Impact of Ring-Fusion on the Excited State Decay Pathways of N-Annulated Perylene Diimides

N-Annulated perylene diimide (PDI) dimers offer added advantages over conventional (non-annulated) dimers or S/Se-annulated dimers due to the two extra positions of functionalization that they provide, which can be used to improve the solution processability, impact the packing and self-assembly of the molecules in thin films, and tune the absorption and emission energies. In this work about N-annulated thiophene π-bridged PDI dimers, we present a unique strategy to use ring-fusion to highlight the different excited state decay pathways taking place in these systems. Two analogous N-annulated PDI dimers were investigated, whose structures show a rotatable single-bond connection with the thiophene π-bridge (M1) relative to a rigid fused π-bridge connection (M1F). We show that ring-fusion of the M1F dimeric system yields ultrafast intramolecular charge transfer and charge separation, relative to the unfused dimer. For M1F—the fused N-annulated thiophene π-bridged PDI dimer—the charge separation rate increased by over 1 order of magnitude compared to the charge separation rate of M1. Also, ring-fusion opened up the triplet excited state deactivation pathway that was not present in the unfused dimer. The rate of triplet production was shown to be dependent on the polarity of the solvent environment for the fused M1F dimer. We were able to track the respective excited state species (PDI*, PDI·⁺, PDI·–, PDI³*) generated upon photoexcitation, using both femtosecond and nanosecond time resolution to obtain a complete description of the entire activation and deactivation pathways. Nonlinear two-photon absorption measurement was used to provide insights into the excited state charge transfer properties of the compounds. The results were able to show that ring fusion leads to increased electronic coupling between the PDI units of the dimer, which in turn led to an increased charge separation rate and dictated the production of long-lived triplet species. Also, this detailed study points out that ring fusion in N-annulated PDIs can provide excellent π-conjugation, increased electronic coupling, and limited aggregation effects.