Sterically induced stoichiometry

Figure 27.2 (a) Symmetry properties for core-shell structures where r,/r2 < 1.20 (b) sterically induced stoichiometry (SIS) based on respective radii (r,) and (r2) core and shell dendrimers respectively (c) Mansfield-Tomalia-Rakesh equation for calculation of maximum shell filling when r,/r2 < 1.20... 

Clearly, this proposed congestion at the critical branched state M, should exhibit (1) sterically-inhibited reaction rates, (2) sterically-induced stoichiometry, and, quite possibly, (3) a critical phase change due to surface cooperativity (association). These phenomena have been observed experimentally for the Starburst PAMAM dendrimers and may prove to be diagnostic probes for this critical branched state in other dendrimer families. 

Sterically induced stoichiometry (SIS) is manifested whenever the summation of dendrimer surface-group volumes and incoming reagent volumes approach available exterior volume surrounding the reactive dendrimer surface groups Z as illustrated in Figure 42. So long as the summation of the volumes and involved in bond formation with Z remains much less than 4/3 7t(r2 — r ), ideal... 

Fig. 42. Sterically induced stoichiometry (SIS) wherein Z = available dendrimer surface groups, R = chemically bound groups derived from a reagent (R )...

Fig. 31 (a) Symmetry properties of core-shell tecto(dendrimer) structures when ri/r2 < 1.20. (b) Sterically induced stoichiometry (SIS) defined shell capacities (A max). based on the respective core and shell radii, when ri/j-2 < 1.20. (c) Mansfield-Tomalia-Rakesh equatirai for calculating the maximum shell-filling value (capacity) (A/max)> when rjr2 > 1.20 [121, 138, 167]... 

Swanson, D. R., Huang, B., Abdelhady, H., and Tomalia, D. A. 2007. Unique steric and geometry induced stoichiometries observed in the divergent synthesis of poly(ester-acrylate/amine) (PEA) dendrimers. New J Chem 31(7), 11. 

The excimer-like fluorescence of the y-CD-pyrene complex with stoichiometry 1 2 shows an extremely intense circular polarization [104,105], 0em = 1-2 X 10 the same phenomenon was observed for 1-ethylpyrene (14). It was concluded that pyrene molecules in the y-CD cavity form the excimer with an asymmetrical twisted configuration the formation of the achiral, symmetric, sandwich-type pyrene excimer present in bulk solutions is probably hampered by the sterically restricted environment. The S-helicity of the two 13 molecules included in y-CD is in agreement with the induced circular dichroism spectra [106]. 

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