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 aperi-xanthenoxanthene (PXX) donor with aN,N-bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) (Ph₄PDI) acceptor to give an orthorhombicPXX-Ph₄PDI⋯DADA⋯ π-stacked co-crystal with a CT transition dipole moment that is perpendicular to the transition moments for S_n← S₀excitation ofPXXandPh₄PDI. Using polarized, broadband, femtosecond pump-probe microscopy, we have determined that selective photoexcitation ofPh₄PDIin the single co-crystal results in CT exciton formation within the 300 fs instrument response time. At early times (0.3 ≤t≤ 500 ps), the CT excitons decay with at^−1/2dependence, which is attributed to CT biexciton annihilation within the one-dimensional ⋯DADA⋯ π-stacks producing high-energy, long-lived (>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.