Magnetic fingerprint of naphthalenediimide-based radical crystals.

While π-conjugated radicals are essential for developing high-temperature organic ferromagnets, the interplay between their spin density distribution, packing modes, and resulting magnetic order remains poorly understood. This study focuses on planar conjugated naphthalene diimide (NDI) anion radical systems characterized by robust π-π interactions. By synergistically modulating NDI substituents and reductive crystallization conditions, we successfully constructed a series of single-crystals exhibiting distinct aggregation modes. Utilizing a combination of electron paramagnetic resonance (EPR), superconducting quantum interference device (SQUID) magnetometry, and DFT calculations, we carried out a deep analysis of how geometric parameters of molecular packing-including slip displacement, interplanar spacing, and twist angles-modulate the magnetic exchange coupling constant (J), and constructed a structure-magnetism correlation map. These crystals exhibit pronounced Mott insulator characteristics and low-dimensional localized magnetism, with magnetic coupling strengths governed by orbital phase overlap integrals, demonstrating high tunability ranging from -397 cm-1 to -69 cm-1. Crucially, our theoretical investigations identify an "accidental orbital orthogonality" phenomenon induced by specific slippage along the short axis, which effectively quenches the kinetic antiferromagnetic term and reveals potential ferromagnetic fingerprint regions. This work establishes clear physical models and molecular precursors for the rational design of functional materials with ferromagnetic coupling through precision crystal engineering.