(638f) Organosilicon Dendrimer Templates For Synthesis Of Nanocage Materials

Recently, we have investigated the synthesis of hollow cage-like particles between 2 and 6 nm in size, which we refer to as nanocages. Because the void space in the cage interior is of molecular dimensions, we hypothesize that molecular processes inside the cage will be dramatically affected by the shell structure and functionality. In fact, we have previously demonstrated molecular size selectivity for 2 nm micelle-directed siloxane nanocages, where interior amine groups bind selectively to ninhydrin probe molecules, while larger porphyrin molecules are excluded by the cyclosiloxane cage windows.[1]

The preparation requires a template for shell crosslinking that can be degraded in the presence of organosiloxane bonds, and then evacuated to leave a hollow cage-like material. In many respects, dendrimers are ideal structure-directing agents for nanocage synthesis, due to their highly monodisperse and symmetric structure, the ability to functionalize the dendrimer periphery for shell crosslinking, and, most remarkably, the possibility of including a functional moiety at the dendrimer core, which would be trapped inside the nanocage after template degradation.

We report here a new degradable dendrimer for nanocage synthesis. An acyloxysilane dendrimer is prepared by hydrosilylation of ethyldichlorosilane onto an olefinic core, followed by heterofunctional condensation of an alkenoic acid. The second-generation dendrimer was synthesized by this method with 16 surface olefin groups, and higher-generation dendritic materials could also be prepared with minor structural defects. These dendrimers were characterized by 1H, 13C, and 29Si NMR, dynamic light scattering (DLS), solution viscometry, and Fourier transform infrared spectroscopy (FTIR). The surface olefin groups can be functionalized with trichlorosilyl groups and crosslinked with disilanols to give organosiloxane and carbosiloxane shells. Because the geminal silyl ester bonds are degraded rapidly by anhydrous methanol, this dendrimer is suitable for the synthesis of acid- or base-sensitive nanocages under anhydrous conditions.

[1] Y.-W. Suh, M. C. Kung, Y. Wang, and H. H. Kung, J. Am. Chem. Soc. 128 (2006) 2776-2777.