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

Abstract

When subjected to a highly strained environment, N-substituted alkyl chain perylene diimides, PDI-C_n (n) = 5–8), reveal a combination of structural and optoelectronic responses directed by the odd–even alkyl chain effect. Their phase transitions, absorption and emission properties have been systematically analyzed using synchrotron and in-lab X-ray diffraction, UV-vis absorption, 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 P. Their distinct polymorphic behavior under high pressure is regulated by the alkyl chain length and conformations, and at high temperatures by the scissor-like movements 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 red-shifting the absorption edge in PDI-C₈ by 84.4 nm GPa⁻¹. These findings provide valuable insights into the mechanism behind structure–property relationships 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.