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.