A PhotoswitchableSolvatochromicDye for ProbingMembraneOrderingby RESOLFTSuper-resolutionMicroscopy**

A switchable solvatochromic fluorescent dyad can be used to map ordering of lipids in vesicle membranes at a resolution better than the diffraction limit. Combining a Nile Red fluorophor with a photochromic spironaphthoxazine quencher allows the fluorescence to be controlled using visible light, via photoswitching and FRET quenching. Synthetic lipid vesicles of varying composition were imaged with an average2.5-foldresolutionenhancement, compared to the confocal images. Ratiometric detection as used to probe the membrane polarity, and domains of different lipid ordering were distinguished within the same membrane.