Interplay between Intermolecular and Intramolecular Singlet Fission in Thin Films of a Covalently Linked Terrylenediimide Dimer

Covalent chromophore dimers having the required energetics can undergo intramolecular singlet fission (SF) in solution; however, in the solid state, intra- and intermolecular SF can compete. Here, the structure and excited-state dynamics of a linear terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer, TDI-Ph-TDI, in which the two TDI molecules are linked via one of their imide nitrogen atoms to a 2,5-di-t-butylphenyl spacer at its 1,4-positions are studied in solution and in thin films to understand the interplay between these two SF mechanisms. TDI-Ph-TDI undergoes slow intramolecular SF in toluene due to weak through-bond interactions and a lack of through-space electronic coupling. The TDI-Ph-TDI dimers in the films are π-stacked, allowing for through-space interactions between neighboring TDI moieties. As a result, TDI-Ph-TDI undergoes intermolecular SF two orders of magnitude faster than intramolecular SF. Using film-processing techniques, the SF dynamics can be modified. The (T1T1) states in the unannealed and thermally annealed films dissociate to form free triplet excitons, whereas a long-lived (T1T1) state with mixed charge-transfer character is observed in a chlorobenzene solvent vapor annealed film. Although intramolecular SF in TDI-Ph-TDI cannot compete with intermolecular SF, the structure of TDI-Ph-TDI has a strong influence on the possible film morphologies and the role of the charge-transfer state in SF.