Disentangling Direct Coupling from Charge-Transfer-Mediated Coupling in Singlet Fission of Weakly Interacting Dimers

The efficiency of the excitation energy transfer between molecules is driven by the coupling of their excited states. However, experimental details about such excitonic coupling often remain elusive, partly because of the involvement of dark states, limiting its characterization. This is most apparent within singlet fission (SF), where a bright singlet state transforms into two triplet states, mediated by dark states: the correlated triplet-pair and charge-transfer (CT) states. We address this lack of direct experimental characterization of excitonic coupling in intramolecular SF (i-SF) by applying transient two-dimensional electronic spectroscopy to spiro-conjugated dimers. Specific modification of the dihedral arrangements of the monomeric units affects i-SF dynamics, resulting from alterations in the excitonic coupling strengths. Our findings support a primarily direct i-SF mechanism, facilitated by weak, yet direct excitonic coupling between singlet- and triplet-pair states, that decreases with increasing dihedral angle, while superexchange coupling via the dark CT states increases. Through these insights, we provide experimental access to the interaction between bright singlet and dark intermediate states, which has previously been characterized primarily through theory.