Exploring the Depth-Dependent Microviscosity inside a Micelle Using Butterfly-Motion-Based Fluorescent Probes

The viscosity distribution of micellar interiors from the very center to the outer surface is dramatically varied, which has been distinguished in theoretical models, yet it remains highly challenging to quantify this issue experimentally. Herein, a series of fluorophore-substituted surfactants DPAC-F (n = 3, 5, 7, 9, 11, 13, and 15) are developed by functionalizing the different alkyl-trimethylammonium bromides with the butterfly motion-based viscosity sensor, N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC). The immersion depth of DPAC units of DPAC-F in cetrimonium bromide (CTAB) micelles depends on the alkyl chain lengths n. From deep (n = 15) to shallow (n = 3), DPAC-F in CTAB micelles exhibits efficient viscosity-sensitive dynamic multicolor emissions. With external standards for quantification, the viscosity distribution inside a CTAB micelle with the size of ∼4 nm is changed seriously from high viscosity (∼190 Pa s) in the core center to low viscosity (∼1 Pa s) near the outer surface. This work provides a tailored approach for powerful micelle tools to explore the depth-dependent microviscosity of micellar interiors.