Macrocyclic Steric Confinement at Heavy‐Atom Sites to Suppress Spin‐Orbit Coupling for Room‐Temperature Spin Transport in Organic Semiconductors

ABSTRACT Electron spin transport in organic semiconductors (OSCs) at room temperature is fundamentally limited by an intrinsic trade‐off between spin diffusion and spin relaxation, arising from spin‐orbit coupling associated with heavy atoms in conjugated backbones. Here, we demonstrate that macrocyclic steric confinement at heavy‐atom sites unlocks a previously inaccessible spin‐transport regime in OSCs, in which efficient spin diffusion and long‐lived spin coherence intrinsically coexist. Employing a previously reported macrocyclic‐confined thiophene‐based OSC as a model system, we show that a covalently linked macrocycle enforces a highly linear conjugated backbone while dispersing spin density away from heavy atoms. This unique combination suppresses spin‐orbit coupling while preserving strong charge delocalization. Thus, the macrocyclic‐confined molecule exhibits a concurrent and marked enhancement in both intrinsic spin diffusion coefficient and spin lifetime in pristine thin films. When implemented in organic spin valves, this molecule delivers a room‐temperature spin diffusion length of 322 nm together with a spin lifetime of 253 µs, setting new benchmarks of both metrics for organic spintronic materials. These findings reveal how macrocyclic steric confinement fundamentally reshapes spin‐orbit coupling in conjugated systems and establish a general chemical principle for reconciling structural order with long‐lived spin coherence in OSCs.