Dendrimers branching

Dendrimers have distinctive properties, such as the ability to entrap small molecules in their core region and very low intrinsic viscosities in solution. Such properties require molecules to have achieved a particular size, and not all molecules with branches radiating from a core are large enough to develop the characteristic properties of true dendrimers. Branched molecules below this critical size are called dendrons and are the equivalent in dendrimer chemistry of oligomers in polymer chemistry. 

The starburst point occurs at a well-defined limit for each dendrimer system, and its occurrence is dependent mainly on (a) the functionality of the core, (b) the multiplicity of the branches and (c) the branch length. However, the volume of the core itself, and the length of the monomer branches also have an influence. The end groups may occupy the outer surface of the dendrimer, or the branches may fold inwards, thus distributing the end-groups within the dendrimer. The factors that control this behaviour of the branches are not fully understood, but include the nature of the solvent and the detailed chemistry of the dendrimer branches. 

Fig.1. Non-activated and activated PAMAM-dendrimers. Schematic diagram of a non-acti-vated (left) and activated dendrimer (middle). The right panel shows a magnification of the dendrimer branches...

Dendrimers are structurally defined branched polymeric molecules and can form good X-ray quality crystals. This is particularly useful in the correlation of structure and properties. Two main synthetic approaches have been used divergent, which is appropriate for smaller dendrimers, and convergent, which is better for larger dendrimers. Branched polysilanes with a structurally defined branched core were described in 1994, but the growth from the core was not controlled and non-uniform structures were obtained.357... 

Several groups have engaged in the study of linear polymer-dendrimer conjugates (i.e. architectural copolymers). These structures combine block copolymer and dendrimer branching features within one molecular architecture [69-71],... 

Except for some regions indicated by arrows, most of the surface consists of small domains of various sizes, as seen in Figure 12.4 (b). Low generation members (i.e. G = 0-3) in the PAMAM dendrimer series appear to exist as floppy open, plate-like molecules in which the dendrimer branches can easily interpenetrate each other and establish intermolecular interactions [19],... 

In order to study the molecular dynamics of the outer segments of a dendrimer, one pyrene moiety was selectively and covalently attached to one dendron of poly(aryl ester) dendrimers by Adams (in total three pyrene molecules per dendrimer) [24]. The fluorescence decay of pyrene in the THF solution of the labeled dendrimers provided details of the pyrene excimer formation, such as the excimer formation rate, the excimer decomposition rate constant and the equilibrium constant of the excimer formation. These parameters were utilized to evaluate the diffusional mobility of the dendrimer branches. 

In most cases, metal ion coordination by a dendrimer takes place by units that are present along the dendrimer branches (e.g., amine, imine, or amide groups) or appended at the dendrimer periphery (e.g., terpyridine, cathecolamide ligands). When multiple identical coordinating units are present, dendrimers give rise to metal complexes of variable stoichiometry and unknown structures. Luminescent dendrimers with a well defined metal-coordinating site have been reported so far [16, 17], and the most used coordination site is 1,4,8,11-tetraazacyclotetradecane (cyclam). 

Fig. 5 Schematic representation of the [Zn(2)2]2+ species in which the dendrimer branches are extending outward...

The energy dissipation in the dendrimer bearing a certain number of pery-lenemonoimide chromophores can be followed by applying time resolved fluorescence techniques. These measurements indicate excimer-like interactions among neighboring perylenemonoimide dyes. Furthermore, two types of motion are detected a rotation of the whole molecule and a fractional motion maybe of the dendrimer branch [68]. 

Like the divergent approach, the convergent method also involves repetition of several basic chemical reactions. However, the reaction cycles are used to synthesize individual dendrons (dendrimer branches) instead of complete dendrimers. The dendrons have a protected focal point which can be activated in the last synthetic step and linked to two or more attachment points of a core molecule. Dendrimers synthesized by either method contain defects, but the problem is less pronounced for materials prepared by the convergent method. 

Opposite to the divergent method, the convergent method [3] consists in the stepwise synthesis of dendrimer branches first. They are then attached to the core yielding the dendrimer. Today the name dendrimer has been extended to... 

Figure 7.6.4. A dendrimer branch self-assembling to hexagonal packing.

In the synthesis of functional dendrimers, interest has hitherto been focussed on variation of the functional core unit or peripheral groups and the resulting effects on the properties of the dendrimer. For a long time, the only function ascribed to the dendritic branches and their repeating units was that of a scaffold linking periphery and core. It was overlooked that, in the interior of the dendrimer scaffold, an individual characteristic (nano)environment can arise which is largely dependent upon the chemical characteristics and the polarity of the repeating units used to construct the dendrimer. Moreover, they can facilitate cascade processes and serve as a platform for cooperative effects between dendrimer branches [37]. 

Such enantiomerically pure chiral compounds, which are optically inactive in the generally investigated UV/Vis spectral range from 200 to 800 nm, can be designated as cryptochiraF according to Mislow et al., since the chiral information is hidden in the molecule (i.e. present in cryptic form) [12a, 20]. Cryptochirality of the dendrimer is explained on the one hand by the pronounced conformational mobility of the dendrimer branches and on the other by the slight electronic difference between the dendritic substituents. 

These results indicate that in the individual dendrimers, interactions take place both in the ground state and in the excited state between the naphthyl units and also between naphthyl and amine units of the dendrimer branches which result in dimer/excimer and charge-transfer/exciplex excited states. 

Anomalous behaviour of the molar rotation depending upon the generation can basically be indicative of the existence of chiral substructures in the dendrimer branches. In many cases such anomalies merely occur as a result of constitutional differences between the chiral building blocks which are exposed to different local environments in the different parts of the dendrimer. CD-spectro-scopic studies can often provide more detailed information about the underlying reasons. 

The kinetics of guest exchange are fast because the dendrimer branches are held well apart by the large cores. 

Fig. 11 Comparative titration of ATP2- with 1, 2 and 3 (6 x 10 5 M) in CH2CI2 CVs before, during and after addition of [ATP] [N(m-Bu)4]2 (left the number of dendrimer branches are indicated inside the waves). Decrease of the intensity of the initial CV wave ( ) and increase of the intensity of the new CV wave ( ) vs. the number of the equiv. of [(n-Bu4)N]2 [ATP] added per cluster branch of 2 (right)...

Fig. 15. Divergent assembly of branch cells into a generation = 3 dendrimer. Branch cell hierarchy is noted in (A), (B) and (C)...

These effects all contribute to the differentiation of dendrimer branch cells during the advancement to higher generations (Fig. 38). A comparison of Tx values for dendrimer interior carbons and surface carbons shows that although the Tj values for interior carbons remain relatively constant with increasing generation, the Tj values for the surface carbons decrease dramatically [148]. 

Fig. 44. The de Gennes model for self-consistent force field operating on a dendrimer branch cell...

---