Photoswitching hydrazones based on benzoylpyridine

Area of Science:

• Organic Chemistry
• Materials Science
• Photochemistry

Background:

• Benzoylpyridine hydrazones are a class of organic compounds with photoswitching capabilities.
• Understanding their photochromic behavior is crucial for developing advanced materials.
• Previous studies on pyridyl-based hydrazones have shown limitations in addressability and isomerization mechanisms.

Purpose of the Study:

• To investigate the photoswitching properties of three accessible benzoylpyridine hydrazones.
• To explore the effect of ultrasound activation on isomer crystallization.
• To elucidate the thermal isomerization mechanisms and factors influencing kinetics.

Main Methods:

• Synthesis and characterization of benzoylpyridine hydrazones.
• Photo-irradiation experiments to induce isomerization.
• Crystallization studies under different conditions (stirring vs. ultrasound).
• Kinetic studies of thermal isomerization in various solvents (e.g., DMSO).
• Computational analysis of transition state geometry and activation entropy.

Main Results:

• Ultrasound activation promotes crystallization of the thermodynamically less stable E isomer at room temperature.
• The studied hydrazones exhibit both P- and T-photochromic behavior, tunable by the benzoyl substituent.
• Improved addressability compared to previously reported pyridyl-based hydrazones.
• Thermal isomerization follows an inversion mechanism, favored by low activation entropy.
• Intermolecular hydrogen bonding plays a significant role in the thermal kinetics of the nitro derivative in DMSO.
• Addition of triethylamine dramatically accelerates Z-isomer thermal isomerization from days to seconds.

Conclusions:

• Benzoylpyridine hydrazones are versatile photoswitches with tunable properties.
• Ultrasound activation offers a novel method for controlling isomer crystallization.
• The identified inversion mechanism and the role of hydrogen bonding provide fundamental insights into hydrazone photoswitch behavior.
• Fast T-type photochromic switching in polar solvents is achievable, opening avenues for practical applications.