The nature of the silica cage as reflected by spectral changes and enhanced photostability of trapped Rhodamine 6G

Rhodamine 6G was embedded in a matrix of silica gel glass by the sol to gel technique. The special features of an inorganic oxide glass as a carrier of an organic dye are discussed in comparison with other solid environments, such as adsorption on powders and within porous glasses, thin films, and doping of plastic matrices. Among the advantages mentioned are photostability of the glass matrix, trapping of the dye molecule, and its total isolation from undesired interactions with its neighboring dye molecules, impurities, and photodecomposition products; nonleachability of the dye; the ability to reach stable very high dye concentrations; reduction of translational, rotational, and vibrational degrees of freedom of the trapped dye; good transparency down to the UV. Embedding R6G in the silica glass enables one to reach high concentrations without undesirable dye aggregation. Stokes shift is larger in the glass than in water. Photostability of the dye is higher in the glass than in water. A remarkable front-face fluorescence stability is observed. These observations, and a critical review of the literature, are used to elucidate the nature of the silica glass cage: it is suggested that it is a hydroxylic polar environment, though somewhat less polar than water. The rigidity of the cage is discussed in terms of required reorientation of the environment around an excited state.