Molecular photoswitches can change their conformation upon irradiation with light and convert this molecular movement into macroscopic changes of sophisticated materials. By combining state-of-the-art azobenzenes with supramolecular low-molecular-weight gelators, based on cyclic dipeptides, photomodulation of hydrogels was achieved. Photochromic amino acids with structures related to the natural L-phenylalanine were an essential element. The supramolecular hydrogels are multi-responsive towards pH, ions, temperature, and, in particular, light. Hereby the gelator F₂-PAP-DKP-Lys can be turned into a sol upon irradiation with green light and within this transition a drug, for example an antibiotic, was released. The further improved supergelator Cl₄-PAP-DKP-Lys₂ is reversibly switchable with red and violet light in the range of 0.2-0.5% in an isotonic Ringer's solution. At a concentration of 1%, the hydrogel consisting of Cl₄-PAP-DKP-Lys₂ remains stable in a boiling water bath. Thus, we envision that the enhanced stability of the supramolecular assembly of Cl₄-PAP-DKP-Lys₂ and orthogonal biocompatibility of F₂-PAP-DKP-Lys, could enable further applications, spanning from regenerative medicine to electronic devices, besides our future vision towards photopharmacology.
