The Doubly Excited State in Singlet Fission

In recent years, a variety of studies have investigated in great depth the details of singlet fission mechanisms in terms of electronic energies and couplings. The nature and properties of the triplet-pair intermediate state or the role of charge separated configurations (CT states) have been extensively explored with the help of theoretical models, ab initio calculations, and experimental characterization. On the other hand, much less is known about the involvement of the localized double electron excitation, that is the D state. In organic conjugated molecules, the D state can be typically described as the HOMO-to-LUMO promotion of two electrons, which makes it inactive to photoabsorption (dark state). In the present study, we tackle the possible roles of the D state by means of electronic structure calculations and the simulation of singlet fission dynamics. Our results suggest that, indeed, the D state can have a significant impact on singlet fission. On one hand, if it lies energetically below the optical exciton it is able to drive the system to singlet fission through direct coupling or via a CT-mediated path. The D state can also act as a mediator in the formation of the triplet-pair state from initial photoactivation. These results should motivate researchers in the field to include this state in their analysis, and help to clarify electronic mechanisms in singlet fission materials.