Stimuli-responsive mesoporous organosilica materials containing pH-sensitive organic dyes

Stimuli-responsive, respectively smart materials are fascinating because their functionality is controlled by an external trigger. There are several reasons why nanostructured and in particular nanoporous materials are valuable for the generation of novel smart materials. The significant presence of interfaces facilitates the sensing event, which is the first step of the stimulus-response chain. In particular in the field of periodically ordered mesoporous silica (PMO) one has learned to equip high-surface area materials with a multitude of organic functionalities. However, the number of mesoporous (organo)-silica materials is still very limited which can be defined as smart materials. In the current paper it is shown that the chemistry of organic, pH-sensitive azo dyes can be transferred to mesoporous organosilica materials. Starting from a PMO material containing aniline it was possible to derive a surprisingly high content of the methyl yellow functionality (60%). This is unprecedented for post-modification routes. The latter result demonstrates the good accessibility of organic groups embedded in PMO materials. In addition, the synthesis of a tailor made sol–gel precursor containing a methyl yellow function bridging between two alkoxysilane groups is presented. This precursor allows the preparation of PMO materials containing 100% of the dye function. Finally, the switching of the optical properties of the materials with the proton concentration (pH-value) as a chemical trigger was investigated in detail.

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► Mesoporous organosilica materials containing pH-indicator dyes are prepared.
► Unprecedented post-modification degrees of 60% are documented.
► The optical properties and switching kinetics are described.
► A method to obtain reliable optical data about dispersions of mesoporous silica is reported.