Observation of an emissive intermediate in a liquid singlet fission and triplet fusion system at room temperature

The photophysical processes of singlet fission and triplet fusion have numerous emerging applications. They respectively involve the separation of a photo-generated singlet exciton into two dark triplet excitons and the fusion of two dark triplet excitons into an emissive singlet exciton. The role of the excimer state, and the nature of the triplet-pair state in these processes have been a matter of contention. Here we carefully analyse the room temaperature time-resolved emission of a neat liquid singlet fission chromophore, 5,12-bis(n-octyldiisopropylsilylethynyl)tetracene. It is demonstrated to exhibit three spectral components: Two that correspond to the bright singlet and excimer states, and a third component that becomes more prominent during triplet fusion. A spectrum consistent with this third compoment is found to be enhanced with magnetic fields, confirming its origins in radiation from weakly-coupled triplet pairs. This magnetically enhanced emission is attributed to the emission from the strongly coupled triplet pair state. These observations serve to unite the view that there is an emissive intermediate in singlet fission and triplet fusion, and that this species is distinct from the broad, unstructured excimer emission.