Singlet Fission in Quaterrylenediimide Thin Films

Singlet fission (SF) creates two triplet excitons following absorption of a photon by two electronically interacting chromophores. Quaterrylene-3,4:13,14-bis(dicarboximide) (QDI) is a strongly absorbing chromophore that readily fulfills the energetic requirements for SF, E(S₁) > 2E(T₁), and thus should undergo rapid and efficient SF. SF was studied in thin films of the QDI derivative N,N-bis(2,6-diisopropylphenyl)-QDI (ArQDI), which undergoes SF in <300 fs to form the correlated triplet pair state, ¹(T₁T₁), which dissociates with a (7.3 ± 1.2 ns)^-1 rate constant. The observed triplet yield for a thin film that has been solvent-vapor annealed with CH₂Cl₂ is 135 ± 20% instead of 200%, which is typically expected of chromophores that undergo ultrafast formation of the ¹(T₁T₁) state. The lower SF yield in ArQDI results from the failure of the ¹(T₁T₁) state to dissociate before returning to the ground state. In contrast to other molecules, like hexacene, which have low triplet energies, the SF rate in ArQDI is not limited by a multiphonon relaxation bottleneck, largely due to the fact that the S-T energy gap in the film is substantially smaller than that measured for monomeric ArQDI. The ability to maintain a favorable S-T energy gap in a film is a design consideration when chromophores are considered for use to enhance solar cell performance.