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 1H 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 (ΔH ≪ 0); the enthalpic driving force is partially compensated by an unfavorable entropy (TΔS < 0). Self-assembly and the host−guest binding are therefore beautiful examples of the nonclassical hydrophobic effect.