Novel aromatic ion--pairs: Synergy between electrostatics and pi-face aromatic interactions.

摘要

This dissertation focuses on the design and study of charged aromatic molecules where weak pi-pi interactions synergize with electrostatic interactions to enhance the overall interaction between aromatic moieties. Each chapter investigates some aspect of this hypothetical synergy between electrostatics and pi-face aromatic cohesion.;The first chapter unveiled the importance of electrostatics in the intramolecular stacking of flexible aromatic molecular templates 1-2Br and 2a. While our previous studies found dicationic molecular template 1-2Br to have intramolecular pi-stacking between electron poor pyridinium and electron rich xylylene moieties, no such stacking interaction was observed in the neutral analog 2a.;Chapter two systematically explored the stacking pattern of electron poor aromatics in the form of oxygen- and / or nitrogen- substituted triangulenium cations, [1(NR)3]+ and [1(O) 3(OH)3]+. As indicated in the chemical literature, triazatriangulenium cations [1(NR)3]+ with N- ethyl (and longer alkyl chains) chains were found to pack as face-to-face dimers. This study found the formation of columnar, face-to-face, n-meric association between aromatic cations in the structures with decreased steric interactions of the side chains in the stacking planes ([1(NMe)3]+ and [1(O) 3(OH)3]+). Similar iso-structural triangulene based aromatic anions, (2)- and (3) 2- didn't indicate any facial interactions in the solid states.;The possible synergy between unit charge electrostatics and pi-face aromatic interactions was explored in aromatic ion pairs 1·2 of triangulene based aromatic cations and aromatic anions. This charge-assisted pi-pi stacking seems to be the novel way of getting strong pi-system interactions where the strongest non-covalent force and the weakest non-covalent force: ionic bonding and pi-stacking respectively synergize together. The pi-pi interaction between ionic aromatics in the solid state was investigated by means of single crystal x-ray diffraction and powder x-ray diffraction (PXRD). The interaction in the solution state was examined by UV-Vis spectroscopy, electrospray ionization mass spectroscopy (ESI-MS) and electrochemical studies. Studies found that optimal synergy was possible only in the ion pairs with no steric interactions of alkyl (or aryl) side chains in the stacking planes (1(O)3·2 & 1(NMe) 3·2) and the interaction was found to be comparable with the strongest radical-assisted pi-stacking described in the chemical literature.;KEYWORDS: Aromatic ion-pairs, MO (Molecular Orbital) calculation, radical assisted pi-stacking, charge-assisted pi-stacking, ion-pairing energy.