Effect of crystal size on triplet pair dynamics in singlet fission of rubrene

Singlet fission proceeds via spin-correlated triplet (TT) pairs and offers promising applications in solar cells, magneto-optical sensors, and quantum technologies. Here, we quantitatively investigated how crystalline morphology influences TT pair dynamics during singlet fission in rubrene orthorhombic crystals by combining experimental and theoretical analyses of the magnetic field dependence of the magnetoluminescence (ML) effect, defined as the magnetic-field-induced change in fluorescence intensity, up to 300 mT at room temperature. Rapidly annealed thin films exhibit an anomalous low-field ML effect, whose maximum reaches -25.2%, and the negative effect persists up to 300 mT. By contrast, millimeter-sized single crystals exhibit a decrease of approximately one order of magnitude in the negative ML effect, accompanied by a low-field shift of the zero-crossing field separating negative and positive ML. The observed results were successfully reproduced by density-matrix simulations based on a four-site model and could be explained by an enhancement in an effective TT pair dissociation rate with increasing crystal size, which shortens the lifetime of the spin-correlated TT pairs undergoing repeated cycles of hopping and spin mixing. The optically detected magnetic resonance measurements at zero field further suggest that long-lived triplet excitons are more readily detected in rapidly annealed films with micrometer-sized crystals than in millimeter-sized single crystals, providing complementary evidence for morphology-dependent triplet localization and transport. These results demonstrate that crystalline morphology provides an effective route to tune TT pair lifetime and spin mixing in rubrene singlet fission, which is crucial for optimizing singlet-fission-based applications.