Hydrogels Self-Assembled from an Azobenzene Building Block: Stability toward UV Irradiation in the Gel and Thin-Film States

Graphical Surprisingly stable : Azobenzene derivatives bearing carboxylic acid units were investigated as hydrogelators. Upon deprotonation/protonation of the carboxylic acids, N -4-azodiphenyl-maleimic acid (ADPMA; see picture) could transform from solution to hydrogel. The ADPMA hydrogel shows typical trans-cis isomerization of the azobenzene unit in solution, yet the hydrogel demonstrates remarkable stability to UV light irradiation in both the bulk gel and thin film states. Azobenzene and its derivatives are widely used as photoresponsive units for the fabrication of photoresponsive smart materials. In this work, two azobenzene derivatives were designed and investigated as hydrogelators, namely N -4-azodiphenyl-maleimic acid (ADPMA) and N -4-azodiphenyl-succinic acid (ADPSA) bearing azobenzene and carboxylic acid segments. In the process of deprotonation/protonation of the carboxylic acids by pH variation, the self-assembly of these two gelators was triggered. ADPMA could transform from solution to hydrogel while the solution of ADPSA formed a precipitate under the same conditions. The solution-gel/precipitate transformation can be repeated by changing the pH. In contrast to the conventional responsiveness to UV-light irradiation for azobenzene-based gels, the ADPMA hydrogel shows typical trans - cis isomerization of the azobenzene unit in solution, yet the hydrogel demonstrates remarkable stability to UV light irradiation in both the bulk gel and thin film states.