron transfer to take place can be studied. This currently presents an objective of our research. Furthermore, dendrimers coated with two types of functionalities at the periphery are currently under intense investigation. The attachment of two different types of dyes at the periphery is of interest in the study of the energy transfer between these dyes (light harvesting antenna). 

Fig. 5. Shape-persistent dendrimer and height micrograph of the monolayer structme on mica obtained by tapping mode SFM [88]...

As a start to a tetralogy on dendrimers, the volumes Dendrimers and Den-drimers II have already appeared in print. This mini-series continues now with the latest volume Dendrimers III and will be completed by the fourth volume Dendrimers IV within the next few months. Volume III offers dendrimers based on novel design concepts leading to highly stiffened and shape persistent dendritic structures as well as to new families of rather soft and floppy dendrimers and focuses on new functional properties and materials aspects. As an example, the question of host-guest interactions with dendrimers, whose existence has been under intense debate for a long time, finds its final - and positive - answer in this volume. As a consequence, dendrimers clearly represent a subset of supramolecular chemistry. 

Moore JS (1997) Acc Chem Res 30 402-413 Shape-persistent molecular architectures of nanoscale dimension... 

Li Z-T, Hou J-L, Li C, Yi H-P. Shape-persistent aromatic amide oligomers new tools for supramolecular chemistry. Chem Asian J 2006 1 766-778. 

Zhu C, Hard C, Lin C, Gitsov I. Novel materials for bioanalytical and biomedical applications environmental response and binding/telease capabilities of amphiphilic hydrogels with shape-persistent dendritic junctions. J Polym Sci Part A Polym Chem 2005 43 4017-4029. 

Traditional theories on chemical combination were based on projecting observable phenomenon at the macroscopic level to the microscopic level. Hence, the mechanical explanations using size and shape persisted well into the 1600s. Alchemical explanations projected human qualities on matter to account for specific reactions and combinations. Substances that combined had a natural attraction or love for one another. Conversely, other substances hated each other and did not combine. 

Phenylacetylene chemistry allows construction of shape-persistent molecular architectures through structural control and monomer diversity.27 Combination of this method with solid-phase techniques enhances the rate at which compounds can be made, especially because automation is possible. Oligomer libraries may also be accessible using this type of chemistry. 

Moore, J. S. Shape-Persistent Molecular Architectures of Nanoscale Dimension, Acc. Chem. Res. 1997, 30,402. 

Fig. 2 Classification of star-shaped three arm mesogens. Flexible (subgroup (i)), semi-flexible (subgroup (ii)) and shape-persistent (subgroup (Hi)) star-shaped molecules. Folding of star-shaped semi-flexible molecules to X- and -shaped mesogens [2], Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

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