Spacing and Site Isolation of Amine Groups in 3-Aminopropyl-Grafted Silica Materials:  The Role of Protecting Groups

Abstract

The relative spacing of amines in 3-aminopropylsilyl-grafted silica is studied by solid-state fluorescence spectroscopy of 1-pyrenecarboxylic acid (PCA) and 1-pyrenebutyric acid (PBA) bound to traditionally prepared, deprotected benzyl- or deprotected 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 spectroscopic data of either PCA or PBA indicate that the number of amine pairs on the surface separated by a distance of 1 nm or less decreases as the total amine loading decreases. Both the intensity ratio of the excimer band to the monomer band (I 470/I 384 or I exc/I mon) and lifetime decay studies of the fluorophore are useful probes of the amine spacing. Separation of amines on the surface can be achieved by either use of a protected synthesis route or through reduction of the concentration of the unprotected 3-aminopropyltrimethoxysilane used in the grafting solution. However, the two routes lead to materials with significantly different average amine spacings. Due to clustering of unprotected amines in solution before grafting or on the surface during the grafting process, amine−amine distances on the surface of materials prepared by an unprotected synthesis are on average smaller than when a protected synthesis is used. With the protected synthesis, evidence suggests that the amines are more isolated, with larger average amine−amine distances when compared to corresponding materials with a similar amine loading prepared via an unprotected synthesis. This is attributed to both the steric influence of the protecting groups and a reduction in silane clustering in solution due to protection of the amines before grafting. Thus, the mechanism of surface amine spacing when using the protection−deprotection strategy appears to involve both of these factors (especially in the case of trityl-spaced samples).

Synthesis–Structure–Property Relationships for Hyperbranched Aminosilica CO2 Adsorbents

“…SBA-15 Synthesis: Mesoporous SBA-15 was synthesized similar to previously reported methods [69][70][71]. In a typical synthesis, 17.9 g of Pluronic 123 (EO-PO-EO block copolymer), 99.4 g of HCl, and 561 g of H 2 O were stirred overnight in a 1 L Erlenmeyer flask.…”

Postsynthetic functionalization of mesoporous silica

“…33 The use of a benzyl instead of a trityl protecting group led to higher grafting densities, approaching those of highly loaded amino-functionalized silicas synthesized by conventional techniques, while still retaining a high degree of site isolation. 34 In an approach more direct than the concept outlined in Figure 2, site-isolated amino groups have also been obtained by reacting a mixture of APTMS and methyltrimethoxysilane with mesoporous silica. Imprinting 36 and site pairing 37 methods have been applied to mesoporous silica to control the relative positioning of functional groups.…”

Temperature Responsive Solution Partition of Organic–Inorganic Hybrid Poly(N‐isopropylacrylamide)‐Coated Mesoporous Silica Nanospheres

“…This kind of multilayer condensation of 3-aminopropylmethoxysilane on a silica surface by the grafting method has been well documented in the literature. [36] The 13 C CPMAS spectrum of RAFT-MSN (Fig. 1b) features several additional resonances, which were assigned to carbons C4-C14 in the trithiocarbonate CTA functionality (Table 1), based on the spectrum of a neat compound of 3-benzylsulfanylthiocarbonylsufanylpropionic acid.…”