Probes of Pore Environment and Molecule−Matrix Interactions in Sol−Gel Materials

The use of molecular probes to study the nature of the local environment around dopants in silicate sol−gel materials is reviewed. The review selectively focuses on probes of pore solvent composition and molecule−matrix interactions using electronic spectroscopy. The sol−gel environment is complex; to systematize the discussion, four regions are defined. The most commonly probed region is the free liquid region where the surroundings of a dopant molecule are similar to those of an equivalent solution. The molecular composition of the mixed solvent, its polarity, and its pH are discussed. The three regions that are most important in affecting the dopant via molecule−matrix interactions are the interfacial region within a few molecular diameters of the pore wall, the pore wall itself, and the constraining regions where the distances between opposite sides of the pores are about the same as the size of the probe molecule. The aspects of molecular mobility that are reviewed are probe molecule rotation, solvent molecule motion, intramolecular motion (conformational changes), and local intermolecular mobility. Finally, a probe of matrix pore shaping is discussed. In all of the examples that are reviewed, the subject is presented from the operational and experimental measurement point of view, i.e., in terms of specific measured properties. The measured quantities are interpreted on the basis of the location of the probe molecule in the various regions of the sol−gel material and as a function of the stage of processing of the material.