Photo-controlled reversible assemblies from rod-coil molecules with azobenzene group

Rod-coil amphiphilic molecules composed of hydrophilic polyethylene glycol chains and an aromatic domain based on biphenyl and azobenzene groups self-assembled into two-dimensional columnar and lamellar nanostructures in the solid state. In the aqueous solution, these molecules produced wormlike micelles, nanoribbons, and large sheets, depending on their chain length. Interestingly, the quenched fluorescence emission of these molecules was observed in methylene chloride; nevertheless, the intensity of fluorescence emission in aqueous solution significantly increased relative to that observed in methylene chloride because the molecular self-assembly strongly suppressed the energy transfer in aqueous solution. Notably, UV/vis illumination resulted in a trans/cis isomerism of the azobenzene unit of these rod-coil molecules. However, these molecules did not respond to light in the liquid crystal phase because the transformation temperature of the liquid crystal phase was higher than the trans/cis isomer critical temperature. In aqueous solution, the trans/cis isomerism of the azobenzene unit induced the conversion of corresponding nanostructures into dispersed wormlike micelles, wormlike micelles and small fragments, and sheets. These results revealed that the trans/cis isomerism of azobenzene could be used as an external stimulus for amphiphilic molecular self-assembling systems to build various stimuli-responsive nanostructures.