Cascade of Multiexciton\nStates Generated by Singlet\nFission

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