Diffusion of single molecules in nanoporous mesostructured materials

iv Laß die Moleküle rasen, was sie auch zusammenknobeln! Laß das Tüfteln, laß das Hobeln, heilig halte die Ekstasen!-CHRISTIAN MORGENSTERN Let the molecules career, leave them to their own confections. Never fuss about corrections ecstasies thou shalt revere!-translation by Sarah Fulford 1 Laisse les molécules faire rage, Quoi qu'elles en secret combinent. Corrections ne plus rumine, Pour sacrées tiens les extases!-traduction par Hans-Werner Kirstein Summary Single-molecule methods play a growing role in materials science because they can reveal structural and dynamic features which are obscured by ensemble averaging in conventional spectroscopic techniques. In this work, such methods were used to study the dynamics of single dye molecules (guests) within different surrounding porous matrices (hosts) using wide-field microscopy and single-molecule tracking. A significant amount of tracking data was collected and sophisticated methods to analyse the data according to diffusion theory were developed. A method was established to directly correlate the diffusion information that is provided by single-molecule trajecto-ries with the images of the porous host systems obtained by transmission electron mi-croscopy (TEM). Furthermore, the results from single-molecule tracking experiments were compared with diffusion measurements using pulsed-field gradient NMR in the same samples. All these investigations provided detailed structural information about the porous host systems as well as a thorough understanding of the diffusional behaviour within the different environments. Two different types of porous silica materials were investigated in this work: Sol-gel glasses and mesoporous thin films. Whereas sol-gel glasses exhibit a broad distribution of pore sizes around a mean value, mesoporous materials have pores with a distinct diameter which are arranged in well defined topologies. Sol-gel glasses with two different mean pore diameters, 3 nm and 22 nm, were investigated using a newly synthesized streptocyanine dye for the single-molecule experiments. In the materials with the bigger pores the dye molecules were found to follow a random motion with an average diffusion coefficient of D M22 =0.72 µm 2 s −1. However, in the material with smaller pores, large inhomogeneities in the diffusion behaviour were detected. A large number of molecules remained confined within regions of different diameters in the range of hundreds of nanometres. Some molecules showed changes between mobile and immobile states. Appropriate methods for the data analysis were devised in order to reveal the inhomogeneities within a single trajectory and to evaluate the changes in the diffusivity quantitatively. For comparison, pulsed-field gradient NMR measurements were done in the same …