Three-Phase Emulsions during Cross-Coupling Reactions in Water.

Fluorescence lifetime imaging microscopy (FLIM) enabled identification of three-phase emulsions during a cross-coupling reaction in water, offering insight into their features and factors driving their formation. Droplet morphology was influenced by surfactant choice, ionic strength through phosphate concentration, and evolution of the reaction medium composition. Spatially resolved, subdroplet imaging characterized two organic phases, with one exhibiting preferential localization of the palladium catalyst. Anisotropy and solvatochromic polarity measurements indicated that the palladium-catalyst-containing organic-droplet core exhibited higher viscosity than the organic shell, whereas the polarity of the two organic phases was indistinguishable within the solvatochromic detection capability. The presence of three-phase emulsions correlated with overall formulation and with faster rates of product generation. These findings provide insight for composition-droplet structure relationships toward optimizing aqueous-phase organic reactions and advancing synthetic organic chemistry in water.