Modulating Excited-State Charge Transfer Character in 1,6,7,12-Tetrakis( p -X-phenoxy)perylene-(3,4:9,10)-bis(dicarboximide) Single Crystals

Abstract Enhancing the excited-state charge transfer character in homogeneous organic solids provides an avenue to promote electron−hole pair generation between identical chromophores, potentially enabling higher efficiencies in organic photovoltaics. Previous studies have demonstrated that chromophore orientation has a significant effect on the propensity for complete charge separation in the solid state, but the interplay between electronic and steric effects within similar crystal packing regimes remains an open question. In this study, we compare single crystals of three 1,6,7,12-tetrakis(p-X-phenoxy)perylene-(3,4:9,10)-bis(dicarboximide)s, where X = Br, CF3, and t-butyl, Br-tpPDI, CF3-tpPDI, and tBu-tpPDI, respectively, which are found to have similar extended packing structures but sub-Å differences in slip-stacking distances between neighboring chromophores. Transient absorption microscopy shows the formation of a mixed Frenkel exciton/charge transfer (CT) excimer in all three derivatives. We find that the crystal structure of CF3-tpPDI possesses a dimer pair with a favorable geometry that accentuates the excited-state polarizability, and thus, this derivative possesses the highest degree of CT character in the mixed state. These results show that the core substituent properties of these PDI chromophores direct excited-state evolution, providing a design criterion for optimizing charge separation in solid-state organic materials.