Aggregation-Controlled Spin Coupling in Rylene Diimide-Based Ferromagnetic Semiconductors

Conspectus Organic ferromagnetic semiconductors (OFMS), which would unite long-range spin order with semiconducting charge transport in a purely organic lattice, offer a potential pathway to transcend the limits of Moore’s law. Yet the field has long faced three coupled obstacles. First, the Curie temperatures of reported systems remain far below room temperature (<10 K), because intermolecular ferromagnetic coupling is weaker than ambient thermal fluctuations. Second, the saturation magnetization is extremely low in the few systems that order near room temperature (∼10–3 emu g–1): only a small fraction of radicals (<1%) adopt ferromagnetic configurations, while most remain nonferromagnetic. Third, charge transport is largely absent, as radical spins are typically localized and intermolecular transport pathways remain underdeveloped. Underlying all three is a single gap: the absence of a mechanistic understanding linking aggregation structure to intermolecular spin coupling. This Account recounts, in the order it unfolded, how our group confronted that gap. Working on rylene diimide-based semiconductors─chosen for their large conjugated cores, tunable substitution, and relatively stable anion radicals─we developed an ionization-induced reconstruction strategy that, unexpectedly, produced room-temperature ferromagnetism in perylene diimide radical aggregates, with a saturation magnetization exceeding 1 emu g–1. Although this corresponds to only ∼8% of the theoretical maximum, it already exceeds previously reported values by 3 orders of magnitude. After impurity-driven artifacts were excluded, orbital-resolved spectroscopy confirmed that the magnetism arises from spin-polarized π electrons. The ferromagnetism, however, appeared only within an exceptionally narrow processing window, which drove us to ask how aggregation structure governs spin coupling. Using single-crystal, molecular-cluster, and periodic models, we established that ferromagnetism requires a specific combination of intermolecular twist and slip that suppresses anti-ferromagnetic exchange, an effect amplified cooperatively in many-body assemblies. Exchange-coupling maps computed as functions of the π-π distance, twist angle, and slip displacement further translate this geometric criterion into a first-generation quantitative design diagram for rylene diimide radical assemblies. Crucially, this understanding revealed that the principal obstacle was not molecular but a processing conflict: in solution, radical generation and ordered assembly are inseparable. We therefore decoupled the two through a topochemical reduction strategy and obtained OFMS films with a room-temperature saturation magnetization exceeding 10 emu g–1 (∼73% of the theoretical maximum), 4 orders of magnitude above previously reported values, together with a conductivity of 1.05 S cm–1. Looking back across these studies, we distill a multiscale structure-spin-function framework: SOMO delocalization at the molecular level sets the available exchange channels, twist and slip at the aggregation level select between antiferromagnetic and ferromagnetic coupling, and decoupling radical generation from assembly at the processing level determines how many spins ultimately join the ordered network. We hope that these insights will stimulate further exploration of organic spin materials and accelerate their development toward spintronic devices.