Spin-Dependent Exciton Transport and Negative Diffusion in Thermally Activated Delayed Fluorescence Crystals

Thermally activated delayed fluorescence (TADF) materials have emerged as a promising class of emitters capable of harvesting triplet excitons via reverse intersystem crossing (RISC), offering a route to 100% internal quantum efficiency. While molecular design strategies have focused extensively on enhancing RISC, the spatial dynamics of excitons in the solid state remain less explored. Here, we report direct visualization of exciton diffusion in a crystal of a donor-acceptor type TADF emitter, MA-TA, revealing a counterintuitive phenomenon of negative exciton diffusion, in which the spatial distribution of excitons becomes narrow over time. Time-resolved photoluminescence spectroscopy and microscopy were employed to track the evolution of singlet and triplet excitons with nanometer-scale resolution. The diffusion coefficient of singlet excitons was found to be two orders of magnitude larger than that of triplet excitons, reflecting distinct transport mechanisms: Förster transfer for singlets and Dexter electron exchange for triplets. Crucially, the mean square displacement (MSD) of excitons initially increased but showed a transient decrease around 60 ns, indicating negative diffusion. To elucidate the origin of this behavior, we developed a kinetic model composed of singlet and triplet excitons, which are interconverted by intersystem crossing (ISC) and RISC processes. The simulation quantitatively reproduces the experimental MSD profile and demonstrates that interconversion between excitons with distinct diffusivities gives rise to negative diffusion, in the absence of nonlinear interactions such as triplet–triplet annihilation. These results provide a new mechanistic basis for controlling exciton transport in solid-state materials and indicate that negative diffusion is a ubiquitous feature in systems where mobile and immobile excitons dynamically interconvert.