Recently, Johansson and co-workers provided the first direct evidence for the existence of nonfluorescent bodipy (4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene) H dimers in double-labeled proteins and fluorescent J dimers in labeled lipid vesicles et al. J. Am. Chem. Soc. 2002, 124, 196), allowing for the calculation of many of the properties of the dimers. Herein, we report on the use of molecular confinement within a sodium silicate derived glass to provide a highly reproducible system wherein nonfluorescent bodipy H dimers can be formed from the free probe essentially quantitatively without any interference from higher-order aggregates or fluorescent J dimers. The formation of the H dimer followed an unexpected first order kinetic process. On the basis of analysis of the fluorescence anisotropy of the entrapped monomer, it was concluded that the H-dimer formation was promoted by adsorption of monomers onto the silica surface (rate limiting step), followed by rapid dimerization. Using exciton coupling theory, it was determined that the H dimer consisted of two strongly coupled monomers that were stacked in a parallel orientation with a distance of 7.6 Å between the monomer units. The transition dipole moment of the monomer was determined to be 26.6 × 10 -30 C m (8.1 D), the emission quantum yield of the H dimer was found to be close to zero, and the Fo ¨rster distance for energy transfer between the monomer and H dimer was calculated to be 56 ( 2 Å. All of these values are in excellent agreement with those determined by Johansson et al.

The fluorescence-based nanosize metrology approach, proposed recently by Geddes and Birch (Geddes, C. D.; Birch, D. J. S. J. Non-Cryst. Solids 2000, 270, 191), was used to characterize the evolution of primary silica particles through the sol-to-gel transition and during aging of sodium silicate (SS) derived silica. In this study, the evolution of silica particles within SS derived silica was examined as a function of pH and glycerol doping through the sol-to-gel transition and up to 10 months after gelation. Time-resolved anisotropy decays were measured for the cationic dye rhodamine 6G, which was strongly adsorbed to the silica nanoparticles, and for the anionic probe pyranine, which provided accurate data on the microviscosity of the internal aqueous solution within sols and gels. The data provide evidence for the presence of nonaggregated primary particles far beyond the gelation point and even after prolonged aging of the resulting silica when aging is done either at low pH or in the presence of glycerol. Both the fraction of stable primary particles and the final size of the primary particles within the aged silica were dependent on the pH and the presence of glycerol. In general, lower pH values (pH 3.5) or the presence of glycerol increased the fraction of primary particles relative to samples at prepared at pH 6.5. Gelation pH did not have an affect on the final particle size, which was 1.5 ± 0.4 nm at both pH 3.5 and 6.5. On the other hand, a smaller particle size (0.9 ± 0.2 nm) was observed for SS sols and hydrogels containing 50% glycerol.