Three-Component Supramolecular System with Multistimuli-Responsive Properties in Water

Hyperbranched polyethylenimine terminated with isobutyramide groups (HPEI-IBAm), 4-(phenylazo)benzoic acid (PABA), and α-cyclodextrin (α-CD) were assembled together at pH≈7 to form the three-component supramolecular complexes that were verified by ¹H and 2D ROESY ¹H NMR spectroscopy. UV/Vis spectrometric titration experiments showed that the content of α-CD in the three-component complexes was less than the feed amount and it was difficult for all the PABA units in the complexes to further form complexes with α-CD. The obtained three-component supramolecular complexes exhibited thermoresponsive properties in water. Increasing the α-CD concentration led to a sharp increase in the cloud point temperature (T_cp) at the beginning, but after the [α-CD]/[PABA] ratio was in the region of 1.3-1.6, the T_cp increased gradually When the concentration of α-CD was low, a higher concentration of PABA led to a lower T_cp, however, the opposite was observed when the concentration of α-CD was high. For the three-component complex, increasing the α-CD concentration at pH≈7 or at pH≈9 led to different T_cp temperatures. In the low α-CD concentration range, adjusting the pH from ≈7-≈9 resulted in an increase in the T_cp, similar but not so pronounced as that of the two-component system of HPEI-IBAm/[PABA]. When the concentration of α-CD was high, adjusting the pH from ≈7-≈9 decreased the T_cp; this observation is different to that of the two-component system of HPEI-IBAm/[PABA]. Reversible trans-to-cis photoisomerization of azobenzene units in the complexes occurred, following irradiation with UV or visible light. Trans-to-cis isomerization of azobenzene units decreased the T_cp. However, this result differed to that of the two-component system of HPEI-IBAm/PABA.