Light-responsive vesicles based on azobenzene containing imidazolium surfactants and sodium oleate

In this work, three light responsive imidazolium surfactants containing azobenzene moiety (CmAZOCnIMB, m = 0, 2, 4 and m + n = 6), namely, 1-[2-(4-phenylazo-phenoxy)-ethyl]-7-methylimidazolium bromide (C0AZOC6IMB), 3-[2-(4-phenylazo-phenoxy)-ethyl]-7-methylimidazolium bromide (C2AZOC4IMB), and 5-[2-(4-phenylazo-phenoxy)-ethyl]-7-methylimidazolium bromide (C4AZOC2IMB), were developed. These surfactants showed strong micellization tendency in the aqueous solution with their critical micelle concentrations (cmc) highly depending on the location of azobenzene moiety. Moreover, they also showed innate light response with excellent reversibility attributing to the trans-and cis-isomerization of azobenzene moiety under UV- and visible-light irradiation, respectively. Phase behavior showed that the addition of CmAZOCnIMB in the sodium oleate (NaOA) aqueous solution could induce the apparent growth of aggregates from spherical micelles to vesicles via wormlike micelles. Moreover, the presence of CmAZOCnIMB also endowed the catanionic systems with excellent light response, i.e., the typically reversible vesicle to multi-connected wormlike micelle network transitions. Static light scattering and dynamic rheological measurements were employed to characterize the turbidity and viscosity variation during the transitions. The morphological changes between vesicles and wormlike micelles were demonstrated by cryogen transmission electron microscopy (cryo-TEM) observation. Moreover, the transition mechanism was illustrated by 1H NMR and 2D Noesy spectra.