Controlling silica surfaces using responsive coupling agents

The surface chemistry of silica was modified using coupling agents capable of participating in oxidation or in the Diels-Alder (and retro Diels-Alder) reactions. The synthesis of the latter coupling agents, using trialkoxysilane groups linked to a cyclopentadiene structure, was achieved by the condensation of sodium cyclopentadienolide with 1-chlorodimethylsilyl-3-triethoxysilylpropane to give 1-cyclopentadienyldimethylsilyl-3-trialkoxysilylpropane. The cyclopentadiene ring in this structure was shown to undergo normal Diels-Alder chemistry with maleimides or maleic anhydride to give 7-(dimethylsilylpropyltrialkoxysilane)-5-norbornene-2,3-dicarboxylic acid anhydride. The retro Diels-Alder reaction of 7-(dimethylsilylpropyltrialkoxysilane)-5-norbornene-2,3-dicarboxylic acid anhydride in solution was not very efficient: the adduct is very stable and only undergoes the retro Diels-Alder reaction at temperatures in excess of 200 °C. Once grafted to the surface, however, the retro Diels-Alder reaction was achieved at a level greater than 90% by use of thermolysis in the presence of free cyclopentadiene. In addition, a polyene-based coupling agent derived from squalene was prepared by hydrosilylation using HMe₂SiOSiMe₂CH₂CH₂Si(OEt)₃. Once grafted to the surface, oxidation by ozone led to ozonides that could be reduced to ketone/aldehyde groups. These in turn could be trapped by functional groups such as hydrazines to make surface-bound hydrazones. With both types of coupling agents, the surface energy and nature of the functional groups bound to the surface could be changed on demand in response to an external stimulus.