Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films

Singlet
exciton fission (SF) in organic chromophore assemblies
results in the conversion of one singlet exciton (S1) into
two triplet excitons (T1), provided that the overall process
is exoergic, i.e., E(S1) > 2E(T1). We report on SF in thin polycrystalline
films of
two terrylene-3,4:11,12-bis­(dicarboximide) (TDI) derivatives 1 and 2, which crystallize into two distinct
π-stacked structures. Femtosecond transient absorption spectroscopy
(fsTA) reveals a charge-transfer state preceding a 190% T1 yield in films of 1, where the π-stacked TDI
molecules are rotated by 23° along an axis perpendicular to their
π systems. In contrast, when the TDI molecules are slip-stacked
along their N–N axes in films of 2, fsTA shows
excimer formation, followed by a 50% T1 yield.