Unraveling two distinct polymorph transition mechanisms in one n-type single crystal for dynamic electronics

Area of Science:

• Materials Science
• Crystallography
• Organic Electronics

Background:

• Cooperative transitions are vital in biological systems for efficient molecular processes, circumventing energetic and entropic barriers.
• These transitions involve concerted molecular displacement in crystals, differing from typical nucleation and growth mechanisms.
• Cooperative transitions are rare in molecular crystals, and their underlying mechanisms remain poorly understood.

Purpose of the Study:

• To investigate the mechanisms behind two distinct thermally activated phase transitions in 2-dimensional quinoidal terthiophene (2DQTT-o-B) crystals.
• To understand the role of alkyl side chains in triggering cooperative behavior in molecular crystals.
• To establish alkyl chain engineering as a method for controlling polymorphic behaviors in organic semiconductors.

Main Methods:

• Analysis of thermally activated phase transitions in 2DQTT-o-B crystals.
• Investigating the relationship between alkyl side chain reorientation and cooperative structural transitions.
• Correlating alkyl chain disorder with nucleation and growth transitions and biradical state formation.

Main Results:

• Alkyl side chain reorientation was identified as the trigger for cooperative behavior, inducing a domino-like molecular tilt.
• A distinct nucleation and growth transition was observed, associated with increased alkyl chain disorder and driven by biradical state formation.
• Two different polymorphic behaviors were identified in 2DQTT-o-B crystals, each linked to specific transition mechanisms.

Conclusions:

• Alkyl chain reorientation is key to initiating cooperative transitions in molecular crystals.
• Controlling alkyl chain disorder is crucial for managing nucleation and growth transitions.
• Alkyl chain engineering provides a rational approach to tune polymorphic behaviors for advanced electronic applications.