Identification of π-Stacking Motifs in Naphthalene Diimides via Solid-State NMR

Organic electronics, featuring π-conjugated small molecules and polymers, have gained significant attention for their potential in flexible, lightweight devices. However, characterization of the ordered, π-stacking domains within these materials using microscopy or X-ray diffraction (XRD) is challenging with complex systems or when crystallography is impractical. This study applied 1D ¹³C multiple cross-polarization magic angle spinning (multiCP/MAS) and 2D ¹H-¹³C heteronuclear correlation (HetCor) solid-state nuclear magnetic resonance (ssNMR) to systematically characterize π-stacking motifs in a series of N,N'-dialkyl naphthalene diimides (NDIs). These techniques were shown to distinguish between the electronic environments attributed to different π-stacking motifs adopted in these NDIs, such that distinct packing types could be identified by their ssNMR "fingerprints" without requiring growth of XRD-quality crystals. Density functional theory (DFT) supported the experimental data by linking observed motifs with calculated chemical shifts and electronic effects due to stack-bonding interactions. These results lay the foundation for systematic ssNMR characterization of π-stacking domains in diverse organic materials, particularly in complex or blended systems where crystallography is challenging.