Abstract
<jats:p>Hierarchical organisation across length scales is a defining feature of living matter but remains challenging to engineer in synthetic soft materials. Here we report a light-controlled, fully reversible association of phospholipid vesicles mediated by an amphiphilic azobenzene photoswitch embedded in the lipid bilayer. In the dark, the E isomer laterally aggregates within membranes to form ordered nanodomains, which promote vesicle-vesicle adhesion and the emergence of micrometer-scale assemblies. Ultraviolet irradiation induces E→Z isomerisation, disrupting both intramembrane aggregates and inter-vesicle assemblies and restoring monodisperse nanometre-sized vesicles. The process is repeatable, fatigue-resistant and tunable through guest loading, vesicle concentration and photon flux. Molecular dynamics simulations reveal how isomer-dependent aggregation and partial bilayer protrusion underpin the coupling between molecular and colloidal scales. This work demonstrates how a molecular photoswitch can amplify nanoscale rearrangements into reversible, light-programmable organisation of soft colloidal matter.</jats:p>