Probing the Interfacial Electronic Density of States of Perylenediimide Thin Films using Electronic Sum-Frequency Generation

Perylenediimide dyes (PDIs) are pigments known for crystallochromy, photophysical stability, and an ability to undergo exciton multiplication through singlet fission. The solid-state PDI band structure is intimately connected to local intermolecular coupling and is well described using a tight-binding Holstein Hamiltonian. Moreover, the intermolecular structure of PDI solids also modulates their ability to transport energy and engage in singlet fission. However, interfaces formed between PDI solids and other materials presents a different environment from the bulk, with potentially different properties. To use PDIs in novel organic-inorganic hybrid devices, the PDI-substrate interface must be understood.

Along with bulk structural and spectroscopic analyses, we probed the interfacial density of states of PDI films using electronic sum frequency generation (ESFG). Our ESFG spectra show a narrowed bandgap (~80 meV) at interfaces formed between PDI solids and oxide substrates, which is qualitatively reproduced by subtle changes in PDI intermolecular structure at the interface induced by strain. These findings also suggest there is an energy gradient that could move excitons towards PDI-substrate interfaces, which could enable several singlet fission-based electronics.