Solvophobically-Driven Oligo(ethylene glycol) Helical Foldamers. Synthesis, Characterization, and Complexation with Ethane-1,2-diaminium

Oligo(ethylene glycols) 1a−h, which are incorporated with one to eight 2,3-naphthylene units,
respectively, have been synthesized and characterized. The conformational changes of the new
oligomers have been investigated in chloroform−acetonitrile binary solvents by the UV−vis, 1H
NMR, and fluorescent spectroscopy. It has been revealed that the naphthalene units in hexamer
1f, heptamer 1g, and octamer 1h are driven by solvophobic interaction to stack in polar solvents.
As a result, compact helical conformations are formed that give rise to a cavity similar to that of
18-crown-6. Shorter oligomers 1b−e exhibit weaker folding tendency. 1H NMR studies reveal that
1f−h are able to complex ammonium or ethane-1,2-diaminium 19, but not secondary ammonium
compounds. The association constants of complexes 1f·19, 1g·19, and 1h·19 in acetonitrile are
determined to be 3.5(±0.4) × 103, 1.0(±0.12) × 104, and 2.5(±0.4) × 104 M-1, respectively, with the
1H NMR titration method. For comparison, hexamer 22, which incorporates six 1,5-naphthylene
units, is also prepared. The UV−vis and fluorescent investigations show that 22 is also able to fold
in polar solvents, but no helical structure can be produced due to mismatch of the stacking
naphthalene units and consequently there is no obvious complexation between 22 with ethane-1,2-diaminium ion. The structures of the longest foldamer 1h and its complex with 19 have been
studied with molecular mechanics calculations. This work represents a new approach to building
folding conformations from flexible linear molecules.