Single Atom Substitution Alters the Polymorphic Transition Mechanism in Organic Electronic Crystals

Understanding the molecular mechanism of polymorphic transition is essential for controlling molecular packing for high-performance organic electronics. Polymorphic transition in molecular crystals mostly follows the nucleation and growth mechanism. We recently discovered a cooperative polymorphic transition in organic semiconductor single crystals driven by bulky side-chain rotation. In this work, we demonstrate that a single atom substitution in the side-chains from carbon to silicon can completely alter the transition pathway from a cooperative transition to nucleation and growth. We reveal that bulkier side-chains become interlocked to inhibit side-chain rotation and thereby hinder molecular cooperativity to lead to the nucleation and growth mechanism. We report the utilities of both types of transitions in organic electronic devices. Nucleation and growth allows kinetic access to metastable polymorphs at ambient conditions for structure–property study. On the other hand, cooperative transition enables in situ and reversible access to polymorphs for rapid modulation of electronic properties while maintaining structural integrity. Using this simple molecular design rule, we can access both polymorphic transition pathways and selectively utilize their advantages in organic electronic applications.