Reversible Mechanical Induction of Optical Activity in Solutions of Soft‐Matter Nanophases

Abstract

Nanophases of J-aggregates of several achiral amphiphilic porphyrins, which have thin long acicular shapes (nanoribbons), show the immediate and reversible formation of a stationary mechano-chiral state in the solution by vortex stirring, as detected by their circular dichroic signals measured by 2-modulator generalized ellipsometry. The results suggest that when a macroscopic chiral force creates supramolecular chirality, it also creates an enantiomeric excess of screw distortions, which may be detected by their excitonic absorption. An explanation on the effect of the shear flow gradients is proposed on the basis of the orientation of the rotating particles in the vortex and the size, shape, and mechanical properties of the nanoparticles.

Structure vs. properties — chirality, optics and shapes — in amphiphilic porphyrin J-aggregates

“…Therefore, we can reasonable assume that the electronic and optical properties of and on the relative orientation of the linear dichroism at 420 nm and 490 nm measured in oriented particles of the aggregates. 6 The linear dichroic measurements performed in mature particles show that the H-band is aligned along the longer 65 particle axis ** (see Supplementary Information), in agreement with the present diffraction data. This corresponds to the general behaviour of aromatic organic compounds, showing a faster crystal growth along the π-stacking direction.…”

“…Some authors had attributed this H-band to independent species of H-aggregates. However, polarized 20 fluorescence spectra of H 4 TPPS 4 J-aggregate solutions 5 as well linear dichroism measurements 6 show the orthogonal arrangement of H-(≈ 420 nm) and J-transitions (≈ 490 nm), i.e. that both excitons are forming part of the same structure.…”

“…Furthermore, in the case of long-shaped particles of the aggregates the use of two-photomodulators polarimetry and 40 Mueller-matrix methods allowed to discriminate between combinations of linear polarizations and true CD in the experimental CD signals. 7 In fact, recent results suggest that the title J-aggregates do have chiral structures 6 and that the detection, or not, of optical activity in samples would depend 45 on whether scalemic or racemic mixtures are finally obtained. This is analog to the case in which achiral compounds form enantiopure crystals as the most stable mesomorph, yielding the racemic mixture of chiral crystals called racemic conglomerates.…”

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

“…However, one must note that the observed CD may originate from not only a true CD signal but also artifactual combinations of linear dichroism (LD), linear birefringence (LB), and circular birefringence (CB) [22]. We have not evaluated their contributions in the observed CD spectra, but Mueller matrix spectroscopy, developed by Ribó et al, may allow their evaluations [23][24][25].…”