Subtle conformational variations induce distinct optical and electron transport properties in naphthalenediimide-based solids

Abstract

Semiconductor layer homogeneity and uniform crystallinity are essential for ensuring reliable and reproducible performance in organic electronic devices such as OFETs, OLEDs, and organic solar cells. However, in solution-processed organic semiconductors—particularly those based on small molecules—the occurrence of polymorphic domains significantly undermines their reliability. In OFETs, in particular, such film inhomogeneities lead to large variations in device parameters. These polymorphic domains arise due to barrierless and competing crystallization pathways, a challenge that must be addressed to achieve consistent device performance. Rigid molecular systems are especially prone to such polymorphism, as their conformational rigidity combined with multiple non-covalent interaction modes can lead to structurally distinct yet energetically similar packing motifs. In this study, we explore the formation and implications of polymorphic forms in a rigid, end-group-substituted naphthalenediimide (NDI)-based n-type molecular semiconductor. To maintain structural rigidity and hence enhanced charge transport, picolyl end groups were employed. We isolated three distinct crystalline polymorphs—P-NDI, B-NDI, and R-NDI—each displaying unique colors and optical characteristics. Single-crystal X-ray diffraction (SCXRD) analysis revealed that these are conformational polymorphs, differing primarily in the torsional angle around the bridging carbon, which was calculated to be 62°, 99°, and 134° for P-, R-, and B-NDI, respectively. These subtle conformational differences significantly influenced the nature and strength of intermolecular interactions. Among the polymorphs, P-NDI exhibited a shorter centroid-to-centroid distance, a moderate π–π overlap, along with a high LUMO orbital overlap integral, and a large excimer-to-monomer emission ratio in its crystalline state. Despite promising charge transport properties, experimental OFET fabrication was limited by the poor solubility of these polymorphs in common organic solvents. This manuscript details the comprehensive structure–optical property–charge transport relationships of these polymorphs and highlights the need for design strategies that address polymorphic behavior in rigid small-molecule semiconductors.