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Effect of Localized Structural Perturbations on Dendrimer Structure

出版	Ohio State University, 2006
URL	http://books.google.com.hk/books?id=CTKwswEACAAJ&hl=&source=gbs_api

註釋Abstract: The synthesis and conformational properties of folded dendrons based on a 2-methoxyisophthalamide (2-OMe-IPA) repeat unit are described. The hydrodynamic properties of dendrons preorganized via the syn-syn conformational preference of 2-methoxyisophthalamide are compared with 2,6-pyridinedicarboxamide (2,6-pydic) analogues. The effect of subtle differences in the nature of the conformational equilibria that exist within the 2-OMe-IPA and 2,6-pydic repeat units on the global structural properties of the corresponding dendrons was explored computationally and by 1H-DOSY NMR spectroscopy and time-resolved fluorescence anisotropy (TRFA) measurements. Whereas the syn-syn preference of the 2-OMe-IPA branched repeat unit is stabilized entirely by intramolecular hydrogen-bonding interactions, this preference in the 2,6-pydic system is a consequence of both intramolecular hydrogen-bonding and dipole minimization effects. This branching unit was used to study the effect of placing photoresponsive groups in the interior and throughout compact helical dendrons. Azobenzene was used as the photochemical trigger to induce global conformational changes of the dendritic architecture. 2D-NOESY NMR indicated a highly compact helical structure when the azobenzenes were in the more thermally stable trans-conformation. Photoisomerization of azobenzene from trans to cis, induced a structural change to the compact nature of the dendrons, as evidenced by an increase in the hydrodynamic volume. This change was followed by UV-vis spectroscopy and 1H DOSY NMR. The activation parameters and rate constants of the thermal back isomerization were found.