A new class of light-responsive fluids based on reversible transitions between wormlike micelles and rodlike micelles is developed. The system is composed of a synthesized light-responsive surfactant, 4-decylazobenzene-4-(oxyethyl)-dihydroxyethylmethylammonium bromide (C(10)AZODEMAB), and a salicylic acid derivative, 5-methyl salicylic acid (5 mS). 5 mS is highly efficient in inducing the micellar growth of C(10)AZODEMAB at a constant concentration, i.e., [C(10)AZODEMAB] = 30 mmol L-1. Viscoelastic fluids are formed in a wide concentration region of 5 mS, and show UV light-induced shining behaviors. More importantly, they can return to their original states after visible light irradiation. Cryogen transmission electronic microscopy (cryo-TEM) and rheology measurements confirm that the light-induced rheological responses are attributed to the reversible transitions between wormlike micelles and rodlike micelles. UV-Vis and H-1 NMR spectra are employed to study the molecular interactions between C(10)AZODEMAB and 5 mS before and after light irradiation systematically, which evidence the critical role of light-induced isomerization between trans-C(10)AZODEMAB and cis-C(10)AZODEMAB during the transition process well.
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