The confined translational and orientational motions of a series of four different perylene diimide (PDI) dye molecules diffusing along one dimension (1D) within individual cylindrical silica mesopores are followed to explore the dependence of these motions on dye structure and charge.  The motions of the dye molecules are observed via simultaneous single molecule tracking and single molecule emission polarization measurements. Uncharged perylene diimide (PDI) dyes were reported previously to exhibit strong orientational confinement as they diffuse within the nanometer-sized 1D pores of cetyltrimethylammonium bromide (CTAB)-templated mesoporous silica. It was concluded the PDI molecules were confined to the most hydrophobic core regions of the surfactant micelles. Charged PDI molecules are expected to behave differently.  In these studies, the motions of cationic and anionic PDI dyes are compared to those of two uncharged PDIs having different alkane tail lengths.  All four dyes exhibit populations that are immobile, along with separate populations that diffuse in either 1D or 2D.  The anionic and cationic PDI dyes exhibit the largest and smallest populations, respectively, of immobile molecules, suggesting that electrostatic interactions between the charged dyes and the cationic surfactant headgroups play a significant role in limiting molecular motion.  The cationic and anionic PDI dyes also exhibit the largest populations of 2D diffusing molecules, suggesting they may more readily pass between the cylindrical micelles and through the pore walls.  Via an analysis of the 1D frame-to-frame step size distributions, all four dyes are shown to exhibit evidence for desorption-mediated diffusion, indicating the dyes move by "hopping" along the pore axis.  All four dyes also emit strongly polarized fluorescence as they move in 1D, indicating they are orientationally confined within the nanochannels.  Investigations of the emission polarization for 1D diffusing molecules indicates the degree of orientational mobility depends on molecular length and is consistent with a confined wobbling of the molecules within the pores.

Single-molecule tracking studies of charge-dependent translational and orientational dye motions within surfactant- and solvent-filled silica mesopores

255th National Meeting (2018)

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