Vortex-Flow-Directed Chiral Macroscopic Ordering of Platelet Nanostructures Formed via the Supramolecular Assembly of Platinum Complexes with Bis(phenylisoxazolyl)benzene

Abstract

To understand the vortex flow-directed circular dichroism (CD) effect observed in homogeneous solutions containing supramolecular structures, the macroscopic order formed by supramolecular structures oriented within a flow must be visualized. In this study, a bis(phenylisoxazolyl)benzene-attached platinum complex was found to self-assemble to form uniform anisotropic platelet nanostructures that are oriented within a flow, thereby generating a chiral macroscopic order that is responsible for CD and linear dichroism (LD) effects only in the vortex flow regime. Cooperative self-assembly of a bis(phenylisoxazolyl)benzene-attached platinum complex via controlled supramolecular polymerization produced anisotropic platelet nanostructures with a narrow polydispersity index. The orientational order parameter of the nanostructures correlated with the flow velocity; thus, the nanostructures were oriented along the flow direction. Furthermore, the vortex flow of the dilute nanostructure solution broke the symmetry of the flow, thereby generating a chiral macroscopic order. As a result, CD and LD effects were observed in the vortex flow regime of the dilute nanostructure solution. These results can be generalized to the formation of chiral macroscopic order in solutions containing anisotropic nanostructures.

1

(a) Molecular structures of 1 and 2. Formation of a supramolecular nanostructure via the self-assembly of 1. (b) Illustration of the helical ordering of nanostructures in the vortex flow induced by stirring in a sample cuvette.

2

(a) UV–vis spectra of 1 (3.0 × 10^–4 M in toluene) upon heating at a rate of 1 K min^–1: (from red to blue) 298, 303, 308, 313, 318, 323, 328, 333, and 338 K. (b) Plot representing the degree of aggregation (α) as a function of temperature. All points were obtained from the UV–vis spectra presented in panel (a). (c) CD spectra of 1 (3.0 × 10^–4 M in toluene) at 298 K without stirring (black curve) and with stirring: (from a–g) 100, 200, 300, 500, 900, and 1000 rpm for CW (red curve) and CCW (blue curve) rotations. (d) LD spectra recorded in toluene at 298 K with stirring at 0 rpm (black curve), 100 rpm (blue curves), 500 rpm (green curves), and 1000 rpm (red curves) for CW (solid curves) and CCW (dashed curves) rotations. (e) CD responses recorded at 444 nm with stirring at 0 rpm (black curves) and 1000 rpm for CW (red curves) and CCW (blue curves) rotation.

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(a) CD spectra of 1 (3.0 × 10^–4 M in toluene) recorded at different temperatures: (from a–i) 298, 303, 308, 313, 318, 323, 328, 333, and 338 K with stirring at 1000 rpm for CW (red curve) and CCW (blue curve) rotations. (b) Plots of the degree of aggregation (α) during CW (red) and CCW (blue) rotation as a function of the temperature. All points were obtained from the CD spectra of 1 (3.0 × 10^–4 M) recorded in toluene.

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(a,c) AFM (5 μm × 5 μm) and (b,d) TEM images of 1 on a mica plate and on a copper grid with lacy carbon. A solution of 1 (3.0 × 10^–4 M in toluene) was spin-coated onto a mica plate for AFM, and drop-cast onto a copper grid with lacy carbon for TEM at: (a,b) 298 K and (c,d) 353 K.

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(a) Schematic illustration of the experimental setup used to measure the absorption anisotropy. (b) Microscopic image of emission from the nanostructures. The red arrows indicate the direction of maximum absorption. (c) An example of the change in emission intensity versus the orientation angle of the excitation light (black). The red line represents a cos² fit of the data. (d) Histogram of the absorption anisotropy.

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(a) Crystal structure of 2 arranged in an antiparallel fashion in the asymmetric unit. The hydrogens are omitted for clarity. The red arrow indicates the direction of the electronic transition dipole at 446.0 nm. (b) Stacked structure of the dimeric forms along the major axis (blue arrow) of the crystal.

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(a–h) Time-lapse CLSM images (82 μm × 82 μm) of a toluene solution of 1 (3.0 × 10^–4 M) under the flow conditions (λ_ex = 488 nm): (a–d) in a velocity of 44 μm s^–1; (e–h) in a velocity of 25 μm s^–1. The red arrow represents the flow direction. The yellow, blue, and green arrows indicate the position of the same particles at each time interval. (i) Definition of the angle θ of the principal axis with respect to the displacement vector from t _n to t _n+1. (j) Plot of the order parameter of the molecular aggregates to the flow direction versus the velocity of the molecular aggregates.

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Photographic images of the ink trails in the flow generated by stirring CW at 600 rpm, in the (a) middle and (b) bottom of a 1 cm sample cuvette. (c) Incident light position during the CD measurement and schematic illustrations of the helically and horizontally ordered nanostructures, and helically displaced nanostructures along the optical path. (d,e) CD and LD spectra of the solution of 1 (3.0 × 10^–4 M) under stirring CW at 1000 rpm and upon shifting the position of the incident light from (a) the center (X = 0 mm) to (h) the bottom (X = 7 mm) by pulling up the cuvette in 1 mm steps. (f,g) Plots of the CD and LD intensities at 452 nm versus the incident light position (X) from panel (c).