For the construction of functional systems, a surprising limited number of primary interactions between molecules is available.  Hydrogen bonds, hydrophobic interactions, ion pairing, aromatic (π, π) interactions and cation-π interactions can be named as basic set, comparable to notes to compose music, letters to write or colors to paint.  The addition of more colors to the chemist's palette is of fundamental importance.  We entered this field when we discovered that synthetic transport systems can serve as unique analytical tools to elaborate on interactions that are otherwise difficult to detect.  With this approach, experimental evidence for the functional relevance of anion-π interactions and halogen bonds was secured.  These interactions are the underrecognized counterparts of cation-π interactions and hydrogen bonds.

Anion-π interactions attract much interest in theory but remain difficult to be seen at work for several reasons.  We realized that naphthalenediimides (NDIs) are ideal to study anion-π interactions because their quadrupole moment is exceptionally positive (the strongest organic π-acids known today, much stronger than TNT, but not explosive).  Increasing transport activity with increasing π-acidity demonstrated that anion-π interactions account for ion transport.  Similar trends were found for calixpyrroles (in collaboration with Pablo Ballester, Tarragona) and calixarenes.

Calixarene scaffolds were attractive because it was possible to switch from anion-π interactions to halogen bonds without global structural changes, by single-atom mutations.  Halogen bonds - strong and directional like hydrogen bonds but more hydrophobic - turned out to be ideal for transport (in collaboration with Pierangelo Metrangolo and Giuseppe Resnati, Milano).  Their transport power is arguably best expressed with trifluoroiodomethane, the smallest possible organic anion transporter, a gas, bp = -22 ºC, that can be bubbled through a solution to turn on transport.  To maximize activities, calixarenes were unrolled into transmembrane halogen-bonding cascades for cooperative anion hopping.

Other exotic interactions explored with synthetic transport systems include anion-macrodipole interactions (with Gilles Guichard, Bordeaux), aromatic donor-acceptor interactions (to open and close ion channels, sensors and photosystems) as well as dynamic covalent bonds (disulfide and hydrazone bridges for sensing and cellular uptake).  Current efforts focus on the application of the lessons learned to catalysis, because there is no reason to believe that, with evidence for ground-state stabilization of anions in hand, anionic transition states could not be stabilized as well.  The first example for anion-π catalysis has just been published.

Introductory Review:  Acc. Chem. Res. 2013, 10.1021/ar400014r.