Cascade of Multiexciton States Generated by Singlet Fission.

Identifying multiexciton states generated from singlet fission is key to understanding the carrier multiplication process, which presents a strategy for improving the efficiency of photovoltaics and bioimaging. Broadband optically detected magnetic resonance (ODMR) is a sensitive technique to detect multiexciton states. Here, we report a dominant species a weakly exchange coupled triplet pair located on adjacent molecules oriented by nearly 90° (V2) under intense light excitation, contrasting to the π-stacked triplet pair under low excitation intensity. The weakly coupled species model precisely reproduces the intricate spin transitions in the Hilbert space of the triplet pair. Combining the magneto-photoluminescence and high-magnetic-field ODMR, we also identify a strongly exchange-coupled state of three triplet excitons formed by photoexcited V2, which manifests through the magnetic-field-induced level crossings between its quintet and triplet manifolds. Our findings demonstrate the microscopic picture of different multiexciton states and the possible transitions among them during exciton fission, which provide insight for the further molecular designs for fission materials.