Selective incorporation of one of the isomers of a photoswitchable molecule in wormlike micelles

Understanding the physical and chemical mechanisms underlying the interactions between giant micelles and photoswitchable molecules is essential because it is not evident why some photoswitchable chromophores with apparently good potential to modify micellar systems' macroscopic properties have a poor performance on UV-irradiation. We report a mechanism that describes the trans-isomer's expulsion embedded in the wormlike micelles when it is transformed into the cis-isomer on UV-irradiation. Consequently, a small change in the rheological properties of the micellar solution is observed on UV-irradiation. As a model system, 4-(phenylazo) benzoate ion as the photoresponsive chromophore and giant cylindrical micelles made of N-Tetradecyl-N,N-Dimethyl-3-ammonio-l-PropaneSulfonate, and Sodium Dodecyl Sulfate, in saline water, are used. The transisomer chmmophore incorporates into the micelles modifying the solution viscosity and viscoelastic spectra, which follows the Maxwell model at low and intermediate frequencies. However, the effect of UV-irradiation on rheological properties is not significant. Micro-rheological experiments are performed with diffusive wave spectroscopy to obtain high-frequency viscoelastic spectra to estimate the characteristic length scales of the wormlike micellar network with the embedded chmmophore to understand its rheological behavior better. Detailed interaction between the chromophore and the micelles is obtained with (HM)-H-1-NMR and H-1 DOSY NMR experiments to determine the chmmophore conformation into the micelles and diffusivity of the isomers.