Ion Pair-π Interactions.
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.
School of Chemistry and Biochemistry, University of Geneva , CH-1211 Geneva, Switzerland.
Kaori
K
Fujisawa
School of Chemistry and Biochemistry, University of Geneva , CH-1211 Geneva, Switzerland.
Marie
M
Humbert-Droz
School of Chemistry and Biochemistry, University of Geneva , CH-1211 Geneva, Switzerland.
Romain
R
Letrun
School of Chemistry and Biochemistry, University of Geneva , CH-1211 Geneva, Switzerland.
Eric
E
Vauthey
School of Chemistry and Biochemistry, University of Geneva , CH-1211 Geneva, Switzerland.
Tomasz A
TA
Wesolowski
School of Chemistry and Biochemistry, University of Geneva , CH-1211 Geneva, Switzerland.
Naomi
N
Sakai
School of Chemistry and Biochemistry, University of Geneva , CH-1211 Geneva, Switzerland.
Stefan
S
Matile
D057846:Cell-Penetrating Peptides / Q000737:chemistry*; D006841:Hydrocarbons, Aromatic / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007477:Ions / Q000737:chemistry; D015394:Molecular Structure
D057846:Cell-Penetrating Peptides; D006841:Hydrocarbons, Aromatic; D007477:Ions