Pressure- and temperature-driven transitions and conformational conversions of n-hexyl substituted perylene diimide (PDI-C6) crystals

Perylene diimides (PDIs) are organic semiconductors with properties suitable for applications in electronic devices. This study presents a comprehensive investigation of PDI-C6, a perylene diimide modified with an n-hexyl alkyl chain at the imide position, under non-ambient conditions, with a focus on the rich diagram of structurally varying phases and their anisotropic strain. Five distinct polymorphs of PDI-C6 have been found. The ambient phase I transforms to phase II at 465 K, which at 521 K transforms to phase III. The perylene cores arranged in parallel stacks in phase I rotate in the scissor-opening motion by 33° in phase II. Under high pressure, phase I transforms to phase IV at 1.22 GPa and to phase V at 1.50 GPa. The transitions between phases I, IV and V stepwise modify the conformation of the n-hexyl substituents. Surprisingly, all five polymorphs are of space-group type P1̄, which is unprecedented as the symmetry of phases usually involves the space-group changes. These findings significantly enhance our understanding of the structural transformations of next-generation electronic materials and highlight the role of crystal engineering in constructing novel organic semiconductors with high sensitivity to environmental stimuli.