Investigating the molecular origin of polymorphism and transition pathways in organic semiconductors with bulky side chains

Polymorphism is the ability for a compound to adopt multiple crystalline packing states. In molecular crystals, where the dominant interactions are van der Waals and weak quadrupole interactions, polymorphism is prevalent. In context of organic electronics, polymorphism offers a new avenue for enhancing device performance as slight changes in crystal packing can cause significant impact on charge carrier mobility. However, polymorphs are often discovered serendipitously, and it remains challenging to understand the molecular origin. Therefore, to harness polymorphism to modulate electronic properties, a molecular-level understanding of the polymorphic transition mechanism and pathways is needed. Although some studies attempt to uncover the molecular origin of polymorphism in molecular crystals, they are system-specific and usually exclude organic semiconductor systems. Here, we investigate the molecular origin of polymorphism in organic semiconductors with bulky side chains by studying their phase transition pathways. We discover a unique cooperative transition with distinct advantages for application in organic electronics, and establish a general molecular design rule to trigger it. We access both transition pathways by a single atom substitution in the molecule and incorporate their advantages to organic electronics. We also determine a specific role of bulky side chains to trigger polymorphism in a systematic study of a series of systems with various side chain environments. We present a discovery of cooperative transition, which has not been observed in organic electronics before. It is rarely observed in molecular crystals, and thus the origin and mechanism are largely unexplored. The transition is analogous to martensitic transition in metals and alloys, and the main characteristics are low transition barrier, ultrafast kinetics, and structural reversibility. We report the discovery of martensitic transition in single crystals of two different organic semiconductors. In situ microscopy, single-crystal X-ray diffraction, Raman and nuclear magnetic resonance spectroscopy, and molecular simulations combined indicate that the rotating bulky side chains trigger cooperative transition. Cooperativity enables shape memory effect in single crystals and function memory effect in thin film transistors. We establish a molecular design rule to trigger cooperative transition in organic semiconductors, showing promise for designing next-generation smart multifunctional materials. Furthermore, we use in situ cross polarized optical microscopy of single crystals complemented with interaction energy calculations to present new experimental evidence of a rare phenomenon that will contribute to expanding the understanding of cooperative transition in molecular crystals. We separate the transition process into three distinct steps where each step corresponds to propagation of structural change in a specific direction. We analyze an initiation stage and two propagation stages from hysteresis and propagation speed during cyclic transitions. We discover the dichotomous role of defects in facilitating the initiation step and hindering the propagation steps of transition. We conclude that cooperative transition shows mixed mechanisms of nucleation and cooperativity. From our molecular design rule of rotation of bulky side chains that can trigger cooperative transition, we next 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. From single crystal X-ray diffraction, Raman spectroscopy, and molecular simulations, 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. We also conduct a systematic study of five organic semiconductors with absence or variations of bulky side chains. From differential scanning calorimetry and cross polarized optical microscopy, we discover polymorphs only in three out of five systems, which contain side chains on both ends of the core. We access 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. The bulky side chains create spatial freedom in the packing, creating an environment more favorable for polymorphism. This proposes a generalizable molecular design rule for triggering polymorphs in organic semiconductors.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste