Organic materials have attracted recent attention in spintronics due to their weak spin-orbit coupling (SOC), long spin relaxation times, and highly designable molecular/crystal structures. Compared with inorganic systems, spin behaviors in organic materials are more sensitive to microstructures and chemical environments, making it feasible to regulate spin effects through structural design.

This review represents the rules and strategies by which structural design in organic functional crystals tunes spin effects. From the perspective of spin dynamics, SOC, exchange interaction, and hyperfine interaction (HFI) jointly determine the characteristics of spin relaxation in organic systems, while their relative importance varies with structural order, carrier density, and transport pathways. In this context, experimental phenomena such as organic magnetic-field effects, spin transport, and organic magnetism can be regarded as collective manifestations of multiple microscopic interactions under specific structural and device conditions rather than direct reflections of a single mechanism. Tailoring interaction strengths and their modes of action via structural design thus constitutes an effective route for regulating spin effects.

Regarding crystal-structure regulation, this review summarizes the effects of crystallinity and crystal parameters on the spin behaviors. Improving crystal order can enhance carrier transport, reduce disorder-induced scattering and local random fields, thereby providing more stable channels for spin transmission. Meanwhile, molecular packing motifs, intermolecular interactions, and molecular curvature are also key factors affecting spin relaxation. Structural isomers, by introducing well-defined conformational differences, offer clean comparative platforms for clarifying how subtle structural variations alter spin-related interactions, and they also inspire further molecular-level approaches for tuning spin effects.

Regarding chemical-composition regulation, this review discusses the impacts of isotope effects, atom substitution, and multicomponent systems on the spin behaviors. Isotopic substitution can modulate HFI via altering nuclear spins, serving as an important means to regulate spin decoherence. Atom substitution changes the molecular electronic structure and local environment, providing flexible pathways to tune SOC and energy-level distributions. Multicomponent systems introduce new localized states, energy-level gradients, and phase interfaces, enabling cooperative interactions that lead to more complex yet tunable spin responses. In addition, the coupling between spin and charge at magnetoelectric heterostructure interfaces can also realize electric-field modulation of spin polarization and spin transport, as well as magnetic-field and optical-field modulation of polarization, further expanding the design space for spin regulation in organic systems.

Moreover, this review represents recent progress on spin-selective effects in chiral material systems and points out that chiral structures provide a solution for controlling spin polarization that differs from conventional magnetic routes. Spin-related responses can be achieved without relying on strong SOC via constructing molecules and crystal structures with stable chiral features, offering some opportunities for developing novel spin-functional devices.