Nanoconfinement and Mass Transport in Silica Mesopores: the Role of Charge at the Single Molecule and Single Pore Levels

Polarization-dependent single molecule tracking was employed to simultaneously probe the translational and orientational diffusion of four perylene diimide (PDI) dyes, having different lengths and charges, within the one-dimensional (1D) nanoscale pores of surfactant-templated mesoporous silica films. The wide-field fluorescence videos acquired reveal that a significant fraction of the molecules follow 1D pathways and exhibit highly polarized fluorescence, consistent with their orientational confinement. Single-frame step size distributions prepared from these data were fit to a new model that accurately describes the distribution for 1D Fickian diffusion in the presence of finite localization precision. Average diffusion coefficients obtained from mean square displacement ( D MSD ) data were 20-100% larger for the two uncharged PDIs compared to the charged PDIs, reflecting electrostatic interactions of the latter with oppositely charged sites on the cationic surfactant headgroups and deprotonated silanol sites on the pore walls. Polarization-dependent tracking data show that the longest uncharged PDI was most strongly confined, while the three shorter dyes were less confined. The cationic PDI produced a wobbling angle distribution that was broader than the others, suggesting it explores more of the pore diameter. The results provide new knowledge on the mechanisms by which the dye molecules interact with the pore-filling medium and the pore surfaces, helping to elucidate the factors controlling the rate of mass transport.