Light-responsive microstructures capable of pyrene monomer fluorescence switching

Abstract

We synthesized an amphiphilic fluorescent compound (PyAzEG) having a pyrene fluorophore directly attached to twisted ortho-alkylated azobenzene (i) to suppress pyrene excimer formation and (ii) to facilitate pyrene monomer fluorescence switching. PyAzEG molecules in a dilute solution underwent azobenzene-based trans ↔ cis photoisomerization under UV and visible light irradiation. When such amphiphilic molecules dispersed in an aqueous solution, they could assemble into spheres through a delicate balance between π–π stacking among aromatic rings and hydrophilic interactions among tetra(ethylene glycol) chains. For a PyAzEG suspension containing micrometer-sized spheres, typical pyrene monomer fluorescence was observed in the range of 370–460 nm, whereas excimer fluorescence did not emerge from the fluorescence spectrum. Monomer fluorescence switching as well as structural transformation from spheres to less packed vesicle-like structures were achieved by changing the wavelength of light. Furthermore, PyAzEG spheres exhibited enhanced fluorescence relative to the monomeric solution, which is probably due to moderate intermolecular interactions among chromophores in a head-to-tail fashion.