Charge-Transfer Biexciton Annihilation in a Donor-Acceptor Co-crystal yields High-Energy Long-Lived Charge Carriers

Organic donor-acceptor (D-A) co-crystals have attracted much interest due to their important optical and electronic properties. Co-crystals having ···DADA··· π-stacked morphologies are especially interesting because photoexcitation produces a charge-transfer (CT) exciton, D•+-A•-, between adjacent D-A molecules. Although several studies have reported on the steady-state optical properties of this type of CT exciton, very few have measured the dynamics of its formation and decay in a single D-A co-crystal. We have co-crystallized a peri-xanthenoxanthene (PXX) donor with a N,N-bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) (Ph4PDI) acceptor to give an orthorhombic PXX-Ph4PDI ···DADA··· π-stacked co-crystal with a CT transition dipole moment that is perpendicular to the transition moments for Sn ← S0 excitation of PXX and Ph4PDI. Using polarized, broadband, femtosecond pump-probe microscopy, we have determined that selective photoexcitation of Ph4PDI in the single co-crystal results in CT exciton formation within the 300-fs instrument response time. At early times (0.3 8 ns) electron-hole pairs in the crystal. These energetic charge carriers may prove useful in applications ranging from photovoltaics and opto-electronics to photocatalysis.