Reimagining singlet exciton fission: Insights from dimer architectures

Singlet fission (SF) is an ultrafast bimolecular photophysical process in which a high-energy singlet excited state redistributes its energy into two lower-energy unbound triplet states, allowing a theoretical triplet quantum yield of up to 200%. Extensive studies over the past decade and a half have shown that SF proceeds through the formation of a spectroscopically detected correlated triplet-triplet pair (TT) intermediate, which subsequently dissociates to form the requisite two independent triplet excitons. Efficient separation and utilization of these triplets requires a detailed understanding of their generation and extraction in molecular aggregates and thin films. However, limited control over film morphology and intermolecular packing often makes it difficult to systematically tune SF rates. In this perspective, we summarize mechanistic insights gained from steady-state and time-resolved spectroscopic investigations of well-defined dimeric chromophore systems. We highlight the key relaxation and loss pathways that emerge subsequent to formation of the TT state, which can lower the effective triplet yield to less than 100% undoing the opportunities that SF presents. We further discuss strategies to promote the dissociation of TT pair into free triplets rather than undergoing annihilation processes, even within such dimeric architectures. Finally, although triplet extraction from dimers is commonly viewed as inefficient, we outline possible approaches for enabling effective harvesting of triplet excitons in relevant timescales from these molecular architectures.