Sequence-dependent generation of pancake π-dimer anions with peptide-flanked perylene diimides

In contrast to weak noncovalent interactions between π-conjugated molecules, the strong multi-centered/two-electron intermolecular π-π interaction referred to as a "pancake" bond results from the constructive intermolecular π-orbital overlap of two singly-occupied molecular orbitals on adjacent chromophores. Pancake bonding is not only interesting as a novel chemical bonding behavior, it is a potential carrier trapping mechanism within organic semiconductor materials. We report on the formation kinetics of pancake π-dimer dianions following photoinduced electron transfer from anthracene carboxylate donors to aggregates of peptide-functionalized perylene diimide (PDI) in buffered aqueous solution. Pancake dimer ion formation is sensitive to the sequence of the solubilizing oligopeptide tethered to the PDI imide positions: positioning an alanine or glycine residue adjacent to PDI stifles or enables, respectively, the frontier-orbital overlaps that stabilize pancake π-π bonding in these species, as corroborated with DFT-computed relative bonding stabilities for PDI dimer dianions. We track photoreduction mechanisms using time-resolved absorption spectroscopy, observing that dimer anions are formed through the interaction of singly-charged monomers with temperature-dependent studies revealing a low energy barrier to π-dimer anion formation. We observe a slower build-up of PDI-peptide anions and π-dimer dianions using 9-anthracene carboxylic acid as the donor in aerated solutions and under "dark" conditions (after irradiation) compared to what is observed with argon-sparged solutions. We attribute this difference to the photooxidation of this donor, which generates long-lived redox-active species that are photoreduced during irradiation and subsequently shuttle electrons to PDI-peptides through a redox cascade that exceeds the duration of light exposure. Our findings illustrate how the structure-specificity of peptide scaffolds can be used to support novel chemical interactions in molecular aggregates.