Recently, we developed a simple methodology for synthesizing aminopropyl-modified silica materials with control over amine spacing [1,2]. Using a protection/deprotection strategy with different sized protecting or “spacing” groups, we created materials with different amine loadings. Probe reaction and spectroscopic tests indicated that these new aminosilica materials were functionally different from traditional aminosilica materials prepared via grafting. Our original hypothesis was that the density of amine sites was controlled by two factors. First, we hypothesized by preventing aminosilanes from hydrogen bonding or “clustering” in solution, we could prevent these pre-grafting amine “clusters” from being transformed into clustered surface species. Thus, we used iminosilanes that could not cluster by hydrogen bonding in solution for grafting. Second, we believed we could use the steric spacing imparted by a bulky protecting group (such as a trityl group) when such species were used as spacers to position the amines on the surface a minimum distance apart. In this work, we probe average amine-amine surface spacing of a variety of aminosilica materials and use the results to reassess our original amine-spacing hypothesis.

The relative spacing of amines in amine-grafted silica is studied by solid-state fluorescence spectroscopy of 1-pyrenecarboxylic acid (PCA) and 1-pyrenebutyric acid (PBA) bound to the traditional and deprotected benzyl- or trityl-spaced aminosilicas. Thermogravimetric analysis and FT-Raman spectroscopy results show evidence that the protected imine can be cleaved to yield the corresponding amine in essentially quantitative yield. The steady-state fluorescence spectroscopy data of either PCA or PBA indicate that as the amines are separated a distance corresponding to the size of the protecting group, excimer formation is either unseen or greatly minimized compared to traditional aminosilica materials. Lifetime decay studies of the fluorophore show that as the amines are separated on the silica, the lifetimes increase as much as 4 times that of the traditional aminosilica loaded with the fluorophore. Studies in materials grafted with excess aminosilane in solution, with a limiting concentration of aminosilane in solution, or the protected aminosilanes in solution are consistent with pre-grafting organization of amines in solution contributes to amine “clustering” on the surface when typical grafting protocols are employed. Thus, the mechanism of surface amine spacing when using the protection-deprotection strategy involves both (i) prevention of solution clustering by amine protection and (ii) steric spacing (especially in the case of trityl-spaced samples).

1. Chem. Mater. 2003, 15, 1132.

2. Langmuir, 2006, 22, 2676.