Fujisawa K Humbert-Droz M Letrun R Vauthey E Wesolowski TA Sakai N Matile S 2015 We report that anion-π and cation-π interactions can occur on the same aromatic surface. Interactions of this type are referred to as ion pair-π interactions. Their existence, nature, and significance are elaborated in the context of spectral tuning, ion binding in solution, and activation of cell-penetrating peptides. The origin of spectral tuning by ion pair-π interactions is unraveled with energy-minimized excited-state structures: The solvent- and pH-independent red shift of absorption and emission of push-pull fluorophores originates from antiparallel ion pair-π attraction to their polarized excited state. In contrast, the complementary parallel ion pair-π repulsion is spectroscopically irrelevant, in part because of charge neutralization by intriguing proton and electron transfers on excited push-pull surfaces. With time-resolved fluorescence measurements, very important differences between antiparallel and parallel ion pair-π interactions are identified and quantitatively dissected from interference by aggregation and ion pair dissociation. Contributions from hydrogen bonding, proton transfer, π-π interactions, chromophore twisting, ion pairing, and self-assembly are systematically addressed and eliminated by concise structural modifications. Ion-exchange studies in solution, activation of cell-penetrating peptides in vesicles, and computational analysis all imply that the situation in the ground state is complementary to spectral tuning in the excited state; i.e., parallel rather than antiparallel ion pair-π interactions are preferred, despite repulsion from the push-pull dipole. The overall quite complete picture of ion pair-π interactions provided by these remarkably coherent yet complex results is expected to attract attention throughout the multiple disciplines of chemistry involved. application/pdf https://sonar.ch/global/documents/149696 eng info:eu-repo/semantics/altIdentifier/doi/10.1021/jacs.5b05593 info:eu-repo/semantics/altIdentifier/pmid/26291550 info:eu-repo/semantics/openAccess Journal of the American Chemical Society. - 2015 Cell-Penetrating Peptides Hydrocarbons, Aromatic Hydrogen-Ion Concentration Ions Molecular Structure Ion Pair-π Interactions. http://purl.org/coar/resource_type/c_6501