Unveiling the sensing mechanism at the molecular level: a DFT study on the disaggregation of perylene diimide radical anion pimers

The radical anion of amide-functionalized perylene diimide (TFPDIOH˙^-) aggregates into a pimer that is stabilized through pancake bonding. In the presence of primary amines, this pimer can undergo disaggregation, offering potential for responsive organic sensors. In this study, density functional theory calculations were employed to elucidate the sensing mechanism, which can be represented as follows: 1/2[TFPDIOH]₂^2- + <i>n</i>BuNH₂ → [<i>n</i>BuNH₂·TFPDIOH]˙^-. Computational results reveal that steric hindrance from the bulky substituents on the amide positions weakens π-stacking interactions, thereby allowing strong hydrogen bonding to induce pimer disaggregation. The phenolic hydroxyl group on the substituent forms a low-barrier hydrogen bond (LBHB) with <i>n</i>BuNH₂, which is characterized by a short N⋯O distance, high <i>ρ</i>_BCP, 3c-4e bonding pattern, and nearly barrierless proton transfer. The electron-withdrawing fluorine atoms on the substituent enhance hydroxyl acidity, further stabilizing LBHB formation. These findings reveal the LBHB-driven disaggregation mechanism and demonstrate that the rational combination of pancake bonding and LBHB interactions offers a novel strategy for developing π-radical-based organic sensors with enhanced sensitivity.