Functionalization of the helix interior can influence binding strength and selectivity. With internal cyano groups, mPE foldamers could bind metal ions such as Ag(I) . Oligo(m-ethynylpyridine) was shown by Inouye and colleagues to fold in the presence of certain monosaccharides . The state of ionization was found later to strongly influence the propensity of these oligomers to bind saccharides and fold . Details can be found in Chapter 7 of this book.
Folding is not limited to meta-connectivity. When three ortho-linked phenylene ethynylene (oPE) oligomers were studied by Tew and colleagues using 1H NMR spectroscopy, oligomers with as few as four repeat units (28-30) were found to adopt folded conformations in acetonitrile . Folding was partly enabled by the 60° internal angle given by the ortho linkage, which made it possible to create a turn with only three repeat units. Electronic effects were also important in these structures, as the oligomer (i.e. 30) consisting of electron-deficient rings and the one (i.e. 29) with both electron-deficient and electron-rich rings folded better than 28, which only contained electron-rich aromatic rings. oPE polymers were synthesized and studied by Khan and Hecht . Optical spectroscopy indicated transition between extended transoid and helical cisoid conformations induced by solvents, providing evidence for solvophobically driven folding in the oPE backbone.
Ethynylhelicene oligomers 31n were studied by Yamaguchi et al. using CD and NMR spectroscopy . Helical conformations were identified in oligomers with n > 7. Solvents had a tremendous effect on the kinetics of the conformational change, with the rate of unfolding correlating well with the polarizability of aromatic solvents. VPO studies indicated the folded helix to be dimeric and the unfolded coil monomeric. Due to different aggregations in the two conformers, the helical conformation could also be promoted by higher concentration of 31n.
Rigid linkers are not always needed to construct foldamers stabilized by non-adjacent aromatic units. Oligonaphthalene 32 prepared by Li, Chen, and others displayed hypochromism in acetonitrile, a solvent that promoted aromatic association, but not in chloroform, a solvent that disrupted aromatic association . Stacked conformation was also supported by upfield-shifted proton signals and appearance of an excimer-like emission band. When the fraction of acetonitrile was increased in a mixture with chloroform, sigmoidal curves characteristic of cooperativity were observed for these foldamers. Interestingly, upon folding, 32 formed a central cavity resembling crown ethers or kryptands and could be used to bind ethane-1,2-diammonium salt in a 1:1 ratio (see also Chapter 7).
The same group also reported 33, which had the donor and acceptor units in two blocks instead of in an alternating order . These oligomers folded like a molecular zipper (see also Chapter 4). The folded conformer was quite stable, dominant in both relatively nonpolar solvents (chloroform with 0.5% CF3COOH) and polar aprotic solvents such as DMSO and DMF. For the longer oligomers (m = n = 2), the orange color from the intramolecular CT complexation was maintained at up to 150 ° C in DMF.
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