Spectroscopic Visualization of Vortex Flows Using Dye‐Containing Nanofibers

Abstract

Which way around? A J‐aggregated zinc porphyrin dendrimer can be used to detect the macroscopic chirality of a vortex. The sign of the circular dichroism response changes quickly upon switching from clockwise (CW) to counterclockwise (CCW) stirring (see picture). The observed chiroptical activity most likely arises from a macroscopic helical alignment of nanofibers formed from the polymeric J‐aggregate.

Experimental aspects of solid state circular dichroism

“…[59][60][61][62] As a caution: conventional stirring of a suspension in a cuvette may create vortices with induced supramolecular optical activity as pointed out recently. [63][64][65][66] Another recent application deals with nanocrystalline benzophenone 67 where CD spectra in Nujol mull and in suspension are compared, the experimental part is however not well described.…”

Stirring competes with chemical induction in chiral selection of soft matter aggregates

“…More striking is the report that the combination of soft aggregation and stirring may lead not only to chiral induction 22,23 but, more importantly, also to sign selection, in both three 24,25 and two dimensions 26,27 . Discussions on the true nature of such an enantioselection in three-dimensional aggregates have raised some controversy [28][29][30][31][32][33] . Recently, d'Urso et al 34 have hinted at the possibility to exert enantioselective control on the supramolecular assembly of achiral components doped with a chiral complex by means of vortex flow.…”

Hydrodynamic Helical Orientations of Nanofibers in a Vortex

“…Linear dichroism (LD) and circular dichroism (CD) spectroscopies allow the visualization of helical alignments in torsional flows of a vortex generated by mechanical rotary stirring of fluids. We initially discovered the helical alignment of a supramolecular nanofiber, composed of a zinc porphyrin derivative bearing dendritic substituents, in the vortex [13,18]. Next, a one-handedly twisted supramolecular nanofiber was found to show unequal LD responses in right-and left-handed vortex flows, owing to unequal helical alignment of the nanofiber [19].…”

“…A supramolecular polymer of zinc porphyrin dendrimer DP (Figure 2a) can helically align in a vortex generated by mechanical rotary stirring of a fluid in an optical cell [13,21]. DP self-assembles to form a J-aggregate in nonpolar solvents by π-stacking interactions and hydrogen bonds involving its carboxylic acid side groups (Figure 2b) [11].…”

“…Compared with, e.g., rheometric shear flows, vortexes are complex, involving many locally different fluid flows. For example, upon mechanical rotary stirring of a fluid in a cuvette, a tight, torsional flow is generated at the rotary center, while the peripheral part of the vortex involves a loose, spiral flow ( Figure 1) [13]. We were prompted to investigate whether these vortex flows can be utilized for controlling the orientations of molecules or molecular assemblies.…”