The odd-even alkyl chain effect on the structure and optoelectronic properties of alkyl-substituted perylene diimide (PDI) derivatives at highly strained environments

When subjected to a highly strained environment, such as high pressure, perylene diimides N-substituted with alkyl chains PDI-Cn (n = 5–8) reveal a combination of structural and optoelectronic responses directed by the odd-even alkyl chain effect. Their phase transitions as well as absorption and emission properties have been systematically analyzed by synchrotron and in-lab X-ray diffraction, UV-Vis, and photoluminescence spectroscopy, complemented by theoretical calculations. Their packing interactions, dominated by π-stacking and weak C-H···O interactions, lead to strikingly similar crystal structures, all belonging to the triclinic space group P1 ̅. Their distinct polymorphic behavior under high pressure is regulated by the alkyl chain length and conformations, and at high temperatures by the scissor-like opening and closing movement of parallel π-stacked molecules. The most susceptible to external stimuli are the even-PDIs, PDI-C6 and PDI-C8, which transform between three phases under high pressure. Compression of π-stacking distances induces a significant bathochromic shift in absorption and emission spectra, most prominently redshifting the absorption edge in PDI-C8 by 84.4 nm GPa-1. These findings provide valuable insights into the mechanism behind structure-property relations for one of the most promising classes of organic semiconductors and underscore the role of the odd-even effect in organic semiconductors combined with the potential of strain engineering for optimizing material performance.