PGSE NMR studies on DAB-organo-rhodium dendrimers: Evaluation of the molecular size, self-aggregation tendency, and surface metal density

PGSE NMR measurements have been carried out for DAB-dendr-(NH2)(n) [n = 4 (Dab4), 8 (Dab8), 16 (Dab16), 32 (Dab32), and 64 (Dab64)] and DAB-dendr-[NH(O)COCH2CH2OC(O)C5H4Rh(NBD)](n) [n = 4 (Rh-Dab4), 8 (Rh-Dab8), 16 (Rh-Dab16), 32 (Rh-Dab32), and 64 (Rh-Dab64)] in CD2Cl2 and CD3OD as a function of the concentration. The hydrodynamic radius (r(H)) and, consequently, the hydrodynamic volume (V-H) of all the species are determined from the measured translational self-diffusion coefficients (D-t). In CD2Cl2, both Dab and Rh-Dab dendrimers show a tendency toward self-aggregation that increases with the generations. In addition, while the radii r(H) for Dab dendrimers is ca. 20-30% higher in CD3OD than in CD2Cl2, the r(H) values for Rh-Dab dendrimers are only slightly influenced by solvent variation. To estimate the Rh-Rh spatial proximity (d(Rh/Rh)) on the surface, the internal radius (r(int)) of the Dab skeleton in Rh-Dab dendrimers was (i) considered equal to that of the Dab dendrimers (model A) or (ii) evaluated assuming that the additional solvent molecules derived from the attachment of Rh to Dab dendrimers were incorporated into the elongated dendritic skeleton (model B). It was found that dRh/Rh decreases from 17.2-19.8 angstrom (Rh-Dab4) to about 14.0 angstrom (Rh-Dab64) with the increase in dendrimer generation.