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

Thermally activated delayed fluorescence (TADF) materials harvest triplet excitons via reverse intersystem crossing (RISC), enabling a 100% internal quantum efficiency. While RISC has been extensively studied, exciton spatial dynamics in solids remain underexplored. We directly visualized exciton diffusion in a crystal of a donor-acceptor TADF emitter, MA-TA, revealing a counterintuitive effective negative diffusion in which the exciton distribution narrows over time. Transient photoluminescence microscopy showed that singlet excitons diffuse 2 orders of magnitude faster than triplets, governed by Förster and Dexter mechanisms, respectively. The mean square displacement initially increased and then transiently decreased at ∼60 ns. A kinetic model incorporating singlet-triplet interconversion via intersystem crossing and RISC quantitatively reproduced the data, demonstrating that dynamic exchange between mobile and immobile excitons causes effective negative diffusion without nonlinear annihilation. The first experimental observation of effective negative diffusion in TADF materials not only extends the range of systems in which this phenomenon can emerge but also provides a mechanistic framework for controlling exciton transport.