Photocontrolled reversible release of guest molecules from coumarin-modified mesoporous silica

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Abstract
Since the discovery1 of MCM-41 more than ten years ago, many investigations have explored the suitability of hexagonal mesoporous silicas for potential practical applications2,3,4. These range from catalysis5,6 and optically active materials7,8 to polymerization science9,10,11,12, separation technology3,13,14 and drug delivery15,16,17,18, with recent successes in the fabrication of hybrid mesoporous organosilicas19,20,21 expected to open up further application possibilities. Because the pore voids of this class of materials exhibit relatively narrow pore size distributions in the range of 2–4 nm in diameter, mesoporous silicas can selectively include organic compounds and release them continuously at a later stage. The functionalization of MCM-41 pore voids with photoactive derivatives22,23,24,25 provides influence over the material's absorption behaviour, but full control over the release process remains difficult. Here we show that the uptake, storage and release of organic molecules in MCM-41 can be regulated through the photocontrolled and reversible intermolecular dimerization26,27 of coumarin derivatives attached to the pore outlets. Successful functionalization requires uncalcined MCM-41 still filled with the template molecules that directed the formation of its pores, to ensure that coumarin derivatives attach preferentially to the pore outlets, rather than their inside walls. We find that this feature and the one-dimensional, isolated nature of the individual pores allow for efficient and reversible photocontrol over guest access to the material's interior.