Observation of Translational Diffusion of Single Terrylenediimide Molecules in a Mesostructured Molecular Sieve

The translational motion of single terrylenediimide (TDI) dye molecules incorporated into the organic part of a surfactant−silica mesostructure of a monolithic M41S host was observed by confocal microscopy. Only isotropic motion was found indicating that the channel-like motion lies below the optical resolution limit of the experiment. An effective diffusion constant was evaluated to be D = 3.72 × 10 -2 μm 2 s -1 . In contrast, for the TDI molecules diffusing in the pure lyotropic hexagonal phase of the same surfactant in water, a one-dimensional anisotropic diffusion with a diffusion constant of D = 2.07 μm 2 s -1 was measured. The decrease in D and the occurrence of isotropic diffusion on the length scale of the optical experiment on going from the lyotropic phase to the mesostructured host are attributed to an increase in viscosity and tortuosity due to the formation of the silica walls around the template micelles in the synthesis of the M41S material. Apart from the mobile fraction, which shows normal diffusion, a stationary fraction of ∼10% of TDI molecules is observed; however, no intermediate cases were detected.