Spectroscopic and Polarization-Dependent Single-Molecule Tracking Reveal the One-Dimensional Diffusion Pathways in Surfactant-Templated Mesoporous Silica

The efficiency and selectivity of catalytic reactions and chemical separations occurring in liquid-filled mesoporous materials are governed by the translational and orientational mobilities and surface interactions of the incorporated reagents and analytes. In earlier studies of these phenomena by single molecule emission polarization (SMEP) methods, perylene diimide (PDI) dyes were shown to exhibit unexpectedly strong confinement as they diffused within the one-dimensional (1D) pores of surfactant-templated mesoporous silica films. Restriction of PDI orientational motions was attributed to their confinement to the most hydrophobic regions of the micelles filling the pores. Unfortunately, no clear evidence for the location of the dye was obtained, and its confinement could also be explained by interactions with the pore walls. In this report, spectroscopic single molecule tracking (sSMT) studies using the polarity sensitive dye Nile Red (NR) are employed to determine the location of the molecules. NR exhibits 1D diffusion, consistent with its confinement to the cylindrical pores. The sSMT data reveal that the majority of NR molecules are found in nonpolar environments having polarities similar to that of <i>n</i>-hexane. SMEP measurements demonstrate that the NR molecules diffuse with their long axes aligned parallel to the long axis of the pores and that they are orientationally confined to ∼0.6 nm diameter pathways within the pores. The diffusion coefficient for 1D diffusing NR is also shown to be ∼10<sup>3</sup>-fold smaller than in bulk solution. Taken together, these results demonstrate that the NR dyes are confined to the hydrophobic cores of the micelles. These studies afford an enhanced understanding of how nanostructuring of the pore-filling medium in solvent- and surfactant-filled mesoporous materials governs the mass transport and surface interactions of incorporated reagents and analytes.