Thermo-, pH-, and Light-Responsive Supramolecular Complexes Based on a Thermoresponsive Hyperbranched Polymer

Hyperbranched polyethylenimine terminated with isobutyramide groups (HPEI-IBAm) was mixed with 4-(phenylazo)benzoic acid (PABA) to form supramolecular complexes through the neutralization reaction between the amino groups of HPEI-IBAm and the carboxylic acid group of PABA, which was verified by ¹H and 2D NOESY ¹H NMR spectroscopy. The obtained supramolecular complexes with a molar ratio of PABA to HPEI-IBAm of ≤8 were soluble in water and exhibited thermoresponsive properties. Their cloud point temperature (T_cp) was sensitive to PABA content, and PABA molecules were exchanged between HPEI-IBAm hosts. The topology of the polymer affected the change in T_cp of the complexes. At pH ∼7, increasing the PABA content decreased T_cp, whereas it caused T_cp to increase at pH ∼9. Reversible trans-to-cis photoisomerization of azobenzene units in the complexes occurred following irradiation with UV or visible light. At pH ∼7, trans-to-cis isomerization of azobenzene units increased T_cp, whereas the opposite occurred at pH ∼9.