Lighting Up Nonemissive Azobenzene Derivatives by Pressure

Area of Science:

• Materials Science
• Organic Chemistry
• Photophysics

Background:

• Pressure-induced emission (PIE) activates luminescence in nonemissive materials.
• Achieving PIE in nonemissive organic materials has been a significant challenge.
• Azobenzene derivatives are known for their photoisomerization properties.

Purpose of the Study:

• To report the first observation of PIE in a nonemissive organic system.
• To investigate the mechanism of PIE in azobenzene derivatives under high pressure.
• To explore the potential for high-efficiency organic light emission via PIE.

Main Methods:

• Synthesis and characterization of azobenzene derivatives (1,2-bis(4-(anthracen-9-yl)phenyl)diazene and 1,2-bis(4-(9H-carbazol-9-yl)phenyl)diazene).
• High-pressure experiments using diamond anvil cells to induce and measure PIE.
• Spectroscopic analysis to correlate molecular conformation, photoisomerization, and luminescence.

Main Results:

• PIE was successfully observed in nonemissive azobenzene derivatives starting at 0.52 GPa.
• Emission enhancement was linked to molecular conformational changes and excited-state properties, not solely isomerization inhibition.
• Complete suppression of photoisomerization occurred at 1.5 GPa, with accelerated emission enhancement up to 3.53 GPa.
• PIE was replicated in a carbazole-containing analogue, confirming general applicability.

Conclusions:

• This study presents the first instance of PIE in nonemissive organic azobenzene derivatives.
• The findings highlight the critical role of excited-state constituents in PIE, challenging conventional understanding.
• This work establishes a novel high-pressure route for studying crystalline-state photoisomerization and developing efficient organic emitters.