Activation and Inhibition of Isomerization of a Cationic Azobenzene Surfactant in the Large Void Space of Polyglycerol Dendron Micelles

Owing to the unique geometric structure of dendritic amphiphiles with voluminous dendrons, their micelles can harbor a large void space, which provides a new research focus and approach for micellar functionalization. In this work, we used the void space to construct a UV responsive micelle system of the mixed dendritic amphiphile (C12-(G3)2) and cationic azobenzene surfactant (C4AzoTAB). The synthesized C12-(G3)2 that possesses double third generation polyglycerol (PG) dendrons and a single alkyl chain is expected to highlight the large void space within the inside of the micelles. Thus, the aims of this work are to achieve the isomerization of C4AzoTAB in situ and to deeply understand the intermolecular interaction in the mixed micelles. The effect of the large void room with a wall decorated with the ether oxygen atoms on the isomerization of C4AzoTAB was studied by isomerization kinetics, conductivity measurements, isothermal titration calorimetry (ITC), and 1H NMR and 2D NOESY spectroscopies. The isomerization behavior of C4AzoTAB in C12-(G3)2 micelles was presented in terms of its kinetic constant, counterionic association, interaction enthalpy, and position and orientation of C4AzoTAB. The results of NMR and conductivity show that the quaternary ammonium group of C4AzoTAB situates on the surface of the mixed micelles with C12-(G3)2 both before and after UV-irradiation, while the position of azobenzene group in C12-(G3)2 micelles depends on its conformation. The C12-(G3)2 micelles can inhibit the UV response of the trans-isomer and activate the thermal relaxation of the cis-isomer, which has a potential application in the field of light-controlled smart nanocarriers.