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

TL;DR

This paper uses solid-state NMR to identify π-stacking patterns in naphthalene diimides, offering a new way to study organic electronics without needing perfect crystals.

Contribution

The study introduces a systematic ssNMR method to characterize π-stacking motifs in organic materials without crystallography.

Findings

ssNMR techniques distinguish π-stacking motifs in NDIs based on electronic environments.

DFT calculations correlate observed NMR fingerprints with chemical shifts and stacking interactions.

The method enables characterization of π-stacking in complex systems where XRD is impractical.

Abstract

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.

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