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(S-1) > 2E(T-1), 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.