Fluorescent Probes of the Molecular Environment within Mesostructured Silica/Surfactant Composites under High Pressure

The molecular environment of the organic component of periodic silica/surfactant composites under elevated pressure is studied using a rigidochromic Re complex as a molecular probe. This amphiphilic organometallic complex is incorporated into the organic component of the silica/surfactant composite during synthesis. Shifts in the luminescence peak position of the Re complex indicate changes in the rigidity or viscosity of the surrounding matrix. In the experiment, under elevated pressure, the luminescence from the complex provides a measure of the changing molecular environment inside the mesophase. The results confirm conclusions from earlier experiments on the mechanical properties of silica/surfactant composites under high pressure: Although an increase in viscosity is observed with increasing pressure, the molecular environment does not become fully rigid until pressures as high as 10 GPa have been reached. This pressure is significantly higher than the solidification point of most pure organic phases and suggests that the mechanical strength of the silica framework provides a protected molecular environment for organic molecules situated within the silica nanopores.