Ultra‐Low Molecular Weight Photoswitchable Hydrogelators

• Published in: Angewandte Chemie 2021-03, Vol.133 (12), p.6838-6844
• Main Authors: Larik, Fayaz Ali, Fillbrook, Lucy L., Nurttila, Sandra S., Martin, Adam D., Kuchel, Rhiannon P., Al Taief, Karrar, Bhadbhade, Mohan, Beves, Jonathon E., Thordarson, Pall
• Format: Article
• Language: English
• Subjects: arylazopyrazole; Chemistry; Gels; Hydrogels; Loss modulus; Low molecular weights; low molecular-weight gelator; Molecular weight; photoswitch; Rhodamine; self-assembly; Sol-gel processes; Transmission electron microscopy
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.202015703

Two photoswitchable arylazopyrozoles form hydrogels at a concentration of 1.2 % (w/v). With a molecular weight of 258.28 g mol−1, these are the lowest known molecular weight hydrogelators that respond reversibly to light. Photoswitching of the E‐ to the Z‐form by exposure to 365 nm light results in a macroscopic gel→sol transition; nearly an order of magnitude reduction in the measured elastic and loss moduli. In the case of the meta‐arylazopyrozole, cryogenic transmission electron microscopy suggests that the 29±7 nm wide sheets in the E‐gel state narrow to 13±2 nm upon photoswitching to the predominantly Z‐solution state. Photoswitching for meta‐arylazopyrozole is reversible through cycles of 365 nm and 520 nm excitation with little fatigue. The release of a rhodamine B dye encapsulated in gels formed by the arylazopyrozoles is accelerated more than 20‐fold upon photoswitching with 365 nm light, demonstrating these materials are suitable for light‐controlled cargo release. Small, photoswitchable (MW 258 g mol−1) arylazopyrazoles form self‐assembled gels in water that can be reversibly cycled between a gel and solution state using two different excitation wavelengths that cause „thinning“ of the gel fibers upon the light induced E to Z isomerisation. These materials could be used for light‐controlled cargo release.