Magnetic Properties of Self‐Assemble Naphthalene Diimide Radical Aggregates

Abstract The concept of creating room‐temperature ferromagnets from organic radicals proposed nearly sixty years ago, has recently experienced a resurgence due to advances in organic radical chemistry and materials. However, the lack of definitive design paradigms for achieving stable long‐range ferromagnetic coupling between organic radicals presents an uncertain future for this research. Here, an innovative strategy is presented to achieve room‐temperature ferromagnets by assembling π‐conjugated radicals into π‐π stacking aggregates. These aggregates, with ultra‐close π‐π distances and optimal π‐π overlap, provide a platform for strong ferromagnetic (FM) interaction. The planar aromatic naphthalene diimide (NDI) anion radicals form nanorod aggregates with a π‐π distance of just 3.26 Å, shorter than typical van der Waals distances. The suppressed electron paramagnetic resonance (EPR) signal and emergent near‐infrared (NIR) absorption of the aggregates confirm strong interactions between the radicals. Magnetic measurements of NDI anion radical aggregates demonstrate room‐temperature ferromagnetism with a saturated magnetization of 1.1 emu g⁻¹, the highest among pure organic ferromagnets. Theoretical calculations reveal that π‐stacks of NDI anion radicals with specific interlayer translational slippage favor ferromagnetic coupling over antiferromagnetic coupling.