Magnetic Properties of Self-Assemble Naphthalene Diimide Radical Aggregates

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 pi-conjugated radicals into pi-pi stacking aggregates. These aggregates, with ultra-close pi-pi distances and optimal pi-pi overlap, provide a platform for strong ferromagnetic (FM) interaction. The planar aromatic naphthalene diimide (NDI) anion radicals form nanorod aggregates with a pi-pi distance of just 3.26 & Aring;, 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 pi-stacks of NDI anion radicals with specific interlayer translational slippage favor ferromagnetic coupling over antiferromagnetic coupling. Numerous naphthalene diimide (NDI) anion radicals are produced through ordinal chemical reduction-oxidation reaction in solution, which are assembled as well-defined nanorod aggregates with ultrastrong pi-pi interactions to form ferromagnetic coupling. Magnetic measurements of these NDI anion radical aggregates exhibit the room temperature ferromagnetic ordering with the saturated magnetization of 1.1 emu g.-1. image