Photostimuli Reach a Selective Intermediate in a Microflow: One-Shot Transformation from a Supramolecular Co-Polymer to a Micro-Disk Structure

Abstract

In supramolecular chemistry, photostimulants are generally combined with a static solution under thermodynamic equilibrium with no time progression. After reaching the thermodynamic state, self-assembly events contain various species–a mixture of component molecules, intermediate species, and completed assemblies–which light reaches uniformly. In this study, snapshot control of supramolecular polymerization was first combined with pinpoint photoirradiation using a microflow system. Employing the azobenzene derivative trans-C3NO as a monomer, a snapshot moment of supramolecular polymerization along a microflow channel was selectively irradiated with UV light at 365 nm in a space-resolved manner, so that the monomers, intermediates (oligomers), or extended supramolecular polymers were selectively exposed to light stimuli. We found that a pinpoint photostimulus to each snapshot moment had a pronounced effect on the kinetic pathway by tuning the timing at which the snapshot moment of cis-C3NO was generated. Upon irradiation in the upstream region, in the very early stages before initiating polymerization, supramolecular polymerization was suppressed by generating a less reactive cis-C3NO monomer. However, photoirradiation does not affect the supramolecular polymers in the downstream region because of their stiff nature. Remarkably, when irradiating the middle stream region involving a soft-natured intermediate species, supramolecular copolymerization occurred through in situ conversion from trans- and cis-C3NO inside the primitive supramolecular polymer. Loose monomer stacking in the primitive aggregate endows it with mechanoresponsiveness. Under the influence of shear force in a Hagen–Poiseuille flow, the resultant supramolecular copolymers containing geometrically different cis-isomers were rolled up and transformed into a micrometer-sized disk-like structure. During the in situ supramolecular copolymerization and transformation to the disk structure, a liquid–liquid interface generated in the laminar flow acted as a template to fix the orientation of the monomers and supramolecular polymers, leading to the uniform disk formation. Furthermore, monomers’ orientation in the aligned supramolecular polymers are fixed on the interface, on which light is always irradiated in an anisotropic manner. This results in both complexity at the molecular level and long-range structural order such as regular rolling up at the micrometer range over the molecular scale. By incorporating the photostimulus system, microflow extends its potential for supramolecular chemistry.