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 (DMSD) 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...