Molecular Length Dependence of Single Molecule Wobbling within Surfactant- and Solvent-Filled Silica Mesopores

The confined orientational motions of fluorescent dye molecules diffusing along one dimension (1D) within individual silica mesopores are investigated by simultaneous single molecule tracking (SMT) and single molecule emission dichroism (SMED) methods. Four perylene diimide (PDI) dyes of different lengths are employed as the probe species. Wobbling angles exhibited by the individual molecules are measured within cetyltrimethylammonium bromide-filled mesopores. The results show a clear dependence on probe molecule length, attributable to confinement of the molecular motions to small cavities within the surfactant- and solvent-filled mesopores. These results are used to obtain quantitative estimates of the accessible cavity diameters. Histograms of these data reveal a broad distribution of cavity sizes. The average cavity diameters are shown to be largely independent of molecular length and yield a global mean value of 1.06 ± 0.03 nm, corresponding to ∼1/3 the physical diameter of the silica mesopores, as estimated from X-ray scattering data. The difference in physical and accessible pore diameters is attributed to confinement of PDI orientational motions by nanostructuring of the solvent/surfactant medium filling the pores. It is proposed that the PDI molecules are confined to the most hydrophobic regions of the surfactant micelles and that formation of a water-rich solvent layer at the silica/surfactant boundary may also contribute. These results will facilitate a deeper understanding of solute–solvent interactions in nanoconfined systems and are relevant to applications of mesoporous silica materials in solution-phase catalysis and chemical separations.