Reversible Single-Crystal-to-Single-Crystal Isomerization of the Azo Photoswitch

Area of Science:

• Materials Science
• Crystallography
• Photochemistry

Background:

• Reversible E ⇄ Z photoisomerization of azo molecules is crucial for photoresponsive materials.
• Achieving this isomerization within single crystals while maintaining crystallinity (single-crystal-to-single-crystal or SCSC) has been a significant challenge due to restricted molecular motion.

Purpose of the Study:

• To directly observe and characterize SCSC E ⇄ Z isomerization in an azobispyrazole molecule.
• To understand the molecular mechanisms and intermediate states involved in the isomerization process within a crystal lattice.

Main Methods:

• Single-crystal X-ray diffraction was employed to monitor the isomerization process.
• Structural analysis of the initial E isomer, the final Z isomer, and key intermediate states.

Main Results:

• Direct observation of SCSC E ⇄ Z isomerization in an azobispyrazole single crystal.
• Identification of a three-step transformation process involving conformation changes, isomerization, and coordinated molecular motions (interlayer gliding, distance adjustment) with minimal displacement.
• The similarity between the E and Z crystal lattices facilitates the observed transformation.

Conclusions:

• The study demonstrates that azo isomerization can occur in bulk crystals without disrupting single crystallinity, challenging previous perceptions.
• This work opens new possibilities for designing and developing advanced light-responsive crystalline materials and devices.
• The captured intermediate structures provide unprecedented insight into the mechanism of solid-state photoisomerization.