Unravelling the intramolecular n → σ* interaction in ultra-electron deficient naphthalenediimides and their radical ions

Abstract

In this work, the role of non-covalent n → σ* intramolecular interactions in bestowing stabilization to exceptionally low-lying LUMO molecules of Naphthalenediimides (NDI) and their radical ions have been investigated utilizing different electronic structure calculation methods at the DFT level of theory and X-ray crystallography. We compared the effect of electron donating groups (EDGs) and electron-withdrawing groups (EWGs), e.g., OMe and F, respectively, at the para-positon of the phenylphosphonium groups integrated at the 2,6-positions of the NDI scaffold on the intramolecular P–O interactions and the evolving electronic effects. The natural bond orbital (NBO) analysis exhibited strong charge transfer from the imide O atoms of the NDI to the phosphorus atom of the phosphonium groups validating a donor–acceptor type of orbital interaction. Atoms in molecule (AIM) analysis also illustrated the strong P–O interaction, as the charge density, ρ(r), and the Laplacian of the density ∇²ρ(r_b) at BCP is within the range of well-known non-covalent interactions. These calculations also revealed long-range electronic communication between the EDGs/EWGs at the phenyl groups and the NDI scaffold.