Optical detection of heterogeneous single molecule diffusion in thin liquid crystal films

Abstract

Lateral diffusion of three different dye molecules (terrylene and two perylene diimides) in 4 to 225 nm thin films of 8CB liquid crystals in the smectic-A phase has been investigated on a single molecule level. The influence of film thickness on tracer diffusion can be qualitatively modeled by a hydrodynamic approach. Molecular tracking experiments as well as fluorescence correlation (FCS) studies reveal the presence of diffusion dynamics which span a range of at least more than one order of magnitude in time, which is much larger than the reported anisotropic self-diffusion observed for 8CB bulk samples. We tentatively assign the heterogeneity to the formation of diffusion limiting domains on a micrometer scale within the 8CB films or at the interfaces.

Influence of mesoscopic structures on single molecule dynamics in thin smectic liquid crystal films

“…The orientational behavior of PDI dye molecules has been discussed recently in more detail. 13 The optical transition dipole moment of PDI is parallel to the long axis of the chromophoric backbone. Therefore in the case of o-PDI the dipole moment is oriented parallel to the LC director.…”

“…One of them (o-PDI: N,N 0 -di-hexadecyl-perylene-3,4,9,10-tetracarboxdiimide) orients with its long molecular axis parallel to the LC director 13 while the other one (no-PDI: N,N 0 -di-propyl-1,6,7,12-tetra-(4-heptyl-phenoxy)-perylene-3,4,9,10-tetra-carboxdiimide) has no preferred orientation in a LC film. The orientational behavior of PDI dye molecules has been discussed recently in more detail.…”

“…Such processes might show up, however, when analyzing probability distributions of single diffusivities. 13 In this approach all squared displacements r 2 between all traced molecules in succeeding frames are calculated. With these data 16 where Dt is the time lag between succeeding frames.…”

“…This implies a temporal resolution of 20 ms and a time scale between 1 s and 20 ms. A much faster time resolution is obtained via fluorescence correlation spectroscopy (FCS) in a confocal setup 13 resulting in a time range of 1-10 À6 s. For FCS measurements the dye concentration was about 10 À8 mol l À1 in toluene. AFM images have been obtained by a Nanowizard (JPK Instruments).…”

“…to NMR experiments for systems in the nanometre regime such as very thin LC films. In a recent paper 13 we have reported on lateral diffusion of single dye molecule tracking in smectic 4-n-octyl-4 0 -cyanobiphenyl (8CB) as a function of film thickness between 5 and 225 nm. The orientation with respect to the LC director has been controlled via properly tailored tracer molecules.…”