1. What are the contributions in this paper?
The authors report that anion−π and cation−π interactions can occur on the same aromatic surface.
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Ion Pair−π Interactions
Kaori Fujisawa,Marie Humbert-Droz,Romain Letrun,Eric Vauthey,Tomasz Adam Wesolowski,Naomi Sakai,Stefan Matile +6 more
TL;DR: It is reported that anion-π and cation-π interactions can occur on the same aromatic surface and their existence, nature, and significance are elaborated in the context of spectral tuning, ion binding in solution, and activation of cell-penetrating peptides.
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Abstract: 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.
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Figures
Figure 6. Covalent system 3a for antiparallel ionpair-π interactions with sulfonate-tetraalkylammonium pairs to explore contributions from π-π interactions and hydrogen bonding. 
Table 3. Computational Data for Parallel and Antiparallel Ionpair-π Interactions.a 
Table 1. Steady-State Spectroscopic Properties of Parallel and Antiparallel Ionpair-π Interactions.a 
Figure 12. Fractional CF emission intensity IF (lex = 492 nm, lem = 517 nm) is shown during the addition of pyrenebutyrate (grey, dotted), 4a (blue, dashed) and 4p (red, solid, 2 µM final concentration) after 50 s (a) and pR (0.25 µM) after 100 s (b) to EYPC-LUVsÉCF (50 mM CF, 10 mM Na+ phosphate, 10 mM NaCl, pH 7.4, 25 ºC; calibrated by final addition of triton X-100 (c)). 
Figure 8. Time-resolved fluorescence decay of 1a (a), 1p (b) and 1o (blue) in CCl4 under neutral (red) and acidic conditions (green). Results from data analysis are listed in Table 2 (entries 1, 3-5). 
Figure 10. Semi-covalent systems 4a and 4p (M+: triethylammonium) for antiparallel and parallel binding of counterion 4c (A-: PF6-) and 5a and 5p (A-: Cl-) for antiparallel and parallel binding of counterion 5c (M+: triethylammonium) by ionpair-π interactions. Shifts in the 1H NMR spectra in response to ion binding are indicated in Hertz (compare Figure 11).
Citations
Functional Naphthalene Diimides: Synthesis, Properties, and Applications
TL;DR: This comprehensive review surveys developments over the past decade in the field of naphthalene diimides and explores their application toward: supramolecular chemistry; sensors; host-guest complexes for molecular switching devices; ion-channels by ligand gating; gelators for sensing aromatic systems; catalysis through anion-π interactions; and NDI intercalations with DNA for medicinal applications.
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Experimental Binding Energies in Supramolecular Complexes
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Anion Receptor Chemistry
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Experimental investigation of anion–π interactions – applications and biochemical relevance
TL;DR: The present feature article summarizes the experimental findings of anion-π studies in the gas phase, solution and in the solid state and highlights the application of this weak intermolecular force in anion recognition, sensing and transport as well as in catalysis.
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Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers.
Paola Peluso,Bezhan Chankvetadze +1 more
TL;DR: This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences.
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