Understanding the Role of Bulky Side Chains on Polymorphism of BTBT-Based Organic Semiconductors

Polymorphism plays a major role in organic electronics, since even the slightest change in packing can modulate electronic properties. Controlling and accessing polymorphs of organic semiconductors are critical, but they are usually discovered by serendipity. A clear understanding of the molecular origin of polymorphism is essential, but system-specific studies are largely prevalent, each system presenting a unique challenge. In this work, we conduct a systematic study of five organic semiconductors with the absence or presence of various bulky side chains. We discover polymorphs only in three out of five systems in their bulk single-crystal forms. The three systems that exhibit polymorphism all contain side chains on both ends of the core. We resolve the crystal structures and analyze key parameters for changes in the packing environment such as density, intermolecular distance, and short contacts. These parameters indicate that the added bulky side chains loosen the molecular packing and that, as the side chains get bulkier, the effect is stronger. Thus, the bulky side chains create a packing environment more favorable for polymorphism. This proposes a generalizable molecular design rule for triggering polymorphs in organic semiconductors.