Thermal Annealing of Singlet Fission Microcrystals Reveals the Benefits of Charge Transfer Couplings and Slip-Stacked Packing

Defects in molecular crystals, created by nonequilibrium crystal packing, can have significant influence over the energetics and dynamics of singlet fission. Though normally thought of as detrimental to the performance of organic semiconductors, in principle, defects could have crystal packings with better energetics and couplings for singlet fission than equilibrium crystal packing structures. Using two-dimensional electronic microscopy and X-ray diffraction, we monitor the population of non-equilibrium slip-stacked geometries in microcrystals of TIPS-pentacene before and after thermal annealing. We find that the presence of these slip-stacked structures enhances the singlet fission yield, as measured by triplet excited state absorption. The slip-stacked geometry matches no known polymorph of TIPS-pentacene, according to X-ray diffraction but indicates that slip-stacked molecules are present as tiny crystalline domains within the larger microcrystal. The data presented here provide evidence that slip-stacked geometries, present as defects in larger TIPS-pentacene microcrystals, enhance singlet fission yields.