An NMR and molecular modeling study of carbosilane-based dendrimers functionalized with phenolic groups or titanium complexes at the periphery.

Dendrimers are modified polymers whose architecture is defined by the presence of a central atom or core with multiple branches. These molecules lend themselves to a variety of architectures and uses, including drug delivery and catalysis. The study of the molecular conformations and shapes of dendritic molecules is necessary but not yet routine. Here we present an NMR and molecular modeling study of a series of carbosilane dendrimers, namely 1G-{(CH2)3[C6H3(OMe)]OH}4 (1), 2G-{(CH2)3[C6H3(OMe)]OH}8 (2), and 2G-{(CH2)3[C6H3(OMe)]O[Ti(C5H5)Cl2]}8 (3). Various two-dimensional NMR techniques were used to completely assign the 1H and 13C resonances of molecules 1-3. This information was used, in conjunction with 1H and 13C spin-lattice relaxation measurements, to assess the chain motion of the molecules. The NMR data were also compared with 1-ns molecular dynamics (MD) simulations of 1 and 2 using the MMFF94 force field. The results indicate that these dendrimers possess a core that is motionally decoupled from the rest of the dendrimer, with flexible arm segments that extend from the core. The addition of eight functionalized titanium groups to the ends of the dendrimer chains of 2 to yield molecule 3 serves to further restrict chain motion.