Promoting singlet fission over exciton self-trapping by altering the packing morphology of anthradithiophene derivatives

When considering the criteria for singlet fission in solid-state thin-film organic photovoltaics, particular packing motifs play a significant role by modulating intermolecular electronic coupling. Using single-crystal ultrafast microscopy methods, we isolate the packing configuration that best promotes endoergic singlet fission.  Specifically, four fluorinated anthradithiophene (diF R-ADT) crystals are grown with differing functionalized derivatives [and packings]:  R = TES [brickwork], TSBS [sandwich herringbone], TDMS [brickwork] and TBDMS [twisted columnar]. In each ADT packing morphology, both the dynamic and spectral signatures of correlated triplet pair state (TT) formation are measured over a broad temperature range (1.5 to 295 K).  By varying the applied B-field (0 to 7 T), the photoluminescence emission and excitation spectra clearly isolate TT-state emission peaks for all ADT-crystals except sandwich herringbone.  Matching the singlet state depletion rate, a corresponding 70 ps rise in the TT population is prominent whenever the probe beam is polarized along the crystal CT-axis.  Conversely, ADT derivatives with a sandwich-herringbone packing morphology show no spectral or dynamic signatures of TT formation due to a preferential exciton self-trapping pathway.