Diffusion of chiral molecules and propagation of structural chirality in anisotropic liquids

Diffusion in nature is usually considered as a smooth redistribution process. However, it appears that the diffusion of chiral molecules and the propagation of chirality may proceed in quite different ways. Indeed, in the present work, unexpected quantization of the spatial concentration of chiral molecules is discovered in self-aligned molecular liquids. It is shown that the interpenetration of two liquids is forming discrete diffusion barrier walls resulting in steplike concentration distribution of chiral molecules in space. The concentration gradient is at least an order of magnitude stronger from both sides of the barrier wall compared to the gradient between those walls. It is also shown that this microscopic diffusion process may be controlled by macroscopic boundary conditions imposed on the host molecular system. Both of those phenomena are related to the collective long-range orientational “elastic” interactions of molecules of the host. The observed phenomena may radically change our understanding of diffusion of chiral molecules, among others, in biological tissue, which contains many examples of self-aligned molecular liquids. This, in turn, has the potential to revolutionize drug design and delivery techniques.

Figure 1

Identity of chiral fluorophore 1. (a) Synthesis of 1 (blue and red parts indicate lipophilic and fluorogenic regions, respectively). (b) X-ray crystal structure of 1 (green dash indicates intramolecular H bond). The structure was deposited at the Cambridge Crystallographic Data Centre (CCDC No. 1518911).

Figure 2

The microscope photography of the contact interface between two liquids (and its neighborhood) in the ground state of the CFM. The extreme left band in the image represents the zone having higher concentration of the CFM. (a) The sandwich is placed between crossed polarizers and a white light source is used to illuminate the sandwich. Inset (top right): example of two parallel horizontal lines. (b) A pass-band interferential filter is added to the setup (the sandwich is still placed between crossed polarizers).

Figure 3

Spatial distribution of the intensity of the fluorescence of the excited CFM in the neighborhood of the interface between two liquids (the CFM is excited here and there are no polarizers). (a) The image of spatial distribution of fluorescence. (b) The intensity distribution of the fluorescence in the direction perpendicular to the contact interface. Images were processed by using imagej software and the intensity was determined by matlab.

Figure 4

Intensity of the fluorescence versus the concentration of the chiral fluorophore.

Figure 5

Effective diffusion coefficient in the case of weak boundary conditions (no rubbing).

Figure 6

Zoom in (a) and out (b) of one horizontal line.

Figure 7

Dependence of the fluorescence intensity upon the application of an electric field in the 8-μm-thick sandwich with uniformly doped CFM (in 5CB) with concentration of 0.0017 g/ml.