Symmetry-Breaking Charge Separation in the Solid State: Tetra(phenoxy)perylenediimide Polycrystalline Films

Generation of electron-hole pairs via symmetry-breaking charge separation (SB-CS) in photoexcited assemblies of organic chromophores is a potentially important route to enhancing the open-circuit voltage of organic photovoltaics. While most reports of SB-CS have focused on molecular dimers in solution where the environmental polarity can be manipulated, here, we investigate SB-CS in polycrystalline thin films of 1,6,7,12-tetra(phenoxy)perylene-3,4:9,10-bis(dicarboximide) having either n-octyl groups (octyl-tpPDI) or hydrogen atoms (H-tpPDI) attached to its imide nitrogen atoms. Structural analyses using various X-ray techniques reveal that while both compounds show π-π stacking in thin films, H-tpPDI is more slip-stacked than octyl-tpPDI and has intermolecular hydrogen bonds to its neighboring molecules. Transient absorption spectroscopy shows that octyl-tpPDI exhibits strong mixing between its singlet excited state and a charge transfer state, yielding an excimer-like state, while H-tpPDI undergoes nearly quantitative SB-CS, making the latter a promising candidate for use in organic photovoltaic devices.