Single molecule emission polarization (SMEP) measurements are used along with single molecule (SMT) data to better understand the confinement of dye molecules diffusing within one-dimensional(1D) surfactant templated silica mesopores.  Mesoporous silica films are prepared by spin casting silica sols templated by cetyltrimethylammonium bromide on microscope coverslip surfaces.  The films are loaded with any one of several different dye molecules during preparation. Cationic, anionic, and uncharged perylene diimide dyes are employed, as is the strongly solvatochromic dye nile red.  The films are subsequently imaged on a widefield fluorescence microscope under a water-ethanol vapor atmosphere that mobilizes the molecules.  Fluorescence videos acquired simultaneously in two orthogonal polarizations reveal that all dyes investigated were orientationally confined, diffusing in a highly aligned state within the cylindrical mesopores.  Quantitative SMEP measurements show that the level of orientational confinement depends on the length of the hydrocarbon tails attached to the perylene diimide core and whether the dyes are uncharged, cationic, or anionic.  The SMT data reveal that confinement of the dye molecules to individual silica mesopores is also dependent on dye charge.  While studies employing the dye nile red indicate that the uncharged hydrophobic dyes are most likely confined to central regions within the pore-filling surfactant micelles, SMT data obtained from charged dyes suggest that some (i.e., anionic dyes in particular) locate near the charged surfactant-silica interface and can more readily cross between neighboring mesopores.  The results of these investigations afford an improved understanding of mass transport phenomena in the tightly confined spaces of surfactant-filled silica nanopores and will have an impact on the applications of these and related materials in chemical separations and catalysis.<br/>

On the confined orientational motions of dyes diffusing in surfactant templated cylindrical silica mesopores

2019 Fall National Meeting

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