Molecular tweezer and clip in aqueous solution:: Unexpected self-assembly, powerful host-guest complex formation, quantum chemical 1H NMR shift calculation

The newly prepared water-soluble naphthalene tweezer 2a and anthracene clip 4a (substituted both with lithium methanephosphonate groups in the central spacer unit) undergo an unexpected self-assembly in aqueous solution. The highly ordered intertwined structures of the self-assembled dimers [2a](2) and [4a](2) were elucidated by quantum chemical H-1 NMR shift calculations. 2a and 4a form extremely stable host-guest complexes with N-methylnicotinamide 8 in methanol and water as well. According to the thermodynamic parameters determined by 1H NMR titration experiments at various temperatures the self-assembly of 2a and 4a and their strong binding to NMNA 8 observed in aqueous solution are enthalpy driven (Delta H << 0); the enthalpic driving force is partially compensated by an unfavorable entropy (T Delta S < 0). Self-assembly and the host-guest binding are therefore beautiful examples of the nonclassical hydrophobic effect.