Ultra-Low Molecular Weight Photoswitchable Hydrogelators

Area of Science:

• Materials Science
• Supramolecular Chemistry
• Photochemistry

Background:

• Hydrogels are water-swollen polymer networks with diverse applications.
• Photoswitchable molecules can alter material properties upon light exposure.
• Developing low molecular weight hydrogelators responsive to external stimuli is a key challenge.

Purpose of the Study:

• To synthesize and characterize novel photoswitchable arylazopyrazole hydrogelators.
• To investigate the light-induced reversible gel-sol transition.
• To demonstrate light-controlled cargo release capabilities.

Main Methods:

• Synthesis of arylazopyrazole derivatives.
• Hydrogel formation studies at varying concentrations.
• Spectroscopic analysis (UV-Vis, NMR).
• Rheological measurements (elastic and loss moduli).
• Cryogenic transmission electron microscopy (cryo-TEM).
• Dye release experiments.

Main Results:

• Two photoswitchable arylazopyrazoles formed hydrogels at 1.2% (w/v).
• These hydrogelators possess the lowest molecular weight (258.28 g/mol) reported for light-responsive hydrogels.
• Light-induced E- to Z-isomerization triggered a gel-to-sol transition with a significant decrease in moduli.
• Cryo-TEM revealed a reduction in sheet width from 29±7 nm to 13±2 nm upon photoswitching.
• Reversible photoswitching was achieved with minimal fatigue using 365 nm and 520 nm light.
• Encapsulated rhodamine B dye release accelerated over 20-fold upon 365 nm light exposure.

Conclusions:

• Photoswitchable arylazopyrazoles represent a new class of low molecular weight hydrogelators.
• These materials exhibit reversible light-induced gel-sol transitions.
• The developed hydrogels are effective for light-controlled cargo release applications.